AnaChem.Notes.(Chap1^J2^J34^J36)_121041.pdf

CHAPTER 1 1A | The Role of Analytical Chemistry THE NATURE OF ANALYTICAL CHEMISTRY Analytical Chemistry applied throughout industry,...

CHAPTER 1 1A | The Role of Analytical Chemistry THE NATURE OF ANALYTICAL CHEMISTRY Analytical Chemistry applied throughout industry, medicine, and all the sciences Examples: A measurement science consisting of a set of powerful ideas concentrations of oxygen and of carbon dioxide are and methods that are useful in all fields of science, determined in millions of blood samples every day and engineering, and medicine. used to diagnose and treat illness o Significance evident in: quantities of hydrocarbons, nitrogen oxides, and carbon NASA’s Mars rover explorations, where analytical monoxide present in automobile exhaust gases are instruments examined the chemical composition of measured to determine the effectiveness of emission- rocks and soil, suggesting Mars was once warm control devices and wet. quantitative measurements of ionized calcium in blood 2004 missions of Spirit and Opportunity rovers serum help diagnose parathyroid disease in humans further used analytical tools like APXS and quantitative determination of nitrogen in foods establishes Mossbauer spectrometer to discover silica and their protein content and thus their nutritional value carbonate deposits. etc. APXS (alpha particle X-ray spectrometer) - measures x-ray radiation and backscattered alpha spectra. These Quantitative analytical measurements also play a vital role in measurements can determine the elemental composition of many research areas in chemistry, biochemistry, biology, geology, the target on which it is docked. physics, and other sciences. Examples: Mossbauer spectrometer - a nondestructive technique which probes a specific element which may occupy one or more quantitative measurements of potassium, calcium, and crystallographic sites, may have one or more electronic sodium ions in the body fluids of animals permits configurations, and may or may not carry a magnetic moment. physiologists to study the role of these ions play in nerve- LIBS (laser-induced breakdown spectrometer) – provide signal conduction as well as muscle contraction and determination of many elements with no sample preparation. relaxation. It can determine the identity and amounts of major, minor, and Chemists unravel the mechanism of chemical reactions through trace elements and can detect hydrated minerals. reaction rate studies. The rate of consumption of reactants or Analysis package contains a quadrupole mass spectrometer, a formation of products in a chemical reaction can be calculated gas chromatograph, and a tunable laser spectrometer. Its goals from quantitative measurements made at precise time intervals. are: Material scientists – quantitative analyses of crystalline a) to survey carbon compound sources germanium and silicon in their studies of semiconductor devices b) search for organic compounds important to life whose impurities lie in the concentration range of 1 x 10^6 to 1 x c) reveal the chemical and isotopic states of several elements 10^-9 percent. d) determine the composition of the Martian atmosphere Archaeologists identify the sources of volcanic glasses e) search for noble gas and light element isotopes (obsidian) by measuring concentrations of minor elements in Both Qualitative and Quantitative information are required in an samples taken from various locations. This knowledge in turn analysis. makes it possible to trace prehistoric trade routes for tools and weapons fashioned from obsidian. Qualitative Analysis All branches of chemistry draw on the ideas and techniques of reveals the identity of the elements and compounds in a analytical chemistry. Analytical chemistry has a similar function sample with respect to the many other scientific fields listed in the diagram (Figure 1-1). Quantitative Analysis indicates the amount of each substance in a sample Analytes are the components of a sample that are determined Data from various spectrometers on the rovers contain both types of information. Gas chromatograph and mass spectrometer incorporate a separation step as a necessary part of the analytical process. APXS and LIBS experiments, chemical separation of the various elements contained in rocks is unnecessary since the methods provide highly selective information. Qualitative analysis is often an integral part of the separation step, and determining the identity of the analytes is an essential adjunct to quantitative analysis. Interdisciplinary nature of chemical analysis makes it a vital tool in medical, industrial, government, and academic laboratories throughout the world. Figure 1-1 In the measurement step, we measure one of the physical properties mentioned in Section 1B. In the calculation step, we find the relative amount of analyte present in the samples. In the final step, we evaluate the quality of the results and estimate their 1B | Quantitative Analytical Methods reliability. We compute the results pf a typical quantitative analysis from two 1C-1 Choosing a Method measurements. The first step in quantitative analysis is selecting a method, which One is the mass or the volume of sample being analyzed. can be challenging and requires experience. The selection Second is the measurement of some quantity that is influence by three main factors: proportional to the amount of analyte in the sample, such as 1. Accuracy Required: Higher accuracy often demands more mass, volume, intensity of light, or electrical charge. This time and resources, so a balance must be struck between the second measurement usually completes the analysis, and we desired accuracy and the available time and money. classify analytical methods according to the nature of this final 2. Number of Samples: When analyzing many samples, investing measurement. time in preparation (such as equipment calibration) is ▪ Gravimetric methods – determine the mass of the analyte or feasible. For fewer samples, a simpler method that minimizes some compound chemically related to it preliminary steps may be better. ▪ Volumetric method – the volume of the solution containing 3. Sample Complexity: The complexity of the sample and its sufficient reagent to react completely with the analyte is components also affects the choice of method. measured ▪ Electroanalytical methods – involve the measurement of 1C-2 Acquiring the Sample such electrical properties as potential, current, resistance, and quantity of electrical charge The next step in quantitative analysis is acquiring the sample that ▪ Spectroscopic methods – based on measurement of the matches the bulk material’s composition. This is challenging interaction between electromagnetic radiation and analyte when dealing with large, heterogeneous materials, like shipment atoms or molecules or on the production of such radiation by of silver ore, where a small sample must accurately reflect the analytes entire 25 tons. ▪ Group of miscellaneous methods [A material is heterogeneous if its constituent parts can be Mass spectrometry – mass-to-charge ratio of molecules distinguished visually or with the aid of a microscope.] Rate of radioactive decay Heat of reaction Sampling also presents challenges in biological contexts, such as Rate of reaction blood gas analysis, where variability and external factors can Sample thermal conductivity affect results. Strict procedures ensure samples remain Optical activity representative and maintain integrity until analysis. Refractive index Sampling is often the most difficult and error-prone step, as the reliability of the final results depends on the quality and representative of the sample. 1C | A Typical Quantitative Analysis 1C-3 Processing the Sample A typical quantitative analysis involves the sequences of steps shown in the flow diagram of Figure 1-2. In some instances, one This is the third step in an analysis and involves preparing the or two of these steps are omitted. For example, if the sample is sample for measurement. While some samples, such as water for already a liquid, we can avoid the dissolution step. pH measurement, require no processing, most samples need preparation. Preparing Laboratory Samples:  Solid Samples: These are ground to reduce particles size, 1C-7 Evaluating Results by Estimating Their Reliability mixed for homogeneity, and often dried to prevent changes in chemical composition due to moisture. Alternatively, moisture Analytical results are incomplete without an estimate of their content can be determined separately. reliability. The experimenter must provide some measure pf the uncertainties associated with computed results if the data are to Liquid Samples: These must be handled carefully to prevent have any value. evaporation or contamination, especially if they contain dissolved gases. Special precautions, like using sealed containers or an inert atmosphere, may be needed. 