Polarographic determination of glycine with a dropping copper amalgam electrode

J. Electroanal. Chem., 66 (1975) 53--65 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

POLAROGRAPHIC DETERMINATION OF GLYCINE WITH A DROPPING COPPER AMALGAM ELECTRODE

J. HERNANDEZ MENDEZ

Department of Analytical Chemistry, University of Valencia, Valencia (Spain) A. SANCHEZ PEREZ and F. LUCENA CONDE

Department of Analytical Chemistry, Unwersity of Salamanca, Salamanca (Spain) (Received 27th January 1975)

ABSTRACT The anodic oxidation of a dropping copper amalgam electrode in presence of dilute solutions of glycine in 0.50 M NaCIO4 has been studied. An anodic wave at --0.28 V (SCE) is observed, yielded by diffusion of glycinate anion in the solution towards the electrode surface. The wave-heights increase with the glycinate concentration (function of glycine concentration and pH value) until the anodic oxidation is controlled by the metal diffusion into mercury. The effect of pH is interpreted by attributing it to the depolarizer effects at glycinate anion even though the zwitterion is present in much larger concentrations. The applicability of anodic oxidation of a dropping copper amalgam electrode in the determination of glycine in the range of concentrations 10--4--10- 2 M with a rigorous control of pH (8.0 < pH < 10.5) is shown. The standard deviation of the proposed method is 4.1% and the minimum concentration determinable is in the 1 x 10 - 4 M range.

INTRODUCTION T h e a n o d i c waves o f a d r o p p i n g m e r c u r y e l e c t r o d e , c o r r e s p o n d i n g t o t h e f o r m a t i o n o f c o m p o u n d s o f m e r c u r y ions, are o f t e n u s e d f o r p o l a r o g r a p h i c determinations of some anions and organic substances of biological importance [ 1,2]. Nevertheless, the anodic oxidation of dropping amalgam electrodes (DAE) has n o t b e e n u t i l i z e d f o r a n o d i c d e t e r m i n a t i o n o f a n i o n s , b e c a u s e t h e a n o d i c dissolution of metal from m e r c u r y has been always considered the polarographic limiting factor [ 3,4]. H o w e v e r , as Z a b r a n s k y [5] a n d S c h u p p e t al. [6] h a v e p o i n t e d o u t t h e a n o d i c d i f f u s i o n c u r r e n t can be l i m i t e d e i t h e r b y t h e c o n c e n t r a t i o n o f amalg a m a t e d m e t a l o r b y t h e c o n c e n t r a t i o n o f t h e l i g a n d in t h e a q u e o u s s o l u t i o n . THEORY T h e a n o d i c o x i d a t i o n o f a m a l g a m a t e d m e t a l a t t h e D A E in t h e p r e s e n c e o f c o m p l e x i n g a g e n t m a y be r e p r e s e n t e d b y o v e r a l l r e a c t i o n

54 M(Hg) + p X ~- MXp + ne

(1)

whenever the rate of formation of the complex MXp be rapid. If the electrochemical system is rapid, the equilibrium potential of reaction (1), according to Nernst equation, is given by the expression: o

RT

RT

Eeq = Ea -- n F In fl + ~

[MXp] In [M(Hg)] [X] *~

(2)

E a being the standard potential of the M'~+ ~ M(Hg) system and fl the overall formation constant of the MXp complex. From eqn. (2): o

RT

KM(Hg)KP + nR FT In

RT

E~q = E a -- n F In/3 + ~

In

(3)

i - - iMX p

(iM(Hg) --/)(ix -- i) p

KMxp

KMtHg), K x , KMX p being the proportionality constants between the limiting current and the concentrations of the respective species. This general equation adopts different particular expressions. (a) Anodic wave in excess of complex agent (there is no complex MXp in solution and the process is controlled by the metal diffusion in the amalgam). RT

Eeq -- E a -- ~

RT

In fl + ~

KM(I-Ig) In g M x p

RT

RT

i

n F [KIP "~- n F In iM(Hg)- -- i

(4)

(b) Anodic wave with a deficiency of the complex agent (there is no complex MXp in solution and the process is controlled by the diffusion of the complex agent in solution). RT

RT

gPx

Eeq -- E a -- ~-~ In/3 + ~-F In K M X p

RT

RT

- F In [M(Hg)] + ~

i

in (ix -- i) p

(5)

