Lecture-1-ChE-401.pdf

INTRODUCTION TO ANALYTICAL CHEMISTRY KEYZA JEAN M. RIVERA Registered Chemical Engineer, BatStateU MS Environmental Engineering, U.P. Diliman CHAPTER I Nature and Applications of Analytical Chemistry Main Divisions of Analytical Chemistry Quantitative Method of Analysis...

INTRODUCTION TO ANALYTICAL CHEMISTRY KEYZA JEAN M. RIVERA Registered Chemical Engineer, BatStateU MS Environmental Engineering, U.P. Diliman CHAPTER I Nature and Applications of Analytical Chemistry Main Divisions of Analytical Chemistry Quantitative Method of Analysis Tools of Analytical Chemistry ChE 401 | Analytical Chemistry 1.1 NATURE AND APPLICATIONS OF ANALYTICAL CHEMISTRY ChE 401 | Analytical Chemistry NATURE OF ANALYTICAL CHEMISTRY Mainly deals with the measurements of the components of a greater sample Separation, identification, and determination of the components In 1984, Friedrich Wilhelm Ostwald proved its role in developmental science. Art and science of determining what matter is and how much of it exists a.k.a. measuring science, composed of potent concepts and techniques that are advantageous in the fields of science, medicine, and engineering ChE 401 | Analytical Chemistry APPLICATIONS OF ANALYTICAL CHEMISTRY Determination of the concentration of O2 and CO2 in blood samples which are useful in diagnosing a patient’s illness Gauging the effectiveness of smog-control devices by allowing the measurement of NOx and CO from automobile during emission test Quantitative measurement of protein content and nutritional value of meals by determining nitrogen content Diagnosis of parathyroid disease by measuring ionized calcium content in blood ChE 401 | Analytical Chemistry APPLICATIONS OF ANALYTICAL CHEMISTRY Analysis of steel during its production permits adjustments in the concentration of such elements such as carbon, nickel, and chromium. Farmers tailor fertilization and irrigation schedules to meet changing plant needs. Performing immunoassay in determining antibodies or antibody-related reagents in formulating medicine for COVID-19 virus Measurement of antigen by using polymerase chain reaction (PCR) test to detect the presence of COVID-19 virus ChE 401 | Analytical Chemistry APPLICATIONS OF ANALYTICAL CHEMISTRY Permits physiologists to study the role of ions in nerve signal conduction as well as muscle contraction by measuring potassium, calcium, and sodium ions in body Chemical reactions can be calculated from quantitative measurements made at equal time intervals Material scientists rely heavily on quantitative analysis of crystalline germanium and silicon to determine impurities, which leads to assessment of its effectivity. Archaeologists identify the source of volcanic glasses (obsidian) by measuring concentration of minor elements in samples. ChE 401 | Analytical Chemistry The relationship between analytical chemistry with respect to many other scientific fields. ChE 401 | Analytical Chemistry 1.2 MAIN DIVISIONS OF ANALYTICAL CHEMISTRY ChE 401 | Analytical Chemistry MAIN DIVISIONS OF ANALYTICAL CHEMISTRY Qualitative Analysis – makes the elements and compounds in the sample identifiable and determines what material are contained in a sample Quantitative Analysis – any method used for determining the amount of a chemical in a sample ChE 401 | Analytical Chemistry QUALITATIVE ANALYSIS TESTS Solubility test – used to determine if solid dissolves in the solvent Precipitation test – used to see if solid formed when two dissolved substances mix pH test – determine the concentration of dissolved [H] ions Flame test – used to see what color is formed when the substance is burned ChE 401 | Analytical Chemistry QUANTITATIVE ANALYSIS CATEGORIES Complete or Ultimate Analysis – The total amount of constituents in the sample is determined. Elemental / Molecular Analysis – refers to the measurement of the amount of each element or molecule Partial Analysis – Only few constituents are determined. Major constituent / Macro Analysis – implies that the constituent is