1D | An Integral Role For Chemical Analysis: Feedback Control Defining Replicate Samples: Systems Analyses are typically conducted on replicate samples to improve Analytical chemistry is often part of a larger process, helping to result quality and reliability. Replicate measurements are improve health, regulate product quality, control environment averaged, and statistical tests are performed to establish their contaminants, or extraterrestrial life. For example, in managing reliability. diabetes, quantitative analysis is used to monitor and control blood glucose levels. Patients determine their actual blood [Replicate samples, or replicates, are portions of a material of glucose state by taking a sample and comparing it to the desired approximately the same size that are carried through an analytical range (below 95 mg/dL). If the levels are too high, insulin is procedure at the same time and in the same way.] administered, and the glucose level is measured again after a Preparing Solutions: delay. This cycle of measuring, comparing, and adjusting is known as feedback loop, which maintains the desired state. Feedback Most analyses are performed on solutions. The solvent should systems like this are used across many fields, from biological dissolve the entire sample, including the analyte, without systems to industrial processes, where chemical analysis is key. causing loss. If the sample is insoluble in common solvents (e.g., silicate minerals, polymers, tissues), harsh chemical process (e.g., strong acids, bases, oxidizers, or high temperature fusion) may be needed to convert it to a soluble form. Once soluble, it must be ensured that the analyte ca n be measured proportionally. If not, additional chemical steps may be required, such as converting manganese to MnO for analysis in steel. Often, interference must be removed before measurements are taken. 1C-4 Eliminating Interferences Eliminating interferences is the step following sample preparation, where substances that could interfere with the measurement are removed. In chemical analysis, properties used for measurement are rarely unique to one chemical species and often apply to a group of elements or compounds. Interfering substances, or “interferents”, can affect the accuracy of the final measurement. To address this, a method must be developed to isolate the analytes from these interferences. There are no universal rules for eliminating interferences, making this one of the most challenging parts of the analytical process. Figure 1-3. Feedback system flow diagram. The desired state is determined, the actual state is measured, and the two states are 1C-5 Calibrating and Measuring Concentration compared. The difference between the two states is used to change a controllable quantity that results in a change in the state All analytical results depend on a final measurement X of a of the system. Quantitative measurements are again performed physical or chemical property of the analyte. This property must on the system, and the comparison is repeated. The new vary in a known and reproductible way with the concentration cA difference between the desired state and the actual state is again of the analyte. Ideally, the measurement of the property is directly used to change the state of the system if necessary. The process proportional to the concentration. provides continuous monitoring and feedback to maintain the cA = kX controllable quantity, and thus the actual state, at the proper level. The text describes the monitoring and control of blood glucose where k is a proportionality constant. With two exceptions, concentration as an example of a feedback control system. analytical methods require the empirical determination of k with chemical standards for which cA is known. The process of determining k is thus an important step in most analyses; this [Feature 1-1, will read in the ppt.] step is called calibrating. 1C-6 Calculating Results Computing analyte concentrations from experimental data is CHAPTER 2 usually relatively easy, particularly with modern calculators or CHEMICALS, APPARATUS, AND UNIT OPERATIONS OF computers. These computations are based on the raw ANALYTICAL CHEMISTRY experimental data collected in the measurement step, the characteristics of the measurement instruments, and the stoichiometry of the analytical reaction. 2A | Selecting and Handling Reagents and Other Chemicals solution of iron (III) chloride can be used, although it’s less effective. The purity of reagents has an important bearing on the accuracy attained in any analysis. It is therefore essential that the quantity Cleaning Procedures: Thoroughly clean all beakers, flasks, or of a reagent with its intended use. crucibles before use. Wash with hot detergent, rinse with tap water, and follow with several rinses using deionized water. 2A-1 Classifying Chemicals Properly cleaned glassware should exhibit a uniform film of water. Reagent Grade Drying Glassware: It is seldom necessary to dry the interior These chemicals meet the minimum standards set by the surface of glassware before use: drying is ordinarily a waste of American Chemical Society (ACS) and are used in analytical work. time at best and a potential source of contamination at worst. Some suppliers list the maximum impurity levels allowed, while Removing Grease Films: Use organic solvent like benzene or others provide actual impurity concentrations. acetone or commercial preparations to eliminate grease films Primary-Standard Grade from the glassware. Primary standards are exceptional purity and are carefully analyzed by the supplier, with the assay listed on the label. They 2C | Evaporating Liquids are often obtained from the National Institute of Standards and Technology, which also provide reference standards for complex Decreasing the Volume of Solutions and Removing Unwanted substances. Species [The National Institute of Standards and Technology (NIST) is the 1. Reducing Solution Volume: To decrease the volume of a current name of what was formerly the National Bureau of solution containing a non-volatile solute, use a ribbed Standards] cover glass to allow vapors to escape while preventing contamination. Avoid using glass hooks and Special-Purpose Reagent Chemicals conventional cover glass, as they are less effective. These chemicals are designed for specific applications, such as 2. Controlling Evaporation: Evaporation can lead to overheating solvents for spectrophotometry and high-performance liquid and bumping, causing solution loss. To minimize this, chromatography. Information relevant to their intended use, like heat gently and use glass beads if permitted. absorbance at specific wavelengths or ultraviolet cutoff, is 3. Eliminating Unwanted Species: provided with these reagents. Chloride and Nitrate Removal: Add sulfuric acid and evaporate until white fumes of sulfur trioxide 2A-2 Rules for Handling Reagents and Solutions appear. Nitrate and Nitrogen Oxides Removal: Use urea in acidic A high-quality chemical analysis requires reagents and solutions solutions. of known purity. A freshly opened bottle of a reagent-grade Ammonium Chloride Removal: Add concentrated nitric chemical can ordinarily be used with confidence; whether this acid and evaporate to a small volume. same confidence is justified when the bottle is half empty Organic Matter Removal (Wet Ashing): Add sulfuric acid depends entirely on the way it has been handled after being and heat until sulfur trioxide fumes appear, then opened. The following rules should be observed to prevent the add nitric acid to oxidize remaining organic matter. accidental contamination of reagents and solutions. All procedures requiring the release of fumes should be 1. Select the best grade of chemical available for analytical performed in a hood for safety. work. Whenever possible, pick the smallest bottle that will supply the desired quantity. Bumping is sudden, often 2. Replace the top of every container immediately after removal violent boiling that tends to of the reagent; do not rely on someone else to do this. spatter solution out of its 3. Hold stoppers of reagent bottles between your fingers; never container. set a stopper on a desk top. 4. Unless specifically directed otherwise, never return excess Wet Ashing is the oxidation of reagents to the bottle to avoid contamination. the organic constituents of a 5. Unless directed otherwise, do not insert tools (e.g., spatulas, sample with oxidizing spoons, or knives) into bottles of solid chemicals; shake or reagents such as nitric acid, tap to dispense the contents, and use a clean spoon if sulfuric acid, hydrogen necessary. peroxide, aqueous bromine, or 6. Keep the reagent shelf and the laboratory balance and neat. a combination of these Clean up any spills immediately, even though someone is reagents. waiting to use the same chemical or reagent. 