(c) Anodic--cathodic wave in excess of the complex agent (there is complex MXp in solution, the cathodic wave is controlled by the complex diffusion and the anodic wave is controlled by the metal diffusion in the amalgam). RT KMtItg) o RT In/3 + In - Eeq = Ea nF -~ KMX p

RT RT i - - iMX p In [X] p + In nF n-F iM(Hg)-- i

(6)

(d) Anodic--cathodic wave with a deficiency of the complex agent {there is complex in solution, the cathodic wave is controlled by the complex diffusion and the anodic wave is controlled by the complex agent diffusion in solution). RT RT n F In ~ + ~ In

Eeq = E~ -

-

--

K~

RT

KMX p

nF

In [M(Hg}] +

RT

n-F

In

i

-

-

iMX p

(ix -- i}p

(7)

From the analytical point of view and for the polarographic determination of complex agent case (b) is the most interesting and will be considered in

55

detail. The E = f(i) equation of the anodic wave obtained with a DAE when the oxidation process is controlled by the diffusion of the complex agent species in solution, from (5), is RT i E = E l l 2 + ~-~ In (ix -- i)p

(8)

2P-lKx RT RT RT RT +7 In [M(Hg)] + n F In [X] p-1 Ell 2 = E a --~ ln /3 + ~-F l n - -

KMXp

(9)

The half-wave potential is a function of the standard potential of the system, of the stability constant of complex formed, of the concentration of the metal in the amalgam and the complex agent in solution, when p ~> 2; and in addition to the proportionality constant between the limiting current and concentrations. These equations agree with those given by Heyrovsky and Kfita [1] and Crow [7] for the anodic waves produced in the oxidation of DME in presence of complex agent for Hg 2÷ ion; if in those equations the term ( R T / n F ) In [M(Hg)] does n o t figure, because [Hg] = 1. The aim of this work is to study the polarographic behaviour of dropping copper amalgam electrode in the presence of dilute solutions of amino acid glycine. The m e t h o d takes on a particular interest because the amino acids do n o t produce reduction waves on DME and all proposed methods are indirect, most of them based on the reduction of the soluble complexes formed by the reaction of amino acids with solid copper salts. EXPERIMENTAL

The DAE utilized is represented in Fig. 1. It is based on the assemblies described both by Sancho et al. [8] and by Birch and Manahan [9]. Amalgam oxidation is avoided by preparing and storing it in an inert atmosphere of nitrogen and maintaining the amalgam subjected to a cathodic potential during the use of the electrode. The amalgam is prepared by the electrolytic reduction of copper nitrate solutions with a Metrohm 211 coulometer. Polarograms were recorded with a Sargent Model XVI polarograph. All potentials were referred to the saturated calomel electrode (SCE). The values of pH, with a precision of 0.01 pH units, were measured with a Metrohm E-388 potentiometer using a combined glass--calomel electrode. The stock of glycine solutions was prepared from the solid Merck product. A conductimetric titration [10] with a copper acetate solution, previously electrogravimetrically standardized, is utilized to normalize the stock glycine solutions. The supporting electrolyte are 0.5 M sodium perchlorate solutions. Polarographic measurements were made at 25°C. The oxygen present in the polarographic cell was removed by introducing pure nitrogen for 5 min. The amalgam drop rate was 3.2 mg s-1 and the drop period 2.6 s drop - 1 .

56

N2

Fig. 1. DAE showing: (A) amalgam compartment, (B) solution with reducible ionic species, (C) anodic compartment in the amalgam preparation, (D) agar-agar plug, (E) connections with external circuit (coulometer or battery), and (F) manometer compartment.

RESULTS AND DISCUSSION

The polarographic behaviour of dropping c o p p e r amalgam electrode in solutions containing copper ions and glycine at pH = 8.25 is shown in Fig. 2. The composite anodic--cathodic wave obtained at --0.28 V corresponds t o r ed u ctio n of copper glycinate c om pl ex (cathodic wave) and to the oxidation o f c o p p e r amalgam to c o p p e r glycinate com pl ex (anodic wave). At 0.00 V the o x id atio n of copper amalgam to aqueous ion occurs; this is the same as th at p r o d u c e d in absence of glycine. When the anodic potential attains the value o f +0.40 V the anodic dissolution of m e r c u r y occurs.

