on the mechanism of oxidation-reduction potential

On the Mechanism of Oxidation-Reduction PotentialAuthor(s): Malcolm DixonSource: Proceedings of the Royal Society of London. Series B, Containing Papers of aBiological Character, Vol. 101, No. 707 (Feb. 1, 1927), pp. 57-70Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/81296 .

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57

On the Mechanism of Oxidation-Reduction Potential.

By MALCOLM DIXON, Ph.D. (1851 Exhibition Senior Student).

(Communicated by Sir Frederick Hopkins, F.R.S.--Received July 6, 1926.)

(From the Biochemical Laboratory, Cambridge.)

The phenomena of oxidation-reduction potential are becoming increasingly important in the study of biological oxidation processes. Unfortunately, however, the application of the results obtained from studies on potential to such processes has been somewhat hindered by the fact that the study of

potential phenomena has up to the present been approached from a point of view which differs considerably from that of the more important theories of the mechanism of biological oxidation processes. The conceptions used in the

development of the two subjects have been different, and the close connection between the two sets of phenomena has thereby been obscured. For instance, Wieland's theory, which has played such an important part in the development of the subject of biological oxidation processes, is founded on the conception of activation and transfer of hydrogen atoms, while Mansfield Clark, in his recent

important and valuable studies on reduction-potential, makes use of conceptions such as electron activity, transfer of " electron pairs," and so on. This absence of " linking up " between the two sides of the subject has, in fact, been brought up as an objection to the Wieland view. For instance, Clark (3, 4) claims that the reduction-potential phenomena prove that the oxidation-reduction process consists essentially in the transfer of an electron pair from reductant to oxidant, and not in the transfer of hydrogen as assumed by Wieland. He regards this as a very serious objection to the Wieland theory, although possibly not sufficient

entirely to disprove it. It must be pointed out, however, that the fact that the electron transfer hypothesis leads to correct results is no proof of its truth until the alternative hypotheses have been shown to yield incorrect results.

In the present communication it is shown that the Wieland view also leads to correct results ; in fact, it is possible to predict the reduction-potential phenomena from the work of Wieland. This at once disposes of one of the main objections to Wieland's theory and renders it possible to relate the mechanism of oxidation-reduction potential with that of biological oxidation- reduction processes.

VOL. CI.-B. F



M. Dixon.

The Hydrogen Electrode.

It will be necessary before considering the work of Wieland to make a few

preliminary remarks about the hydrogen electrode, for reasons which will

appear later. If a metal such as platinum is brought into contact with hydrogen gas the hydrogen is absorbed by the metal. The state of the absorbed hydrogen has been described in various ways-for instance, as a platinum-hydrogen alloy or a hydride, or a solid solution of atomic hydrogen, and in many other ways. These all seem to mean very much the same thing, and it is not necessary here to discuss which is the correct description. At any rate, we can picture the metal surface (with which we are alone concerned) as being more or less

completely covered by hydrogen atoms loosely attached to the platinum surface. The hydrogen in this state, which corresponds to the " active hydrogen " of

Wieland, must not be regarded as atomic hydrogen, since it is in combination with the platinum. The attachment is, however, only loose, so that it does

actually approximate to atomic hydrogen in behaviour. These hydrogen atoms are in equilibrium ((a) in the diagram, p. 60) with molecular hydrogen thus:

2H1 -- H2, and to any surface concentration of hydrogen atoms there corresponds a definite pressure of hydrogen gas. Thus the hydrogen-gas pressure can be used as a measure of the concentration of the hydrogen atoms. Applying the law of mass action to the equilibrium we get

[I]2 1kl[H2]

where [H] is the surface concentration of hydrogen atoms and [H2] the concentration of molecular hydrogen; or since [H2] is proportional to P, the

hydrogen-gas pressure, we may write

[H] =k2AP.

