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Chapter 6. The Platinum Microelectrode Method for Soil Aeration Measurement

Chapter 6. The Platinum Microelectrode Method for Soil Aeration Measurement

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236



D. S. MCINTYRE



the gas phase, but measurements of air-filled porosity, O2 and C 0 2 gas

concentrations and diffusion rates, as well as air permeability, gave only

very limited success. On the grounds that “evaluation of conditions at

the interface between the root surface and the soil system presents the

greatest possibility of ascertaining the influence of soil aeration on plant

growth” and that “active root surfaces are covered with water films” so

that “some method of measuring oxygen diffusion . . . through the liquid

phase to a reducing surface similar to that of a plant root, would be of

hndamental importance,” Lemon and Erickson (1 952, 1955) introduced

the platinum microelectrode method. It was said to measure in situ the

rate of oxygen diffusion through soil solution to a simulated root in the

form of a thin platinum wire, at which O2was electrochemically reduced.



B. BASIS OF THE METHOD A N D APPLICATION

Adapted from biological techniques for tissue and blood O2 content,

in which it is referred to as the “oxygen cathode” (Davies, 1962), the

method involves measurement of the electric current caused by the reduction of O2 at the surface of the cylindrical wire electrode, which is

usually 4-5 mm long, with diameter in the range 0.4-1.2 mm, but most

commonly about 0.5 mm. The microelectrode, suitably mounted, is

pushed into the soil and cathodized with respect to a standard reference

electrode. The ensuing current decreases with time as shown in Fig. la.

14



(a)



12



10



4



5



+



8



C



!i



3



0



6



4



2



0



0



2



3

Time (min)



4



5



6



I

a - 21%

b - 157%

c - 10 2%

d - 4.1%



0



1



0



"

01



"



'



02



" " '

03

04



(b)



'



05



~



' ~ '

06

07



~



00



'



~



0.9



Applied voltage (volts-ve vs SCE)



25



-



20



-



I



4



a



1



; 15

?!



-



3



0



I0



-



5-



3

Cathode voltage (volts v e v s S C E )



FIG. 1. Microelectrode current relations for reduction of O2 in porous media. (a) Current-time relations for various unsaturated media. (b) Current-voltage relations for watersaturated media: 0 , sand: X. clay suspension: 0, glass beads 18p median diameter: letters

u-e refer to concentration of Or in equilibrium with saturating solution; SCE, saturated

calomel electrode. (c) Current-voltage relations for an unsaturated sandy loam; I, current

as a function of applied voltage; 11, current as a function of effective voltage.



238



D. S. MCINTYRE



By plotting the current at a certain time against the applied voltage, current-voltage relationships can be established. For a clay suspension,

soil paste, or water-saturated porous medium, a relation of the type shown

in Fig. l b is obtained. Such a relation implies that if a voltage within

the plateau region is applied to the microelectrode, the current will be

independent of voltage, and limited only by the rate of diffusion of O2 to

the electrode surface. Current at time t is given by



where & = Oo flux at time r at the surface of an electrode of radius a.

Oxygen JIux (traditionally but incorrectly called oxygen diffusion rate,

or ODR) may therefore be determined by measuring the current, usually

after attainment of a quasistationary state at 4 or 5 minutes, and by use

of the working equation



where i, = current in microamps, M = molecular weight of 0 2 (= 32

glmole), n = number of equivalents/mole of O2 (= 4), F = the Faraday

(= 96,500 coulombslequiv),A =area of the electrode in cm2.

Since introduction of the method, it has been used extensively in

studies of the effect of aeration on plant and root growth and the behavior of soil organisms (see Stolzy and Letey, 1964a,b). Interpretation

of results has been based on the model depicted in Fig. 2, which is considered to represent soil conditions near a root, or the microelectrode,

in unsaturated soil (Wiegand and Lemon, 1958, 1963; Lemon and

Kristensen, 1961; Kristensen and Lemon, 1964; Letey and Stolzy,

1967). For this model it is assumed that the flux is limited by the diffusion

rate of O2 in solution to give current-voltage relations of the type in

Fig. 1 b. The oxygen flux for such a coaxial cylindrical model is given by



where D = the effective diffusion coefficient for O2 in the soil solution;

Cb = O2 concentration at the liquid-gas interface at mean radius b;

C , = O2concentration at the electrode surface, at radius a.