Effect o f p H on composite anodic--cathodic waves The nature of com pl e x species in solution is pH-dependent. This can be seen in the anodic--cathodic waves obtained with solutions containing Cu u+ and glycine. The results are shown in Fig. 3. In acid media an anodic--cathodic wave is obtained at 0.00 V corresponding to Cu u÷ + 2e ~ Cu(Hg) process. The Cu 2÷ ion is u n c o m p l e x e d with glycine. Above pH = 4.0 (approx.) the waves b e c o m e increasingly drawn o u t and b e c o m e separated in t w o waves

57

Hg-

..... --

CulGIyc}2

i.i;pA 0

I

I ÷ 0.Z

I

-.(It.

EIV(SCE)

-i/~ A

Fig. 2. T y p i c a l p o l a r o g r a m o f 1 X 10 - 3 tool 1-1 o f Cu(Hg), 4 x 10 - 4 M o f Cu 2+ a n d 2 X 10 - 3 M o f glycine in 0.5 M NaC104 at pH 8.25.

Fig. 3. E f f e c t o f pH o n c o m p o s i t e a n o d i c - - c a t h o d i c waves o b t a i n e d w i t h 5 X 1 0 - 4 m o l 1-o f Cu(Hg), 5 X 10 - 4 M o f Cu 2+ a n d 1 × 10 - 2 M o f glycine in 0.5 M NaC104. T h e pH values are i n d i c a t e d o n e a c h p o l a r o g r a m .

58

-Logo /

:f

.~,.o~.:.~ . . . .

3 /* 5

/

/

//

CuZ.

...... _ _ _

.',o,.,----_

~ Cu(~)*

////C~Olyc)Z'~ \ \

/

6 7 8

/

\\

\\x

x\\

x

9 10 11 12 13 I

2

3

/*

5

6

7 8

9

10 11 12 13

pH

Fig. 4. Nature of c o m p ] e x i n g 1 × 1 0 _ 4 M of Cu 2+ and 1 X ered.

a g e n t s p e c i e s and complex species in function 1 0 _ 3 M of glycine. The formation of Cu20 is

of pH for not consid-

at higher p H values: t h e c a t h o d i c wave c o r r e s p o n d i n g t o the r e d u c t i o n o f c o p p e r g l y c i n a t e c o m p l e x , Cu(glyc)2 + 2e ~ C u ( H g ) a n d the a n o d i c w a v e c o r r e s p o n d i n g t o t h e o x i d a t i o n o f a m a l g a m t o u n c o m p l e x e d ions, C u ( H g ) -* Cu 2+ + 2e. In t h e c a t h o d i c - - a n o d i c scan b o t h t h e processes m e n t i o n e d occur. In t h e r a n g e 8--9, t h e h e i g h t o f a n o d i c wave o b t a i n e d at - - 0 . 2 8 V is pHd e p e n d e n t , b e c o m i n g as high as t h e wave c o n t r o l l e d b y d i f f u s i o n o f m e t a l in m e r c u r y . This c o m p o s i t e wave is d u e to t h e f o l l o w i n g process: Cu(glyc)2 + 2e # C u ( H g ) + 2 glyc. This result m a y be e x p l a i n e d c o n s i d e r i n g t h e n a t u r e o f c o m p l e x species as a f u n c t i o n o f p H (Fig. 4) o b t a i n e d f r o m the e q u i l i b r i u m c o n s t a n t s f o r the m e t a l c o m p l e x a n d ligand i o n i z a t i o n equilibria listed in Table 1 [11].

Effect o f p H on anodic waves In t h e p r e c e d i n g e x p e r i m e n t it was o b s e r v e d t h a t t h e a n o d i c w a v e at - - 0 . 2 8 V a p p e a r s o n l y at values o f p H higher t h a n 8, increasing its h e i g h t a c c o r d i n g t o the g r o w t h o f t h e p H o f glycine s o l u t i o n . A s e q u e n c e o f p o l a r o TABLE 1 Reaction

Log K

H+(glyc)- + H+ ~ H+(glyc) H Glyc-- + IT+ ~ H + ( g l y c ) Cu 2+ + glyc-- ~ Cu(glyc) + C u ( g l y c ) + + glyc-- ~ Cu(glyc)2

