611ProblemSet112010

611ProblemSet112010 - CHEMISTRY C611 ELECTROANALYTICAL...

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Unformatted text preview: CHEMISTRY C611: ELECTROANALYTICAL CHEMISTRY Fall, 2010 Problem Set 11 As background for this problem set, the following information is provided. When an ion- selective electrode is used to determine the activity of a cation or anion, the voltage of the cell employed for the measurement can be expressed as follows: 0.05915 Em” = constant : 10g [flunkmwn] n where n is the charge on the particular cation or anion of interest and aunkmwn is the activity of the cation or anion of interest. When a cation is being determined, a plus (+) sign is used before the logarithmic term; and, when an anion is being determined, a minus (—) sign is used before the logarithmic term. I. A sodium-selective membrane electrode was fabricated from a glass composed of l 1% NazO. 18% A1203, and 71% SiOz. Then, the sodium-selective electrode and a saturated calomel reference electrode were dipped into an aqueous sodium chloride solution of unknown concentration to produce the following galvanic ceil: Ag i it'tthlts]= NaCl {1.110 M) | ”sodium-selective glass membrane | solution of unknown pNa || HgIClgts). KClts] | 1 lg When the voltage of the cell was measured, a value of 0.2537 volt was obtained. Then, when the unknown solution was replaced with a sodium chloride solution having a known sodium ion activity of 2.63 X it] 3 M, the voltage ofthe cell was found to be 0.1612 volt. Calculate the sodium ion activity in the unknown solution. 2. A liquid ion-exchange membrane electrode useful for the determination of ammonium ion was prepared by impregnation of a Millipore filter with a saturated solution of T2% nonactin and 28% monactin dissolved in tris(2-ethylhexyl)phosphate. A silver—silver chloride electrode was placed behind the ammonium-selective membrane, and a saturated calomel electrode was combined with the ammonium ion electrode to make the following galvanic cell: Ag | AgClts], Cl" (0.0100 M) | nonactimmonnctin membrane | unknown NH.+ solution ll HggCIlzts), KClts} | Hg When two different solutions having ammonium ion activities of 3.57 x 10"3 M and 1.47 X 10—5 M were placed in the cell. the observed voltages were 0.2415 and 0.1003 volt. respectively. What is the ammonium ion activity ofa sample solution that yields a cell voltage of0.1349 volt? 3. Suppose that, for the preceding problem, it was discovered that the “constant" in the Nernst equation near the top ofthe page had a different value for each ofthe two different known ammonium-ion activities. Explain why this might occur. What strategy would you employ to handle this situation in order to obtain the Nl—Ii' activity in the unknown sample solution? ...
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