7. Observe local regulations concerning the disposal of surplus reagents and solutions. 2B | Cleaning and Marking of Laboratory Ware 2D | Measuring Mass Marking Containers: When performing chemical analyses in In most analyses, an analytical balance must be use to obtain duplicate or triplicate, ensure each sample vessel is clearly highly accurate masses. Less accurate laboratory balances are marked. Use semipermanent pencil marking on etched areas of also used for mass measurements when the demands for glassware or special marking inks for porcelain surfaces, which reliability are not critical. can be baked into the glaze for permanence. Alternatively, a 2D-1 Types of Analytical Balances [A servo system is a device in which a small electric signal causes a mechanical system to return to a null position.] Analytical Balance Configurations and Features of Electronic Analytical Balances Has a maximum capacity that ranges from 1 g to several kilograms and a precision at maximum capacity of at least 1 part in 105. ▪ Macrobalance – the most common type of analytical balance; it has a maximum load of 160 to 200 g and a precision of 0.1 mg. ▪ Semimicroanalytical Balance – has a maximum load of 10 to 30 g and a precision of 0.01 g. ▪ Microanalytical Balance – has a maximum load of 1 to 3 g and a precision of 0.001 mg, or 1 𝜇𝑔. Evolution of the Analytical Balance The analytical balance has significantly evolved over the past few Electronic analytical balances come in two configurations as decades: shown in Figure 2-3: 1. Traditional Equal-Arm Balance: Featured two pans attached 1. Pan Configurations: to a beam pivoting on a central knife edge. Weighing was tedious and time-consuming, requiring manual addition Pan Below the Cell (Figure 2-3a): This design offers of standard masses to balance the object being higher precision by positioning the pan below the cell, weighed. allowing for limited movement and preventing torsional 2. Single-Pan Balance (introduced in 1946): Offered a forces from affecting the balance’s alignment. significant improvement in speed and convenience over Top-Loading Design (Figure 2-3b): Provides the traditional balance. It quickly became the preferred unencumbered access to the pan while maintaining choice in most laboratories. precision comparable to the best mechanical balance. 3. Electronic Analytical Balance: This modern balance, which 2. Key Features: lacks a beam and a knife edge, offers superior speed, Automatic Taring: Allows the balance to display zero with convenience, accuracy, durability, and features like a container on the pan, enabling taring up 100% of its computer control and data logging. It is now replacing capacity. the single-pan balance in laboratories, making the Dual Capacities and Precisions: Some balances offer mechanical single-pan balance nearly obsolete. dual modes, switching from a microbalance capacity to a semimicrobalance (30 g) with increased precision to 2D-2 The Electronic Analytical Balance 0.01 mg, effectively making them two balances in one. 3. Ease of Use: Operation of an Electronic Analytical Balance Modern electronic balances are designed for speed and An electronic analytical balance uses a servo system to measure simplicity, often controlled by a single touch-sensitive mass. The balance consists of: bar that powers the instrument, calibrates it against a standard mass, or zeros the display. This use-friendly 1. Structure and Function: The pan sits above a hollow metal design enables reliable mass measurements with cylinder surrounded by a coil that fits over a cylindrical minimal training or experience. permanent magnet’s inner pole. An electric current in the coil creates a magnetic field, levitating the pan, indicator [ A tare is the mass of an empty sample container. Taring is the arm, and any load on the pan. process of setting a balance to read zero in the presence of the 2. Null Position Mechanism: When the pan is empty, the current tare.] is adjusted so that the indicator arm remains at a null position. Placing an object on the pan causes it to move downward, increasing the light striking a photocell in the null detector. The resulting current from the photocell is amplified and fed into the coil, generating a stronger magnetic field to return the pan to its original position. 3. Servo System: This system maintains the null position by using a small electric current to compensate for the added mass. 4. Mass Measurement: The current required to keep the pan and object in the null position is directly proportional to the mass of the object and is readily measured, digitized, and displayed. 5. Calibration: Involves using standard mass to adjust the current so that the correct mass is displayed. [To levitate means to cause an object to float in air.] 2D-3 The Single-Pan Mechanical Analytical Balance Components of a Mechanical Balance This method ensures precise mass measurements by balancing the pan through incremental adjustments and measuring the Mechanical balances, both equal-arm and single-arm, share beam’s deflection. several fundamental components: Precautions in Using an Analytical Balance 1. Beam and Knife Edges: The lightweight beam, which forms the core of the balance, An analytical balance is a delicate instrument that you must is supported by a central knife edge (A) on a flat surface. handle with care. Consult with your instructor for detailed A second knife edge (B), located near the left end, supports instructions on weighing with your particular model of balance. the beam on a planar surface inside a stirrup that connects Observe the following general rules for working with an analytical the pan to the beam. Both knife edges are made of very balance regardless of make or model: hard materials, like agate or synthetic sapphire, to reduce 1. Center the load on the pan as well as possible. friction and improve performance. 2. Protect the balance from corrosion. Objects to be placed on [The two knife edges in a mechanical balance are prism-shaped the pan should be limited to nonreactive metals, nonreactive agate or sapphire devices that form low-friction bearing with two plastics, and vitreous materials. planar surfaces contained in stirrups also of agate or sapphire.] 3. Observe special precautions for the weighing of liquids. 4. Consult the instructor if the balance appears to need 2. Pan and Masses: adjustment. The left end of the beam has a pan for holding the object 5. Keep the balance and its case scrupulously clean. A camel’s to be weighed. A set of masses, secured by hangers, can hair brush is useful for removing spilled material or dust. be lifted mechanically one at a time using knobs on 6. Always allow an object that has been heated to return to room balance case. temperature before weighing it. The right end of the beam holds a counterweight to 7. Use tongs or finger pads to prevent the uptake of moisture by balance the pan and masses on the left. dried objects. 3. Arrest Mechanisms: Beam Arrest: Lifts the beam, preventing the central knife 2D-4 Sources of Error in Weighing edge from touching its surface and freeing the stirrup from Correction for Buoyancy⁵ the outer knife edge. This mechanism protects the bearings from the damage when objects are added or A buoyancy error will affect data if the density of the object being removed. weighed differs significantly from that of the standard masses, Pan Arrest: Supports the mass of the pan and its contents, due to differing buoyant forces in air. preventing oscillation. Both arrests are controlled by an external lever and should be engaged when the balance is not in use. 4. Air Damper (Dashpot): Mounted near the end of the beam opposite the pan, the air damper uses a piston within the cylinder to quickly bring the beam to rest by resisting motion through air expansion and contraction. 