The mean thickness of the assumed liquid film separating the electrode

from the gas phase is given by (b - a ) ; it will obviously depend on the

moisture content of the soil. Although there must be some liquid con-



SOIL AERATION BY PLATINUM MICROELECTRODE



Platinum

surface



239



- Water



\

sol I particles



Water film



(bl



Platinum

electrode



FIG.2. The model which is assumed in order to explain microelectrode behavior. (a)

Particles and solution separating the electrode from gas-filled pores (after Kristensen and

Lemon, 1964). (b) Coaxial cylindrical model with water film of mean thickness 6r separating

the electrode from gas-filled pores.



tinuity between the working electrode and the reference electrode, this

model was considered to represent the physical conditions sufficiently

well. In operation of the method, it is assumed that cb is the equilibrium

concentration in solution for the particular partial pressure of O2 in the

gas phase, and C, = 0. Putting ( b - a ) = 6 r , Eq. (3) can then be written



and 6 r has been called the “apparent mean liquid path length” (Kristensen

and Lemon, 1964) and “mean filmthickness” (Letey and Stolzy, 1967).

In general the method has been applied to soil-plant studies in two

ways. First, using Eq. (2) there have been attempts to correlate plant and

organism behavior with O2flux, or to determine a flux that is critical for



240



D. S. MCINTYRE



root growth.’ Second, by application of Eqs. (2) and (4), values of 6 r

critical to root growth have been determined (Wiegand and Lemon,

1958, 1963; Kristensen and Lemon, 1964; Stolzy and Letey, 1964a,b;

Letey and Stolzy, 1967). The last authors have measured a value of

20 x I 0-8 g ern+ min-’ as the minimum for root extension of many plants,

and subsequently determined equivalent critical values of 6r for specified

porosity and O2conditions. It should be noted that as fa,, is dependent

on the radius of the electrode, and on the time of reading the current,

these must be standardized for comparison of results.

Lemon and Erickson (1 955) realized that the “boundary conditions

around an electrode are not strictly similar to those around an exploring

root” but stated that “electrode measurements have suggested that

boundary conditions around an active root are more important to the

rate of oxygen supply than was previously realized.” The apparent dependence, from this model, of oxygen flux on the effective diffusion coefficient in the solution, and on the concentration gradient across the

water film, meant that the flux should be dependent on three main soil

properties. They are moisture content, structure of the soil next to the

electrode, and the O2 concentration in the gas-filled pores. As the last

would be related to gaseous diffusion in the soil, the method gave promise

of integrating the effects of those properties which control soil aeration

and the rate of 0 2 uptake by roots. However, recent investigations of the

method have shown that in unsaturated soils the current-voltage relations

of the microelectrode do not show a plateau. In general the relations are

of the form shown in Fig. lc. Such a relationship suggests that the

process governing the rate of O2 removal by the microelectrode is not

diffusion of O2through a water film. It throws considerable doubt on the

diffusionmodel of Fig. 2, and has important implications in the interpretation of measurements made with the diffusion model in mind. Covey and

Lemon (1962) in an analysis based on the coaxial cylindrical model,

showed divergence of theoretical and experimental results for periods

greater than 3 minutes. They concluded that the electrode (and plant root)

geometry does not strictly meet the boundary conditions of the model.

However, it can be replaced by another model (shown in Fig. 6) based

on consideration of the electrode kinetics of a microelectrode inserted in

unsaturated porous media (three-phase system).

Although the microelectrode method is basically an electrochemical

one, it was used widely for soil-plant studies without a thorough appraisal

of the electrochemical principles which govern its response in three-phase

It has been tacitly assumed that the rate-controlling process for O2 uptake by a root is

also diffusion through an encompassing film of water.



SOIL AERATION BY PLATINUM MICROELECTRODE



24 I



media. In two-phase media, it has often been demonstrated experimentally that transport rate of 0 2 has an overwhelming influence on

current limitation. For three-phase media it has been assumed that the

response is similarly governed. Hence kinetics of the reaction, and the

effect of transport rate of the product (OH-) from the electrode, were not

initially investigated in three-phase media, while soil properties such as

pH and electrical resistance were thought to have little influence on

electrode response. Subsequent more critical investigations and analyses

(Kristensen, 1966; Mclntyre, 1966b, 1967),have suggested that through

lack of knowledge of the fundamental electrochemicalprinciples involved,

the results of many workers are not universally applicable.



