Analytical Chem istry - DePauw University

346 Analytical Chemistry 2.0for Cu 2+ . Preserving samples by first adding H 2 O 2 and then addingHNO 3 , eliminated the determinate error. Explain their observations.17. In a particular analysis the selectivity coefficient, K A,I , is 0.816. Whena standard sample with an analyte-to-interferent ratio of 5:1 is carriedthrough the analysis, the error in determining the analyte is +6.3%.(a) Determine the apparent recovery for the analyte if R I = 0. (b) Determinethe apparent recovery for the interferent if R A = 0.18. The amount of Co in an ore is to be determined using a procedure forwhich Fe in an interferent. To evaluate the procedure’s accuracy, a standardsample of ore known to have a Co/Fe ratio of 10.2:1 is analyzed.When pure samples of Co and Fe are taken through the procedure thefollowing calibration relationships are obtainedSSCoFe= 0.786×m= 0.669×mwhere S is the signal and m is the mass of Co or Fe. When 278.3 mg ofCo are taken through the separation step, 275.9 mg are recovered. Only3.6 mg of Fe are recovered when a 184.9 mg sample of Fe is carriedthrough the separation step. Calculate (a) the recoveries for Co and Fe;(b) the separation factor; (c) the error if no attempt is made to separatethe Co and Fe before the analysis; (d) the error if the separation stepis carried out; and (e) the maximum recovery for Fe if the recovery forCo is 1.00, and the maximum allowed error is 0.05%.19. The amount of calcium in a sample of urine was determined by a methodfor which magnesium is an interferent. The selectivity coefficient,K Ca,Mg , for the method is 0.843. When a sample with a Mg/Ca ratioof 0.50 was carried through the procedure, an error of –3.7% was obtained.The error was +5.5% when using a sample with a Mg/Ca ratioof 2.0. (a) Determine the recoveries for Ca and Mg. (b) What is theexpected error for a urine sample in which the Mg/Ca ratio is 10.0?20. Using the formation constants in Appendix 12, show that F – is aneffective masking agent for preventing a reaction between Al 3+ andEDTA. Assume that the only significant forms of fluoride and EDTAare F – and Y 4- .21. Cyanide is frequently used as a masking agent for metal ions. Its effectivenessas a masking agent is better in more basic solutions. Explainthe reason for this pH dependency.CoFe