Now, if the platinum surface is immersed in an aqueous solution the surface

hydrogen atoms can also go into solution as hydrogen ions, and are in equilibrium with the hydrogen ions in the solution (equilibrium (b) in the diagram). They are thus electromotively active, and their electrolytic solution pressure will, of course, be proportional to their surface concentration; i.e.,

p=k3[[H] or p k=kVP.

Substituting in the general electrode equation of Nernst, namely, RT c

E = - log we get

E RT [[H] E = - log kV/P'

58



i/lVechanism of Oxidation-Reduction Potential.

where E is the absolute potential of the platinum, R is the gas-constant, T is the absolute temperature, n is the valency of the ion, and F is the

Faraday constant; or, expressing the potential, as is usual, with reference to the normal hydrogen electrode, we may write

RT [H'] Eh =-- log /7.

This is, of course, the fundamental equation of the hydrogen electrode. After these introductory remarks we now turn to the consideration of a

certain aspect of the work of Wieland.

Wieland's Work on Platinum.

Wieland (6, 7) found that when platinum or palladium black, i.e., a preparation of the metal in a state of fine division and presenting a large surface, was

brought into contact with solutions of a number of oxidisable substances, such as hydroquinone, indigo white and other leuko-dyes, hydrazobenzene, sugars, etc., the platinum became laden with hydrogen and the hydroquinone, etc., was simultaneously oxidised. The hydroquinone, for instance, tends to pass over into its oxidised form, quinone, with loss of hydrogen atoms, which are taken up by the platinum. At the end of the experiment, the solution contains

quinone, and on separating and heating the platinum hydrogen gas is given off.

On the other hand, Wieland showed that the hydrogen could again be taken

up from the platinum by a number of reducible substances (" hydrogen accep- tors "), such as quinone, methylene blue and other reducible dyes, etc., which

thereby became reduced. The process in these cases is, in fact, reversible. Wieland used these facts in developing his theory of oxidation-reduction by hydrogen transfer.

Now, on reading Wieland's work, one is struck by the fact that there would

appear to be a general correspondence between the substances which give up hydrogen to platinum and those which give reduction potentials, and similarly those which take up hydrogen from hydrogenised platinum give oxidation

potentials. For instance, hydroquinone, leuko-dyes, hydrazobenzene, sugars, etc., all give reduction potentials, while quinone, methylene blue, etc., give oxidation potentials. Aldehydes, on the other hand, were found by Wieland not to give up hydrogen to the metal, and they do not give any potential. This

correspondence, although at present based upon somewhat limited data, suggested the view developed in this paper.

F2

59



M. Dixon.

It appears then that the hydrogen atoms on the platinum surface are in

equilibrium in yet a third way, which can be represented as

AH2 - A + 2 H,

where H represents a hydrogen atom on the platinum surface (equilibrium (c) in the diagram).

We may represent the whole system diagrammatically as follows:-

H/" - H {(a) ,H

Pt/HH'- (b)

A-H -= H>A (C)

/ +A

where AH2 represents some molecule analogous to hydroquinone.

Derivation of the General Reduction-Potential Equation.

We may now pass on to consider how these facts are capable of explaining the phenomena of reduction-potential. Of the three equilibria represented in the diagram, it is clear that (b) is the only one which can directly affect the

potential of the platinum, since the others are non-ionic. We may regard the platinum as a hydrogen electrode in which the hydrogen pressure is con-

trolled by equilibrium (c), or, in other words, by the amounts of oxidant and

reductant in solution.

Now, if we increase the amount of reductant (AH2) in solution, more hydrogen atoms will be transferred from this to the platinum, so that the surface con-

centration [H] will rise. In consequence, more hydrogen ions will pass from

the electrode into the solution, and the potential of the electrode will become

more negative. Conversely, an increase in the concentration of oxidant (A) will have the opposite effect, and the electrode will become more positive. An

increase in the acidity of the solution will, of course, drive back hydrogen ions on to the electrode surface, and make it more positive, and vice versa.