in high concentration Trace Analysis – constituents to be determined are in low concentration ChE 401 | Analytical Chemistry Types of analysis in Types of constituents sample size (g) and analyte level ChE 401 | Analytical Chemistry 1.3 QUANTITATIVE METHOD OF ANALYSIS ChE 401 | Analytical Chemistry QUANTITATIVE METHOD OF CHEMICAL ANALYSIS Gravimetry – uses the mass of the analyte or some compound related to it Volumetry – also called titrimetry; measures the volume of the solution containing sufficient reagent to react completely with the analyte Electroanalytical – involves the measurement of electrical properties such as potential, current, resistance, and quantity of electrical charge Spectroscopy – based on the interaction of electromagnetic radiation with the analyte atoms or molecules, or by the analyte’s production of such radiation Miscellaneous groups of method – measurements of such quantities such as mass- to-charge ratio of molecules by mass spectrometry, rate of radioactive decay, heat of reaction, thermal conductivity, optical activity, refractive index ChE 401 | Analytical Chemistry OVERVIEW: STEPS OF QUANTITATIVE ANALAYSIS 1. Choosing a method This is a crucial first step in any quantitative investigation that calls for both experience and intuition. The desired level of accuracy is one of the initial factors to be taken into account throughout the selection process. The quantity of sample to be analyzed is the second factor connected to the economic factor. The choice of method is always influenced to some extent by the complexity of the sample and the quantity of sample components. ChE 401 | Analytical Chemistry OVERVIEW: STEPS OF QUANTITATIVE ANALAYSIS 2. Acquiring the samples – An analysis must be conducted on the sample that has the same composition as the majority of the material. If the individual components of a substance, such as animal tissue, soil, or coal, can be identified visually or through the use of a microscope, the substance is heterogeneous. A sample weighing about one gram will be used for the assay. Assay is a process of determining how much of a given sample is the material indicated by its name. 3. Processing the sample – Under most circumstances, the sample must be processed in any of a variety of different ways. ChE 401 | Analytical Chemistry STEPS OF QUANTITATIVE ANALAYSIS 1. Preparing laboratory samples – A solid laboratory sample is ground to reduce particle size, combined to guarantee homogeneity, and kept for varying amounts of time. 2. Defining replicate samples – Replication raises the standards of the findings and adds a level of dependability. 3. Preparing solutions – The majority of analyses are carried out in solutions of a sample created with an appropriate solvent. 4. Eliminating interference – These are species that alter the final measurement by boosting or attenuating the quantity being measured, leading to errors in analysis. ChE 401 | Analytical Chemistry STEPS OF QUANTITATIVE ANALAYSIS 5. Calibrating and measuring concentration – The final measurement X of the analyte’s physical or chemical characteristic determines the outcome of every analytical test. This property must change in a predictable or measurable way as the analyte's concentration changes (CA). Ideally, the concentration would be directly proportional to the property's measurement. 𝑪𝑨 = 𝒇 𝑿 = 𝒌𝑿 where k is the proportionality constant. With only two exceptions, analytical methods call for the empirical calculation of k using known chemical standards for CA. Thus, determining k is a crucial step in the majority of analyses; this step is known as calibration. ChE 401 | Analytical Chemistry STEPS OF QUANTITATIVE ANALAYSIS 6. Calculating results – These are done by the stoichiometry of the analytical reactions, the parameters of the measurement devices, and the actual experimental data gathered. 