C. REVIEWOUTLINE

Because the method is being widely used in agronomic, ecological, and

sometimes plant physiological experimentation, any limitations that are

inherent in the method because of the electrochemical and physical

principles governing its behavior, should be examined. Therefore the

remainder of this review will examine (1) the electrochemistry from the

point of view of the reactions at the electrode, electrode kinetics, and

transport processes; (2) the effect of the environment on the behavior of

the microelectrode; (3) physical effects of insertion of the electrode on

this environment. The proposed models will be reviewed and related to

experimental results. Finally, conclusions will be made regarding the usefulness of the platinum microelectrode method and the reliability of

measurements made with it. Details of the experimental method have

been published several times previously (Birkle et al., 1964; Letey and

Stolzy, 1564; Stolzy and Letey, 1964a; Lemon and Erickson, 1952,

1955; van Doren and Erickson, 1966; van Doren, 1958) and will be

mentioned here only when results are dependent on the particular technique adopted. Neither will the problems of experimental use, such as

extreme variability occurring in field conditions, be discussed; they have

been treated by the workers mentioned above. This review is concerned

rather with the fundamental principles underlying operation of the electrode in soil, as a system of controlled potential electrolysis of oxygen,

and how those principles affect application of the method to soil-plant

studies.

II.



Electrochemistry



Electrochemical analysis is important in determining which process

controls the current resulting from O2 reduction at the working electrode.



242



D. S. MCINTYRE



It is necessary to know the nature of the reaction, its products, and their

subsequent behavior, the influence of transport processes and of pH in

determining electrode response. One can then deduce, using experimental

evidence, likely rate-controlling processes in soils where a variety of

conditions may be met. The object is to determine in particular whether

transport of 0 2 ,or some other process, is dominant in controlling the rate

of reduction of 0 2and

, hence the current. Previously it has been assumed

that transport rate of O2 is the dominant rate-controlling process in both

root uptake and in the electrochemical reduction of 0 2 .

Evidence regarding the apposite reactions, and their rate in relation to

transport processes, comes mainly from solution electrochemistry. This

evidence indicates that when a platinum electrode having surface properties similar to those expected with the microelectrode, is used in

solution, enhancement of transport of 02 by forced or even by natural

convection, may alter the current controlling process from one of O2

supply rate to that of reaction rate. Similarly in porous media, it is known

that for water-saturated conditions in which the nearest gas-liquid interface is the soil surface, the current is controlled by the rate of diffusion of

O2 along very small concentration gradients. However as moisture is

lost and air enters pores near the electrode, the opportunity exists for

concentration gradients between the nearest gas-liquid interface and the

electrode to be orders of magnitude greater than in saturated soils. This

could then make possible a very high transport rate of O2 which may

make the reaction rate the current-controlling process over parts of theelectrode.

It is important also to recognize the influence of pH, for the voltage at

which H+ ion reduction begins is dependent on the pH of the solution

surrounding the electrode. As well, pH may affect the type of reaction

occurring, and the reaction rate. Solution electrochemistry indicates that

the pH has little effect on decomposition voltage2 of H+ until the pH

decreases to about 4 (see later). This value is not often found in measurement on soils, but it must be remembered that soil pH is measured on an

extract, and that the measured value decreases as the soil-water ratio increases. Thus the in situ pH value will be less than that measured on an

extract. It will decrease with water content, and also with an increase in

the salt concentration. Moreover the decomposition voltage for H+

zDecomposition voltage is defined by Potter (1961); here it is taken to mean that electrode voltage at which measurable current first occurs. Characteristic voltages for reduction

or oxidation are usually expressed in terms of half-wave potential (EI/z),but the decomposition voltage is more pertinent in the present treatment.



SOIL AERATION BY PLATINUM MICROELECTRODE



243



depends as well on 02 concentration (Oden, 1962) and on current density

(Potter, 196 1).



A.



REACTION



The net reactions commonly accepted as occurring at the platinum

microelectrode have been (Lemon and Erickson, 1955; Oden, 1962;

Stolzy and Letey, 1964a):

(a) for acid media (Kolthoff and Lingane, 1952) or at pH < 5 (Charlot

ef al., 1962)

O2 + 4 H + + 4e--



2H20



(5)



(b) for alkaline media (Kolthoff and Lingane, 1952) or at 5 < pH < 12

(Charlot et al., 1962)

Op



-



+ 2 H 2 0 + 4e-



40H-



(6)



From the theoretical and experimental investigations, Oden (1 962) considers that reaction (2) is relevant to a lower pH limit of about 3.5-4.0

for air-saturated solutions. Therefore we will consider only this reaction,

as soil pH normally falls within the limits specified.