5. Protective Enclosure: The balance is enclosed in a case with doors to protect from air currents, allowing for high precision (discrimination between small mass differences, such as less than 1 mg). Weighing with a Single-Pan Balance In a single-pan balance, the beam is initially horizontal with no object on the pan and all masses in place. When the pan and beam arrests are disengaged, the beam can freely rotate around the knife. 1. Placing the Object: The object is placed on the pan, causing the beam’s left end to move downwards. 2. Restoring Balance: Masses are systematically removed from These errors can be corrected using a specific equation that the beam until the imbalance is less than 100 mg. accounts the densities of the object, the standard masses, and 3. Measuring Deflection: The angle of the beam’s deflection the air. from its original horizontal position is directly proportional to the remaining mass needed to restore balance. 4. Optical System Measurement: An optical system measures this angle. A reticle with a scale (0 to 100 mg) is mounted on the beam. A beam of light passes through the scale and is magnified, projecting onto a frosted glass plate for easy Where: reading. 5. Reading Position: A vernier scale allows for readings to the W1 = corrected mass of the object nearest 0.1 mg. W2 = mass of the standard masses dobj = density of the object dwts = density of the masses or ±0.05 g. Many are electronic and feature taring and digital readout for ease of use. Additionally, triple-beam balances, dair = density of the air displaced by them; dair has a value of though less precise, offer simplicity, durability, and affordability. 0.0012 g/cm3 They have three calibrated scales for mass measurement and are For objects with a density of 2 g/cm3 or greater, buoyancy errors suitable for many weighing tasks despite their lower precision are usually minimal (less than 0.1%), making corrections often compare to top-loading balances. unnecessary. However, for low-density solids, liquids, or gases, [Use auxiliary balances for weighings that do not require great buoyancy effects are significant and require correction. Standard accuracy.] masses used for calibration typically have densities between 7.8 and 8.4 g/cm3, with 8 g/cm3 being sufficient for most purposes. For higher accuracy, specific balance specifications should be consulted. 2E | Equipment and Manipulations Associated with Weighing Solids often absorbs moisture from air, which can alter their mass, especially when they have a large surface area, like powdered samples. To avoid errors due to humidity, samples are typically dried before analysis. This involves heating the sample or container to a constant mass through a repeated cycle of heating, cooling, and weighing. This process is done until successive masses are consistent 0.2 to 0.3 mg, ensuring that the sample has reached a stable mass and that any chemical or physical changes during heating are complete. 2E-1 Weighing Bottles Solids are typically dried and stored in weighing bottles, which come with a ground-glass portion on the outside, preventing contact Temperature Effects with the sample and avoiding Weighing an object at a temperature different from its sample loss from the glass surroundings can introduce significant errors. Two main factors surface. Plastic weighing bottles contribute to this problem: convention currents in the balance are also available and offer greater case create a buoyant effect on the pan and object, and warm air durability compared to glass trapped in a closed container weighs less than cooler air. These bottles. factors can make the object’s apparent mass lower by up to 10 or 2E-2 Desiccators and Desiccants 15 mg for typical items like porcelain crucibles or weighing bottles. To avoid such errors, heated objects must always be Oven drying is commonly used to remove moisture from solids allowed to cool to room temperature before weighing. but is unsuitable for substances that decompose or do not release water at oven temperatures. Dried materials are often Other Sources of Error stored in desiccators while cooling to prevent moisture uptake. Porcelain or glass objects can sometimes develop static charges Desiccators contain a drying agent like anhydrous calcium that cause balance readings to become erratic, especially in low chloride, calcium sulfate, anhydrous magnesium perchlorate, or humidity. This charge can dissipate spontaneously after a short phosphorus pentoxide, and their lids have ground-glass surfaces lightly coated with grease for an airtight seal. To avoid disturbing the sample, the desiccator lid should be removed or replaced with a sliding motion, and slight rotation and downward pressure should seal it. Care must be taken with heated objects in desiccators; pressure changes from waring or cooling can break the seal or create a vacuum, leading to sample loss or contamination. Allow partial cooling before sealing and occasionally break the seal during cooling to relieve vacuum pressure. Hygroscopic materials should be stored in tightly covered containers, while most other solids can be stored uncovered in desiccators. time. To prevent or correct this issue, a low-level radioactive source or a faintly damp chamois can be used to neutralize the static charge. Additionally, the optical scale of a single-pan mechanical balance should be routinely checked for accuracy using a standard 100-mg mass, particularly when the full scale range is used. 2D-5 Auxiliary Balances In analytical laboratories, less precise balances than analytical ones are widely used for their speed, durability, and convenience. Top-loading balances are particularly useful, accommodating loads from 150 to 25,000 g with varying precision, such as ±1 mg  bottle immediately, and then weigh the bottle again, including any remaining solid. Repeat this process for each sample, and determine the masses by weighing the difference. 2E-6 Weighing Liquids The mass of a liquid is typically determined by difference. For noncorrosive and relatively nonvolatile liquids, the liquid is transferred into a pre-weighed container with a snugly fitting cover, such as weighing bottle, and the container’s mass is subtracted from the total mass. For volatile or corrosive liquids, the liquids is sealed in a pre- weighed glass ampoule. The ampoule is heated, and its neck is immersed in the sample; as it cools, the liquids is drawn into the bulb. The neck is then sealed with a small flame. After cooling to room temperature, the ampoule, along with any glass removed 2E-3 Manipulating Weighing Bottles during sealing, is weighed. The ampoule is then broken and Heating at 105 ͦC to 110 ͦC for one hour typically sufficient to transferred to an appropriate container. A volume correction for remove moisture form the surface of most solids. To dry a the glass may be needed if the liquid is transferred to a volumetric sample, a weighing bottle should be placed in a labeled beaker flask. with ribbed cover glass, which protects the sample from contamination while allowing air circulation. Crucibles containing precipitates that can be dried by heating can also be handled this 2F | Filtration and Ignition of Solids way. The beaker must be clearly marked for identification. 2F-1 Apparatus To prevent contamination, avoid touching dried objects with your fingers, as oils and moisture from the skin can transfer to them. Simple Crucibles Instead, use tongs, chamois finger cots, clean cotton gloves, or Simple crucibles are containers made from materials like strips of paper, to handle dried objects during weighing. porcelain, aluminum oxide, silica, and platinum, which maintain a consistent mass and are mainly used to convert precipitates into a measurable form. The solid is filtered and ignited in the crucible. Crucibles made from nickel, iron, silver, and gold are used for high- temperature fusions but can suffer mass changes and contamination due to reactions with the atmosphere or sample. The most suitable crucible should be chosen to minimize interference in analysis. Filtering Crucibles Filtering crucibles serve as both containers and filters, using a vacuum to speed up filtration. They are quicker than using filter paper. 