THE
ELECTROCHEMICAL
OF NATURAL
CHARACTERISTICS
REDOX CELLS
M. Whitfield’
CSIRO
Division
of Fisheries and Oceanography,
Cronulla, N.S.W. 2230, Australia
P.O. Box 21,
The discharge characteristics
of a range of natural rcdox cells have been monitorccl.
The
equilibrium
cxchangc current densities ( i,,) have been calculated and compared with the
The results indicate that instrumental
current drains imposed by modern clectromctcrs.
loading should bc negligible,
but the small values of i,, observed may result in the slow
This effect may not bc so serious in reduced scdiattainment of a steady-state potential.
ments where the Eh is closely correlated with sulfide activity.
In other cases some of the
irrcgularitics
in electrode behavior may be removed by discharging
the ccl1 in situ and
mcnsrning its rcchargc characteristics.
INTRODUCTION
Although Eh mcasurcmcnts2 have been
used for many years to charactcrizc marine and freshwater sediments (cl;. see
Mortimer 1941, 1942; Zo,Bell 1946) it is only
recently that the mechanistic limitations of
thcsc mcasurcmcnts have been trcatcd in
detail (Stumm 1965). Morris and Stumm
( 1967, p, 279) stated that
since natural waters are generally
in a dynamic rather than an eauilibrium
condition.
even the concept of a single oxidation-reduction potential
characteristic
of the aqueous
environment
cannot bc maintained.
At best.
measurements can reveal an Eli value appli:
cable to a particular
system or systems in
partial chemical equilibrium
and then only if
the systems are electrochemically
reversible
at the electrode surface at a rate that is rapid
compared with the electron drain or supply
by way of the measuring elcc,trode.
In a rcccnt paper (Whitficld
I969) I
attcmptcd to show that the ope~ntional use
of Eh as a parameter to charactcrizc the
estuarine cnvironmcnt is not dcpcndcnt on
the interpretation
of this parameter as a
true oxidation-reduction
-potential and that
useful and self-consistent results can bc
obtained if suitable cxpcrimcntal
prccauL Present address:
Marinc
Biological
Association, The Laboratory,
Citadel I-Iill, Plymouth PLl
2PB, England.
2 Ela is the potential, expressed on the standard
hydrogen
scale, generated
at a bright platinum
electrode immersecl in the sample.
LIMNOLOGY
AND
OCEANOGRAPIIY
Cons arc taken. This empirical approach
to the use of Eh measurements rests hcavily on the assumption that the rate of clcctron exchange at the clcctrodc surface is
sufficiently
large to maintain a stcadystate potential in the face of the current
drain or load imposed by the measuring
instrument
(see the last section of the
quotation from Morris and Stumm).
If
this is not so then drifting and unsteady
potentials will bc observed even if the
environment is not disturbed at a11 by the
insertion of the probe.
The equilibrium
cxchangc current dcnsity ( if) ) gives a measure of the rate of the
clcctron cxchangc reaction; its significance
in the interpretation
of Eh mcasurcments
was discussed by Stumm ( 1965) and by
Morris and Stumm (1967). They concluded
that a threshold value of about 1O-7 A cin2
for i0 is the minimum that can bc cxpectcd
to sustain a steady-state potential.
Further, they indicated that, of the rcdox sysems significant in natural waters, only the
Fc2+-Fe3-k and Mn” t-Mn’-‘- couples
arc capable oE sustaining cxchangc currents of
this order. Thcsc couples would only be
significant in cnrichcd waters where the
dissolved iron or mangancsc contents are
grcatcr than lo-‘) M. These conclusions
h aVC been given some cxpcrimcntal support
by the work of Bohn ( 196s) and of
Peshchcvitsky ct al. ( 1967).
This paper dcscribcs expcrimcnts
de-
383
MAY
1972,
V. 17(3)
384
M.
WIIITPIELD
signed to determine equilibrium
exchange
current densities in a variety of natural
redox cells. The same experiments also
permit quantitative
assessment of the interesting possibility of using such cells as
energy producers. For example, the “builtin” potential difference (up to 660 mV)
across the sediment-water
interface could
be used to generate power by immersing
one array of inert electrodes in the mud
and exposing a second array to the overlying water. Since the electrodes are not
consumed in the cell reaction, such an
array could function indefinitely
provided
that the system was ahowed to (and was
able to) recharge periodically
by switching off the load. The feasibility of such a
system can be assessed by observing the
behavior of such cells under load. Here
again the exchange current density is
critical.