A good deal of controversy exists among electrochemists as to the

exact reaction. Apart from specific details, two substantially different

courses have been proposed (Williams, 1966). The first involves production of H 2 0 2and its subsequent reduction, so that the reaction in

more detail becomes



+ 2eH 2 0 2+ 2eO2+ 2 H 2 0 + 4e-



O2 + 2H,O



--



20H-



+ H202



20H-



-



40H-



(7)



The alternative hypothesis suggests that electroreduction of oxygen

proceeds at a metal electrode by the formation and subsequent decomposition of surface oxides. In this case the overall reaction is of the

form:



+ 2M

+ 2 H 2 0 + 2e2MOH + 2eO2+ 2 H 2 0 + 4e0 2



2M0



--



+2 M 0



+



2MOH 2 0 H 2M 2 0 H -



+



40H-



244



D. S. MCINTYRE



The third step in the reaction is rate-controlling when platinum electrodes are used. Both reactions require 4 electrons for the reduction of a

molecule of 0 2 and

, reaction (8) shows formation and destruction of an

oxide and hydroxide of the metal.

Support for the presence of H 2 0 2 comes from qualitative measurements by Laitinen and Kolthoff (1941), and quantitative measurement by

Delahay ( I 950), during controlled electrolysis for long periods. Sawyer

and Interrante (1961) considered that H Z 0 2was the primary reduction

product at a prereduced electrode and proposed a mechanism invoking

formation of platinum hydroxides during the reaction. Kozawa ( 1964)

concluded that the proportion of H d 0 2formed electrochemically depended on the surface conditions of the platinum electrode, such as

presence of oxide and/or adsorption of ions present in solution, and that

the H 2 0 2 was decomposed catalytically. Further evidence of H 2 0 z

formation was presented by Muller and Nekrassow (1965), who made

measurements using a rotating disc electrode with a guard ring. They

found that the presence of oxides of platinum retarded the reduction of

O2to H 2 0 2(but not reduction to OH-), in alkaline solution, and that this

reaction occurred rapidly only on oxide-free surfaces. The surface oxide,

however, accelerated reduction of H202 by catalytically decomposing it.

Other authors, mainly from purely electrochemical analysis, are of the

opinion that the reaction proceeds without the production of H 2 0 z .

Lingane ( 196 1) from chronopotentiometric measurements concluded

that HeOr was not produced, and Riddiford (1961) on the basis of

analysis of various data was of the same opinion. In a recently published

book Hoare ( I 968) states that H 2 0 2is always formed by reduction of 0 2

at a cathode, but that at a current density less than

A/cm2 (an

approximate current of 6 microamps for the electrodes in common usage)

catalytic activity of platinum is sufficient to reduce the H 2 0 2at the same

rate as it is formed. For a greater current density HEOzaccumulates.

Impurities can lower the catalytic activity and hence the required minimum current density for HZO2accumulation. Further reviews and references can be found in Williams (1966), Rickman et al. (1968), and Rickman (1966).

The results have been obtained by a variety of techniques with variations in electrode pretreatment (and hence electrode surface properties),

electrolyte nature and pH, temperature, current density, quantity of

electricity passed, time of treatment, and intensity of stirring. It appears

that the reaction may take different routes dependent on different combinations of these parameters. However, whatever the route taken, it

appears necessary that for the reaction to proceed platinum oxides or



SOIL AERATION BY PLATINUM MICROELECTRODE



245



hydroxides must be present or formed during the reaction. Many of the

differences would be resolved if H 2 0 2 is catalytically decomposed, for

some arguments regarding its absence are based on interpretation of

current-voltage-time relationships where small quantities may not be

detected. Rickman et al. ( 1968) conclude that, in measurements with the

platinum microelectrode, H 2 0 2is formed regardless of the state of oxidation of the electrode, but that the rate of its catulyric decomposition depends on the presence of an “active” oxide. An irreversibly formed oxide

can exist on platinum from aging of a clean electrode, and Rickman et ul.

(1968) consider that this inhibits the rate of decomposition of H 2 0 2

which can then be lost from the surface into the bulk solution, as suggested by Oden ( 1962).

