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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 timedependent term seen in the classic
Cottrell equation for linear diffusion and that the second term inside the {} marks is a
steadystate term arising From diffusion to the spherical or hemispherical Ltltramicro~
electrode. Assume that you wish to do a potentialstep 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 steadystate 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 steadystate contribution to the current become
100 times the difﬁrsioncontrolled 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 socalled 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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 Fall '10
 DennisG.Peters

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