F = 96485 coul/mole e^- name_______________

EXAM III, FALL 1999

1. A 0.0150 M solution of Fe(CN)₆^-4 is to be oxidized to Fe(CN)₆^-3 by constant potential electrolysis (potentiostatic coulometry). E° (Fe(CN)₆^-3 + e^- Û Fe(CN)₆^-4) is +0.36 v. What voltage must be applied to the working electrode to decrease the ferrocyanide to 1.0 x 10^-6M? The reference electrode is an SCE with a potential of +0.242 v.

2. Al⁺³ is to be determined indirectly by constant current coulometry. A sample containing aluminum was treated with excess quinoline, Q, to precipitate Al(Q)₃^.. The precipitate was filtered, washed and dissolved in 50 ml of acid 0.20 F NaBr solution. This solution was then subjected to constant current electrolysis to generate bromine at the working electrode: 2Br^- Þ Br₂ + 2e^-. The bromine in turn reacted with the quinoline: 2Br₂ + Q Þ QBr₂ + 2 HBr . The Al sample required 734.4 seonds for titration at a current of 0.00482 amps. How many mg of Al were present in the sample?

3. A.The analysis described in problem 2 did not require a coulometer, which would have been necessary for constant potential coulometry. Why not?

B. While the method described in 2 required a method of detecting the end point, constant potential coulometry would not need end point detection. Why not?

4. A. Distinguish between linear scan voltammetry and differential pulse polarography.

B. Why are stipping methods more sensitive than other voltammetry procedures?

C. Why are residual currents smaller with pulse methods than with linear scan (DC) methods?

5. A. In linear sweep voltammetry, how would you check for simple diffusion control for the process Ox + ne Û Red?

B. How would you check for reversibility in pulse polarography?

C. In cyclic sweep voltammetry, how could you show that Red was (or was not) involved in a following chemical reaction such as Red à Z?

D. Sketch the amperometric titration curve with two polarized electrodes expected for the titration of I₃^- with S₂O₃^-2 . In this case, only the I₃^-/I^- couple is reversible at the electrodes; thiosulfate shows no reactions at the electrode surface.

6. I. List or describe the variables that cause peak broadening in chromatography for each the following terms:

A. Longitudinal Diffusion.

B. Eddy Diffusion.

C. Resistance to mass transfer in the stationary phase.

D. What is the difference between ion chromatography and ion pair chromatography?

7. A.Two compounds are separated with a resolution of 0.75 on a 30 cm column. What column length would be required to achieve a resolution of 1.50, assuming that other parameters do not change?

B. The following data were obtained from a gas chromatograph of naphthalene on a 15 meter OV17 column: t_m = 3.50 min; t_R = 5.30 min.

A. What is the linear flow rate, u, in cm/sec?

B. Find the retention (capacity) factor, k’. (Do you need the value of the compressibility factor?)

C. What other information do you need, to find the partition coefficient for naphthalene on this column?

8. It is desired to effect a separation of a series of aromatic compounds on a non-polar stationary phase. When acetonitrile (P’ = 5.8) was employed as a mobile phase, a capacity factor, k’, of a significant compound was found to be 19.

A. What polarity should the mobile phase have to yield a k’ of 5?

B. What volume fraction of tetrahydrofuran (P’ = 4.0) should be added to acetonitrile to achieve the desired polarity?