Qualitatively then at any rate the effects expected are in agreement with

actual measurements of reduction-potential. It may be mentioned here in parenthesis that, as there is a definite pressure

of hydrogen gas in the platinum, there will always be a slight diffusion of this

60



Mechanism of Oxidation-Reduction Potential.

from the electrode into the solution. The hydrogen pressures are, however, in general very small, so that the amount diffusing will also be very small; and, moreover, there is good reason for believing that reaction (c), in the systems we are considering here, is rapid in comparison with the diffusion process, so that

any effect of diffusion is negligible. We shall see later, however, that this is

probably not always the case, but here we shall assume that the effect of such diffusion upon the hydrogen atom concentration is negligible.

We must now see how far the quantitative deductions on this view harmonise with the known facts of reduction-potential. Equilibrium (c) can be written:-

AH2f - A + 2H,

where H denotes a hydrogen atom on the platinum surface. Applying the mass-action law to this we get:-

[AH2] = K'[A][H]2. We saw earlier that

[H] = k2VP,

so that we obtain

P K I H21 [Al2] [A]

Now the hydrogen-electrode equation given above can be re-written

IRT 1 RT Eha- 2F log p F . pH,

where pH has its usual significance; and, substituting, we get

RT [A] RT E ~- log K[AH2] -~. p

RT 1 RT [AH2] RT 2F log K

- 2F log [A]

-

PH'

or, writing Eo for the first (constant) term,

RT [AAH2] RT E, Eo - R

log [A] F .pH.

But this is the ordinary accepted reduction-potential equation for systems of the general type AH2 + A, for example, the hydroquinone-quinone system, etc.

Thus, starting from Wieland's observations, and making only the simplest assumptions, we arrive at the general reduction-potential equation, which is

61



M. Dixon.

identical with that obtained on the electron-transfer theories. Had we known

nothing of the reduction-potential phenomena we might, in fact, have predicted them from the experiments of Wieland.

The Effect of Ionisation.

In cases where one or both of the constituents of the system ionise, the equation must be modified to take into account the effect of the ionisation on the potential. For example, if the molecule AH2 dissociates into A" ions and hydrogen ions, the concentration of undissociated AH2 molecules ([AH2] in the equation) will no longer be equal to the total substance AH2 added.

Clark assumes that in all cases two hydrogen ions must be dissociated from the molecule, and that only the double anion A" is active, giving up two electrons to the electrode. The opposite assumption will be made here, namely, that

only the unionised molecules are active, and that they act by transferring two

hydrogen atoms to the electrode in accordance with Wieland's theory. The hydroquinone system may be taken as a typical example. Hydroquinone

ionises as follows:-

H2Q - HQ' + H1

and

HQ,Q" +H*.

By mass action

K = [HQ'] [H'] K = [Q"] [H'] [H2Q] [HQ']

where K1, K2 are the respective ionisation constants. Writing So and ST for the concentration of total oxidant (quinone) and reductant (hydroquinone), respectively, using Clark's symbols, we have:-

So= [Q] S, = [H2Q ] + Q'] -- [Q"].

Solving the three equations for [H2Q] we get:-

[H2Q]>K = K K+1

[H]2 [I1] +

_H H-] 1

62




and substituting in the general equation given in the last section and re-arranging we obtain:

RT `S, T RT /KIK, Ki RT . Eh= Eo- - log S-+ 2F log [H.]2l [ 1)-- F H 2F so 2F_ \[HP]2

K1 +1

RT S, RT I K1K2 K RT - 2

lo + 2F log ( 2 1) + A log [E-1] 2F so 2F [H.]2 I] 2F

RT~T2`3~!7 s RT KjK2 H 0]2 zEo

- ilogr + RT log 23 + 11121 ? 2F ?

So + 2F g L([H]2 + [H ] + ) [ ]

: Eo - R T log (KK2 4- K1[H] + [HI]2). 20 o- 2-F

Now this last equation is the identical equation obtained by Clark (1) for

this system, and verified experimentally. It will be seen, therefore, that the

theory has again led to quantitatively correct results. Other and more com-

plicated cases have also been worked out, but it is unnecessary to give details

here, as it will appear later that the two theories are bound to give the same

results. The above example will suffice to illustrate the method of procedure.