2.34 9.60 8.07 6.90

59

Fig. 5. Effect of pH on anodie waves. Polarograms obtained with 2.2 X 10-3 mol 1-1 of Cu(Hg), 0.5 M NaC104, 1.87 X 10-4 M glyeine (left) and 5.58 X 10-3 M glycine (right). The pH values are indicated on each polarogram. grams was carried o u t with different glycine concentrations and d i f f e r e n t pH values. In Figs. 5 and 6 t h e results are shown, where the pH d e p e n d e n c e of wave-heights can be observed. The waves began to be complicated with insoluble Cu20 f o r m a t i o n at pH value a b o u t 10.5 and more alkaline solutions, when the glycine c o n c e n t r a t i o n is small, theoretically justified in the potential--pH diagram (Fig. 7). For greater concentrations of glycine the first anodic wave becomes as high as the anodic wave controlled by diffusion o f Cu in Hg. All these results only can be explained if t h e y are attributed to the depolarizer effects at glycinate anion. It is well known that the amino acid species are different for distinct values o f pH: cation, H ÷ (glyc)H, in acid media, zwitterion, H+(glyc) - , in neutral media; and anion, g l y c - , in alkaline media. The anion c o n c e n t r a t i o n begins to be i m p o r t a n t at pH 8 and depends on the glycine c o n c e n t r a t i o n and the pH value. In Fig. 8 the wave-heights for different values o f glycinate anion concentration are plotted. The free glycinate anion c o n c e n t r a t i o n is calculated by knowing the total c o n c e n t r a t i o n o f glycine and the pH value. The graphic representation is a straight line with a plateau which corresponds to a limiting current by the c o n c e n t r a t i o n of Cu metal in the amalgam. These results are in excellent agreement with the Pearlmutter and Stuehr [14] results; t h e y have shown from relaxation techniques the preference o f Cu 2. for the glycinate anion though the zwitterion is present in m u c h larger concentration.

60

/

22

o f

o~°E/V

II It

.0/,5

18

Cu(Olyc)°

Cu2"

I I

Cu(Olyc)z

CuIOH}

II

*030

I

14 * 0.15 10

000 CuiNg)

~

-0~5 I

/ 8

"

"030 I I

9

10

11 pH

i

i

i

i

i

i

J

i

L

i

i

i

1 2 3 ~, 5 5 7 B 9 10 11 12 13pH

Fig. 6. Effect of pH on anodic waves. Plots of wave-heights against pH for different glycine concn. (A) Absence of glycine; (q)) 1.87 x 10 - 4 M; ( v ) 4.67 x 10 - 4 M ; ( o ) 9.34 x 10 - 4 M; (~7) 1.87 x 10 - 3 M; (~) 9.34 x 10 - 3 M. Fig. 7. Potential--pH equilibrium diagram for the system copper--amalgam--water in presence of glycine. The calculations are made according to Pourbaix [ 12 ] and the standard potential of Cu(Hg) from Kolthoff and Lingane [ 1 3 ] . [Cu 2+ ] = 1 X 10 - 4 M, [ g l y c i n e } = 1 X 10 - 3 M a n d [ C u ( H g ) ] = 1 X 10 - 3 m o l 1 - 1 .

Influence of buffer substances Knowing that the wave-height is dependent on the pH value, it was thought necessary to fix the pH with a buffer substance. The results were n o t satisfactory because the amalgam oxidation in presence of carbonate, borate and ammonia t o o k place at potentials near those of the oxidation in presence of only glycinate anion. The inconvenience of n o t being able to use buffer substances can be eliminated by the buffer capacity of glycine itself at pH values near pK~ {9.60), due to the transformation o f the zwitterion in anion as is explained below.

Influence o f glycine concentration on anodic waves Figure 9 illustrates h o w the heights of the anodic waves increase as the glycine concentration does for three pH values. As was pointed o u t before, the height of the wave is proportional to the anion concentration and at con-

61

/z

2C

10

/

/

/?

