the equilibrium distribution coefficient (D)values from Figure 1 were converted to D,values for every concentration of nitric acid studied and are given in Table 1. It is clear from Table I that D. values obtained from the peak volumes of the eluted activities and those calculated from distribution coefficients are in good agreement. ACKNOWLEDGMENT
The authors are grateful to Frederick Nelson of Oak Ridge National Laboratory for his comments on this paper.
G. W.,. Higgins, I. R., Roberts, J. T., in ‘‘Ion Exchange Technology,” F. C. Nachod, J. Schubert, e&., p. 419, Academic Press, New York, 1956. (10) Strelow, F. W. E., ANAL. CHEM. (9) Parker,
LITERATURE CITED
(1) Bhatki, K. S., Adloff, J. P., Radwchim. Acta 3, 123 (1964). (2) Bhatki, K. S., Ephraim, D. C., Inddian J . Chem. 4 (6), 261 (1966). (3) Doering, R. F., Tucker, X. D., Stang, J., J. Inorg. Nucl. Chem. 15, 215 (1960). (4) Goldin, A. S., Velten, R. J., Frishkorn, G. W., ANAL.CHEM.31, 1490 (1959). (5) Hamaguchi, H., Ikeda, N., Iwasa, A., Radioisotopes 13, 377 (1964). (6) Lerner, M., Rieman, W., ANAL. CHEM.26, 610 (1954). (7) Levin, V. I., Meshcherova, I. V., Marygina, A. B., Sarvetnikov, 0. E., Radwkhimiya 5 ( l ) , 37 (1963). (8) Macasek, F., Cech, R., Chem. Zvesti 19, 107 (1965).
37, 106 (1965). (11) Susuki, Y., Intern. J . A p p l . Radiutam Isotopes 15, 599 (1964).
A. T. RANE K. S. BBATKI~ Tata Institute of Fundamental Research Colaba, Bombay 5 BR India. 1 Correspondence regarding this publication may be addressed to this author.
Determination of Adsorbed Cobalt and Iron Ethylened initrilotetraacetate Com plexes on Platin urn Electrodes by Thin Layer Electrochemistry SIR: Previous studies in these laboratories in which diffusion chronopotentiometry was employed as the measuring technique ( 1 , 2) produced evidence that both cobalt(II1)-EDTA and cobalt(I1)-EDTA anions are extensively adsorbed on bright platinum electrodes. Quantitative measurement of the amount of each complex adsorbed was not attempted because of the poor definition of the waves and the possibility that the data contained some contribution from oxide film formation or supporting electrolyte reduction. When bromide or iodide ion was added to the solutions, the adsorption of the cobalt complexes appeared to be prevented, presumably because of the preferential adsorption of the halides. The general utility of the thin layer electrochemical technique for the quantitative study of adsorption has been established recently (7, 8) and we have applied i t to the cobalt-EDTA system to check the earlier results and to provide a quantitative measure of the extent of the adsorption. The ironEDTA complexes were also examined for possible adsorption. EXPERIMENTAL
The thin layer electrode was the micrometer type previously described ( 5 ) . The area of each face was 0.3175 sq. cm. The electrode was cleaned by alternate potentiostatic oxidation (1.2 volts us. SCE) and reduction (0.0 volts us. SCE) while a stream of oxygen was flowing across the free 1F electrode faces. After the cleaning cycle the electrode was maintained a t 0.4 volt us. SCE for several minutes until any residual current decayed to less than 1 Ha. Both linear potential sweep (8) and integral chronoamperometric [potential
step with current integration (S)] techniques were employed in the manner previously described. All solutions were deaerated with and stored under prepurified nitrogen. The electrode was enclosed in a polyethylene tent which was constantly swept with nitrogen that had been equilibrated with water at the laboratory temperature. Reagents. A 0.1F solution of cobalt(1IbEDTA ICOY-~) was nrepared ‘by mixing equimolar quantities of COS04~7HzOand Na2H2Y .2J+O in a I F pH 7 phosphate buffer solution. CrvstallineNa CoY.4H90 (11) was prepired as follows: An aqueous solution of B ~ ( C O Y ) ~ . ~ Hprepared ZO, according to the published procedure (9), was treated with a slight excess of NazS04, the resulting BaS04 removed by filtration, and the filtrate added to a large volume of anhydrous ethanol. The resulting precipitate was recrystallized from a n aqueous solution to give large violet crystals which were dried for 24 hours a t 50’ C. Purity of the crystals was confirmed by thin layer electrochemical analysis of weighed portions. Attempts to prepare pure NH4FeY HzO by the published procedure (8) were unsuccessful. However, NaFeY. 