2E-4 Weighing by Difference Weighing by difference is a simple method for determining a series of sample masses. First the bottle and its contents are weighed. One sample is then transferred from the bottle to a container; gentle tapping of the bottle with its top and slight rotation of the bottle provide control over the amount of sample removed. Following transfer, the bottle and its residual contents are weighed. The mass of the sample is difference between the two weighings. It is essential that all the solid removed from the Sintered-glass (also called fritted-glass) crucibles come in weighing bottle be transferred without loss to the container. different porosities (fine, medium, and coarse ̶ marked f, m, and c) and can withstand temperatures up to 200°C, while quartz, 2E-5 Weighing Hygroscopic Solids unglazed porcelain, and aluminum oxide crucibles tolerate higher temperatures. Gooch crucibles have a perforated bottom and Hygroscopic substances, which quickly absorb moisture from the were traditionally filtered with asbestos, now replaced by glass air, require careful handling. To weigh such substances, use a mats, which can endure temperatures over 500°C and are less separate weighing bottles for each sample. Place the required hygroscopic than asbestos. amount of sample in each bottle, heat them for an appropriate time, then quickly cap and cool them in a desiccator. After cooling, briefly open each bottle to relieve any vacuum, then weigh it. Transfer the contents to a receiving vessel quickly, recapping the Filter Paper Ashless paper is a crucial filtering medium in analytical chemistry, made from cellulose fibers treated with hydrochloric and hydrofluoric acids to remove impurities, followed by neutralization with ammonia. However, residual ammonium salts may affect certain analyses, such as the Kjeldahl method for nitrogen. Ashless paper tends to absorb moisture from the atmosphere, so it must be destroyed by ignition to weigh the collected precipitate. It typically leaves less than 0.1 mg of residue, which is negligible. Various porosities of ashless paper are available, and it can help filter gelatinous precipitates like 2. Washing: After decantation, wash liquid is added to the beaker hydrous iron (III) oxide, although clogging may still occur. Mixing and mixed thoroughly with the precipitate. The solid is allowed to ashless paper pulp with the precipitate can reduce clogging. settle, and the wash liquid is decanted through the filter. This washing step may be repeated several times depending on the precipitate. Washing most of the precipitate before transferring it to the filter results in a more thoroughly washed precipitate and a faster filtration process. 3. Transfer: The bulk of the precipitate is moved from the beaker to the filter using directed streams of wash liquid, with the stirring rod guiding the flow of material onto the filtering medium. Any residual precipitate that clings to the beaker walls is dislodged with a rubber policeman (a piece of rubber tubing crimped on one end and fitted onto a stirring rod). The last traces of hydrous oxide Heating Equipment precipitates can be wiped from the beaker walls with small pieces of ashless paper, which are then ignited along with the main filter Precipitates can often be weighed directly after being brought to paper. a constant mass in a low-temperature drying oven. These ovens are electrically heated, maintaining temperatures to within 1°C, Precautions during Filtering: with maximum temperatures ranging from 140°C to 260°C. Many precipitates are effectively dried at around 110°C. Efficiency is To prevent creeping (spreading of precipitate over wetted improved by forced air circulation or using pre-dried air under surfaces), filters should not be filled more than three-quarters partial vacuum. Microwave ovens are popular for significantly full. Adding a small amount of nonionic detergent like Triton reducing drying times, drying slurry samples in 5-6 minutes X-100 to the supernatant or wash liquid can also help compared to 12-16 hours in conventional ovens, and also minimize creeping. speeding up drying times for silver chloride, calcium oxalate, and [Creeping is the process in which a solid moves up the side of a barium sulfate precipitates. wetted container or filter paper.] A heat lamp can dry a precipitate collected on ashless paper and Gelatinous precipitates should be completely washed before char the paper, which can then be ignited in a muffle furnace. drying. If allowed to dry prematurely, they may crack and Laboratory burners such as Meker, Tirrill, and Bunsen are used for shrink, making further washing ineffective. intense heat, with the Meker burner achieving the highest temperatures. Heavy-duty electric furnaces, or muffle furnaces, Following these steps and precautions ensures efficient filtration maintain controlled temperatures above 1100°C, requiring safety and minimizes the loss or contamination of the precipitate. precautions like long-handled tongs and heat-resistant gloves for handling. 2F-3 Directions for Filtering and Igniting Precipitates 2F-2 Filtering and Igniting Precipitates Preparation of a Filter Paper Preparation of Crucibles Folding and Seating Filter Paper in a Funnel: A crucible used to convert a precipitate to a form suitable for To properly fold and seat filter paper in a 60-degree funnel, follow weighing must maintain a constant mass within the limits of these steps, as shown in Figure 2-13: experimental error during drying or ignition. To achieve this, the crucible is thoroughly cleaned, which can be conveniently done by 1. Fold the Paper in Half: Fold the circular filter paper exactly in backwashing on a filtration train for filtering crucibles. The half to create a semicircle (step a). Firmly crease the fold. crucible is then subjected to the same heating and cooling 2. Fold Again: Fold the semicircle again into a quarter-circle shape regimen required for the precipitate. This process is repeated until (step b). Firmly crease this second fold as well. a constant mass is achieved, defined as consecutive weighings that differ by 0.3 mg or less. 3. Tear a Corner: Tear off a small triangular piece from one of the corners parallel to the second fold (step c). This creates a small Filtering and Washing Precipitates hole in the center of the folded paper, which helps the paper seat 1. Decantation: This involves carefully pouring off the supernatant better in the funnel. liquid from the precipitate without disturbing the solid. The liquid 4. Open into a Cone: Open the paper so that the untorn quarter is passed through the filter while the precipitate remains in the forms a cone shape (step d). beaker where it was formed. This step helps to speed up filtration by preventing the filter pores from clogging with the precipitate 5. Fit into the Funnel: Place the cone-shaped paper into the 60- too early. A stirring rod is used to direct the flow of the liquid, and degree funnel. Press along the second fold to make the paper any remaining drops are collected back into the beaker. conform to the funnel's interior shape (step e). 6. Seat the Paper: Dampen the filter paper cone with water from a By following these steps, you minimize the risk of tearing the filter wash bottle. Then, gently pat the paper with your finger to ensure paper and ensure that the precipitate is safely transferred to the it adheres tightly to the funnel's surface (step f). This prevents air crucible for further processing or weighing. from leaking between the paper and the funnel and ensures that the stem of the funnel will fill with an unbroken column of liquid. Ashing Filter Papers Properly folding and seating the filter paper helps maintain an efficient filtration process by preventing air leaks and ensuring a smooth flow of liquid through the filter. When using a heat lamp to ash a filter paper, the crucible is placed on a clean, nonreactive surface like a wire screen covered with aluminum foil. The lamp is positioned about 1 cm above the crucible rim and turned on to allow charring without much attention. Adding a drop of concentrated ammonium nitrate solution can accelerate the process. Any remaining carbon can then be removed with a burner. When using a burner, more attention is needed because the higher temperatures can cause mechanical loss of precipitate if the moisture is expelled too quickly or if the paper ignites. The hot carbon from the burning paper can also reduce some precipitates, which may complicate reoxidation. To minimize these problems, the crucible should be positioned to Transferring Paper and Precipitate to a Crucible allow air access, with a clean cover available to extinguish any Transferring Filter Paper and Precipitate to a Crucible: flames. After filtration and washing, the filter paper and its contents must Begin heating with a small flame, be carefully transferred from the funnel to a crucible that has gradually increasing the already been brought to constant