METHODS
The investigations
must cover a wide
range of conditions since Eh measurements have been used to characterize the
from
whole spectrum of environments
well-aerated surface waters to stagnant reduced sediments (e.g. Baas Bccking et al.
1960; Krumbein and Garrels 1952). In addition the nature of the sediments will
vary from location to location and, in view
of the complexity of natural redox systems,
this may have a crucial influence on the
reproducibility
of Eh measurements.
The simplest way to span this wide
range is to consider the characteristics of
natural redox cells of the form
Inert metal
anode
1 reduced
layer
oxidized
1ayer
1 inert metal
cathode
cell I,
rather than to use the conventional
Eh measurement,
Reference
electrode
1 reference
solution
sample
solution
[ inert
ccl1 for
metal
cell II.
When cell I is allowed to discharge
through an external load (e.g. a standard
resistance)
the flow of current through the
cell, and hence through the external circuit, will be opposed by activation barricrs at the surfaces of the inert electrodes.
To enable the electrons to surmount these
barriers, a potential is established within
the cell in opposition to the equilibrium
(or steady state) potential; this is known
as the current-produced
overpotential (q).
Once a new steady state has been achieved
to balance the imposed current drain the
net currents that flow through the anode
and through the cathode must be equal.
Naturally, the magnitude of the steadystate current (I) and the overpotential (r])
wiI1 be controhed by the highest activation barrier to be overcome; i.e. the equilibrium exchange current density (io) measured
with cell I should represent the lowest
value that might be expected in the range
of environments spanned by the oxidized
and reduced layers. Thus cell I can be
used to give a limiting value for the exchange current (at least in terms of the
order of magnitude) that might be experienced over a wide range of environmental
conditions.
If water in equilibrium
with
the atmosphere is always used in the oxidized layer then variations in the nature
of the reduced layer can be used to assess
the effects of differing sediment compositions. The loading characteristics of cell
I will also illustrate directly the energyproducing potentialities
of natural redox
systems.
The cells studied represent the systems
air-saturated water-anoxic
water ( cells A
to C, Table 1) and air-saturated waterreduced sediments ( cells D and E ) . Cells
A to C were designed to contain fairly
high concentrations of degraded organic
material (the water in the vicinity of the
clcctrodcs was discolored by material released from the putrefying substrate), and
cells D and E were representative examples of the usual field situation.
Cells A to C were established in l-liter
measuring cylinders by laying the ingredients of the solid substrate in the base
of the cylinder to a depth of about 24
cm. The inert metal electrodes were then
placed in position: one set 5 cm from the
CIIARACTERISTICS
TABLE
1.
OF
Summary
NATURAL
REDOX
385
CELLS
of redox cells studied
Inert electrode
Cell
type
A
B
C
D
E
Reduced
substrate
Oxidized
substrate*
Freshwater
Freshwater
Freshwater
Seawater
Seawater
* In equilibrium
with
Seawater
enriched
above reduced mud
with minced meat
Reduced mud
Mud at roots of Zostera
Geometric
surfnce
mm (cm2)
TYP
bed
Platinum mesh
Platinum sheet
Gold sheet
Platinum mesh
Platinum mesh cathode
Platinum sheet anode
13.0
2.3
6.0
13.0
13.0
6.0
the air.
lip of the cylinder and the other set 1 cm
above the sediment surface. Freshwater
( 300 ml) was then added carefully so as
not to disturb the sediment layer. Afterwards, 700 ml of fresh seawater at the
same temperature were added slowly via
a PVC tube with its tip just above the
sediment-water
interface. In this way, a
sharp stratification
was established, enabling stagnant conditions to build up
fairly rapidly in the water layer immediately above the sediment surface. These
cells have a useful life of 7-10 days before
the stratification
is broken by the stirring
action of gas bubbles released from the dccaying substrate, Similar cells have been
prepared by Skopintsev et al. ( 1967)) who
have shown that the cells are capable of
recovering quite rapidly from an intermittent load.
