D. Harvey - Modern Analytical Chemistry (794078), страница 66
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As shown in Example 7.17and Figure 7.26, the extraction efficiency for a divalent cation increases fromapproximately 0%–100% over a range of only 2 pH units. Furthermore, achelating agent’s metal–ligand formation constant varies substantially betweenmetal ions. As a result, significant differences arise in the pH range over whichdifferent metal ions experience an increase in extraction efficiency from 0% to100% (Figure 7.27).1400-CH07 9/8/99 4:04 PM Page 223Chapter 7 Obtaining and Preparing Samples for Analysis223(c)100(a)90(b)(d)(e)(f)Percent extracted80706050403020Figure 7.27100–4–202468101214pHPlot of extraction efficiency for selectedmetals using dithizone in CCl4.
The metalions are: (a) Cu2+; (b) Co2+; (c) Ni2+; (d) Sn2+;(e) Pb2+; and (f) Cd2+.EXAMPLE 7.18Using Figure 7.27, explain how an aqueous mixture of Cu2+, Pb2+, and Cd2+can be separated by extraction with dithizone in CCl4.SOLUTIONFrom Figure 7.27 we see that a quantitative separation of Cu2+ from Pb2+ andCd2+ can be accomplished if the aqueous phase is buffered to a pH of less than5.5. After the extraction is complete, the pH can be buffered to approximately9.5, allowing the selective extraction of Pb2+.Liquid–liquid extractions using ammonium pyrrolidine dithiocarbamate(APDC) as a metal chelating agent are commonly encountered in the analysis ofmetal ions in aqueous samples.
The sample and APDC are mixed together, and theresulting metal–ligand complexes are extracted into methyl isobutyl ketone beforeanalysis.7H Separation Versus PreconcentrationTwo frequently encountered analytical problems are: (1) the presence of matrixcomponents interfering with the analysis of the analyte; and (2) the presence of analytes at concentrations too small to analyze accurately.
We have seen how a separation can be used to solve the former problem. Interestingly, separation techniquescan often be used to solve the second problem as well. For separations in which acomplete recovery of the analyte is desired, it may be possible to transfer the analytein a manner that increases its concentration. This step in an analytical procedure isknown as a preconcentration.Two examples from the analysis of water samples illustrate how a separationand preconcentration can be accomplished simultaneously.
In the gas chromatographic analysis for organophosphorous pesticides in environmental waters, the analytes in a 1000-mL sample may be separated from their aqueous matrix by a solidphase extraction using 15 mL of ethyl acetate.23 After the extraction, the analytes arepresent in the ethyl acetate at a concentration that is 67 times greater than that inpreconcentrationThe process of increasing an analyte’sconcentration before its analysis.1400-CH07 9/8/99 4:04 PM Page 224224Modern Analytical Chemistrythe original sample (if the extraction is 100% efficient). The preconcentration ofmetal ions is accomplished by a liquid–liquid extraction with a metal chelator. Forexample, before their analysis by atomic absorption spectrophotometry, metal ionsin aqueous samples can be concentrated by extraction into methyl isobutyl ketone(MIBK) using ammonium pyrrolidine dithiocarbamate (APDC) as a chelatingagent.
Typically, a 100-mL sample is treated with 1 mL of APDC, and extracted withten mL of MIBK. The result is a ten-fold increase in the concentration of the metalions. This procedure can be adjusted to increase the concentrations of the metalions by as much as a factor of 40.7I KEY TERMSbreakthrough volume (p. 196)composite sample (p. 186)coning and quartering (p. 199)convenience sampling (p.
185)dialysis (p. 206)distribution ratio (p. 216)extraction (p. 212)grab sample (p. 185)gross sample (p. 193)in situ sampling (p. 186)judgmental sampling (p. 184)laboratory sample (p. 199)masking (p. 207)masking agent (p. 208)Nyquist theorem (p. 184)partition coefficient (p. 211)preconcentration (p. 223)purge and trap (p. 214)random sample (p. 183)recovery (p.
202)sampling plan (p. 182)separation factor (p. 203)size-exclusion chromatographystratified sampling (p. 185)systematic–judgmentalsampling (p. 184)systematic sampling (p. 184)supercritical fluid (p. 215)(p. 206)7J SUMMARYAn analysis requires a sample, and how we acquire the sample iscritical. To be useful, the samples we collect must accurately represent their target population. Just as important, our sampling planmust provide a sufficient number of samples of appropriate size sothat the variance due to sampling does not limit the precision ofour analysis.A complete sampling plan requires several considerations,including the type of sampling (random, judgmental, systematic,systematic–judgmental, stratified, or convenience); whether to collect grab samples, composite samples, or in situ samples; whetherthe population is homogeneous or heterogeneous; the appropriatesize for each sample; and, the number of samples to collect.Removing a sample from its population may induce a changein its composition due to a chemical or physical process.