The details of the reaction are of interest from two points of view, rate

and efficiency of the reaction. Some authors (e.g., Charlot et al., 1962)

consider that reduction of H 2 0 2is a slow reaction, particularly at low pH

values ( < 10). Therefore its presence may make the reaction rate low,

particularly if its decomposition rate is diminished by the lack of sufficient

oxides on the electrode (which applies to most microelectrode measurements in soils). Efficiency refers to the relation between current and

amount of O2 reduced. Oden (1962) found from electrolysis of O2 in

solution, that on the basis of a 4-electron reaction, total charge passed

could account for the loss of 02,making the reaction 100% efficient;

similar conclusions were reached by Willey and Tanner (1961). Slow

decomposition of H 2 0 2could, however, reduce efficiency by allowing

loss from the electrode surface due to diffusion. In soils, adsorption by,

and reaction with, soil constituents could further reduce efficiency (Oden,

1962). As both variation in pH and state of the electrode surface apparently affect the decomposition rate of H 2 0 z , there could be variability in current from one soil to another, when measurement is made

with the platinum microelectrode.

B.



REACTION

RATE



The reaction rate should be considered in relation to the rate of supply

of the reactant, and whether the former is likely to be the currentlimiting process. On a “bright” platinum surface (as distinct from a

platinized surface) the reaction rate is apparently highly dependent on

the surface conditions. For such an electrode four initial conditions are

recognized in general: ( 1) preoxidized, anodized (or chemical treatment,

e.g., HNO& (2) prereduced, cathodized (or chemical treatment, e.g.,

H 2 S 0 4 ) ;(3) abraded (clean); (4) aged-oxidized (any of the above three



246



D. S. MCINTYRE



surfaces allowed to stand in water or air). Another condition has been

introduced by Black and Buchanan ( I966), viz. (5) cathodized in phenolphthalein solution. Surfaces (2), (4), and ( 5 ) have been shown by these

authors to behave similarly in air-saturated two-phase systems.

Preoxidized electrodes apparently allow the greatest reaction rate

(Lingane, 196 1; Sawyer and Interrante, 196 1 ; Rickman, 1966; Rickman

et al., 1968). Lingane (1961) considers that at least one-tenth of the

surface area of the electrode should be covered with freshly formed

oxide in order to maintain the reaction rate at a maximum in air-saturated

solution. The current strength will then depend on the diffusion rate of

02,but diffusion control is lost if the coverage is less than one-tenth of

the surface area, and reaction rate becomes the current-controlling

process. However, the utilization of preoxidized electrodes for soil

measurements is both awkward and time consuming. Moreoever, in soils

the electrode surface is abraded by insertion and then cathodized for 4 or

5 minutes before the current is read. From Lingane’s investigations, the

deduction can be made that such treatment is sufficient to remove any

oxides previously present.

The actual initial condition of the surface is hard to determine for, as

well as the treatment of the electrode mentioned above due to its normal

use, mild abrasion may be used before each measurement. Sometimes the

electrode may also be “aged oxidized” to some extent. Sawyer and

Interrante (1961) showed that a reduced electrode in solution without

voltage applied, is oxidized in a few seconds, but that the oxide is removed by cathodization for 2 to 3 minutes at a current density of 0.02

mA/cm2 (1-2 p A for the platinum microelectrode normally used). Presumably the same changes would occur with a freshly abraded electrode

also and give rise to a standard surface.

Rickman et al. ( 1 968) and Rickman ( 1 966) quoted evidence to show

that an aged-oxidized surface inhibits 0 2 reduction and is hard to remove

except by abrasion or strong chemical treatment. Black and Buchanan

(1966) have found from electrolysis in sand saturated with 0.5 M KCI

solution, that the aged-oxidized surface gives similar results to a prereduced electrode. Rickman et al. (1968) and Rickman (1966) also

demonstrated by controlled potential electrolysis and chronopotentiometric measurements in a clay suspension, that formation and removal of

oxides occurred on a platinum microelectrode. The presence of oxides

increased the reaction rate (and hence current) as well as affecting the

decomposition potential of 02.They were unable to demonstrate that the

state of oxidation of the electrode significantly affected current, and

reached the conclusion that diffusion rate of O2 in a suspension was so



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Chapter 6. The Platinum Microelectrode Method for Soil Aeration Measurement

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