Cell D was prepared by placing about
500 ml of a silty, reduced sediment in the
bottom of a e-liter beaker and adding 1.5
liters of fresh seawater. One set of electrodes was immersed to a depth of 2-3 cm
in the mud and the other to a similar
depth in the overlying
seawater. The
sediment samples used in cells A-D were
taken from the area of reduced, silty sediment in Gunnamatta Bay, Sydney (Whitfield 1969 ) ,
Cell E was established in situ by inserting a cylindrical platinum sheet electrode
to a depth of 10 cm in the silt underlying
a Zostera bed. The platinum mesh cathode was suspended from a float in the
overlying water. The mesh clcctrodcs used
in all cells were cleaned by cathodic elcc-
trolysis in 0.001 M sulfuric acid for 15 min
followed by rinsing in distilled water. The
sheet electrodes were rubbed with a fine
emery cloth before cathodic cleaning.
The cells were left overnight to attain a
stable potential (drift less than 5 mV/hr)
and to equilibrate
to room temperature
(20 +- 2C). The resistance box (R, Fig. 1)
was then adjusted to the desired value and
the cell allowed to discharge through the
load by depressing the switch ( S ). The
discharge curve was recorded continuously
on the potcntiomctric
recorder (sensitivity,
500 mV full-scale deflection;
recording
speed 50 mm/min)
and potentials were
read off at frequent intervals from the
voltmeter and written on the chart. After
a stable potential had been attained (drift
less than 0.1 mV/min)
the load was disconnected and the recharging charactcristics of the ccl1 were measured in the same
way. A stable potential was normally attained within 2-4 min of switching in the
load and the ccl1 potential would spring
back to its initial value when a similar
time had elapsed after the load had been
disconnected.
RESULTS
AND
DISCUSSION
Most measurements were made on cells
of type A (Table 1) because they were
considered to represent a worst case for
the reproducibility
of Eh measurements,
The rcduccd phase is very rich in organic
material and the clectrodc potential more
likely to be disturbed by water movements in the vicinity of the electrode than
386
0.8
(a)
Recorder
-i>
I
E. 0.4 u
.!5
+I>
A
4pd
II
2.:
0
A’2
I II
II
,
,
,
,
,
I
(b) Voltmeter
1.6
-i
>
E
M
.%
+I>
0.8
FIG. 1. Circuit for measuring loading characteristics of natural redox cells. For cells A to D
( Table 1) ( b ) was a Dynamco DM2022 digital
voltmctcr.
For cell I3 (a) was omitted and (b)
was a Keithlcy
model 610 battery-opcratcd
electrometer.
is the case for interstitial
water in the
scdimcnts.
The charge and discharge curves for all
the cells fit an equation of the form
v = t/+1 + ht),
(1)
where V is the cell potential at time t and
a and b arc constants. Response curves of
platinum electrodes immcdiatcly
after insertion into a sample oE reduced mud appear to follow a similar relationship (e.g.
Whitfield
1969). However, the behavior
of thcsc curves becomes erratic after 10 or
15 min, probably bccausc of enhanced bacterial action in the small sample container.
The steady-state potential approached dur-
100
t (sets)
Fro. 2. Charging
and discharging
curves for
cell A plotted
according
to equation
(4).
A.
Charging curves for experiments A8C ( O), A9C
and AlOC ( 0 ). B. Discharging
curves.
(A>
Numbers on the graphs correspond with the data
points for cell A listed in Table 2.
ing charge or discharge ( V, ) and the half
time of the process (tlh, time when V = V,
/2) can be obtained from the relationships
V, = 1/b
(2)
t1/$= 1cc/h 1.
(3)
Equation ( 1) has the same form as the
equation used to describe the response of
an ion-sclectivc electrode to a sudden step
in the concentration of the selcctcd ion
( Mullcr 1969)) and it also bears a formal
rcscmblancc to the Langmuir adsorption
isotherm and the Michaelis-Mentcn
cquation for enzyme activation kinetics.
CIIARACTERlSTICS
TABLE
(1)
Code
A
1D
2D
3D
4D
5D
6T>
7D
8D
9D
1011
11D
12D
13D
14D
8C
(;J
‘v3’
(9 x 10-q
n
(1nV)
101
102
105
110
125
150
1,300
53
33
9
75
5
20
53
Open
cimii t
9c
1OC
11c
12c
13c
G”’
m
(mV)
2.