For thisreason, samples are collected in inert containers and are often preserved at the time of collection.When the analytical method’s selectivity is insufficient,it may be necessary to separate the analyte from potentialinterferents. Such separations can take advantage of physicalproperties, such as size, mass or density, or chemical properties. Important examples of chemical separations include masking, distillation, and extractions.In a solid-phase extraction the analytes are first extractedfrom their solution matrix into a solid adsorbent.
After washing to remove impurities, the analytes are removed from theadsorbent with a suitable solvent. Alternatively, the extractioncan be carried out using a Soxhlet extractor.In a liquid–liquid extraction, the analyte (or interferent) is extracted from one liquid phase into a second, immiscible liquidphase. When the analyte is involved in secondary equilibrium reactions, it is often possible to improve selectivity by carefully adjusting the composition of one or both phases.1400-CH07 9/8/99 4:04 PM Page 225Chapter 7 Obtaining and Preparing Samples for Analysis225Experiments7K Suggested EXPERIMENTSThe following set of experiments introduce students to the important effect of sampling on the qualityof analytical results. Each experiment is annotated with a brief description of the principles that itemphasizes.Bauer, C.
F. “Sampling Error Lecture Demonstration,” J.Chem. Educ. 1985, 62, 253.This short paper describes a demonstration suitable foruse in the classroom. Two populations of corks are sampledto determine the concentration of labeled corks. The exercisedemonstrates how increasing the number of particlessampled improves the standard deviation due to sampling.Clement, R. E.
“Environmental Sampling for Trace Analysis,”Anal. Chem. 1992, 64, 1076A–1081A.Sampling of a large population (n = 900) of coloredcandies (M&M’s work well) is used to demonstrate theimportance of sample size in determining the concentrationof species at several different concentration levels. Thisexperiment is similar to the preceding one described by Bauerbut incorporates several analytes.collect a sufficient amount of data in a single laboratoryperiod. The overall variance of the analysis is partitioned intocomponents due to the method, to the preparation ofsamples, and to sample collection. The validity of equation7.5 is also evaluated.Kratochvil, B.; Reid, R.
S.; Harris, W. E. “Sampling Error in aParticulate Mixture,” J. Chem. Educ. 1980, 57, 518–520.In this experiment the overall variance for the analysis ofpotassium hydrogen phthalate (KHP) in a mixture of KHPand sucrose is partitioned into that due to sampling and thatdue to the analytical method (an acid–base titration). Byhaving individuals analyze samples with different % w/wKHP, the relationship between sampling error andconcentration of analyte can be explored.Guy, R. D.; Ramaley, L.; Wentzell, P. D. “An Experiment inthe Sampling of Solids for Chemical Analysis,” J. Chem. Educ.1998, 75, 1028–1033.Lochmuler, C.
“Atomic Spectroscopy—Determination ofCalcium and Magnesium in Sand with a Statistical Treatmentof Measurements” published on the web athttp://www.chem.duke.edu/~clochmul/exp4/exp4.html.This experiment uses the molybdenum-blue method todetermine the concentration of phosphate in aphosphate/sodium chloride mixture. Flow-injection analysisis used to increase the speed of analysis, allowing students toThis experiment introduces random sampling. Theexperiment’s overall variance is divided into that due to theinstrument, that due to sample preparation, and that due tosampling.The following experiments describe homemade sampling devices for collecting samples in the field.Delumyea, R. D.; McCleary, D.
L. “A Device to CollectSediment Cores,” J. Chem. Educ. 1993, 70, 172–173.Directions are provided for preparing and using a simplecoring device using PVC pipe. This experiment also details aprocedure for determining the weight percent of organicmaterial in sediments as a function of depth.Rockwell, D. M.; Hansen, T. “Sampling and Analyzing AirPollution,” J. Chem. Educ. 1994, 71, 318–322.Two simple air samplers are described as well as their usefor determining particulates in air.Saxena, S.; Upadhyay, R.; Upadhyay, P.
“A Simple and LowCost Air Sampler,” J. Chem. Educ. 1996, 73, 787–788.This experiment describes the construction of an airsampler using an aquarium pump, a flow meter, a filterholder, and bottles that serve as traps for analytes.Applications include the determinations of SO2, NO2,HCHO, and suspended particulate matter.Shooter, D. “Nitrogen Dioxide and Its Determination in theAtmosphere,” J.
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