The Relation between the Two Theories.

A fuller comparison of the mechanisms by which a substance such as hydro-

quinone produces a reduction potential on the two theories will assist in making clear their relation to one another. On Clark's electron transfer theory the

hydroquinone must first lose two hydrogen ions by dissociation as a dibasic

acid. The doubly-charged anion then reacts with the electrode by transferring to it two electrons, thus becoming converted into quinone and making the

potential of the electrode more negative. The balance of the equilibrium between the electrode and the hydrogen ions is thereby upset, so that in accord-

ance with the hydrogen-electrode equation there is bound to be a passage of

hydrogen ions from the solution to the electrode. The net result is therefore a

passage of hydrogen ions and of electrons from the hydroquinone to the

platinum. On the present theory, on the other hand, there is a direct transfer of hydrogen

atoms from the hydroquinone to the platinum, followed of course by a re-adjust- ment of the equilibrium with hydrogen ions.

Now a hydrogen atom consists of a hydrogen ion plus an electron, so that on

thermodynamical grounds it is clear that the two theories must give the same

results. The essential difference between the two mechanisms is that on Clark's

63



M. Dixon.

view the hydrogen ions and the electrons are transferred separately, whereas here it is assumed that they are transferred together as hydrogen atoms.

It may therefore be concluded that the reduction-potential phenomena do not furnish a proof that the reaction takes place by the transfer of " electron pairs," since they follow equally well on the assumption of hydrogen-atom transfer. Clark's objections to the Wieland theory are therefore shown to be groundless.

It should be pointed out here in passing that this linking up not only enables us to explain the reduction-potential phenomena on the Wieland view, but it

also enables us to explain the observations of Wieland on the electron-transfer

view. For instance, on this view the addition of hydroquinone to a suspension of platinum black would produce a negative charge on the metal, by electron-

transfer from the hydroquinone. This charge would cause the passage of

hydrogen ions from the solution to the metal, and the disappearance of these

ions would cause a further ionisation of the hydroquinine, and so on. The

platinum would therefore become laden with hydrogen, which would have been

derived from the hydrogen ions.

As the evidence at present does not appear to permit of a definite decision

as to which mechanism is the true one, it would be dangerous to adopt one view

to the exclusion of the other. The writer's personal view is that both actions

probably take place. For instance in the case of hydroquinone, under conditions

where ionisation is slight (e.g. in acid solution) the main action may be supposed to be a transfer of hydrogen atoms from the non-ionised molecules H2Q; in

alkaline solution, where it is mostly present as Q" we may suppose a loss of two

electrons, while at reactions where the singly charged ion HQ' is the main

constituent the process may be assumed to consist in a loss of one hydrogen atom and one electron.

In certain cases the electronic theory appears to become somewhat far-

fetched and artificial, particularly in those cases where no ionisation has been

detected. For instance hydrazobenzene gives a high reduction-potential, but

it has no acid properties whatever. Yet we have to assume, on the electronic

view, that the only active form of the molecule is that in which, not one, but

both of the hydrogen atoms attached to the nitrogen atoms have been dis-

sociated as hydrogen ions. The amount of this dibasic acid ion must surely be

infinitesimal under ordinary conditions. Clark himself has drawn attention to

other similar cases. On the other hand, the Wieland theory would become

equally artificial in any case where the ionisation of the hydrogen atoms

involved was so strong as to be practically complete.

64



Mechanismn of Oxidation-Reduction Potential.

The rH Scale.

Clark (2) has recently introduced a very convenient scale of "reducing power," analogous to the pH scale of acidity. The physical basis and justifica- tion of this scale is brought out very clearly by the view put forward in the

previous sections. According to Clark's notation the rH value of a solution is defined as log 1/P, where P as before is the pressure of hydrogen gas in a platinum electrode in equilibrium with the solution, which is, of course, calculated from the potential by means of the hydrogen electrode equation. The complete analogy between the pH scale and the rH scale will be clearly seen if the two .sets of equations are worked out side by side.

