Page 117 - Modern Analytical Chemistry
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              100    Modern Analytical Chemistry


              18. To test a spectrophotometer for its accuracy, a solution of  Atmospheric  2.31017  2.30986  2.31010  2.31001
                 60.06 ppm K 2 Cr 2 O 7 in 5.0 mM H 2 SO 4 is prepared and  Origin:  2.31024  2.31010  2.31028
                 analyzed. This solution has a known absorbance of 0.640 at  Chemical  2.30143  2.29890  2.29816  2.30182
                 350.0 nm in a 1.0-cm cell when using 5.0 mM H 2 SO 4 as a  Origin:  2.29869  2.29940  2.29849  2.29889
                 reagent blank. Several aliquots of the solution are analyzed
                                                                    Explain why these data led Rayleigh to look for and discover Ar.
                 with the following results
                                                                 23. Gács and Ferraroli reported a new method for monitoring the
                   0.639  0.638  0.640  0.639  0.640  0.639  0.638                      25
                                                                    concentration of SO 2 in air. They compared their method
                 Determine whether there is a significant difference between  with the standard method by sampling and analyzing urban
                 the experimental mean and the expected value at a= 0.01.  air from a single location. Air samples were collected by
              19. Monna and co-workers studied the use of radioactive isotopes  drawing air through a collection solution for 6 min. Following
                 as a means of dating sediments collected from the bottom of  is a summary of their results with SO 2 concentrations
                               23
                 lakes and estuaries. To verify this method they analyzed a  reported in microliters per cubic meter.
                 208 Po standard known to have an activity of 77.5 decays/min,  Standard
                 obtaining the following results                  method: 21.62  22.20  24.27  23.54  24.25  23.09 21.02
                 77.09   75.37   72.42   76.84   77.84   76.69    New
                                                                  method: 21.54  20.51  22.31  21.30  24.62  25.72 21.54
                 78.03   74.96   77.54   76.09   81.12   75.75
                                                                    Using an appropriate statistical test, determine whether there
                 Determine whether there is a significant difference between
                                                                    is any significant difference between the standard and new
                 the mean and the expected value at a= 0.05.
                                                                    methods at a= 0.05.
              20. A 2.6540-g sample of an iron ore known to contain 53.51%
                                                                 24. The accuracy of a spectrophotometer can be checked
                 w/w Fe is dissolved in a small portion of concentrated HCl
                                                                    by measuring absorbances for a series of standard
                 and diluted to volume in a 250-mL volumetric flask. A
                                                                    dichromate solutions that can be obtained in sealed
                 spectrophotometric method is used to determine the
                                                                    cuvettes from the National Institute of Standards and
                 concentration of Fe in this solution, yielding results of 5840,
                                                                    Technology. Absorbances are measured at 257 nm and
                 5770, 5650, and 5660 ppm. Determine whether there is a
                                                                    compared with the accepted values. The results
                 significant difference between the experimental mean and the
                                                                    obtained when testing a newly purchased
                 expected value at a= 0.05.
                                                                    spectrophotometer are shown here. Determine if
              21. Horvat and colleagues investigated the application of atomic  the tested spectrophotometer is accurate at a= 0.05.
                 absorption spectroscopy to the analysis of Hg in coal fly ash. 24
                 Of particular interest was the development of an appropriate  Standard:  1  2  3      4      5
                 procedure for digesting the samples in order to release the Hg  Measured
                 for analysis. As part of their study they tested several reagents for  absorbance:  0.2872 0.5773  0.8674  1.1623  1.4559
                 digesting samples. Results obtained with HNO 3 and with a 1 + 3  Accepted
                 mixture of HNO 3 and HCl are shown here. All concentrations  absorbance:  0.2871 0.5760  0.8677  1.1608  1.4565
                 are given as nanograms of Hg per gram of sample.
                                                                 25. Maskarinec and associates investigated the stability of volatile
                                                                                                  26
                        HNO 3 :  161  165  160  167  166            organics in environmental water samples. Of particular
                 1 + 3 HNO 3 –HCl:  159  145  140  147  143  156    interest was establishing proper conditions for maintaining
                                                                    the sample’s integrity between its collection and analysis. Two
                 Determine whether there is a significant difference between
                                                                    preservatives were investigated (ascorbic acid and sodium
                 these methods at a= 0.05.
                                                                    bisulfate), and maximum holding times were determined for
              22. Lord Rayleigh, John William Strutt (1842–1919) was one of  a number of volatile organics and water matrices. Results (in
                 the most well-known scientists of the late nineteenth and  days) for surface waters follow.
                 early twentieth centuries, publishing over 440 papers and
                                                                                            Ascorbic   Sodium
                 receiving the Nobel Prize in chemistry in 1904 for the
                                                                                              acid     bisulfate
                 discovery of argon. An important turning point in the
                 discovery of Ar was Rayleigh’s experimental measurements   methylene chloride  77       62
                 of the density of N 2. Rayleigh approached this experiment in  carbon disulfide  23     54
                 two ways: first by taking atmospheric air and removing any  trichloroethane   52        51
                 O 2 and H 2 that was present; and second, by chemically
                                                                     benzene                   62        42
                 producing N 2 by decomposing nitrogen-containing
                                                                     1,1,2-trichloroethane     57        53
                 compounds (NO, N 2 O, and NH 4 NO 3 ) and again removing
                                                                     1,1,2,2-tetrachloroethane  33       85
                 any O 2 and H 2 . His results for the density of N 2 , published in
                                                                     tetrachloroethene         41        63
                 Proc. Roy. Soc. 1894, LV, 340 (publication 210), follow (all
                 values are for grams of gas at equivalent volume, pressure,  toluene          32        94
                 and temperature).                                   chlorobenzene             36        86
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