7. Evaluating result by estimating their reliability – If the experimenter wants the data to have any meaning, they must quantify the uncertainties surrounding the computed results. ChE 401 | Analytical Chemistry 1.4 TOOLS OF ANALYTICAL CHEMISTRY ChE 401 | Analytical Chemistry TOOLS OF ANALYTICAL CHEMISTRY Analytical chemistry is a core set of operations and equipment that is necessary for laboratory work in the discipline and that serves as a foundation for its growth and development. The purity of reagents has an important bearing on the accuracy attained in any analysis. ChE 401 | Analytical Chemistry CHEMICAL CLASSIFICATION Reagent-grade – Chemicals range from 96 to 98 percent purity, or nearly ACS grade purity. Primary standard-grade – The provider has thoroughly examined these reagents, and the assay is printed on the label of the container. Secondary standard-grade – Purity has been established by chemical analysis and serves as reference material for titrimetric methods. Special purpose reagent chemicals – Included among these are solvents for spectrophotometry and high-performance liquid chromatography. ChE 401 | Analytical Chemistry IMPORTANT REQUIREMENTS OF PRIMARY STANDARD 1. High purity 2. Atmospheric stability 3. Absence of hydrate water so that the compound of the solid does not change in variation and humidity 4. Modest cost 5. Reasonable solubility in the titration medium 6. Reasonably large molar mass so that the relative error associated with weighing the standard is minimized ChE 401 | Analytical Chemistry RULES FOR HANDLING REAGENTS AND SOLUTIONS 1. Select the best grade of chemical available for analytical work. 2. Put back the top of every container immediately after removal of the reagents; do not rely on someone else to do this. 3. Hold the stoppers of reagents between your fingers; never set a stopper on a desktop. 4. Unless specifically directed otherwise, never return any excess reagent to a bottle. 5. Unless directed otherwise, never insert spatulas, spoons, or knives into a bottle that contains a solid chemical. 6. Keep the reagent in the shelf and the laboratory balance clean and neat. 7. Observe local regulations concerning the disposal of surplus reagents and solutions. ChE 401 | Analytical Chemistry CLEANING AND MARKING OF LABORATORY WARE 1. Flasks, beakers, and some crucibles have small etched areas on which semi-permanent markings can be made with a pencil. 2. Special marking inks are available for porcelain surfaces. The marking is baked permanently into the glaze by heating at a high temperature. 3. An organic solvent, such as benzene or acetone, may be effective in removing grease. Saturated solution of FeCl3 ChE 401 | Analytical Chemistry EVAPORATING LIQUIDS Evaporation is frequently difficult to control because of the tendency of some solutions to overheat locally. To reduce the risk of bumping, which could result in a partial loss of solution, careful and gentle heating will be used. Bumping is reduced by using glass beads. Arrangement for the evaporation of liquid ChE 401 | Analytical Chemistry DURING EVAPORATION, SOME UNWANTED SPECIES CAN BE ELIMINATED LIKE: 1. Chloride and nitrate can be removed from a solution by adding sulfuric acid and evaporating until copious white fumes of sulfur trioxide are observed. 2. Urea is effective in removing nitrate ion and nitrogen oxides from acidic solutions. 3. Ammonium chloride is best removed by adding concentrated nitric acid and evaporating the solution to a small volume. 4. Ammonium ion is rapidly oxidized when it is heated; the solution is then evaporated to dryness. ChE 401 | Analytical Chemistry WET ASHING This is a process of adding sulfuric acid and heating to the appearance of sulfur trioxide fumes. Nitric acid can be added toward the end of heating to hasten the oxidation of the last traces of organic matter. Organic constituents can be eliminated from a solution. ChE 401 | Analytical Chemistry MEASURING MASS Analytical balance – tool used to measure mass with a maximum capacity ranging from 1 gram to a few kilograms and precision of at least 1 part in 105 up to 106 at full capacity You must consult with your instructor