By definition rH = -log P pH = -log [H].

The two equilibria are

AH2- A + H AH A/ A' + H'.

Applying the mass law to these we get

PK[ll1 [l]= lK [AH] P K [A] [ A'] '

and taking logarithms we get

rH= pK +- log _[] pH = pK' [- log [A --

where A denotes either oxidant or acid molecule, pK means log 1/K, K is the oxidation-reduction equilibrium constant (expressing the affinity of oxidant for hydrogen), and K' is the dissociation constant of the acid.

The last two equations represent the familiar dissociation or titration curve, and it can easily be shown that the laws which apply to pH in connection with acid-base equilibrium apply equally to rH in connection with oxidation-reduction

equilibrium. The behaviour of strong and weak acids and bases, buffers, indicators, etc., all find an exact counterpart in oxidation-reduction equilibria.

Of course, a small rH value corresponds to a high reducing power, and in general it may be said that a system giving a small rH is bound to reduce one which has a larger rH, and vice versa. At any rate, this is true at the electrode surface, but we must not lose sight of the fact that potential measurements can only give information about what is happening at the electrode surface, and it is possible that in special cases the platinum might activate one of the constituents, or in other words act as a catalyst. In such cases the reaction

65



M. Dixon.

might not proceed in the solution in the absence of a catalyst. Actually, however, it appears that in general reactions take place in solution in accordance with the rH data, and the practical value of, for instance, the " rH indicators" introduced by Clark depends upon this fact.

When the rH equals 0 the hydrogen pressure becomes atmospheric, and the gas begins to come off the electrode in bubbles. In the hydrogen electrode as ordi-

narily used of course the rH = 0. On the other hand, when the H =

about 41 the electrode evolves oxygen gas. There is always a balance between

hydrogen atoms and hydroxyl groups at the electrode surface, and there is, in

consequence, always an oxygen pressure in addition to the hydrogen pressure. Below rH 20 the hydrogen pressure will predominate, and above this value the

oxygen pressure. We can, however, deal throughout with the hydrogen and

neglect the oxygen, just as we speak of the pH of a strong alkali, although the

hydrogen ion concentration is negligible in comparison with the hydroxyl ions.

Electrodes of different Metals.

Electrodes of different metals are, in general, found to give the same potential when immersed in solutions of reversible oxidation-reduction systems such as

those we have been considering. Clark, for instance, has found that a quin-

hydrone solution gives identical potentials with electrodes of platinum, gold,

palladium and mercury. This might perhaps be expected, for, although the affinities of the various metals for hydrogen differ very greatly, this may be supposed to affect all the equilibria in the same way, a metal with a high affi-

nity for hydrogen being less ready to part with hydrogen atoms, either to the

oxidant, or as hydrogen gas, or as hydrogen ions, than one with a low affinity.. Thus, a given concentration of oxidant and reductant in the solution will be

in equilibrium with a given hydrogen pressure and will correspond to a given

electrolytic solution pressure whatever the metal; although, of course, the surface

concentration of hydrogen atoms in equilibrium with these will naturally be

greater in the case of a metal with a high affinity for hydrogen. Of course, it is

assumed that the metal does not react chemically with the solution.

The hydrogen pressure therefore, and consequently the rH, is a true measure

of the reducing power of a solution, being independent of the metal used.

In passing, it may be pointed out that it seems difficult to explain on the

electronic theory why different metals should give the same potentials in such

systems, as although the " electron activity" of the solution will remain constant

that of the electrode will vary with the metal used.

66



Mlliechanisrm of Oxidation-Reduction Potential.

The Sulphydryl System.

There is one reduction-potential system which differs fundamentally from the reversible oxidation-reduction systems we have considered hitherto, namely that formed by the sulphydryl compounds, cysteine and reduced glutathione, which was studied by Dixon and Quastel (5). These sulphydryl compounds are easily oxidised to the corresponding disulphide compounds, with loss of

hydrogen atoms: 2 R.SH --> R. S. S. R. + 2H.

