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POLAROGRAPHY WITH STATIONARY ELECTRODES
Oak Ridge National Laboratory
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POLAROGRAPHY WITH STATIONARY ELECTRODES
By L. B. Rogers
Mixtures of ions which react rapidly with mercury usually cannot be analyzed polarographically
with the dropping mercury electrode. However, these polarographic analyses appear to be feasible
using solid electrodes made from noble metals. The general features of their behavior are covered
in this report. My collaborators in carrying out this work were A. F. Stehney, R. B.
H. H. Miller.
Polarographic reactions are fundamentally electrolytic processes although polarography
name attached to the use of micro electrodes to obtain current-voltage curves for analytical purposes
was first proposed in 1922. Until just before the last war, polarographic studies were confined almost
entirely to the use of the dropping mercury electrode. This was natural because many elements are
readily reduced from one oxidation state to another and because mercury with its high hydrogen over-
voltage enables one to examine many of these cathode reaction potentials to -1.5 v or more (vs a
saturated calomel electrode). However, the reactions which can be studied are usually limited to
those which occur at a potential more negative than the one at which mercury itself begins to dissolve.
Therefore, we have examined the possibility of making polarographic analyses in the anodic region by
substituting a platinum electrode for the dropping mercury electrode. Platinum and other noble
metals having high oxygen overvoltages are, for anodic studies, in a position comparable to that of
mercury for cathodic studies (see Figure 1).
The range of potential covered by each type of electrode
is altered by changes in the acidity of the solution.
It might be advisable at this time to point out the incompleteness of the common conception of
the terms anode and cathode. In an electrolytic cell containing two similar platinum electrodes at
different potentials, one of them is of necessity more negative than the other. If a reaction such ai
Ag+ + e + Ago
is possible, one would expect it to take place faster though not necessarily exclusively
at the electrode having the higher concentration of electrons, i.e., the more negative potential. One
may draw an analogous picture for oxidation in such a cell. In contrast to this set-up, the polaro-
graphic cell usually consists of one electrode, dipping into the cell, whose potential is measured
through a salt bridge against a reference electrode.
The potential on the polarizable electrode within
the polarographic cell can be varied continuously from strongly positive to strongly negative. This
electrode is not in competition with another during a reaction. Thus in a polarographic cell, an elec-
trode might be sufficiently negative for reduction to take place while its potential would make it an
anode in the usual electrolytic cell. Therefore,
anodicc" reductions and cathodicc" oxidations are
quite possible in polarography. Thus, if one wishes to study the reduction of some of the stronger
oxidizing agents such as permaganate, dichromate, ceric, argentous, cupric, and ferric ions, one is
forced into studies of the anndic repinn. All nf these innn are nannil~ v nriinned at zero potential bv
the dropping mercury electrode unless they are in the form of stable complexes which, being more
difficult to reduce, require more negative potentials.
To date, several stages of development have been reached in studies of anodic reactions using
stationary electrodes of noble metals.
as 1900 electrolytic studies of the reduction of per-
manganate showed that
as the potential became less positive a reduction current began to flow,
increased with further lowering of potential, and finally flattened off in a manner exactly like the
polarographic diffusion current. Other investigators showed later that many redox pairs behaved
similarly, their efforts being aimed primarily at the study of reversibility of reactions or of over-
voltage of different metals. The third stage was marked by the use of the polarograph by Walen and
Haissinski to record the following electrolytic data automatically rather than by tediously recording
1.0 0.5 0 -0.5 -1.0 -1.5
VS SATURATED CALOMEL ELECTRODE
Ranges of potential covered by platinum and dropping mercury electrodes in
In 1940, Laitinen and Kolthoff pointed out the usefulness of stationary platinum electrodes for
quantitative polarography, but they used a tedious method which enabled them to proceed slowly under
conditions approaching equilibrium. Each point on the polarographic curve was obtained by approach-
ing a given potential slowly and then waiting
to 3 minutes for the deposition current to reach a con-
stant value. To obtain ten points for a curve required a minimum of thirty minutes.
Laitinen and Kolthoff also reported experiments with a rotating platinum electrode which had two
advantages over stationary electrodes. A constant diffusion current was reached at once, and, for a
given concentration of ion, the current was much larger.
Thus, the rotating electrode shortened the
ary electrode. The sole disadvantage of the rotating electrode appeared to be the greater difficulty in
adapting it to the analysis of small volumes of solution.
