3140
NOTES
Catalytic Polarographic Current of a Metal Complex. IX.'p2 The Effect of Lithium
would substantiate indirectly anion coadsorption mechanism for the other electrolytes.'*,g
Hexafluorophosphate Supporting Electrolyte on the Nickel(I1)-o-Phenylenediamine System
Experimental Section
by Hubert C. MacDonald, Jr., and Harry B. Mark, Jr.
Discussion
Department of Chemistry, the University of Michigan, Ann Arbor, Michigan 48104 (Received August 18, 1969)
A relationship between the modified Koutecky rate parameter,ld,3 A, and the potential of the outer Helmholtz plane, l 2 OHP potential, q0,has previously derived
Recent studies of the polarographic prewave which is observed to arise a t potentials anodic to the main hexaaquonickel(I1) wave in the presence of o-phenylenediamine(0PDA) have shown that this prewave is the result of the reduction of a 1:1 nickel(I1)-OPDA complex.lb~d,eFurther study of the effect of the concentration and nature of supporting electrolyte upon the prewave, Xi? effects, showed that the complex is formed a t the electrode surface and not in the "bulk') of the s o l ~ t i o n . ~ ~ , ~ , The " ~ , ganalysis , ~ , ~ of the data for all systems investigated, however, showed that the quantitative correlation of t l i e ~ r y ~ and e ~ gexperimental ~~ data indicated that the charge of the cation involved in the chemical reaction preceding electron transfer was 1.l d , f This result was surprising as the theory of the effect of the potential within the Helmholt double layer on the rate of chemical reaction a t the electrode surface should be related to the bulk charge of the ion(s) involved in the chemical r e a ~ t i o n . ' * ~ g ~ A~value ~ 5 of x = $2 was expected for the reaction of the hexaaquonickel(I1) ion and neutral OPDA at a pH of 7.6 On extensive study of the adsorption of OPDA by differential capacitance measurements as a function of the nature and concentration of the supporting electrolyte indicated that anion coadsorption was a possible explanation.'g~~ It v a s felt that a study of the nickel(I1)-OPDA prewave in the presence of an anion of the supporting electrolyte which was known to exhibit a minimum tendency toward specific adsorption might provide more direct evidence for an anion coadsorption mechanism. Although alkali metal fluoride salts have been shown to exhibit a negligible tendency toward specific adsorption a t a mercury electrode,S they cannot be employed in this study as the fluoride ion exhibits a strong complexing effect toward the nickel(I1) ion. However, the hexafluorophosphate salts, LiPF6 and KPF,, exhibit a similar lack of specific adsorption of the anion on and do not form complexes with nickel(I1). Thus a study of the nickel(I1)-OPDA prewave current as a function of the lithium hexafiuorophosphate concentration
+
T h e Journal of Physical Chemistry, Vol. 74, No. 16,1970
All experimental methods and procedures employed in this investigation have been described previously. l b r d t e
where lcr is the forward rate constant of the complexation reaction a t the electrode surface, Kh is a Henry's law constant, [L]O is the bulk ligand concentration, x is the stoichiometric coefficient for the reaction, xis the average bulk charge of the ion(s) taking part in the chemical reaction at the surface, and the other symbols have their usual significance. The dependence of the prewave current on the nickel(I1) concentration showed that the (1) For other papers of this series see: (a) H. B. Mark, Jr., and C. N. Reilley, J . Electroanal. Chem., 4, 189 (1962); (b) H . B. Mark, Jr., J. Electroanal. Chem., 7, 276 (1964); (c) H. B. Mark, Jr., L. R . McCoy, E. Kirowa-Eisner, and H. C. MacDonald, Jr., J . Phys. Chem., 72, 1083 (1968); (d) L. R. McCoy, H . B. Mark, Jr., and L. Gierst, J . Phys. Chem., 72, 4637 (1968); (e) H. B. Mark, Jr., and L. R. lMcCoy, Re*. Polarogr., 14, 122 (1968); (f) L. R . McCoy and H. B. Mark, Jr., J . Phys. Chem., 73, 953 (1969); (g) L.R. lMcCoy and H. B. Mark, Jr., ibid., 73, 2764 (1969). (2) This research was supported in part by the National Science Foundation, Grant No. NSF GP-9307. (3) J. Koutecky, Collect. Czech. Chem. Commun., 18, 597 (1953). (4) L. Gierst, Trans. S y m p . Electrode Processes, 109 (1959). (5) J. Dandoy and L. Gierst, J . Electroanal. Chem., 2, 116 (1961). (6) The possible formation in the bulk solution of 1:l complexes of nickel(I1) with the anion of the supporting electrotype or OHwas