Analytical Chem istry - DePauw University

Chapter 4 Evaluating Analytical Data137Before the titration, samples of the steel were dissolved in acid and thechromium oxidized to Cr(VI) using peroxydisulfate. Shown here arethe results ( as %w/w Cr) for the analysis of a reference steel.16.968 16.922 16.840 16.88316.887 16.977 16.857 16.728Calculate the mean, the standard deviation, and the 95% confidenceinterval about the mean. What does this confidence interval mean?18. Ketkar and co-workers developed an analytical method for determiningtrace levels of atmospheric gases. 20 An analysis of a sample containing40.0 parts per thousand (ppt) 2‐chloroethylsulfide yielded the followingresults43.3 34.8 31.937.8 34.4 31.942.1 33.6 35.3(a) Determine whether there is a significant difference between theexperimental mean and the expected value at a = 0.05. (b) As part ofthis study a reagent blank was analyzed 12 times, giving a mean of 0.16ppt and a standard deviation of 1.20 ppt. What are the IUPAC detectionlimit, the limit of identification, and limit of quantitation for thismethod assuming a = 0.05?19. To test a spectrophotometer’s accuracy a solution of 60.06 ppm K 2 Cr 2 O 7in 5.0 mM H 2 SO 4 is prepared and analyzed. This solution has an expectedabsorbance of 0.640 at 350.0 nm in a 1.0-cm cell when using5.0 mM H 2 SO 4 as a reagent blank. Several aliquots of the solutionproduce the following absorbance values.0.639 0.638 0.640 0.639 0.640 0.639 0.638Determine whether there is a significant difference between the experimentalmean and the expected value at a = 0.01.Many of the problems that follow require accessto statistical tables. For your convenience,here are hyperlinks to the appendices containingthese tables.Appendix 3: Single-Sided Normal DistributionAppendix 4: Critical Values for the t-TestAppendix 5: Critical Values for the F-TestAppendix 6: Critical Values for Dixon’s Q-TestAppendix 7: Critical Values for Grubb’s Test20. Monna and co-workers used radioactive isotopes to date sedimentsfrom lakes and estuaries. 21 To verify this method they analyzed a 20 8Postandard known to have an activity of 77.5 decays/min, obtaining thefollowing results.77.09 75.37 72.42 76.84 77.84 76.6978.03 74.96 77.54 76.09 81.12 75.75Determine whether there is a significant difference between the meanand the expected value at a = 0.05.20 Ketkar, S. N.; Dulak, J. G.; Dheandhanou, S.; Fite, W. L. Anal. Chim. Acta 1996, 330, 267–270.21 Monna, F.; Mathieu, D.; Marques, A. N.; Lancelot, J.; Bernat, M. Anal. Chim. Acta 1996, 330,107–116.