NURUL AUNI ZAINAL ABIDIN FACULTY OF APPLIED SCIENCE UITM NEGERI SEMBILAN

1. CHAPTER 4 BASIC REQUIREMENTS FOR ANALYSIS NURUL AUNI ZAINAL ABIDIN FACULTY OF APPLIED SCIENCE UITM NEGERI SEMBILAN

2. Sampling Sampling is the process to get a representative and homogeneous sample. Representative means that content of analytical sample reflects content of bulk sample. Homogeneous means that the analytical sample has the same content throughout.

3. The analysis may be classed as: • MethodSample Weight (mg)Sample Volume (L) i) Meso >100>100 ii) Semimicro 10 – 100 50 - 100 iii) Micro 1 – 10 < 50 iv) Ultramicro < 1 • Classification of the constituents in a sample i) Major> 1 % ii) Minor0.1 – 1 % iii)Trace< 0.1 % iv) Ultratracein the range of few parts per million • or less.

4. Sampling • Deciding how to obtain a sample for analysis depend on: • i. The size of the bulk to be sampled. • ii. The physical state of the fraction to be analyzed. • (solid, liquid, gas) • iii. The chemistry of the material to be assayed. • (Nothing can be done that would destroy or alter the identity or quantity of the analyte)

5. 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 the analysis sample.

6. Steps involved in sampling bulk material

7. Sampling Solids • Inhomogeneity of the material, make sampling of solids more difficult. • The easiest way to sample a material is grab sample– the sample taken at randomand 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. APR 2008

8. Coning and Quartering Method of Sampling Solids • This process is continued until the gross sample is small enough to be transported to the laboratory. OCT 2006

9. Cone and Quarter 

10. Sampling Liquids • Liquid samples are homogeneous and are much easier to sample. • The gross sample can be relatively small. • If liquid samples are not homogeneous, and have only small quantity, they can be shaken and sampled immediately. OCT 2006

11. Sampling Liquids • Sampling techniques will depend on the types of liquid. i) Large volume of liquids (impossible to mix) ii) Large stationary liquids (lakes, rivers) iii) Biological fluids

12. Sampling Gases • 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. OCT 2006

13. Sample Storage and Preservation 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 samples.

14. Sample Storage and Preservation 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.

15. Modern balances are electronic. They still compare one mass against another since they are calibrated with a known mass. Common balances are sensitive to 0.1 mg. Fig. 4.1. Electronic analytical balance.

16. Weighing bottles are used for drying samples. Hygroscopic samples are weighed by difference, keeping the bottle capped except when removing the sample. Fig. 4.2. Weighing bottles.

17. A weighing dish or boat is used for direct weighing of samples. Fig. 4.3. Weighing dish.

18. Volumetric flasks are calibrated to contain an accurate volume. See the inside back cover of the text for tolerances of Class A volumetric glassware. Fig. 4.4. Volumetric flask.

19. Volumetric pipets accurately deliver a fixed volume. A small volume remains in the tip. Fig. 4.5. Transfer of volumetric pipets.

20. Measuring pipets are straight-bore pipets marked at different volumes. They are less accurate than volumetric pipets. Fig. 4.6. Measuring pipets.

21. Syringe pipets precisely deliver microliter volumes. They are commonly used to introduce samples into a gas chromatograph. Fig. 4.7. Hamilton microliter syringe.

22. These syringe pipets can reproducibly deliver a selected volume. They come in fixed and variable volumes. The plastic tips are disposable. Fig. 4.8. Single-channel and multichannel digital displacement pipets and microwell plates.

23. A 50-mL buret is marked in 0.1 mL increments. You interpolate to 0.01 mL, good to about ±0.02 mL. Two readings are taken for every volume measurement. Fig. 4.9. Typical buret.

24. Position the black field just below the meniscus. Avoid parallax error by reading at eye level. Fig. 4.10. Meniscus illuminator.

25. Place the flask on a white background. Place the buret tip in the neck of the flask while your swirl. Fig. 4.11. Proper technique for titration.

26. Use these for quantitative transfer of precipitates and solutions, and for washing precipitates. Fig. 2.20. Wash bottles: (a) polyethylene, squeeze type; (b) glass, blow type.

27. Defining replicate samples • Replicate samples 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.

28. Preparing Solutions of the Sample • A solvent is chosen that dissolves the whole sample without decomposing the analyte. • Sources of error : i) Incomplete dissolution of the analyte. ii) Losses of analyte by the volatilization. iii) Introduction of analyte as a solvent contamination. iv) Contamination from the reaction of the solvent with vessel walls. OCT 2007

29. SAMPLE PREPARATION AND DISSOLUTION • 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 3: i) Dry ashing– Performed at a high temperature (400 – 700 oC) in a muffle furnace. Atmospheric O2 serves as the oxidant, that is organic matter is burned off, leaving inorganic residue. ii) Oxidative ashing iii) Wet digestion – A method for the decomposition of an organic material, such as resins or fibers, into an ash by treatment with nitric or sulfuric acids.

30. DRY ASHING • 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 T more than 500 oC, especially if chlorine is present (blood and urine samples). Pt crucible are preferred for lead for minimal retention losses. If an oxidizing material (Mg(NO3)2) is added to sample, the ashing efficiency is enhanced.

31. DRY ASHING • 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 T to prevent rapid combustion and foaming.

32. WET DIGESTION • Usually use combination of acids to achieve a complete dissolution. • A small amount (5 mL) of H2SO4 is used with larger volumes of HNO3 (20 to 30 mL). • Usually performed in aKjeldahlflask. • HNO3 destroys the bulk of organic matter, but it does not get hot enough to destroy the last traces. • 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. • At this point, the solution gets very hot, H2SO4 acts on the remaining organic material. APR 2007

33. WET DIGESTION • If the organic matter persists, more HNO3 may added. • Digestion is continued until the solution clears. • All digestion procedures must be performed in a fume hood.

34. Eliminating Interferences • Interferences are substances that prevent direct measurement of the analyte and must be removed.

35. Standard solutions •  Definition: Standard solutions are solution whose concentrations are known to a high degree of accuracy. • Characteristics: i) Maintain its concentration over a long period of time (months or years) after preparation. This eliminates the need for restandardization. ii) Must be able to undergo rapidly, stoichiometric, and complete reaction with the analyte.

36. A standard solution can be prepared in either of two ways: 1. A primary standard is carefully weighed, dissolved, and diluted accurately to a known volume. Its concentration can be calculated from this data. 2. A solution is made to an approximate concentration and then standardized by titrating an accurately weighed quantity of a primary standard. • Types of standard solutions: i) Primary standard ii) Seconday standard

37. Primary Standard •  Definition: A compound of highest purity and it is used to determine, directly or indirectly, the concentration of the standard solution for a titration. • Ideal primary standards for volumetric titration should have the following characteristics: i) Highest purity, up to 99.99% (0.01 to 0.02% impurity). ii) Stability, the substance should stable at room conditions or during heating and does not react with constituents of the atmosphere. iii) Free from hydrated water and should be nonhygroscopic. iv) Soluble in titration medium. v) High formula weight to minimize weighing errors. vi) Easily available at reasonable cost.

38. Primary Standard • The number of primary standards available is very limited e. g. oxalic acid (H2C2O4.2H2O), sodium carbonate (Na2CO3), calcium carbonate (CaCO3), sodium chloride and arsenic trioxide.

39. 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 its not a primary standard. • NaOH cannot be weighed in open air because it is highly hygroscopic. APR 2008

40. Secondary Standard • 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.