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Unformatted text preview: CHEMISTRY C611: ELECTROANALYTICAL CHEMISTRY Fall, 2010 Problem Set 8 . For chronopotentiometry. the Sand equation relates the transition time (I) to the constant current (i) and the bulk concentration (0“). A cathodic chronopotcntiogram obtained with a planar platinum electrode in l M hydrochloric acid containing aqua! concentrations of iron(lll) and copperfll) is shown below. (a) Write all of the electrode reactions which are occurring at the platinum cathode during each of the following time periods: 0 d l < T] (ii) 1:; < t < T2 (iii) 1:2 < t < I} (iv) l 3) “[3 (b) If the diffusion coefficients are identical for all species in the solution, what should be the relative sizes of 1:1, 1:3, and 1:3? Show your calculations. (I—Iint: No formulas other than the Sand equation itself are needed.) Data: Fe“ + e Fa” ; a“ = +0.78 Vvs. SHE (:u2+ + ZCI‘ + e :4- Cuth: a“ = +0.46 VvsSHl'i CuClz + e‘ --—* Cu 4.- 2Cl‘ : [5" = +0.18 Vvs. SHE 2. According to Bard and Faulkner (equation 5.3.]. page fill. the current—time behavior for a spherical or hemispherical ultramicroclcctrode is described by the relation r = rampant/{floats} + pm}- where r1, is the radius ofthc spherical or hemispherical ultramicroelectrode. Note that the first term inside the {} marks is the time-dependent term seen in the classic Cottrell equation for linear diffusion and that the second term inside the {} marks is a steady-state term arising From diffusion to the spherical or hemispherical Ltltramicro~ electrode. Assume that you wish to do a potential-step experiment with a hemispher- ical ultramicroelectrode having a radius (r0) of 10 um. In addition, suppose that the electroactive species ofinterest has a diffusion coefficient (D) 015.0 x 10—6 cm2 3'1. ( a) At what value of time (t) will the steady-state contribution to the current still be no more than 1% ofthe timesdependent (Cottrell) contribution to the current? (b) At what value of time (I) will the steady-state contribution to the current become 100 times the diffirsion-controlled contribution to the current? to) At what value of time (i) will these two contributions be equal? 3. U sing the results from problem 2: calculate the thickness of the so-called Nernst diffu- sion layer (a) at the times corresponding to parts (a). (b), and (c) from the following equation a = razor)” . . . . . . . . . 7 _ . where D is the dttfusron coeffiment ot the electroactwe SpCCtCS m cm“ s I, and r 1s the time in seconds. ...
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