for detailed instructions on weighing with your particular model of balance. ChE 401 | Analytical Chemistry PRECAUTIONS ON USING A BALANCE Center the load on the pan as well as possible. Protect the balance from corrosion. Objects to be placed on the pan should be limited to non-reactive metals, non-reactive plastics, and vitreous materials. Consult with the instructor if the balance appears to need adjustment. Keep the balance and its case clean. A camel hairbrush is useful for removing spilled material or dust. Always allow an object that has been heated to return to room temperature before weighing it. Use tongs or finger pads to prevent uptake of moisture by dried objects. ChE 401 | Analytical Chemistry TYPES OF ANALYTICAL BALANCE Analytical Balance Capacity / Operation Precision 1. Macro 160-200 g ±0.1 mg 2. Semi-micro 10-30 g ±0.01 mg 3. Micro 1-3 g ±0.001 mg or 1𝜇g 4. Equal arm Up to 200 g 1 g sensitivity 5. Single pan Usually 110-160 g ±0.1 mg 6. Electronic Usually 100-600 g 0.1-0.001 mg 7. Auxiliary Top loading 15—200 g ±1 mg Up to 25,000 g ±0.05 g Triple beam 2,610 g 0.1 g ChE 401 | Analytical Chemistry SOURCES OF ERROR IN WEIGHING 1. Buoyancy When the things being weighed have a significantly different density than the masses, a weighing error occurs. Equations can provide buoyancy correction for electronic balances. ChE 401 | Analytical Chemistry SOURCES OF ERROR IN WEIGHING 𝑑𝑎𝑖𝑟 𝑑𝑎𝑖𝑟 𝑊1 = 𝑊2 + 𝑊2 − 𝑑𝑜𝑏𝑗 𝑑𝑤𝑡𝑠 Where 𝑊1 is the corrected mass of the object, 𝑊2 is the mass of the standard masses, 𝑑obj is the density of the object, 𝑑𝑤𝑡𝑠 is the density of the masses, and 𝑑𝑎𝑖𝑟 is the density of the air displaced by them and has a value of 0.0012 g/cm3. ChE 401 | Analytical Chemistry SOURCES OF ERROR IN WEIGHING 2. Temperature A large inaccuracy will occur when an object’s temperature differs from that of its surroundings. A heated object must have enough time to cool to normal temperature in order to reduce mistakes. a. Convection currents within the balance case exert a buoyant effect on the pan and object. b. Warm air trapped in a closed container weighs less than the same volume at a lower temperature. ChE 401 | Analytical Chemistry SOURCES OF ERROR IN WEIGHING Both inaccuracies will involve lower item masses, which could be as little as 10 or 15 mg for a normal porcelain filtering crucible or weighing container. (A) porcelain filtering crucible, (B) weighing bottle containing 7.5 g KCl ChE 401 | Analytical Chemistry SOURCES OF ERROR IN WEIGHING 3. Static charge When a porcelain or glass object develops static charge, especially in a low humidity environment, a low-level radioactive source in the balance will produce enough ions to relieve the charge. A little damp chamois can also be used to clean the item. ChE 401 | Analytical Chemistry EQUIPMENT & MANIPULATIONS IN WEIGHING 1. Weighing bottle This is a simple tool for drying solids. A ground glass section of a cap-style is seen on the left, and as it is not in contact with the contents. There is no chance that the sample will afterwards fall to the surface and be lost. ChE 401 | Analytical Chemistry EQUIPMENT & MANIPULATIONS IN WEIGHING 2. Desiccator and desiccants Dried materials are kept in desiccators while cooling to reduce moisture absorption. Use a sliding motion to open or close desiccator’s lid to reduce the possibility of disrupting the sample. By applying light rotation and downward pressure on the lid’s position, an airtight seal is created. ChE 401 | Analytical Chemistry EQUIPMENT & MANIPULATIONS IN WEIGHING Component of desiccators (left) and desiccator containing weighing botte (right) ChE 401 | Analytical Chemistry EQUIPMENT & MANIPULATIONS IN WEIGHING 3. Filtration and ignition of solids Simple crucibles: These transform a precipitate into an appropriate weighing form at constant mass. Filtering crucibles: These act as both containers and filters. A vacuum is employed to speed up the filtration, and a variety of rubber adaptors can be used to create a tight seal. These can be