mass. Since ashless paper has temperature as moisture is very low wet strength, it must be handled delicately to prevent released and the paper begins to char. Thin wisps of smoke are tearing. Here’s how to safely perform this transfer: normal, but a significant increase indicates the paper may flash, requiring a temporary pause in heating. Any flame should be 1. Partially Dry the Filter Paper: Allow the filter paper to dry slightly extinguished immediately with a crucible cover, which may need while still in the funnel. This reduces the risk of tearing during cleaning later to ensure no precipitate remains. Once smoking removal. ceases, heating can be intensified to remove the residual carbon 2. Flatten the Cone: Carefully draw the triple-thick portion of the before the final ignition of the precipitate in a muffle furnace, filter paper across the top edge of the funnel (step a). This action where a reducing atmosphere must be avoided. flattens the cone shape along its upper edge (step b). Using Filtering Crucibles 3. Fold the Corners Inward: Fold the corners of the filter paper A vacuum filtration train is used when a filtering crucible can be inward toward the center (step c). This step secures the used instead of paper. The trap isolates the filter flask from the precipitate within the folded paper. source of vacuum. 4. Fold the Top Edge Over: Fold the top edge of the paper over itself (step d), further enclosing the precipitate securely. 5. Transfer to the Crucible: Gently ease the folded filter paper and its contents into the crucible (step e). Make sure to position it so that the bulk of the precipitate is near the bottom of the crucible.  accuracy. Therefore, volumetric measurements are typically standardized to 20°C. Since most laboratories maintain temperatures close to 20°C, temperature corrections for aqueous solutions are generally unnecessary. For organic liquids, however, their higher coefficient of expansion may require corrections for 2F-4 Rules for Manipulating Heated Objects temperature changes of as little as 1°C. Careful adherence to the following rules will minimize the 2G-3 Apparatus for Precisely Measuring Volume possibility of accidental loss of a precipitate: Volume may be measured reliably with a pipet, a buret, or a 1. Practice unfamiliar manipulations before putting them to use. volumetric flask. 2. Never place a heated object on the benchtop; instead, place it Volumetric equipment is marked by the manufacturer to indicate on a wire gauze or a heat-resistant ceramic plate. not only the manner of calibration (usually TD for "to deliver" or 3. Allow a crucible that has been subjected to the full flame of a TC for "to contain") but also the temperature at which the burner or to a muffle furnace to cool momentarily (on a wire gauze calibration strictly applies. Pipets and burets are ordinarily or ceramic plate) before transferring it to the desiccator. calibrated to deliver specified volumes, whereas volumetric flasks are calibrated on a to-contain basis. [Glassware types include Class A and Class B. Class A glassware is manufactured to the highest tolerances from Pyrex, borosilicate, or Kimax glass. Class B (economy ware) tolerances are about twice those of Class A.] Pipets Pipets are used to transfer accurately known volumes of liquid. Common types include: 1. Volumetric (or transfer) 4. Keep the tongs and forceps used to handle heated objects pipets: Deliver a fixed volume (0.5 to 200 mL) and are often color- scrupulously clean. In particular, do not allow the tips to touch the coded for identification. They are filled to a calibration mark, and benchtop. any residual liquid remaining in the tip is not blown out. 2. Measuring pipets: Allow delivery of various volumes up to their maximum capacity (0.1 to 25 mL). These pipets are also filled to 2G | Measuring Volume a calibration mark, and the handling of the residual liquid varies The precise measurement is important to many analytical by type. methods as the precise measurement of mass. 3. Handheld Eppendorf micropipets: Deliver adjustable microliter 2G-1 Units of Volume volumes. The volume is controlled by a pushbutton that operates a spring-loaded piston. Liquid is drawn into a disposable plastic The unit of volume is the liter (L), defined as one cubic decimeter. tip, and the tip is emptied by depressing the pushbutton further. The milliliter (mL) is one one-thousandth of a liter (0.001 L) and These pipets provide adjustable volumes, with accuracy is used when the liter represents an inconveniently large volume depending on the user's skill, and should be calibrated for critical unit. The microliter (𝜇𝐿) is 10-6 L or 10-3 mL. work. [liter = one cubic decimeter; milliliter = 10-3] 2G-2 The Effect of Temperature on Volume Measurements The volume of a liquid and its container changes with temperature. Glass, commonly used for volumetric measuring devices, has a small coefficient of expansion, so temperature- induced volume changes in the container are usually negligible in standard analytical work. However, dilute aqueous solutions have a coefficient of expansion of about 0.025% per °C, meaning that a 5°C temperature change can significantly affect measurement 4. Automatic pipets: Designed for repeated delivery of specific volumes. Modern versions are motorized and computer- controlled, capable of functioning as pipets, dispensers, burets, and diluters. The desired volume is input via a keypad and Volumetric Flasks displayed on an LED panel, with maximum volumes ranging from 10 to 2500 µL. Volumetric flasks, available in capacities ranging from 5 mL to 5 L, are typically calibrated to contain a specific volume when filled to an etched line on the neck (Figure 2-20). They are used for preparing standard solutions and diluting samples to a fixed volume before pipetting. Some volumetric flasks are also calibrated to deliver a specified volume and feature two reference lines on the neck. To deliver the stated volume, the flask is filled to the upper line. Burets Burets, similar to measuring pipets, allow for the delivery of any volume up to their maximum capacity, with greater precision than pipets. A buret consists of a calibrated tube for holding titrant and a valve to control its flow. The type of valve can significantly affect the buret's performance: 1. Pinchcock Valve: This simple valve uses a close-fitting glass bead inside rubber tubing. Liquid flows past the bead only when the tubing is deformed. 2G-4 Using Volumetric Equipment 2. Glass Stopcock: This valve uses a lubricant between the ground-glass surfaces of the stopcock and the buret for a liquid- Volume markings on volumetric equipment are precisely blazed tight seal. However, bases and other solutions can cause the by the manufacturer. To ensure accurate measurements, the stopcock to freeze if left in place for extended periods, laboratory equipment must be equally clean. Clean glass surfaces necessitating thorough cleaning after each use. support a uniform liquid film, whereas dirt or oil disrupts this film, 3. Teflon Valve: Teflon valves are commonly used because they indicating an unclean surface. are resistant to most reagents and do not require lubrication. Cleaning A brief soak in a warm detergent solution typically removes grease and dirt that cause water breaks. Prolonged soaking should be avoided to prevent the formation of a rough ring at the detergent/air interface, which can impair the equipment's effectiveness. After cleaning, rinse the apparatus thoroughly with tap water and then with three to four portions of distilled water. Drying volumetric ware is usually unnecessary. Avoid Parallax Figure 2-22 In volumetric measurements, the top surface of a liquid in a To dispense an aliquot accurately: narrow tube shows a curvature known as the meniscus. It is standard practice to use the bottom of this meniscus as the 1. Draw Liquid: (a) Draw a small amount of liquid into the pipet. reference point for calibrating and reading the equipment. To 2. Wet the Pipet: (b) Wet the interior surface by tilting and rotating determine this more accurately, an opaque card or piece of paper the pipet. Repeat this process two more times. can be held behind the graduations. 3. Transfer Liquid: (c) While holding the pipet's tip against the [A meniscus is the curved surface of a liquid at its interface with inside surface of the volumetric flask, allow the liquid level to the atmosphere.] descend until the bottom of the meniscus aligns with the line When reading the volume, ensure your eye is level with the liquid etched on the pipet's stem. surface to avoid parallax error. Parallax occurs when the 4. Remove Excess: (d) Remove the pipet from the flask and tilt it meniscus appears either smaller or larger than its actual value if (e) to draw a small amount of liquid up into the pipet. viewed from above or below, respectively (Figure 2-21). 