Our interest in the anodic region of potential and the desirability of using small volumes of solu-
tion prompted us to combine and to extend the work of Laitinen and Kolthoff and that of Walen and
Haissinski. To date we have examined the behavior only of stationary electrodes but half-wave po-
tentials obtained for these electrodes should be essentially the same as those for the more versatile
rotating electrodes. After our work had been in progress for several months, other investigations
with stationary electrodes came to our attention. Last fall Dr. O. H. Mueller reported that he had
used automatic recording with stationary electrodes for his studies on mixtures of quinone and hydro-
quinone. By flowing the solution past the electrode he approached conditions similar to those obtained
with a rotating electrode in which equilibrium is reached instantaneously. One would expect automatic
recording to be feasible under these conditions. A short time ago, Dr. L. A. Matheson mentioned that
he had carried out some unpublished experiments several years ago using stationary electrodes but
that his studies had not proceeded beyond the preliminary stage.
We have compared the limits of reliability of data obtained with stationary electrodes using the
usual automatic recording apparatus with data obtained manually according to the method of Laitinen
and Kolthoff. The reliability was judged by: (1) the constancy of the half-wave potential, (2) the re-
producibility of the diffusion current for a given concentration, and (3) the linearity of the relation
between diffusion current and concentration. We have studied the variations introduced by using differ-
ent rates of change of potential, by increasing the area of the electrode, and by stirring the solution.
We selected silver ion because of its known electrochemical simplicity and reversibility, and because
we expected that a reduction involving precipitation might introduce complications resulting from
changes in the surface of the electrode which would be quite small if a soluble ion were produced
(i.e., Ag+-Ag0 vs Fe3+-Fe2t.
In our studies, we have used three different polarographs, Sargent Models XII and XX and a manual
set-up consisting of a potentiometer, slide-wire and Rubicon galvonometer. Our polarographic cell was
conventional in that the potential of the polarized electrode was measured through an agar bridge
saturated with potassium nitrate against a large saturated calomel electrode. Our polarographic
solution was 0.1M potassium nitrate at pH 4 containing 5 x 10-4M silver nitrate.
In general, the curves had rounded maxima whose height appeared to increase with current (i.e.,
from higher concentrations of silver ions, or from larger electrodes). (See Figure 2.) These maxima
did not appear to be of the same nature as those encountered with the dropping mercury electrode
and this impression was strengthened by finding that the presence of gelatin appeared to have no ef-
fect on them. They probably result from the time-lag in reaching diffusion equilibrium. Attempts to
study these maxima by varying the rate of polarization were unsuccessful because duplicate runs
sometimes showed differences in behavior ranging from the usual maximum to the extreme of no
maximum at all.
Although we were unable to reach a definite conclusion concerning the effect of the rate of polar-
ization on the height of the maximum, we did find that the faster rates produced markedly larger
diffusion currents (see Table 1). However, the half-wave potential appeared to be essentially un-
changed regardless of the rate of polarization or the direction of polarization.
+0.6 +0.5 +0.4
I I 1
Figure 2. Reaction of silver ion at a stationary platinum electrode; recorded automatically.
Effect of rate of polarization on diffusion current.
Rate of polarization (Ma)
(mv/sec) Quiet solutions Stirred solutions
Manual 3.2 usually no id; 7.8
1.46 3.0 7.8 8.4
2.92 3.9 ----
4.38 4.1 16.8 17.6
Our studies which began with the micro platinum electrode of Laitinen and Kolthoff extended to
include those up to two sq cm in area. As expected, a roughly linear relation was found between elec-
trode area and diffusion current for a given solution, but our data indicated that the behavior of elec-
trodes became increasingly erratic with increased size. If actual unknowns were to be analyzed, it
appeared to be more reliable to calibrate each electrode with a known solution rather than to depend
upon calculations based on the measurement of area and current from another electrode. Despite the
a 5 /-
Figure 3. Relation between concentration of silver ion and diffusion current for 1 mm electrode
somewhat lower accuracy found for larger electrodes, the larger diffusion current which results from
the increased area enables one to examine a lower range of concentration. This is closely analogous
to the findings of Laitinen and Kolthoff concerning the better applicability to more dilute solutions of
a rotating micro electrode as compared to a stationary one. Therefore, in order to cover a wider
range of concentration (10-2 to 10-5M) one should have at least two sizes of electrodes (see Figures
3, 4, and 5).