3H20 (4) was successfully obtained by the following modified procedure: A fresh precipitate of Fe(OH), was prepared by adding excess aqueous ammonia to a neutral solution of FeS04 in the presence of atmospheric oxygen. The precipitate was thoroughly washed by decantation and then treated with slightly less than the stoichiometric quantity of ethylenediaminetetraacetic acid, H4Y, and the pH of the mixture adjusted to about 8.5 with NaOH. The remaining Fe(0H)a was removed by filtration and the filtrate adjusted to pH 5 by addition of acetic acid and sodium acetate in appropriate quantities to give a total acetate concentration of 18’ and a concentration of F e y - of about 0.1F. \
,
After several days’ standing, such solutions yielded large brown-colored crystals of NaFeY .3H20which were collected by filtration, washed with ethanol and ether, and air dried. Purity of the crystals was confirmed by thin layer electrochemical analysis of weighed portions. Solutions of Fey-* were prepared by potentiostatic reduction of F e y - at a large platinum gauze electrode in the absence of oxygen. Other chemicals were reagent grade and were used without further purification. The water was triply distilled; the second distillation was from alkaline permanganate. The special micropipets employed have been previously described (8). RESULTS AND DISCUSSION
COY and COY-’. The single-cycle thin layer current potential curve for reduction of COY- followed by oxidation of COY-2 in 1P NaC104 supporting electrolyte is curve d in Figure 1. The great irreversibility of this couple observed in the earlier studies ( I , 2 ) persists even under thin layer conditions, although the much lower current densities prevailing in the thin layer cell produce anodic shifts in the potential at which COY- is reduced. Addition of iodide ion to the solution causes the electrode reactions to become more nearly reversible, as shown in curve B in Figure 1. This effect is believed to result from the preferential adsorption of iodide ion on the electrode leading to desorption of the cobalt complexes which, when adsorbed, interfere with the electrode reactions ( I , 2 ) . This ability of iodide ion both to desorb the cobalt complexes and to render the electrode reactions more reversible was exploited to measure VOL 38, NO. 1 1 , OCTOBER 1966
1601
i
-
i , microampere
4
B
i, microo.mpere
- 22
- 11
*E
- -2 W Figure 1. Single-cycle thin layer current potential curves for cobalt(ll1) and cobalt(l1)-EDTA A
B
Noiodide-1.77pl.of 1.00mFCoY-in l F ( p H 7 ) phosphate buffer With iodide-1.40 pl. of 1 .OOmF COY- in 1 F NaClOa (pH 5) plus 1 .OO pi. of
2.00 mF KI Sweep rote was 2 mv./sec. negative current
Potentiols are volts
SCE.
VI.
the extent of adsorption of the cobalt complexes. T o do this the following procedure was employed : The cleaned electrode was rinsed with excess of a solution containing 1F phosphate buffer (pH = 7) and millimolar COY- or COY-2. The micrometer gap was closed to provide a volume of 1.77 pl. and the expelled reactant solution was removed from the vicinity of the crevice with a capillary pipet. The gap was then opened to a width of 2 to 3 mm. (the 1.77 pl. of cobalt solution adhered, without loss, to one of the electrode faces a t this point) and exactly 1.00 pl. of a 2 mF K I solution was pipetted onto the face of one electrode, where i t adhered. The gap was then set to contain a volume of exactly 2.77 pl. Connection was made to the capillarytipped salt bridge and the total amount of COY- or C o y F 2 in the crevice was determined by the integral chronoamperometric technique (6, 8 ) .
Figure 2. Single-cycle thin layer current potential curve for Fe(lll)-and Fe(ll)-EDTA 1.77 pl. of 1 .OOmF FeY - in 1 F No&Od (pH 5). Sweep rate was 2 rnv./rec. Potentials are volts VI. SCE. Cathodic current is negative current. The dashed line is for an iron-free solution
The amount of adsorbed COY- or was calculated from Equation 1 (8)
Q
No. of trials
1 2
4 4
=
nFVCo
+ 2nFAr
(1)
where Q is the charge that is consumed in the reduction or oxidation of COY- or COY+, respectively, n = 1, F = 96,500 coulombs per equivalent, Ti is the thin layer volume setting before the addition of the iodide, C" is the concentration of cobalt complex employed (1 m M in these experiments), A is the area of each electrode face, and r is the amount of adsorbed reactant in moles /sq. cm. The experimental results are sum-
Table 1.