sintered/fritted glass and Gooch crucible (perforated bottom). Filter paper: It is a very important filtration medium. Ashless paper is used for igniting samples. ChE 401 | Analytical Chemistry COMPARISON OF FILTERING MEDIA FOR GRAVIMETRIC ANALYSES Gooch Crucible, Porcelain Aluminum Characteristic Paper Glass Crucible Glass Mat Crucible Oxide Crucible Speed of filtration Slow Rapid Rapid Rapid Rapid Convenience and ease Troublesome, Convenient Convenient Convenient Convenient of preparation inconvenient Max ignition temp, °C None > 500 200-500 1100 1450 C has reducing Chemical Reactivity Inert Inert Inert Inert properties Several Several Several Several Porosity Many available Available Available Available Available Convenience with Unsuitable; filter Unsuitable; Unsuitable; Unsuitable; Satisfactory gelatinous precipitates clogs filter clogs filter clogs filter clogs Cost Low Low High High High ChE 401 | Analytical Chemistry HEATING EQUIPMENT Low temperature drying. It runs between 1400°C and 260°C, with an acceptable drying temperature of 110°C. Forced circulation was used to dry solids, but another advancement was the flow of pre-dried air through an oven built to function in a partial vacuum. Microwave laboratory ovens are currently quite popular since it shortens drying cycles. It uses electromagnetic waves in the drying of the sample. An ordinary heat lamp can be used to dry a precipitate that has been collected on ashless paper and to char the paper as well. ChE 401 | Analytical Chemistry HEATING EQUIPMENT Burners are practical place to get a lot of heat. The greatest temperature that can be reached is determined by the burner’s design and the fuel’s ability to burn. Meker – provides the highest temperature Tirrill – for safe continuous stream of flammable gas Bunsen – produces a single open gas flame Electric furnace (muffle) – capable of maintaining controlled temperatures of 1100℃ or higher ChE 401 | Analytical Chemistry MEASURING VOLUME 1. Pipettes These permit the transfer of accurately known volumes from one container to another. A volumetric or transfer pipette delivers a single or fixed volume between 0.05 and 200 mL, color coded by volume for convenience in identification and sorting. Measuring pipettes are calibrated in convenient units to permit delivery of any volume up to a maximum capacity of 0.1 to 25 mL. ChE 401 | Analytical Chemistry MEASURING VOLUME (a) volumetric, (b) Mohr, (c) serological, (d) Eppendorf micropipette, (e) Ostwald-Folin, (f) Lambda ChE 401 | Analytical Chemistry CHARACTERISTICS OF PIPETTES Type of Available Name Function Type of Drainage Calibration Capacity, mL Volumetric TD Delivery of fixed volume 1-300 Free Mohr TD Delivery of variable volume 1-25 To lower calibration line Serological TD Delivery of variable volume 0.1-10 Blow out last drop Serological TD Delivery of variable volume 0.1-10 To lower calibration line Ostwald-Folin TD Delivery of fixed volume 0.5-10 Blow out last drop Wash out with suitable Lambda TC Containment of fixed volume 0.001-2 solvent Lambda TD Delivery of fixed volume 0.001-2 Blow out last drop Delivery of variable or fixed Tip emptied by air Eppendorf TD 0.001-1 volume displacement ChE 401 | Analytical Chemistry MEASURING VOLUME 2. Burettes A burette can achieve a degree of precision that is significantly higher than a pipette can. This is made up of valve system for controlling the flow of titrant and a calibrated tube to retain the titrant. (a) glass-bead valve, b) Teflon valve ChE 401 | Analytical Chemistry MEASURING VOLUME 3. Volumetric flask These are produced in sizes ranging from 5 mL to 5 L, and they are often calibrated to hold a particular volume when filled to a line etched on the neck. ChE 401 | Analytical Chemistry READING VOLUMES OF LIQUID When a liquid is contained in a small tube, the top surface displays a distinct curve or meniscus. It is customary to calibrate and operate volumetric equipment by using the meniscus’ bottom as the point of reference. Holding an opaque card or piece