5. Wipe the Tip: (f) Wipe the tip with a lintless tissue. [Parallax is the apparent displacement of a liquid level or of a pointer as an observer changes position. Parallax occurs when an 6. Dispense Liquid: (g) Hold the pipet vertically and allow the object is viewed from a position that is not at a right angle to the liquid to flow into the receiving flask until just a small amount object.] remains inside the tip and a drop is on the outside. 7. Final Touch: (h) Tilt the flask slightly and touch the pipet’s tip to the inside of the flask. When this is done, a small amount of liquid will remain in the pipet. Do not remove this remaining liquid, as the pipet is calibrated to deliver its rated volume with this residual liquid. Cleaning To clean a pipet: 1. Draw Detergent Solution: Use a rubber bulb to draw detergent solution up to 2 to 3 cm above the calibration mark. 2. Drain and Rinse: Drain the detergent solution and rinse the pipet with several portions of tap water. Inspect for any film breaks and repeat this cleaning step if necessary. 3. Final Rinsing: Fill the pipet with distilled water to about one third of its capacity. Carefully rotate the pipet to ensure the entire interior surface is wetted. Repeat this rinsing step at least twice to ensure thorough cleaning. Measuring an Aliquot 2G-5 Directions for Using a Pipet [An aliquot is a measured fraction of the volume of a liquid sample.] Liquid is drawn into a pipet through the application of a slight vacuum. The mouth should never be used for suction because of To accurately use a pipet for sampling: the risk of accidentally ingesting the liquid being pipetted. 1. Wet the Pipet: Use a rubber bulb to draw a small volume of the Instead, a rubber suction bulb (Figure 2-22a) or a rubber tube liquid to be sampled into the pipet, thoroughly wetting the entire connected to a vacuum source should be used. interior surface. Repeat this with at least two additional portions. 2. Fill the Pipet: Carefully fill the pipet to a level somewhat above the graduation mark (Figure 2-22). 3. Arrest the Flow: Quickly replace the bulb with a forefinger to stop the liquid flow (Figure 2-22b). Ensure there are no bubbles or foam in the liquid. 4. Wipe and Adjust: Tilt the pipet slightly from the vertical and wipe the exterior to remove adhering liquid (Figure 2-22c). Touch the pipet tip to the wall of a glass vessel (not the receiving container) and slowly allow the liquid level to drop until the bottom of the meniscus aligns with the graduation mark. 5. Drain into Receiver: Place the pipet tip well within the receiving vessel and allow the liquid to drain. When free flow ceases, rest the pipet tip against the inner wall of the receiver for 10 seconds (Figure 2-22d). 6. Withdraw the Pipet: Withdraw the pipet with a rotating motion Rotate the buret to completely wet the interior. to remove any liquid adhering to the tip. Do not blow or rinse the Drain the liquid through the tip. small volume remaining inside the tip into the receiving vessel Repeat this process at least two more times. (Note 2). 3. Fill the Buret: Fill the buret well above the zero mark. Notes: 4. Remove Air Bubbles: 1. The liquid can best be held at a constant level if the forefinger is faintly moist. Too much moisture makes control Rotate the stopcock rapidly. impossible. Allow small amounts of titrant to pass through the tip to 2. Rinse the pipet thoroughly after use. free any air bubbles. 2G-6 Directions for Using a Buret 5. Adjust Liquid Level: A buret must be scrupulously clean before it is used; in addition, Lower the liquid level to the zero mark or slightly below it. its value must be liquid-tight. Allow it to drain for about 1 minute. Cleaning 6. Record Initial Volume: Note the initial volume reading, estimating to the nearest 0.01 mL. Thoroughly clean the tube of the buret with detergent and a long brush. Rinse thoroughly with tap water and then with distilled This process ensures accurate measurement and avoids air water. Inspect for water breaks. Repeat the treatment if bubbles that can affect the titration results. necessary. Titration Lubricating a Glass Stopcock Figure 2-23 shows the recommended technique for handling a To properly grease and maintain a glass stopcock: stopcock during titration. Grip the stopcock as illustrated to keep it securely in place. Ensure the buret tip is inside the titration flask. 1. Remove Old Grease: Use a paper towel to carefully remove all Add the titrant in about 1 mL increments, swirling or stirring old grease from both the stopcock and its barrel. Ensure both constantly. As you near the endpoint, reduce the increment size parts are completely dry. and add the titrant dropwise. Once close to the endpoint, rinse the container walls. After the titration, let the titrant drain from the 2. Apply Grease: Lightly grease the stopcock, avoiding the area buret for at least 30 seconds before recording the final volume to near the hole. the nearest 0.01 mL. 3. Insert and Rotate: Insert the stopcock into the barrel and rotate it vigorously with slight inward pressure. 4. Check Lubrication: The contact area between the stopcock and barrel should appear nearly transparent. The seal should be liquid-tight. No grease should have worked its way into the tip. Proper lubrication ensures smooth operation and prevents leaks. Notes: 1. Grease films that are unaffected by cleaning solution may yield to such organic solvents as acetone or benzene. Thorough washing with detergent should follow such treatment. The use of silicone lubricants is not recommended; contamination by such preparations is difficult-if not impossible-to remove. 2. So long as the flow of liquid is not impeded, fouling of a buret Notes; tip with stopcock grease is not a serious matter. Removal is best accomplished with organic solvents. A stoppage during 1. When dealing with an unfamiliar titration, workers often a titration can be freed by gentle warming of the tip with a prepare an extra sample solely to understand the endpoint and lighted match. estimate the titrant needed. This sample is not carefully 3. Before a buret is returned to service after reassembly, it is titrated but is used to gain insights into the process, which can advisable to test for leakage. Simply fill the buret with water ultimately save time. and establish that the volume reading does not change with 2. To add increments smaller than one drop, let a small volume time. of titrant accumulate on the buret tip, then touch the tip to the flask's wall. This partial drop will mix with the rest of the liquid Filling as described in Note 3. To prepare and use a buret: 3. Instead of rinsing the flask toward the end of a titration, you can tilt and rotate the flask to ensure that any drops adhering 1. Close Stopcock: Ensure the stopcock is closed. to the inner surface are incorporated into the bulk of the liquid. 2. Condition the Buret: 2G-7 Directions for Using a Volumetric Flask Add 5 to 10 mL of the titrant. Before use, volumetric flasks should be washed with detergent and Table 2-3 assists with buoyancy corrections and provides factors thoroughly rinsed. Drying is usually unnecessary but can be done to convert the mass of water at temperature T to its corresponding by clamping the flask upside down and using a glass tube volume at that temperature or at 20°C. Corrections for buoyancy connected to a vacuum line to speed up the process if needed. related to stainless steel or brass masses and for changes in water and glass container volumes are included in these factors. Direct Weighing into a Volumetric Flask To prepare a standard solution directly, add a known mass of solute to a volumetric flask, using a powder funnel to avoid losing any solid. Rinse the funnel thoroughly and include the washings in the flask. If heating is required to dissolve the solute, first weigh the solid in a beaker or flask, dissolve it by heating with the solvent, and let the solution cool to room temperature. Then, transfer the solution quantitatively to the volumetric flask as described in the next section. Qualitative Transfer of Liquid to a Volumetric Flask Place a funnel in the neck of the volumetric flask and use a stirring rod to guide the liquid from the beaker into the funnel. Tip