(5)
1‘h
(see)
OF
NATURAL
Summary*
of data from
(7)
(0)
SE x 102
REDOX
(nk)
(8)
387
CELLS
cells
(10)
i
(nA m-2)
(y)
&,
(PA)
87.1
88.0
87.1
88.0
84.8
76.3
17.0
169.6
179.7
331.0
88.0
385.5
239.0
167.3
-
(CLW)
0.13
0.13
0.14
0.14
0.14
0.15
0.058
0.24
0.18
0.17
0.098
0.13
0.19
0.25
-
303
291
301
300
300
310
314
317
293
300
300
300
295
317
113
114
116
119
125
138
149
263
111
77
39
86
25
62
115
289
2.0
2.5
3.0
2.5
2.5
2.5
1.0
2.5
2.5
2.5
2.5
3.0
2.0
3.0
7.5
0.64
0.21
0.58
0.25
0.45
0.90
0.72
0.16
0.66
7.52
0.70
1.16
0.97
2.03
1.02
174
172
169
163
150
139
25
177
211
249
202
263
226
173
-
39.5
40.5
41.3
43.3
47.8
51.7
91.3
38.5
26.7
13.5
29.8
8.7
21.5
39.9
-
1.13
1.14
1.13
1.14
1.10
0.99
0.22
2.20
2.33
4.28
1.14
5.00
3.10
2.17
-
78
38
86
24
62
287
296
286
276
282
9.0
10.0
8.5
9.0
8.0
1.04
0.79
1.25
2.24
1.52
-
-
-
-
-
399
337
327
280
232
253
-
13.0
26.8
29.2
9.7
17.2
9.3
-
1.67
2.30
2.55
0.33
0.52
0.54
-
248
338
378
0.10
0.28
0.34
1D
2D
3D
41)
511
6D
1C
2c
3c
36
53
53
99
93
50
Open
circni t
587
626
664
310
280
280
66
122
135
60
122
135
30
48
27
428
459
45s
9.0
8.5
8.5
3.5
2.0
3.0
19.0
12.0
9.5
C
1D
2D
3D
4D
10,000
8,311
4,911
1,411
322
220
200
161
221
169
136
69
13.5
3.5
2.5
4.5
8.03
1.08
2.16
7.49
101
51
64
92
68.6
76.7
68.0
42.8
2.2
2.0
2.8
4.9
D
ID
211
3D
4D
5D
50
93
9
99
1
462
436
444
440
462
156
209
55
209
8
6.0
4.5
4.5
5.0
-
3.25
1.93
4.82
2.58
-
306
227
389
231
454
33.8
48.0
12.7
47.5
1.7
3.12
2.25
6.12
2.11
8.0
242
174.5
474
163.5
621
0.49
0.47
0.34
0.44
0.064
E
1D
2D
3D
111
61
11
165
165
165
110
85
26
-
-
55
80
139
66.8
51.5
15.8
0.99
1.39
2.69
104.2
146.2
248.4
0.11
0.12
0.061
T3
51.2
23.5
21.7
19.7
14.7
11.3
17.0
13.0
34.7
E
80
x
x
x
x
lo-?
lo-?
lo-”
lo-?
3.56
3.24
4.53
7.94
0.010
0.025
0.015
4.9
3.4
3.8
3.4
x
x
x
x
lo-”
lo-”
lo-’
lo-:’
* All symbols usccl in columns ( 2) to ( 11) are explained in the text. Column l--Suffix
D refers to n discharging
cell ancl suffix C to R charging cell. Prefix refers to cell type (Table 1). Column 3-V(,
values based on leveling off
of recorder trace. Columns 7 and S-For
cells A and B, V, vnlucs nr.e estimated from the appropriate
discharge curves
and not from column 3. Cells A and C were 4 days old. Cells Bl to n3 were 1 clay old and cells R4 to BG were 3
clays old.
Equation
( 1) was cxpresscd in the form
(t/V)=a+ht,
(4)
and the data wcsc fitted to this equation
by the method of lcast squares (Table 2
colu~l~s
4-6). The fit was exccllcnt for
cells A, C, and D and the values obtained
for V, on discharging agreed within a few
millivolts with the values
cstimatcd from
the lcvcling-off of the rccordcr trncc nftcr
388
M.