This system is anomalous in that (a) the potential percentage reduction curve is not S-shaped, but logarithmic, and (b) the oxidised form has no effect on the

potential. The equationwhich Dixon and Quastel found to express the behaviour of the system was

RIT RT E = Eo - - log [R. SH]- -

. pH.

These facts can only mean, in our view, that hydrogen atoms can be transferred from the sulphydryl group to the electrode, but that the disulphide compound is quite incapable of taking hydrogen back again from the electrode. The system is, therefore, essentially an irreversible one.

The behaviour of the disulphide group is interesting. It seems to require a certain critical rH value before it can be reduced at all. For instance, the potential measurements show that it is not reduced by the electrode at, say, rH 13 (for glutathione), but if the rH of the electrode is lowered (i.e., the

hydrogen pressure increased) by applying a negative potential to the electrode from an external source a point is reached where it is reduced. In other words, the disulphide group can be reduced electrolytically. Again, zinc dust suspended in neutral solution has an rH small enough to reduce indigo carmine-that is to say, an rH less than 8, but it is quite unable to reduce the disulphide group. If, however, the rH is made still smaller, by increasing the hydrogen ion concentration of the solution, the disulphide compound becomes reduced.

To return to the sulphydryl system, however, it is clear that since the process is irreversible it cannot be in equilibrium, and must be treated kinetically. If the hydrogen were transferred irreversibly to the electrode, and there were no compensating process, we should expect the hydrogen pressure to rise steadily to very high values. The potential measurements show, however, that this is not the case, and it is therefore clear that we must look for some compensating process tending to remove hydrogen from the electrode. The most likely one would seem to be the diffusion of hydrogen gas away from the

67



M. Dixon.

electrode. This, as has been pointed out above, is bound to occur in fact, but it will not appreciably affect the potential given by those systems, such as those we have considered, which react very rapidly with the electrode.

It may, however, well have an effect in this system. The --SH group is, to

judge from experiments on rates of reduction, peculiar in being a very much more slowly reacting group than those substances which form the reversible

systemis of the type studied by Clark, so that it is possible that diffusion may not be slow in comparison with the transfer of hydrogen to the electrode.

Now assuming that this is the actual compensating process, it is clear that, when the rate of supply of hydrogen by the sulphydryl compound is equal to

the rate at which hydrogen gas leaves the electrode, a steady state will be

reached, and a constant potential will be obtained. The system will auto-

matically adjust itself to this steady state, as the surface concentration of

hydrogen atoms will increase or decrease until it is attained.

The process supplying hydrogen to the electrode may be represented thus--

2R. SH - R. S . S. R + 2H,

and its velocity will therefore be proportional to [RSH]2. The slowest part of

the compensating process will probably be the reaction

2H -- H2

in accordance with what would appear to be the most satisfactory theory of the phenomena of over-voltage. The rate of the compensating process will,

therefore, be proportional to [H]2. Now, when the steady state is reached, these two rates will be equal, so that

[R SH]2 K= '[H]2.

We saw in the section on the hydrogen electrode that

p - k3 [H]

so that substituting in the Nernst equation we get

p RT, [H'] dRT E -I E RT log

[H] and E - log 11S] F kJ] log k'[R.SH]'

which may be re-written

RT3C RT Eh- Eo - log [R .SH] H,

which is the experimental equation previously given for this system. It may be thought that in deriving this equation a thermodynamic equation

68



Mlechanism of Oxidation-Reduction Potential. 69

has been applied to a system which is not in equilibrium, but this is not so.

The surface concentration of hydrogen, [HI], which is determined by irreversible

processes, was calculated from kinetic considerations; and the thermo-

dynamic equation was applied only to the relation between the surface hydrogen atoms and the hydrogen ions in solution, which is an equilibrium.