Id/~A

/

6O

/

~' ,/

/ /.v

/

,,

/

/

/

/

,/

o

//

,/

//

/

,°

/ /°

o ~ o - -

,

~

,2 ,/

2PO

I

I

I

4

6

8

i

i

40

60

I 10 i

8

12

16

I03c/M

80 1 0 4 c / M

Fig. 8. Plots of wave-heights against glycinate concentration obtained for different glycine concn, and different pH values. [Glycine ] : (0) 1.87 X 10 - 4 M; (v) 4.67 X 10 - 4 M; (~) 9.34 X 10--4M; (v) 1.87 X 10 - 3 M; (~) 9.34 X 10 - 3 M. Fig. 9. Plots of wave-heights vs. g]ycine concentration for three pH values. (o) pH = 8.64; (v) pH = 9.60; (<>) pH = 10.32.

stant pH values proportional to the amino acid concentration. There is difficulty in giving a constant pH value by adding sodium hydroxide to the glycine solution before scanning the polarographic register, b u t this inconvenience is avoided by making a complete Id--PH register for each concentration of glycine and interpolating a fixed pH value, it n o t being necessary to run the polarogram at a given pH value.

Influence of amalgam concentration A series of polarograms were carried o u t with constant concentration of glycine at pH = 9.70, and different concentrations of copper in the amalgam. In Fig. 10 the graphic representation of wave-heights obtained is shown; while the wave caused by the glycinate anion is practically of constant height, the total height of the two anodic waves Cu(Hg) + 2 g l y c - -* Cu(glyc)2 + 2 e Cu(Hg) -* Cu 2+ + 2 e -

El/2 = --0.30 V E1/2 = 0.00 V

grows linearly with the copper concentration in mercury. There is an upper

62

o/

ld/p A 6C

o/ /

cII

I 2

~1--

I

l 5

103 r-cu (Hg)] rno~ 1-1

Fig. 10. Plots of wave-heights vs. amalgam concentration. (()) First anodic wave (--0.28 V); (o) the two anodic waves. [Giycine] = 1.87 X 10-3 M and pH = 9.70. limit at [Cu(Hg)] = 5 × 10 -~ m o l l - 1 due to the solubility of Cu in Hg; above this value the amalgam stops being a true solution and the dropping o f the electrode is made difficult.

Procedure for the polarographic determination of glycine The co p p er amalgam having been obtained according to the description in the experimental part of this work, is made to drop through a polarographic capillary. The DAE, the reference electrode, the pH electrode and an entrance and exit tube for the nitrogen are placed in the polarographic cell. Sodium perchlorate, potassium nitrate and o t h e r salts t hat do n o t react with c o p p e r ions can be used as supporting electrolyte. The straight line Id--C is obtained in the following manner; for each amino acid c o n c e n t r a t i o n the recording is made at a different pH value obt ai ned by successive addition of sodium h y d r o x i d e (the solution is h o m o g e n i z e d by N 2 bubbling and the error of dilution is avoided by adding sodium h y d r o x i d e at least a h u n d r e d times more c o n c e n t r a t e d than the amino acid). The pH is measured before and after each polarographic scan, and practically does n o t vary. A table of I d - p H values is obtained by measuring the wave-heights. F r o m the graphic representation for each c o n c e n t r a t i o n of glycine used is obtained a family of curves. A pH value is selected and the different heights are interpolated graphically; in this way we obtain points of the Id--c straight line.

63

2E

Id/JJ ~ 22

18

9

10

11

pH

Fig. 11. Plots of I d vs. pH different concentrations of glycine. (m) Absence of glycine; (~) 1.86 X 10 - 4 M, (v) 3.72 X 10 - 4 M; (~) 5.58 X 10--4M, (@) 7.44 X 10 - 4 M; (+) 9.34 X 10--4M;(D) 1.86 X 1 0 - - 3 M ; ( X ) 3.72 X 10--3M;(e) 5.58 X 10--3M;(O) 9.34 X 10--3M~ (o) 14.0 X 10--3M. 2.1 X 10 - 3 mol 1-1 [(Cu(Hg)]; 0.50 M[NaC104].

28

1~fiA 2~

/

20

/

/ 2

i

i

I

i

i

4

6

8

10

12

i

14 1 0 3 c / M

Fig. 12. Graphic representation of I d vs. glycine concentration. The I d values have been obtained interpolating at pH 10.00 in Fig. 11.