WHITFIELD
“Tafel”
or “high field”
plot for the
FIG. 3.
discharge of all cells ( equation 7 ) . A- 0 ; B0; C-A;
D-V;
E-V.
Data points representing the first discharge of each cell are marked
with a horizontal
bar.
200 set of discharge. The agreement was
not so good with cell B but the values of
V, obtained were within 5 mV of the
values estimated from the chart. Some of
the raw data for cell A are plotted in Fig.
2; the discharge curves fan out from a
common origin whereas the data for recharging curves fall closely about a single
straight line. These data also illustrate
well the closeness of fit to equation ( 1) as
specified in Table 2 ( column 6).
The internal
consistency of the half
times for each cell suggests that the same
process is being monitored during each
discharge. The half times for cells A and
B are the same (about 2.5 set) after 3
days running whereas the value for fresh
cells is much greater (about 8.5 set) . The
half time for recharging is consistently
longer than that for the discharge process
in cells A and B; the equilibrium potential
predicted from the straight-line fit is consistently lower than that actually observed
after about 5 min recharging time. This
suggests that two separate processes may
be responsible for fixing the cell potential;
one associated with an electron exchange
reaction, being monitored by the charge
and discharge curves and the other associated with the presence of adsorbed species
on the electrode surface. This may be
coupled with the observation that the first
discharge of any of the cells after an
overnight
charge gives an anomalously
large overpotential (see Fig. 3). The asymmetry of the half times is consistent with
the suggestion of Stumm (1965) and Bohn
(1968) that the platinum electrode records
a mixed potential in natural media.
From the data obtained from equation
( 1) it is possible to calculate the currentproduced overpotential
(7 = V, - V,), the
voltage efficiency of the cell ( E = 100 *V,
/VO), the steady-state current (I = V,/R) ,
the steady-state current density (i = I/electrode area), and the wattage output in the
steady state (W = Vw2/R = V,.?‘). These
data arc listed in Table 2 (columns 7-11).
For an energy-producing
cell the overpotential (q) may be related to the current
density (i) by the Butler-Volmer equation
which in its simplest form can be written as
i = i,[e(l-B)h
-e-L%],
where p is a parameter dependent
electrode reactions and
k = nF/RT
= 16.7nV-2
(5)
on the
at 25C,
(6)
where n is the stoichiometric
number of
electrons associated with the overall cell
reaction and the other terms have their
usual significance. If, under load, the system is pulled well away from equilibrium
then q is large and the second term on the
right-hand side of equation (5) tends to
zero so that
i = ioe(l-8)We
(7)
Equation (7) is known as the “high field’
This equation has been
approximation.
CHARACTERISTICS
OF
shown to hold for values of 7 greater than
120 mV for a single electron process (Bockris and Reddy 1970) and the threshold
potential is lower for multielectron
processes, Therefore a plot of 7 vs. log i for
my data should give a straight line and the
intercept on the current axis will give the
equilibrium
exchange current density ( i0 )
for the overall cell reaction.
The data (Fig. 3) confirm that the high
field approximation
is obeyed within the
limits of experimental
error.
Such an
agreement would not be observed if thcrc
were gross inhomogcneities
in the electrode surface or if the properties of the
electrode surface changed appreciably during the course of the experiments (Ives
and Janz 1961). All the cells give cquilibrium current densities below the threshold
value of 10m7A cm-2 suggested by Stumm
(1965) and by Morris and Stumm (1967).
The cell using gold electrodes is particularly poor with an i0 value of 10-O A cm-2.
This explains why gold electrodes have
behaved poorly in natural media (Barnes
and Back 1964; Whitfield
1969) and is
consistent with the relative behavior of
platinum and gold in the hydrogen elcctrode (Bockris and Rcddy 1970). In the
cells using platinum electrodes the i0 values for the cylinder cell ( cells A and B )
and the mud cell ( cells D and E ) are
self-consistent
despite the different
surface areas and electrode types involved
(Table 1).