of paper behind the graduations will help you determine this minimum with more precision. The eye must be at the level of the liquid surface to avoid an error due to parallax, a condition that causes the volume to appear smaller than its actual value if the meniscus is viewed from above and larger if the meniscus is viewed from below. ChE 401 | Analytical Chemistry CLEANING OF APPARATUS Pipette. Draw the solution 2 to 3 cm over the calibration mark using a rubber bulb. Fill the pipette in various portions with tap water. Examine the flange breaks, then repeat this step. Finally, carefully spin the pipette after filling it with distilled water to about 1/3 of its capacity. Rinse this area at least twice more. Burette. Clean the burette tube completely with detergent and a broad brush. Rinse completely with distilled water after using tap water. Look for water breaks. Repeat the procedure if necessary. ChE 401 | Analytical Chemistry CLEANING OF APPARATUS Volumetric flask. The grease and grime that cause water breaks may typically be removed with a quick soak in a warm detergent solution. It is best to avoid prolonged soaking since a harsh detergent air interface is likely to develop into an area or ring. The equipment needs to be completely rinsed with tap water after cleaning it, and then it needs to be rinsed with three or more parts of distilled water. ChE 401 | Analytical Chemistry SAFETY IN THE LABORATORY ChE 401 | Analytical Chemistry SAFETY IN THE LABORATORY At the outset, learn the location of the nearest eye fountain, tire blanket, shower, and fire extinguisher. At all times, wear proper eye protection due to the risk of serious and permanent eye damage. Do not use contact lens. Avoid getting liquids on your skin. In the event of such contact, drench the affected area with a lot of water right away. Never perform an unauthorized experiment. Never work alone in the laboratory. Be certain that someone is always within earshot. ChE 401 | Analytical Chemistry SAFETY IN THE LABORATORY Never bring food or beverage inside the laboratory. Always use a bulb or other device to draw liquid into a pipette. Wear adequate foot covering (no sandals). Use hair net and coat properly. Be extremely tentative in touching objects that have been heated. Always fire-polish the ends of freshly cut-glass tubing. Never attempt to force glass tubing through the hole of a stopper. Use fume hoods whenever toxic or noxious gases are likely to evolve. Notify your instructor immediately in the event of an injury. ChE 401 | Analytical Chemistry ERRORS IN CHEMICAL ANALYSIS ChE 401 | Analytical Chemistry ERRORS IN CHEMICAL ANALYSIS Error refers to the difference between the measured value and the true “known” value. This denotes the estimated uncertainty in a measurement or experiment. Usually, faulty calibrations or standardizations are those which frequently cause errors. Measurement errors are an inherent part of the world we live. Hence, it is impossible to do an experiment with a free error. ChE 401 | Analytical Chemistry ERRORS IN CHEMICAL ANALYSIS The central value of a set should be more reliable than any of the individual results. An analysis of the variation in data allows us to estimate the uncertainty associated with the central result. Results from six replicate determinations of in aqueous samples of a standard solution ChE 401 | Analytical Chemistry ERRORS IN CHEMICAL ANALYSIS Mean (x): This is the most frequently employed central value indicator. The sum of replicate measurements is divided by the total number of measurements in the set to produce the mean, often known as the arithmetic mean or the average: σ𝑁 𝑖=1 𝑥𝑖 x̄ = 𝑁 where 𝑥𝑖 represents the individual values of x making up the set of N replicate measurements. ChE 401 | Analytical Chemistry ERRORS IN CHEMICAL ANALYSIS When repeated data are organized by rising or decreasing value, the median is the midway outcome. Both values greater and smaller than the median are equally common. The median can be calculated immediately for results with an odd number of results. The middle pair’s mean is used when the number is even. ChE 401 | Analytical Chemistry EXAMPLE 1 Calculate the mean and median for the data shown. 