Stirring the solution completely eliminated the maximum and, as one would expect, increased the
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under our conditions. However, in similar studies of overvoltage in which stirred solutions were
employed, much better precision was obtained. Rotating electrodes also give better agreement and
appear to be easier to control although they are more difficult to set up initially than a simple stirrer.
The studies which have been discussed up to this point were made on the silver nitrate solution
described at the beginning. Half-wave potentials for ions other than silver which are reduced at a
stationary platinum anode are shown in Table
Curves for permanganate and dichromate are shown
in Figures 6 and 7
Although our studies are not complete, we have evidence that, in every
linear relation between diffusion current and concentration should be found. Our studies of these ions
together with others falling in the anodic region have been examined by us in some detail and will be
reported at some time in the future.
Before attempting to interpret results obtained with stationary electrodes by continuous record-
ing one should recognize several factors which will have important bearing upon the results. First, it
is essential that the electrodes be quickly and reproducibly cleaned, and this can be done easily by
chemical methods. However, an electrode can sometimes be cleaned following a run by simply chang-
ing (usually reversing) its potential.
The selection of the proper potential is sometimes difficult
because the use of extreme potentials for cleaning often produces waves having different diffusion
currents and sometimes different half-wave potentials (see Figure 8).
These variations appear to
result from the presence of films of oxygen or hydrogen which were produced on the electrode during
Finally, one must face the fact that if a reaction
is irreversible, potential alone may
not be able to accomplish the cleaning job.
2. Half-wave potentials of some ions reduced at a stationary platinum anode.
The question of irreversibility is also important when half-wave potentials are recorded since,
in instances analagous to those known for the dropping mercury electrode, a wave found at a certain
potential using one direction of polarization may be displaced
as 1 or 2 tenths of a volt-or
may even be absent altogether--when the direction of polarization is reversed (Figure 9). Likewise,
one should not expect that all half-wave potentials obtained with platinum will necessarily agree with
those obtained with the mercury electrode. The source of the disagreement undoubtedly arises from
the difficulty in determining whether or not an electrode
is truly inert.
Lastly, the fact that maxima are the rule, rather than the exception means that one is more
limited than usual in obtaining reliable diffusion currents and good definition of waves when two com-
ponents of a mixture have half-wave potentials which are nearly alike. Better results might be ob-
Figure 6. (1) (2) Reduction of MnOj in 0.1M H2SO4, (3) (5) reduction of MnO in 0.1M NaOH, (4)
oxidation of Mnr+ in 0.1M H2S04. All solutions were 10-3M Mo.
+0.9 +0.8 +(a3 O -0.3
+ .2 +0.9 +0.6 +0.3 0
8. Cleaning electrode
A alkaline MnO4
B ReO4 in H2SO4
Very positive potential (+1.5 v)
--- At 0.2 v more positive than starting potential
+4.2 +0.9 +0.6 +0.3 0 -0.3
In concluding this discussion, some of the advantages of solid electrodes might be mentioned
briefly. As mentioned earlier, a knowledge of the behavior of solid electrodes enables one to obtain
very rapidly information which
is useful, not only to polarographers, .but also to those interested in
carrying out electrolyses. Users of the Sargent Model XX Polarograph in particular may find some
satisfaction in the fact that one does not encounter huge oscillations in the record but instead a
smooth and comparatively unambiguous line. Of real importance to all polarographers are the pos-
sibilities for rapidly examining many inorganic and organic compounds in the anodic region. Further-
more, solid electrodes
as opposed to dropping mercury electrodes allow analyses to be made under
otherwise difficult or impossible conditions, namely, in fused salts or in nonaqueous solvents (liquid
ammonia) in low temperatures. As pointed out earlier, our studies dealt with stationary electrodes
because they seem to be most suitable for anodic studies involving micro volumes of solution. How-
ever, extension of our information to rotating electrodes should be simple, and the use of rotating
electrodes whenever the volume of solution permits should give an increase in sensitivity and in
definition of waves.
1. Heyrovsky, J., Polarographie, Springer, 1941.
2. Kolthoff, I. M. and J3. J. Lingane, Polarography, Interscience, 1941.
3. Mueller, O. H., American Chemical Society Meeting, September 1946.
4. Matheson, L. A., Private communication.
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