It may be argued that the results reprcsent a concentration rather than an activation overpotential; in other words, the
energy barrier controlling electron transfer
is caused by slow diffusion of the reactants
to the electrode surface rather than by a
sluggish reaction there. If this were true
then erratic potentials would bc obtained
unless a consistent degree of stirring or
stagnation were maintained while the mcasurements were being made. This would
make the operational use of Eh in field
studies unrealistic.
For a transport controlled reaction, the concentration ovcrpotential (qo) and the current density (i) arc
related by an equation of the form
NATURAL
REDOX
389
CELLS
I
0.1
-“I
I
0.2
(Volts)
FIG.
4. Plot of current density (i) vs. overpotential
( 7 ) for cell A indicating
that r is an
activation
rather than a concentration
overpotential (see equation 8).
i = i,(l-ea’lc)
(8)
(e.g. see Bockris and Reddy 1970). If i is
plotted against Q, a limiting current density is approached as qC becomes more
negative. A plot of i vs. 7 for cell A shows
no sign of a limiting current density and is
characteristic of an activation overpotential ( Fig. 4). A comparison of the curves
in Fig. 3 indicates that this is true for all
the cells studied. The potential measured
here is therefore not diffusion controlled.
This might bc expected because of the
extremely low current fluxes generated in
the cells.
To assess the errors involved in poten-
390
M.
WIIITFIELD
tiomctric mcasurcmcnts we can consider
equation ( 6) when the system is close to
equilibrium
( 7 is very small) ; i is then rclatcd to ‘1 by the “low field” approximation,
i=i()kTj.
(9)
If we consider a small current Ai passing
through the system this will gcncratc an
ovcrpotcntial
Aq so that,
Ai =
i,,karj.
(10)
If WC need to know the ccl1 potential to
the ncarcst 10 mV (the limit of rcproducibility of field mcasurcmcnts ) then the
maximum current to be drawn by the
measuring instrument is about 1O-9 A for
each square ccntimctcr of clcctrodc surf act. This may bc compared with the offset current of a typical pH mctcr (lo-IL A)
and that of the Kcithlcy 610C electrometer
used in earlier measurcmcnts ( 10-l” A:
Whitfield
1969). High input impedanccmeasuring devices have been used successfully to measure the potentials
of
metal oxide elcctrodcs having cxchangc
current densities of the order of 1O-7 A
cm-2 (Ives and Janz 1961). Thcrcfore thcrc
is no reason, from the point of view of
instrumental
loading, why fairly rcproduciblc
potential
measurements
should
not bc made. However, Eh measurements
should not bc made with a manually balanced potentiometer
or with a sclf-balancing potcntiomctric
recorder. From the
point of view of polarization,
the cells
must bc trcatcd as high impedance sources
( Barnes and Back 1964).
With such low current densities the
steady-state potential may bc attained only
slowly. This is often the case in oxidizing
environments where equilibration
times as
long as 8 hr have been recorded (Barnes
and Back 1964). The i. values rcportcd
hcrc represent the lowest values to bc cxpccted in the rcdox range spanned by cell
I and more favorable conditions might
prevail in a reducing cnvironmcnt.
The
relationship between sulfide activity and
Eh in reducing marinc and cstuarinc scdimcnts (Berner 1963; Kryukov et al. 1962;
Skopintsev ct al. 1967; Whitficld 1969) indi-
FIG. 5. Plot of current density (i) vs. voltage
efficiency
(E) for cells A ( 0 ), C (A),
D (O),
and E (v).
V a1ucs for the ordinate of cell C
have been multiplied
by 100 to bring them on
scale.
catcs that a fairly simple reaction controls
the potential of the platinum electrode,
and steady-state potentials arc achieved
fairly rapidly if cart is taken to prcvcnt
undue disturbance of the sample. Further
studies using cell II with the platinum
elcctrodc immersed in a reducing cnvironmcnt would help to dcfinc more rigorousIy the limits of reproducibility
of Eh
mcasurcmcnts under these conditions.
Where slow attainment of the steady
state is a problem a discharge-charge procedure may bc used together with equations (1) and (2) to estimate the final
potcntia1. Such a proccdurc has been suggcstcd by Mullcr (1969) for use with high
resistance mcmbranc
clcctrodes
where
similar problems arc cncountercd.