Expt. no.
Cathodic current is
Q (total), pc. 208 f 3 189 f 5
marized in Table I. Substantial adsorption of both COY- and COY-2 was found, in agreement with the previous chronopotentiometric evidence ( I , 9 ) . Because both the adsorbed COYand had to be desorbed by iodide in order to be measured in these experiments, it was necessary to show that iodide, in the concentration range used, desorbed the cobalt complexes completely. That this was the case was demonstrated by the experiments summarized in Table 11. In these experiments an exactly measured volume of the cobalt complex was pipetted into the cleaned cell, the gap closed to this volume to allow the cobalt complex to
Adsorption of COY- and COY-2 on Platinum COY- REDUCTION Q (blank), gc. 18 f 1 18 f 1
Q (theory),
Q, (total) Q (blank), pc.
for no absorption, pc.
190 f 4 171 f 6
171 171
r from Equation
1,
moles/cm.2 X 1010 3.1 f 0.4 0 f l
COY-2 OXIDATION 200 i 6 171 4 . 7 f 0.9 30 f 3 230 f 3 171 30 f 3 172 f 8 0 1.2 202 i 5 Experimental conditions: In experiments 1 and 3 the electrode faces were rinsed with reactant solution (1mM COY- or COY-*in 1F phosphate buffer, pH = 7) and then filled with 1.77 pl. of reactant solution; the gap was then opened to a width of 2-3 pl. and 1.0 pl. of 2 mm. KI added from a micropipet. The gap was set to 2.77 gl. and the total amount of COY- or COY+ in the crevice was determined. In experiments 2 and 4 the electrode faces were first rinsed with 2mM iodide solution, next with reactant solution, and then 3 4
4 4
*
were treated by the same procedure. The potential step was 0.25 to -0.05 volt us. SCE in experiments 1 and 2; 0.00 to 0.30 volt us. SCE in experiments 3 and 4. The values of Q (blank) were obtained by repeating the appropriate procedures except that the phosphate supporting electrolyte solution was substituted for the reactant solution in each case.
1602
0
ANALYTICAL CHEMISTRY
~~
Table II.
Exp. no. 1
No. of trials 6
2
5
Desorption of COY- and COY+ from Platinum by Iodide COY- REDUCTION Q (total) Q (blank), MC. Q (blank), pc. Q (total), pc. 14 f 2 92 f 5 106 f 3
Q (theory), for no adsorption, pc. 91
COY-2 OXIDATION 115 f 6
24
3z
2
91 f 8
91
Experimental conditions: 0.94 pl. of reactant solution (1mM COY- or COY-2 in 1F phosphate buffer, pH = 7 ) waa added to the clean cell from a micropipet and the gap adjusted to contain exactly this volume; the gap was then opened to 2-3 mm. and 0.94 pl. of a 2mM KI solution was added from a second micropipet. The gap was adjusted to 1.88 pl. and the COY- or COY-* present in the cavity determined. The potential step waa 0.25 to -0.05 volt vs. SCE in experiment 1 ; 0.0 to 0.30 volt us. SCE in experiment 2. The values of Q (blank) were obtained by repeating the above procedure except that the phosphate buffer solution was substituted for the reactant solution in each case. Table 111.