If we take into account the fact that the - SH group ionises in alkaline solution we get, by the application of the methods illustrated previously, the modified equation

RT RT Eh - Eo - - log S, + - log (K[H'] -+ [H']2

where K is the ionisation constant. This equation is in full agreement with the experimental results of Dixon and Quastel, but at present the data are insufficient for the accurate determination of K. For cysteine it appears to be,

very roughly, 2 X 100- In such systems as the present one we must define carefully what is meant

by rH. To have any useful physical meaning it must equal log I/P, where P is the hydrogen-gas pressure which would be in equilibrium with the actual

hydrogen-atom concentration [H]. It must not be defined with reference to

P', the actual pressure of hydrogen gas in the electrode, which will not be equal to P in such systems. The rH so defined, which is the value calculated from the potential, is then analogous to that of the other systems considered.

The view of this system put forward here indicates that the reduction

potential (and rH) of - SH compounds, as measured by, e.g., a gold electrode, is not a measure of their true reducing power, since if the compensating process could be stopped much higher hydrogen concentrations, and therefore smaller rH values, would be produced. A number of facts confirm this. For instance, measurements with a gold electrode indicate that a N/500 solution of reduced

glutathione has an apparent rHl of about 13. I have found, on the other hand, that such a solution is capable of reducing indigo carmine to completion, so that its true rH, if one may use the phrase, must be smaller than 8. The same

applies to certain other dyes. Further, if our view is correct, we should get a much higher hydrogen

concentration, and therefore a more negative potential and a lower rH, if we used an electrode of a metal in which the rate of the compensating process is very small; in other words, if we used a metal with a high hydrogen over-

voltage. I have been able (in conjunction with Dr. Kodama) to verify this

prediction experimentally. Mercury has a high hydrogen over-voltage, and




does not react chemically with the solution, so that one would predict that a

mercury electrode would be negative to a gold electrode in a solution of a sulphydryl compound. Measurements were made of the difference of potential between gold and mercury electrodes in the same solution of reduced glutathione. The solution was kept stirred by a stream of nitrogen, which also removed

any traces of oxygen. I found that the mercury was about 200 millivolts

negative to the gold-a very large difference. The potential readings were

remarkably steady and constant, and to judge from only a few experiments the mercury electrode behaved in a similar manner to the gold when the

pH and - S concentrations were varied. When quinhydrone solution was

placed in the vessel instead of the glutathione solution the same mercury and

gold electrodes gave no difference of potential. The rH of the glutathione solution calculated from the potential given by the mercury electrode was about 7, which is in agreement with the indigo experiment mentioned above. The disulphide form was found not to affect the potential given by the mercury electrode, so that the critical rH for its reduction must be below 7.

The above facts afford good support to the view of the sulphydryl system

put forward in this section. A satisfactory explanation has hitherto been

lacking.

Summary. 1. It is shown that the phenomena of oxidation-reduction potential can be

predicted from the work of Wieland, and a theory in accordance with Wieland's

theory of oxidation is developed and extended in various directions.

2. An explanation of the anomalous behaviour of the sulphydryl system is

suggested.

The writer wishes to express his sincere thanks to Prof. Sir F. G. Hopkins for his interest and encouragement.

REFERENCES.

(1) Clark, 'The Determination of Hydrogen Ions,' Baltimore (1922).

(2) Clark, 'U. S. Pub. Health Reports,' Reprint No. 826 (1923).

(3) Clark, 'U. S. Pub. Health Reports,' Reprint No. 1017 (1925).

(4) Clark, Chem. Reviews,' vol. 2, p. 127 (1925). (5) Dixon & Quastel, ' Journ. Chem. Soc.,' vol. 123, p. 2943 (1923). (6) Wieland, 'Ber. d. Deutsch. Chem. Ges.,' vol. 45, p. 484 (1912). (7) Wieland, 'Ber. d. Deutsch. Chem. Ges.,' vol. 46, p. 3327 (1913).

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on the mechanism of oxidation-reduction potential

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