A reproducibility
of +25 mV is quite adequate
for the operational USC of Eh values since
useful pictures can bc built up by rounding off the Eh values to the nearest 50 mV
CIIARACTIZRlSTICS
OF
FiATURAL
REDOX
391
CELLS
‘0
t (mins)
i (pAcm-2)
FIG. 0. Plot of power output (W) vs. current
density (i) f or cells A (01,
C (A>, D (V),
and aband E (v).
V a1ues for the ordinate
scissa of cell C have been multiplied
by 100 and
50 respectively
to bring them on scale.
and drawing contours at lOO-mV intervals
( Whitfield 1969) .
As might be expected from Fig. 3 the
cells studied here do not rate very highly
as power sources, Howcvcr, a brief look
at their characteristics will help to confirm
the internal consistency of the data prcscntcd here. Data for cell B will be omitted since the energy-producing
properties
of the cylinder cell vary considerably with
age ( Table 2). Since the data for cell B
were taken at different times in the cell
lift no coherent picture can bc cstablishcd, although
measurements B4D to
B6D agree fairly well with those for cell
A. The voltage efficiency of all cells incrcascs as the current density decreases
( Fig. 5) and therefore the output wattage
passes through a maximum at some intcr-
FIG. 7. Behavior of ccl1 A when
a prolonged
discharge.
subjected
to
mediate current density (Fig. 6). This
pattern of behavior is consistent with that
obscrvcd with other energy-producing cells
and the shape of the curves in Fig. 5 supports the assumption (used in the dcrivation of equation 5) that ohmic polarization
within the cell is ncgligiblc.
The current
density and output power scales for the
gold elcctrodc (cell C) have been cxpandcd
considerably.
The maximum power available from cell D is 0.48 ,uW at 300 n,A
cm-2. When one of the cells was discharged for a prolonged period ( Fig. 7)
the hyperbolic decay curve was followed
for about 80 min and the potential then
fell to a low value (20 mV in this cast)
which was maintained at least for a furthcr 4 hr. Consequently some switching
dcvicc would bc necdcd cvcn to maintain
the peak output of 0.5 pW. A few such
cells in series might bc able to cnergizc an
electronic wristwatch!
392
M.
WHITFIELD
CONCLUSIONS
By discharging
a natural redox cell
through an external load it is possible to
obtain an estimate of the equilibrium
exchange current density ( iO) accompanying
the cell reaction. The i0 value is a measure of the rate of the overall cell reaction
at the electrodes and it controls the polarizability of the cell, i.e. the magnitude of
the overpotential
( 7) produced by the
passage of current. Ideally potentiometric
measurements should be made with a nonpolarizable cell so that small currents generated by the measuring instrument will
not disturb the cell potential.
The present measurements indicate that i0 is small
( <O.l ,X-LAcm-2) in the cells studied and
that the cell potential is probably a mixed
potential resulting from a series of irreversible reactions. Nonetheless, the cells
behave reproducibly
and their behavior
can be described by relatively simple elcctrochemical equations. In addition, the i0
values observed for cells with a mud base
were large compared with the input offset
currents of modern electrometers. Taken
together these observations indicate that
reproducible Eh measurements can be obtained if the cell potential is controlled by
a relatively simple series of reactions and
a steady-state potential can be attained
fairly rapidly.
There is already evidence from widely
separated regions that the potential of a
bright platinum electrode is controlled by
the couple So( rhombic) /HS-( aq. ) under
reduced conditions
(Berner 1963; Kryukov et al. 1962; Skopintsev et al. 1967;
Whitfield
1969). It has also been suggested that under mildly oxidizing conditions the platinum electrode either reflects
the irreversible oxygen potential (Cooper
1937) or that it behaves as a pH electrode
of the second kind via the formation of
Pt( OH)2 (Pcshchevitsky et al. 1967). To
rationalize the wide spectrum of opinion
over the utility of Eh measurements it
would be valuable to consider these rcactions in greater detail. In addition, in situ
measurements of i. using the approach described here could bc used to assess the
feasibility of Eh measurements. The practice of discharging the cel1 through a load
and plotting the recharge characteristics
as a function of time might also help to
reduce the effects of adsorbed organic
material and poorly poised redox systems
and enhance the reproducibility
of Eh
measurements.
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