CHM256 Chapter 4: Basic Requirements for Analysis PDF

CHAPTER 4 BASIC REQUIREMENTS FOR ANALYSIS Prepared by Nur Nadirah  CONTENT ⮚ Sampling ⮚ Techniques of sampling (SOLID, LIQUID, GAS) ⮚ Sample storage ⮚ Dissolution of samples inclusive both dry and wet methods ⮚ Elimination of interferences  SAMPLING ✔ Sampling is the most cr...

CHAPTER 4 BASIC REQUIREMENTS FOR ANALYSIS Prepared by Nur Nadirah  CONTENT ⮚ Sampling ⮚ Techniques of sampling (SOLID, LIQUID, GAS) ⮚ Sample storage ⮚ Dissolution of samples inclusive both dry and wet methods ⮚ Elimination of interferences  SAMPLING ✔ Sampling is the most critical aspect of an analysis. ✔ The significance and accuracy of the measurements can be limited by the sampling process. *ACCURACY & REALIBILITY OF THE RESULT Depends on : Proper Transport of collection of sample from Proper selection sample point of of lab sample (sampling) collection to lab *BEFORE DO SAMPLING CONSIDER : POPULATION, SAMPLE ✔ *SAMPLING: Act of collecting samples to produce meaningful information. ✔ The ease or complexity of sampling will depend on the nature of the sample. ✔ Is a process to get a representative and homogeneous sample. ✔ Representative means that the content of analytical sample reflects content of bulk sample. ✔ Homogeneous means that the analytical sample has the same content throughout. *PURPOSE OF SAMPLING: To obtain a representative sample of the whole sample that can be taken to the laboratory for chemical analysis and the result obtained will be accurate. *Differentiate between population, sample and sampling? CLASSIFICATIONOFANALYSIS Sample Weight (mg) Sample Volume (μL) i) Meso >100 >100 ii) Semimicro 10 – 100 50 - 100 iii) Micro 1 – 10 < 50 iv) Ultramicro 1% ii) Minor 0.1 – 1 % iii) Trace < 0.1 % iv) Ultratrace: in the range of few parts per million 6 or less. SAMPLING TECHNIQUE Deciding how to obtain a sample for analysis depends on: 1) The size of the bulk to be sampled. 2) The physical state of the fraction to be analyzed (solid, liquid, gas). 3) The chemistry of the material to be assayed (properties of the sample) 🙠🙠 (Nothingcan be done that would destroy or alter the identity or quantity of the analyte)  Obtaining a representative sample is the first step of an analysis. The gross sample is several small portions of the sample. This is reduced to provide a laboratory sample. An aliquot of this sample is taken for analysis sample. In the clinical laboratory, GROSS SAMPLE is usually satisfactory for use as a sample because it is not large and homogeneous. (blood and urine) STEPS INVOLVED IN SAMPLING BULK MATERIAL Identify the population from which the sample is to be obtained. Collect a gross sample that is truly representative of the population being sampled. Reduce the gross sample to a laboratory sample that is suitable for analysis.  SAMPLING SOLID Inhomogeneity of the material, make sampling of solids more difficult.  The easiest way to sample a material is: GRAB SAMPLE: the sample taken at random an assumed to be representative. For reliable results, it is best to take 1/50 to 1/100 of the total bulk. The larger the particle size, the larger the gross sample should be. The gross sample must be reduced in size to obtain a …laboratory sample  CONING AND QUARTERING This process is continued until the gross sample is small enough to be transported to the laboratory  *EXAMPLE OF QUESTION Explain the method of sampling for coal sample. It is important to determine whether the coal sample is solid. So describe the sampling method for solid. Sampling solid Using the cone and quarter method Divide a pile of material into quarter. Take a sample from each quarter of the pile and crush these sample and form into a smaller conical pile. Flatten the conical pile and cut into equal quarters. Two opposite quarters are chosen at random. Crush the quarter further. The whole steps are repeated until a laboratory samples obtain.  EXAMPLE OF TOOL FOR SOLID SAMPLING TABLET SAMPLER Designed to take a sample of tablets and capsules as containers are filled. TUBULAR SAMPLER Taking samples from soft and SCOOPER semi-solid materials. A stainless steel scoop with a capacity of 5 oz,148 ml. Perfect for sampling powder, soil, gravel, and other materials. SAMPLING LIQUID ⮚ Liquidsamples are homogeneous and are much easier to sample. ⮚ The gross sample can be relatively small. ⮚ Ifliquid samples are not homogeneous, and have only small quantity, they can be shaken and sampled immediately.  