off the last drop of liquid from the beaker with the stirring rod. Rinse both the stirring rod and the interior of the beaker with distilled water, transferring the washings to the volumetric flask. Repeat the rinsing process at least twice more. Diluting to the Mark After transferring the solute, fill the flask halfway and swirl to help dissolve it. Add more solvent, mix well, and bring the liquid level close to the mark. Allow it to drain for about 1 minute, then use a medicine dropper for any final adjustments. Firmly stopper the flask, invert it several times to ensure thorough mixing, and transfer the solution to a storage bottle that is either dry or rinsed with the solution from the flask. Note: 2H-1 General Directions for Calibration If the liquid level surpasses the calibration mark, you can still save Before calibrating volumetric ware, ensure it is free of water the solution by correcting for the excess volume. Place a self-stick breaks. Burets and pipets do not need to be completely dry, but label at the meniscus location, empty the flask, and refill it to the volumetric flasks should be thoroughly drained and dried at room etched mark with water. Use a buret to measure the additional temperature. The calibration water should be in thermal volume needed to bring the meniscus to the labeled mark. This equilibrium with its surroundings. This is best achieved by drawing additional volume must be added to the nominal volume of the the water ahead of time, monitoring its temperature, and waiting flask when calculating the solution's concentration. until it stabilizes. For calibration, an analytical balance can be used, but weighings to the nearest milligram are generally sufficient for all but the 2H | Calibrating Volumetric Glassware smallest volumes. A top-loading balance is often more convenient. Weighing bottles or small, well-stoppered conical flasks can be Volumetric glassware calibration involves measuring the mass of used to collect the calibration liquid. a liquid (typically distilled or deionized water) and correcting for buoyancy, as the density of water differs from that of the masses Calibrating Volumetric Pipet used. The calibration process includes: To calibrate the volumetric ware: 1. Buoyancy Correction: Apply Equation 2-1 to correct the raw weighing data for buoyancy. 1. Determine the Empty Mass: Weigh the stoppered receiver to the nearest milligram. 2. Volume Calculation: Divide the corrected mass by the density of the liquid at the calibration temperature (T) to determine the 2. Transfer Water: Use a pipet to transfer a portion of temperature- volume. equilibrated water to the receiver. 3. Temperature Correction: Adjust this volume to the standard 3. Weigh the Receiver and Content: Weigh the receiver with the temperature of 20°C using the method described in Example 2-2. water to the nearest milligram. 4. Calculate Mass Delivered: Find the mass of water delivered by subtracting the empty mass of the receiver from the total mass. 5. Calculate Volume: Use Table 2-3 to convert the mass of water into volume. 6. Repeat and Analyze: Repeat the calibration several times, calculate the mean volume delivered, and determine the standard deviation. Calibrating a Buret transcribing data from other sources, as this increases the risk of misplacing or incorrectly recording crucial information, To calibrate the buret: which could compromise the experiment. 1. Prepare the Buret: Fill the buret with temperature-equilibrated [Remember that you can discard an experimental measurement water, ensuring no air bubbles are trapped in the tip. Allow about 1 only if you have certain knowledge that you made an experimental minute for drainage and then adjust the liquid level to the 0.00-mL error. Thus, you must carefully record experimental observations mark. Remove any adhering drop by touching the tip to the wall of in your notebook as soon as they occur. a beaker. Wait 10 minutes and recheck the volume; if the stopcock is tight, the volume should remain constant. 2. Each entry or series of entries in the notebook should have a clear heading or label. For example, if recording the masses of 2. Weigh the Flask: Weigh a 125-mL conical flask with a rubber empty crucibles, use a heading like "Empty Crucible Mass" and stopper to the nearest milligram. label each crucible with a corresponding number or letter to match the recorded masses. 3. Date each page of the notebook as it is used. 3. Transfer Water: Slowly transfer approximately 10 mL of water to 4. Never attempt to erase or obliterate an incorrect entry. Instead, the flask at about 10 mL/min. Touch the tip to the wall of the flask cross it out with a single horizontal line and locate the correct to remove any adhering drop. After 1 minute, record the delivered entry as nearby as possible. Do not write over incorrect volume and refill the buret. Weigh the flask with its contents to the numbers; with time, it may become impossible to distinguish nearest milligram. The difference between this mass and the initial the correct entry from the incorrect one. mass gives the mass of water delivered. [An entry in a laboratory notebook should never be erased but 4. Calculate True Volume: Use Table 2-3 to convert the mass of should be crossed out instead.] water delivered to the true volume. Subtract the apparent volume (recorded volume) from the true volume to find the correction 5. Never remove a page from the notebook. Draw diagonal lines needed. Repeat this calibration until the agreement is within ±0.02 across any page that is to be disregarded. Provide a brief mL. rationale for disregarding the page. 5. Test Over Full Range: Starting from the zero mark, deliver about 2I-2 Notebook Format 20 mL to the receiver and test the buret at 10-mL intervals over its Consult your instructor about the preferred format for keeping the entire range. Plot the correction needed as a function of the volume laboratory notebook. One common convention is to record data delivered. This plot will help determine the correction for any and observations consecutively on each page as they occur. The volume interval. analysis is then summarized on the next available page spread (left Calibrating a Volumetric Flask and right facing pages). The first of these two pages should include the following entries: Weigh the clean, dry flask to the nearest milligram. Then fill to the mark with equilibrated water and reweigh. With the aid of Table 2- 1. The title of the experiment ("The Gravimetric Determination of 3, calculate the volume contained. Chloride"). Calibrating a Volumetric Flask Relative to a Pipet 2. A brief statement of the principles on which the analysis is based. To calibrate a volumetric flask using a pipet for accurate aliquoting: 3. A complete summary of the weighing, volumetric, and/or 1. Transfer Aliquots: Using a 50-mL pipet, carefully transfer ten 50- instrument response data needed to calculate the results. mL aliquots into a dry 500-mL volumetric flask. 4. A report of the best value for the set and a statement of its 2. Mark the Meniscus: Once all aliquots are transferred, mark the precision. meniscus location with a gummed label. Cover the label with varnish for permanence. 3. Dilute to the Mark: Dilute the solution in the flask up to the label. This procedure allows the pipet to deliver a precise one-tenth aliquot of the solution from the flask. 4. Recalibration: If using a different pipet, recalibrate accordingly, as calibration is specific to each pipet. 2I | The Laboratory Notebook A laboratory notebook should be permanently bound with consecutively numbered pages (or manually numbered before use). It is important to record measurements and observations clearly, avoiding overcrowding. Reserve the first few pages for a table of contents, which should be updated as entries are made to keep track of the information efficiently. 2I-1 Maintaining a Laboratory Notebook The second page should contain the following items: 1. Record all data and observations directly into the notebook using ink. While neatness is important, avoid achieving it by 1. Equations for the principal reactions in the analysis. 2. An equation showing how the results were calculated. 3. A summary of observations that appear to bear on the validity 34A | Real Samples of a particular result or the analysis as a whole. Any such entry must have been originally recorded in the notebook at the time the Analyzing a simple sample is usually easier than analyzing observation was made. complex materials because fewer variables need to be controlled,