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diffusion coefficient at 50O0C is approximately
2 x lou6 cm2/sec. At faster scan rates, however,
the i vs vl” plots frequently exhibited upward
1’
curvature, particularly at platinum and platinumrhodium
indicator electrodes. It is believed that
this deviation from the Randles-Sevcik equation is
caused in part by adsorption of uranium on the
surface of the electrode. Conway4 postulated that
for a charge transfer examined by t.he voltage sweep
method there are three contributions to the timedependent
current: (1) a non-Faradaic current
C,, dv/dt associated with charging or discharging
of the ionic double layer, (2) a pseudo-Faradaic
current C dv/dr associated with the change of
extent of coverage by adsorbed species formed or
removed in the electrochemical step, and (3)
Faradaic current iF associated with any net
reactions which can occur within the range of
potential scans employed. Therefore the net
current it that is recorded as a function of time
during a potential sweep is
it
C,, dv/dt + C dv/dt t i, .
Since dv/dt is the scan rate (v) and iF -.= kv1/2,
it can be written as
where k 1.75 x ~O’~I~’~AD‘/~C. Corrections for
adsorption plus charging effects, when encountercd
at fast scan rates, were made by plotting i / v ~ ’ ~
P
vs vl”, where the slope reflects adsorption
phenomena and the intercept contains the Faradaic
term. The diffusion coefficient for U(1V) calculated
from the intercept was in good agreement with the
value from the linear i vs v1l2 plots.
P
In chronopotentiometry, adsorption of reactant
causes the iOr1I2 product, where i, - current
density (amp/cm2), to increase as 7 (transition
time) decreases; this effect was observed for
uranium at short transition times (