Sampling depends on the types of liquids: ✔ LARGE VOLUME OF LIQUIDS (IMPOSSIBLE TO MIX) - If in a pipe, after passing through a pump when they have undergone the most thorough mixing ✔ LARGE STATIONARY LIQUIDS (LAKES, RIVERS) -“thief”sampler, which is a device for obtaining ⮚ SURFACE aliquots at different levels ⮚ MIDDLE ⮚ BOTTOM ✔ BIOLOGICAL FLUIDS The timing of sampling biological fluids is very important (blood composition varies before and after meals) Aliquot: sample or a portion of the total amount of a solution EXAMPLE OF LIQUID SAMPLING A special sampling bottle used to collect LIQUID samples Liquids mix by diffusion is slowly and must be shaken to obtain a homogeneous mixture If water sample is taken from the river, then the water samples is collected at the SURFACE, MIDDLE and at the BOTTOM of the river bed. If liquid is in a large container, the liquid should be stirred first before the samples are taken at the top, middle and at the bottom of the container. Thief sampler  SAMPLING GAS Tend to be homogeneous.  Large volume of samples is required because of their low density. > Air analysis: Use a `Hi-Vol’ sampler that is containing filters to collect particulates. >Liquid displacement method: The sample must has little solubility in the liquid and DOES NOT REACT with the liquid > Breath sample: The subject could blow into evacuated bag Hi-Vol’ sampler Breath sampler Liquid displacement method METHOD OF SAMPLING GRAB SAMPLING An actual sample of air is taken in a flask/bottle/bag or suitable container. CONTINUOUS/INTEGRATED SAMPLING Gas or vapour removed from the air over a measure time period & concentrated by passage through a solid or liquid sorbent.  SAMPLE STORAGE & PRESERVATION 1. Samples storage purpose: There is a time gap between when the sample is taken and the actual analysis is being carried out. 2. For LIQUIDS samples, make sure that it is kept in bottles with stoppers. 3. ACIDIC LIQUID samples can be stored in glass containers. 4. Whereas BASIC LIQUID samples are in plastic containers. 5. SOLID SAMPLES are easier to keep and have less chance of being adulterated by foreign matters. Sometimes it can also get absorbed or adsorbed to the wall of the container.  An important aspect of the sampling process Samples are preserved to prevent from: ✔ Decomposition ✔ Precipitation of metals from water samples ✔ Loss of water from hygroscopic material ✔ Loss of volatile analytes from water sample (volatile organic compounds)  Preparing a laboratory sample Converting the sample to a useful form: ❖ Solids are usually ground to a suitable particulate size to get a homogeneous sample. ❖ Dry the samples to get rid of absorption water.  Purpose of drying solid samples To ensure that the exact weight is obtained during the QUANTITATIVE chemical analysis. How it is done ?  Solid samples dried in oven at 105 - 110oC for 1-2 hours.  Plant and tissue samples dried by heating. Problems associated with drying of samples : 1. Samples might decompose at high temperature. 2. Some samples are sensitive to heat, therefore drying can be carried out in a desiccators.  DEFINING: REPLICATE SAMPLES ❖ Replicatesamples are always performed unless the quantity of the analyte, expense or other factors prohibit. ❖ Replicate samples are portion of a material of approximately the same size that is carried through an analytical procedure at the same time and the same way.  PREPARING SOLUTIONS OF THE SAMPLE A solvent is chosen that dissolves the whole sample without decomposing the analytes SOURCES OF ERROR  Incomplete dissolution of the analyte.  Losses of analyte by the volatilization.  Introduction of analyte as a contamination.  Contamination from the reaction of the solvent with vessel walls.  SAMPLE PREPARATION & DISSOLUTION METHODS SAMPLE DISSOLUTION is the digestion or mineralization of a sample to render it soluble and to destroy organic matter that may interfere with the recovery of the analyte. Sample dissolution procedures can be divided into: 1. Dry ashing 2. Wet digestion DRY ASHING The sample is slowly combusted at a high temperature (400 – 700oC) in a muffle furnace. Atmospheric O2 serves as the oxidant where organic matter is burned off, leaving behind inorganic residue that is soluble in dilute acid. Oxidizing aids may be employed.  