19.4 + 19.5 + 19.6 + 19.8 + 20.1 + 20.3 𝑚𝑒𝑎𝑛 = 𝑥 = 6 Because the set contains an even number of measurements, the median is the average of the central pair: 19.6 + 19.8 𝑚𝑒𝑑𝑖𝑎𝑛 = = 19.7 𝑝𝑝𝑚 𝐹𝑒 2 ChE 401 | Analytical Chemistry ERRORS IN CHEMICAL ANALYSIS Precision: This is the reproducibility of measurements or the similarity of outcomes. Typically, a measurement’s accuracy is just repeating the measurement on replicate samples. Standard deviation, variance, and coefficient of variation are frequently used to characterize the precision of a collection of repeated data. These are based on the deviation from the mean 𝒅𝒊 , which measures how far a particular result 𝒙𝒊 deviates from the mean. ChE 401 | Analytical Chemistry ERRORS IN CHEMICAL ANALYSIS ഥ 𝒅𝒊 = 𝒙𝒊 − 𝒙 Accuracy: The error, which expresses accuracy, shows how near a measurement is to the true or accepted value. Absolute error: In the measurement of a quantity x is given by the equation 𝑬 = 𝒙𝒊 − 𝒙𝒕 Where 𝒙𝒕 is the true or accepted value of the quantity. ChE 401 | Analytical Chemistry ERRORS IN CHEMICAL ANALYSIS Relative error: 𝐸𝑟 is a more useful quantity than the absolute error. The percent relative error is given by the expression: 𝑥𝑖 − 𝑥𝑡 𝐸𝑟 = 𝑥 100% 𝑥𝑡 19.8 − 20.0 𝐸𝑟 = × 100% = −1% 20.0 ChE 401 | Analytical Chemistry ERRORS AND UNCERTAINTY The discrepancy between your response and the correct one is what meant by error. Comparison of data from carefully replicated tests makes determining the precision of a measurement simple. Results can be accurate without being exact, and precise without being accurate. ChE 401 | Analytical Chemistry EXAMPLE 2 Absolute error in the micro- Kjeldahl determination of nitrogen. Each dot represents the error associated with a single determination. Each vertical line labeled (x i - xt) is the absolute average deviation of the set from the true value. (Data from C.O. Willits and C.L. Ogg, J. Assoc. Offic. Anal. Chem., 1949, 32, 561. With permission). ChE 401 | Analytical Chemistry ERRORS AND UNCERTAINTY Analyst 1 obtained relatively high precision and high accuracy. Analyst 2 had poor precision but good accuracy. Analyst 3 are surprisingly common. The precision is excellent, but there is significant error in the numerical average for the data. Analyst 4 shows that both poor precision and accuracy. ChE 401 | Analytical Chemistry TYPES OF ERRORS 1. Random or indeterminate error Data are more or less symmetrically dispersed around a mean value as a result of error. Note that analysts 1 and 3 have considerably less data scatter and random error than analysts 2 and 4. Therefore, the precision of a measurement typically reflects the random error in that measurement. 2. Systematic (or determinate) error A data set’s mean can deviate from the expected value due to error. The systematic error of the first figure is approximately –0.2 ppm Fe. The data of analysts 3 and 4 reveals systematic error of around 0.7% and 1.2% nitrogen, compared to the results of analysts 1 and 2, which have negligible systematic error. ChE 401 | Analytical Chemistry TYPES OF SYSTEMATIC ERRORS a. Instrumental errors: caused by non-ideal instrument behavior, by faulty calibrations, or by use under inappropriate conditions. b. Method errors: arise from non-ideal chemical or physical behavior of analytical systems. c. Personal errors: result from the carelessness, inattention, or personal limitations of the experimenter. 3. Gross error The most common cause when utilizing an instrument is human error. For instance, analytical results will be poor if some precipitate is lost before weighing. After a weighing bottle’s empty mass is established, touching it with your fingers will result in a high mass reading for any solid that is weighed in the contaminated bottle. ChE 401 | Analytical Chemistry THANK YOU! Any questions?