Analytical Chem istry - DePauw University

Chapter 3 The Vocabulary of Analytical Chemistry45Spectroscopy (Chapter 10) and electrochemistry (Chapter 11), inwhich an optical or electrical signal is proportional to the relative amountof analyte in a sample, are examples of concentration techniques. Therelationship between the signal and the analyte’s concentration is a theoreticalfunction that depends on experimental conditions and the instrumentationused to measure the signal. For this reason the value of k A in equation3.2 must be determined experimentally.Since most concentration techniques relyon measuring an optical or electrical signal,they also are known as “instrumental”techniques.3DSelecting an Analytical MethodA method is the application of a technique to a specific analyte in a specificmatrix. We can develop an analytical method for determining the concentrationof lead in drinking water using any of the techniques mentionedin the previous section. A gravimetric method, for example, might precipitatethe lead as PbSO 4 or PbCrO 4 , and use the precipitate’s mass as theanalytical signal. Lead forms several soluble complexes, which we can useto design a complexation titrimetric method. As shown in Figure 3.2, wecan use graphite furnace atomic absorption spectroscopy to determine theconcentration of lead in drinking water. Finally, the availability of multipleoxidation states (Pb 0 , Pb 2+ , Pb 4+ ) makes electrochemical methods feasible.The requirements of the analysis determine the best method. In choosinga method, consideration is given to some or all the following designcriteria: accuracy, precision, sensitivity, selectivity, robustness, ruggedness,scale of operation, analysis time, availability of equipment, and cost.3D.1 AccuracyAccuracy is how closely the result of an experiment agrees with the “true”or expected result. We can express accuracy as an absolute error, eor as a percentage relative error, %e re = obtained result - expected resultobtained result − expected result% e =×100rexpected resultA method’s accuracy depends on many things, including the signal’s source,the value of k A in equation 3.1 or equation 3.2, and the ease of handlingsamples without loss or contamination. In general, methods relying on totalanalysis techniques, such as gravimetry and titrimetry, produce results ofhigher accuracy because we can measure mass and volume with high accuracy,and because the value of k A is known exactly through stoichiometry.Since it is unlikely that we know the trueresult, we use an expected or acceptedresult when evaluating accuracy. For example,we might use a reference standard,which has an accepted value, to establishan analytical method’s accuracy.You will find a more detailed treatment ofaccuracy in Chapter 4, including a discussionof sources of errors.