EXAMPLE SIMPLE DRY ASHING ❖ No chemical aids. ❖ Pb, Zn, Co, Cr, Mo, Sr, Fe traces can be recovered with little loss by retention and volatilization. ❖ Usually a porcelain crucible can be used. ❖ Example: Lead is volatilized at temperature more than 500oC, especially if chlorine is present (blood and urine samples). - Platinum (Pt) crucibles are preferred for lead for minimal retention losses. - If an oxidizing material (Mg(NO3)2) is added to sample, the ashing efficiency is enhanced. IF THE SAMPLE ARE LIQUIDS AND WET TISSUES: ✔ The sample are dried on a stream bath or by gentle heat before they are placed in a muffle furnace. ✔ The heat from the furnace should be applied GRADUALLY UP to full temperature to prevent rapid combustion and foaming. ✔ After dry ashing is complete, the residue is usually leached from the vessel with 1 or 2 mL concentrated or 6 M HCl and transfer to a flask or beaker for further treatment.  DRY ASHING ADVANTAGES DISADVANTAGES Simplicity  Volatilization of elements  Losses of analyte by retention Free from contaminations on the walls of the vessels. since few or no reagents  Adsorbed metals on the are added vessel may in turn contaminate future samples.  2-4 hour are needed for dry ashing (long hour needed) WET ASHING A method for the decomposition of an organic material such as resins or fibers, into an ASH by treatment with a boiling oxidizing acid or mixture of acids. The acids oxidize organic matter to CO2, H2O and other volatile products, which are driven off, leaving behind salts or acids of the inorganic constituents. WET ASHING/DIGESTION Principle: 1. Usually use combination of ACIDS to achieve a complete Dissolution. 2. Mixture of HNO3 and H2SO4 is often used. 3. A small amount (5 mL) of H2SO4 is used with larger volumes of HNO3 (20 to 30 mL). 4. Usually performed in a Kjeldahl flask. 5. HNO3 destroys the bulk of organic matter, but it does not get hot enough to destroy the last traces. Principle: 6. It is boiled off during the digestion process until only H2SO4 remains and dense, white SO3 fumes are evolved and begin to reflux in the flask. 7. At this point, the solution gets very hot, H2SO4 acts on the remaining organic material. 8. If the organic matter persists, more HNO3 may be added. 9. Digestion is continued until the solution clears. 10. All digestion procedures must be performed in a FUME HOOD.  WET ASHING/DIGESTION ADVANTAGES DISADVANTAGES  Superior in term of  Introduction of impurities RAPIDITY from the reagent necessary  Freedom from loss of for the reaction. analyte by retention 31  ELIMINATING INTERFERENCE  Interferences are substances that prevent direct measurement of the analyte and MUST BE REMOVED.  May included separation steps: i. Precipitation ii. Chromatography iii. Distillation iv. Dialysis v. Extraction into an immiscible solvent 32  CHAPTER 4 BASIC REQUIREMENTS FOR ANALYSIS PART 2 Prepared by Nur Nadirah  CONTENTS ⮚ Standards ⮚ Properties of primary and secondary standards ⮚ Preparation of standard solutions: primary and secondary ⮚ Calculations of required amount of reagents/standards ⮚ Storage and dilution of stock solution. Preparation of serial dilutions  WHAT IS STANDARD SOLUTION? A reagent solution of ACCURATELY KNOWN CONCENTRATION is called a STANDARD SOLUTION 2 A STANDARD SOLUTION can be prepared in either of two ways: ❖ PRIMARY STANDARD Is carefully weighed, dissolved, and diluted accurately to a known volume. Its concentration can be calculated from this data. ❖ SECONDARY STANDARD Thhe solution is made to an approximate concentration and then standardized by titrating an accurately weighed quantity of a primary standard.  WHAT IS STOCK SOLUTION? Stock solution is a concentrated solution that will be DILUTED to some lower concentrated for actual use. Stock solutions are used to save preparation time, conserve materials, reduce storage space, and improve the accuracy with which working solutions are prepared.  WHAT IS WORKING SOLUTION? Working Solution is a name given to a chemical solution made for actual use in the lab, usually made from diluting or combining stock or standard solutions.  WHAT IS PRIMARY STANDARD SOLUTION?  PRIMARY STANDARD REAGENT is highly purified compound that serves as a reference material in all volumetric/ titrimetric method.  A PRIMARY STANDARD SOLUTION can be prepared by dissolving an accurately weighed amount of the solid reagent in a solvent and diluted to an accurately known volume of solution using a volumetric flask.  