64 As an illustrative e x a m p l e we s h o w Figs. 11 and 12 in w h i c h t h e I d - - P H a n d

Id--c r e p r e s e n t a t i o n s w e r e o b t a i n e d f o r c o n c e n t r a t i o n s b e t w e e n 1 . 8 6 × 10 - 4 M and 14 × 10 - 3 M o f glycine using a c o n c e n t r a t i o n o f Cu in t h e a m a l g a m o f 2.2 × 10 - 3 m o l 1-1 a n d i n t e r p o l a t i n g at p H = 10.00. A q u i c k e r m a n n e r t o o b t a i n t h e I d - p H values is t o scan the d i f f e r e n t amp e r o g r a m s at a c o n s t a n t p o t e n t i a l value, in t h e d i f f u s i o n p l a t e a u o f t h e wave. In this w a y t h e t y p i c a l d e v i a t i o n o f t h e m e t h o d was c a l c u l a t e d , realizing t e n series o f d e t e r m i n a t i o n s at d i f f e r e n t p H values at E = - - 0 . 1 5 V with a 4.57 × 10 - 4 M glycine solution. T h e s t a n d a r d d e v i a t i o n is 4.1% at p H 10.00. On t h e o t h e r h a n d t h e m i n i m u m glycine c o n c e n t r a t i o n w h i c h p r o d u c e s an a n o d i c wave clearly d i f f e r e n t i a b l e o f t h e w a v e p r o d u c e d b y O H - ions is o f t h e r a n g e 1 × 1 0 - 4 M.

Diffusional characteristics of the anodic waves obtained T h e t h r e e m o s t i m p o r t a n t criteria t o e l u c i d a t e t h e d i f f u s i o n a l c h a r a c t e r o f a p o l a r o g r a p h i c process are t h e d e p e n d e n c e o f w a v e - h e i g h t s on (1) t h e d e p o larizer c o n c e n t r a t i o n , (2) t h e h e i g h t o f m e r c u r y , a n d (3) t h e t e m p e r a t u r e , as c o n s e q u e n c e o f Ilkovic e q u a t i o n [1]. T h e linear d e p e n d e n c e o f w a v e - h e i g h t s o f g l y c i n a t e c o n c e n t r a t i o n has b e e n s h o w n already. T h e d e p e n d e n c e o f d i f f u s i o n c u r r e n t on s q u a r e - r o o t o f t h e a m a l g a m reservoir h e i g h t i n c l u d i n g t h e r e a d i n g o f m a n o m e t e r w i t h t h e n i t r o g e n pressure a b o v e t h e a m a l g a m is linear. T h e t e m p e r a t u r e d e p e n d e n c e o n t h e d i f f u s i o n c u r r e n t is also linear. T h e t e m p e r a t u r e c o e f f i c i e n t is 0 . 0 0 6 5 ° C - 1 , in g o o d a g r e e m e n t w i t h the usual value f o r d i f f u s i o n processes. ACKNOWLEDGEMENT O n e o f t h e a u t h o r s (A.S.P.) is i n d e b t e d t o t h e G e n e r a l D i r e c t i o n o f S u p e r i o r E d u c a t i o n and R e s e a r c h f o r t h e f e l l o w s h i p assigned.

REFERENCES 1 H. Heyrovsky and J. KfJta, Principles of Polarography, Academic Press, New York, 1966, p. 172. 2 D.R. Canterford and A.S. Buchanan, J. Electroanal. Chem., 44 (1973) 261. 3 N.H. Furman and W.C. Cooper, J. Amer. Chem. Soc., 72 {1950) 5667. 4 Y. Okinaka, I.M. Kolthoff and T. Murayama, J. Amer. Chem. Soc., 87 (1965) 423. 5 Z. Zabransky, Collect. Czech. Chem. Commun., 24 (1959) 2426. 6 O.E. Schupp, T. Youness and J.I. Watters, J. Amer. Chem. Soc., 84 (1962) 505. 7 D.R. Crow, Polarography of Metal Complexes, Academic Press, London, 1969, p. 173. 8 J. Sancho, J. Albadalejo and A. Ar6valo, Anales Real Soc. Esp. Fis. Quim. (Madrid), 52-B {1955) 455. 9 C.G. Birch and S.E. Manahan, Anal. Chem., 39 (1967) 1182. 10 M. Pascal, Chim. Anal. (Paris), 38 (1956) 201.

65 11 L. Sillen and A: MarteU, Stability Constants, Special Publication No. 17, The Chemical Society, London, 1964, p. 377. 12 M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, London, 1966, p. 384. 13 I.M. Kolthoff and J.J. Lingane, Polarography, Interscience, New York, 1952, p. 227. 14 A.F. Pearlmutter and J. Stuehr, J. Amer. Chem. Soc., 90 (1968) 858.