considered to explain the z = +l. However, this value of z was found to be independent of the nature of the supporting electrolyte anion and of pH changes between 6.0 and 7.5.1d1e,ftg Also spectrophotometric studies did not show any significant differences on changing the anion of the supporting electrolyte.'b~g (7) The coadsorption was thought to arise from a a-electron interaction of the aromatic ring of the ligand with the electrode surface. This interaction results in an electron deficiency on the solution side of the adsorbed ligand. Thus an anion would tend to adsorb on the ring of the adsorbed OPDA. (8) D. C. Grahame, Chem. Rev., 41, 441 (1947). (9) L. G. SillBrf,and A. E. Martell, "Stability Constants of Metal Ion Complexes, The Chemical Society, Burlington House, London, 1964, p 262. (10) F. C. Anson, private communication of unpublished results, 1969. (11) R. Payne, J . Amer. Chem. SOC.,89, 489 (1967); K . S.Whiteley, Ph.D. Thesis, University of Birmingham, England, 1957; D. J. Barclay, J . EZectroanaZ. Chem., 19, 318 (1968). (12) The OHP is assumed here to be approximately equal to the reaction plane of the chemical complexation reaction. Thus, the OHP potential, 10, is essentially equal t o the reaction plane potentia1.ld
3141
NOTES [LiPFb], M 4.01
hi2t]=1~10"3
-0.01
0.025 en
-1.5
[LiPFd, M E
E
2.0
-
Ir '
-OD4
-a06
-ao
-0.08
-a01
-0.12
- 014
Y O
9
Figure 2. Dependence of the rate parameter on the O H P potential. Current measured at -0.8 V(sce).
-E, Volts
VI.
SCE.
Figure 1. Dependence of prewave on t h e concentration of lithium hexafluorophosphate. [OPDA] = 2 x 10-4 M , PH 7.0.
reaction was first order with respect to this ion. The variation of the rate parameter as a function of the OPDA concentration showed that the reaction was first order with respect to this species also. The same results were obtained in the studies with other electrolytes; it was therefore thought that the overall reaction mechanism was the same for LiPF6 as previously demonstrated for other electroIytes.ldtflg In order to determine the value of x , it is simply necessary to maintain a constant bulk concentration of OPDA and nickel(I1) and vary the potential of the OHP, q0,by varying the concentration of LiPFe.ld84 Under these conditions eq 1 reduces to log X
=
xF RT
constant - --!Po
(2)
As the value of qobecomes more negative (a decrease in LiPFe concentration cathodic to the electrocapillary ey 2 predicts that the rate parameter maximum),1dNrt8 and, hence, the limiting current of the prewave will increase as is observed in Figure 1. The rate parameter, A, can be determined experimentally from the polarographic data of Figure 1 according to the following relationship i/id = F(h) (3) where i is the limiting current of the prewave and id is the diff usion-limited current of nickel(I1). Typical plots of log h vs. qol 3 are shown in Figure 2. The slope
of the line is in agreement with a value of x = + 2 as one would expect for the reaction of the hexaaquonickel(I1) ion with the neutral OPDA. This result also lends some support to the conclusion that the coadsorbed anion does act as a partner in the surface reaction as suggested previously for the other anion^.^ This would account for the "anomalous" value for x of +1 found in the studies with other anions as the electrolyte. (13) Actual to values for Pli'6- have not been determined. Anson, however, has found that the adsorption characteristics of PFa- are essentially identical with F-10. Thus to values for F- as given by Russell's tables" were used here. (14) C. D.Russell, J . Electroanal. Chem., 6,486 (1963).
The Empirical Shielding Parameter Q and Trisubstituted Benzenes by G. Socrat'es Department of Polymer Science, Brunel University, London, England (Received October 1.6,1969)
Anomalous ortho proton and fluorine chemical shifts have been observed for chloro-, bromo-, and iodo-substituted ben~enes.l-~These anomalies cannot be explained by the magnetic anisotropy of the halogens.6 (1) H. Spiesecke and W. G. Schneider, J . Chem. Phys., 35, 731 (1961). (2) H.S. Gutowsky, D. W. McCall, B. R. McGarvey, and L. H. Mayer, J . Amer. Chem. Soc., 74, 4809 (1952). (3) A. J. R. Bourns, D. G. Giles, and E. W. Randall, Proc. Chem. Soc., 200 (1963). (4) N. Boden, 3. W. Emsley, J. Feeney, and L. H. Sutcliffe, Mol. Phys., 8, 133 (1964). (5) 3. S. Martin and B. P. Dailey, J . Chem. Phys., 39, 2722 (1963).
The Journal of Physical Chemistry, Vol. 74,No. 16,1970