Solution prepared using the primary standard will give primary standard solution.  EXAMPLE: Preparation of Na2CO3  WHAT IS SECONDARY STANDARD SOLUTION?  Secondary standard is a solution that is standardize against another standard solution.  If primary standard solution is used to standardize another solution, so after standardization the second solution is called secondary standard solution. PROPERTIES OF PRIMARY STANDARD MATERIAL/REAGENT  A primary standard must exhibit HIGH PURITY typically 100 ± 0.05 %.  A primary standard must exhibit HIGH STABILITY, preferably for years under proper storage conditions. It must be stable with respect to both spontaneous decomposition and reaction with moisture or air.  Primary standard preferably SHOULD NOT contain water of hydration or not hygroscopic.  High equivalent weight (MORESTABLE) is preferable when used for standardization solutions.  The primary standard must NOT BE COSTLY.  The primary standard should BE EASILY AVAILABLE.  The primary standard must BE READILY SOLUBLE PRIMARY STANDARD EXAMPLES The number of primary standards available is very limited. Example: ▪ oxalic acid (H2C2O4.2H2O), ▪ potassium hydrogen phthalate, ▪ sodium carbonate (Na2CO3), ▪ calcium carbonate (CaCO3), ▪ potassium dihydrogen phosphate, ▪ potassium hydrogen tartarate ▪ sodium chloride ▪ arsenic trioxide.  APR 2008 Why not use HCl or NaOH as the primary standard?  A primary standard should essentially available in pure form, stable towards light and heat and react in a stoichiometric proportion.  HCl is a gas which is dissolved in water to form the solution the concentration expressed is very approximate so HCl is not a primary standard.  NaOH cannot be weighed in open air because it is highly hygroscopic.  WHAT IS SECONDARY STANDARD SOLUTION?  A less pure substance whose composition is reliably known.  The purity or the concentration of a secondary standard must be established by careful stoichiometric analysis, usually against a primary standard.  Examples: HCl, HNO3, NaOH, KMnO4 and AgNO3.  DILUTION Dilution ⮚ Dilution - whenever you add solvent to a solution. ⮚ Adding solvent results ina solution of lower concentration. ⮚ You can calculate the concentration of a solution following a dilution by applying this equation: McVc = MdVd where M is molarity, V is volume, and the subscripts c and d refer to the concentrated and diluted values. 1  EXAMPLE How many milliliters of 5.5 M NaOH are needed to prepare 300 mL of 1.2 M NaOH? Solution: 5.5 M x V1 = 1.2 M x 0.3 L V1 = 1.2 M x 0.3 L / 5.5 M, ∴V1 = 0.065 L = 65 mL So, to prepare the 1.2 M NaOH solution, you pour 65 mL of 5.5 M NaOH into your container and add water to get 300 mL final volume Preparation of serial dilution The repeated dilution of a solution to amplify the dilution factor quickly. STANDARDIZATION STANDARDIZATION is a process to determine the concentration of a solution by titrating with a primary standard or with a solution of known concentration  To determine the concentration of impure titrant  Can be determined by titrating with a primary standard dissolved in distilled water. Ltitrant a (mol primary std) OR USE OTHER SIMPLE METHOD – BY COMPARING NO OF MOL STANDARD REAGENT & TITRANT 1. WRITE BALANCED CHEMICAL EQUATION 2. COMPARE NO OF MOL 3. FIND MOLARITY EXAMPLE An approximate solution of 0.100 M HCl was prepared by diluting the HCl stock solution. It was then standardized by titrating with 0.1905 g primary standard, Na2CO3. The titration reaction is follow: Na2CO3 + 2HCl 2NaCl H2O + CO2 The titration requires 36.10 mL of HCl. Calculate the molar concentration of HCl. RMM for Na2CO3 is 105.99 g/mol. Solution: 2 mol HCl = 1 mol Na2CO3 mol Na2CO3 = 0.1905 g/105.99 gmol-1 = 0.00179mol = 0.00179 mol Na2CO3 x 2 mol HCl/ 1 mol Na2CO3 (36.10 mL/1000mL) MHCl =0.09956M EXERCISE Ans: 0.104 M Pure (AR grade) anhydrous sodium carbonate (Na2CO3) is used as a standard to determine the concentration of a solution of HCl. The standardized HCl was then used to determine the concentration of a solution of NaOH. (RMM for Na2CO3 is 105.99 g/mol) ▪Mass Na2CO3 transferred to the titration flask 0.2222 g ▪Titre of HCl used in the Na2CO3 titration 24.53 mL ▪Aliquot of NaOH solution 25.00 mL ▪Titre of HCl used in the NaOH titration 16.75 mL Calculate the concentration of the NaOH solution Answer: Need to find MHCl = 0.1709 M the molarity of HCl MNaOH = 0.1145 M first END