Search for Adsorption of Fey- and Fey-2 on Platinum
be adsorbed, the gap opened. and an exactly measured aliquot of iodide was added, and the amount of cobalt complex in the cell determined in the usual way. The fact that all of the cobalt complex initially added is recovered in this process demonstrates that the iodide leaves none adsorbed and unreactive. The values of r reported here are somewhat smaller than the previous chronopotentiometric data suggest [no values for r were calculated in the previous study but if any of the usual approximate models (IO) are applied to the data, values of 25 to 90 X 10-10 mole/sq. cm. for r result] and it seems likely that the chronopotentiometric transition times were seriously lengthened by the overlap of COY- reduction with supporting electrolyte reduction in the iodide-free solutions. We regard the thin layer values for r as the most reliable because they could be obtained in iodide-containing solutions in which the reduction and oxidation of CoYand are well separated from the background reactions. Fey- and FeyT2. I n contrast with the corresponding cobalt complexes, the iron(II1) and iron(I1)-EDTA complexes behave reversibly at platinum electrodes in the absence of any added iodide ion. A typical thin layer current potential curve for this couple is shown in Figure 2. The peak currents (3.39 pa. cathodic and 3.37 pa. anodic) agree well with each other
Q (total) Q (blank), pc. 170 f. 5
Q (theory),
for no Q (blank), pc. Reactant solution trials Q (total), pc. adsorption, pc. 1mM F e y - in 1F pH 5 6 190 f 4 20 f 1 171 1 acetate buffer 1mM F e y b 2in 1F pH 5 6 184 f 9 21 f 1 163 f 10 2 171 acetate buffer 3 1mM F e y - in 1F 4 187 f 3 15 f 0 . 5 172 f 3 171 NazS04-NaHSOd,pH 5 4 1mM F e y - in 1F 7 189 f 8 16 f 1 173 f 9 171 NaC1OrHCl0~, pH 5 Experimental conditions: Thin-layer volume was 1.77 pl. Potential step was 0.05 to -0.25 volt us. SCE in experiments 1, 3, 4, and -0.25 to 0.05 volt us. SCE in experiment 2. Q blank was obtained in the corresponding chelatefree supporting electrolyte with the same potential steps. Expt. no.
No. of
and with the theoretical value of 3.33 pa. (6).
The possibility of adsorption of F e y - or FeY+ was examined with the potential step integral technique in thin layer, and the results are summarized in Table 111. The data show that neither F e y - or are adsorbed on the platinum electrodes. Similar experiments in which the electrodes were maintained at potentials between +0.6 and +O.l volt us. SCE during the washing and filling steps gave identical behavior. There is thus a clear difference in behavior between the cobalt and the iron chelates of EDTA. Although speculations on the origin of this difference (for example, differing electronic structure, differing tendencies toward hepta-coordination, etc.) are not difficult to provide, neither are they very compelling, and we regard i t as an interesting observation that remains to be rendered expectable. CONCLUSIONS
The ability of the thin layer technique to provide quantitative adsorption data for systems which thwart most other electrochemical techniques is clearly demonstrated by the CoY--CoY-2 couple. The idea of trapping an adsorbate within the thin layer by prewashing and then desorbing it by addition of a preferentially adsorbed substance (for example, iodide ion) may prove of general utility in studies of systems in which the adsorbed reactant
interferes with, or completely prevents, the reaction of unadsorbed reactant; for example, methanol or carbon monoxide on platinum. LITERATURE CITED
(1) Anson, F. C., ANAL.CHEM.36, 520, flQfu1. (2) Anson, F. C., J . Electrochem. SOC. 110, 436 (1963). (3) Brintzinger, V. H., Thiele, H., Muller, U., Z. Anoro. Alloem. Chem. 251. 285 ” (1943). (4) Hoard, J. L., Smith, G. S.,Lind, \--__,.
M., “Advances in the Chemistry of the Coordination Compounds,” S. Kinschner, ed., p. 299, Macmillan, New York, 1961. (5) Hubbard, A. T., Anson, F. C., ANAL. CHEM.36. 723 (19641.
(6) Zbid., 38, 58 (1966). (7) Hubbard, A. T., Anson, F. C., J . Electroanal. Chem. 9, 163 (1965). (8) Hubbard, A. T., Osteryoung, R. A., Anson, F. C., ANAL. CHEM.38, 692 (1966). (9) “Inorganic Syntheses,” T. Moeller, ed., Vol. V, p. 186, McGraw-Hill, New York, 1957. (10) Tatwawadi, S. V., Bard, A. J., ANAL.CHEM.36, 2 (1964). (11) Weakliem, H. A., Hoard, J. L., J . A m . Chem. SOC.81, 549 (1959). ARTHURT. HUBBARD FREDC. ANSON Gates and Crellin Laboratories of
Chemistry California Institute of Technology Pasadena, Calif. WORK supported in part by the U. S. Army Research Office (Durham). A.T.H. is an NSF Predoctoral Fellow; F.C.A. is an Alfred P. Sloan Research Fellow. VOL 38, NO. 1 1 , OCTOBER 1966
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