SHORT COMMUNICATIONS 177

from thermal data of the corresponding pure metal chlorides, i.e.

EM* -- Ecl*= AG*(MC1,)/nF at 727~C (103 K)

Considering the measurement uncertainties, a substantial agreement is obtained between our results and those of Flengas and Ingraham. As the precision of their determinations was not indicated, we calculated by the least squares method, with their experimental data 1, the standard potential and the Nernst slope for copper:

* * Ecu - E o = ( - i. 165 __+ 0.030) V and

2.3 RT/F = (187_ 14) mV

Moreover, by considering the values of AG*/nF for the pure metal chlorides, we can observe that the activity coefficient fM is near to unity for the noble metals, leading us to suppose that the solution of their chlorides is nearly "perfect", while for the others the formation of complexes can be postulated (fu ~ 1).

Laboratoire de Recherches de Chimie Analytique, FacultO des Sciences de Paris, ENSCP, Paris (France)

Richard Combes Jacques Vedel

Bernard Tr6millon

1 S. N. FLENGAS AND T. R. INGRAHAM, Can. J. Chem., 36 (1958) 1103. 2 S. N. FLENGAS AND Z. R. INGRAHAM, J. Electrochem. Soc., 106 (1959) 714. 3 H. A. LAITINEN AND C. H. LIU, J. Am. Chem. Soc., 80 (1958) 1015. 4 H. A. LAITINEN AND J. W. PANKEY, J. Am. Chem. Soc., 81 (1959) 1053. 5 G. DELARUE, Th~se, Paris, 1960; Rapport CEA, 1961, No. 1847. 6 S. SENDEROEF AND A. BRENNER, J. Electrochem. Soc., 101 (1954) 31. 7 J. HLADIK, M. SAUNIER AND G. MORAND, J. Chim. Phys., 64 (1967) 378. 8 M. HANSEN, Constitution of Binary Alloys, Metallurgy and Metallurgical Series, 2nd edn., McGraw-

Hill, New York, 1958. 9 G. BRUHAT, Electricitd, 6th edn., Masson, Paris, 1956, p. 287.

l0 J. SURUGUE, Techniques Gdnkrales du Laboratoire de Physique, Vol. 2, Edition CNRS, Paris, 1962, p. 677. 11 W. J. HAMER, M. S. MALMBERG AND B. RUBIN, J. Electrochem. Soc., 103 (1956) 8.

Received March 17th, 1970 J. Electroanal. Chem., 27 (1970) 174 177

Effect of specific adsorption of anions on polarographic reduction

Polarographic analysis of 1-nitropropane in aqueous solution reveals that at any pH and for any buffer solution the reduction is always irreversible and involves 4 electrons 1- 3. The polarographic wave was therefore analysed 4 using Koutecky's method 5 which enables the log kf,h vs. E diagram to be obtained from the experimental current and potential values and, when the reaction is controlled by a single step, the values of c~n a and kf°h to be determined from the slope of the curve and by extra- polation at E (NHE) = 0. kf°h is the rate constant of the process and ~n a is the electronic transfer coefficient. Results showed that the reduction of 1-nitropropane is controlled by a single reaction step and that the values of the kinetic parameters depend on the pH in all the solutions examined. Only in the case of 10-1 M HC1 did analysis of the

J. Electroanal. Chem., 27 (1970) 177 180

178 SHORT COMMUNICATIONS

wave yield a non-linear plot of log kf, h vs. E, in accordance with the results of Suzuki and Elving 6 who attribute this behaviour to a multi-step reaction mechanism with fairly similar kinetic constants. In view of the singularity of the result and in attempt to clarify the mechanism, experiments were carried out in H2SO4 at the same pH, but the log kf, h vs. E plot proved to be straight in accordance with the results obtained for Mcllvaine buffers (Table 1). The disagreement found in the behaviour of the two

TABLE 1

EFFECT OF SOLUTION pH ON E½ AND KINETIC PARAMETERS OF I-NITROPROPANE POLAROGRAPH1C REDUCTION

Solut ion ~n, -E½/V --log(k°,h/cm s-1)

10-1 N H2SO 4 0.57 0.620 6.52 pH = 2.2 0.55 0.668 6.80 pH = 3.1 0.56 0.670 6.70 pH =4.1 0.50 0.720 6.80 pH = 5.1 0.72 0.800 9.60 pH=6 .1 0.73 0.844 10.15 pH=7 .1 0.79 0.872 11.15 pH=8 .1 0.90 0.890 12.70

acids stressed the need for clarification of the different reduction behaviour. Measure- ments were therefore carried out in solutions containing anions of different E~ and different specific adsorption. Figure 1 shows the plots of log kf.h vs. E obtained for solutions of 1-nitropropane in HC1, H2SO4, H2SO4+KC1, H2SO4+KBr and H2SO,+NaI.

Table 2 shows the values of the kinetic parameters : El/2, the initial discharge potential of the nitro-derivative and the values of E~ corresponding to each anion 7

2oi

E

4 . 2 , 5

v

-3,(3

~3,5

4,0° -~.s~o -;.6oo -;.6so 47°0 ElY vs. S.E.E.

Fig. 1. Behaviour of log kf.h vs. E (V vs. SCE) for i-nitropropane polarographic reduction : Cc~mNo2 = 5 × 10-" M. (&) 10-1 N HE1, (A) 10-1 N H2SO4, (11) 10-1 N H2SO4 + 10- t N KCI, (O) 10-1 N H2SO,t + 10 -1 N KBr, (1:3) 10 - t N H2SO4+ 10 -x N Nal.

J. Electroanal. Chem., 27 (1970) 177-180

SHORT COMMUNICATIONS 179

TABLE 2

VALUES OF THE KINETIC PARAMETERS: E~, THE INITIAL DISCHARGE POTENTIAL (Ei) OF THE 1-NITROPROPANE POLAROGRAPHIC REDUCTION AND THE VALUES OF E z CORRESPONDING TO EACH ANION

Supportin 9 electrolyte - E z / V - E½/V - E I / V ¢m a log(k°h/cm s - 1)

10-1 N HC1 0.520 0.605 0.450 0.50 5.89 10 - t N H2SO 4 0.510 0,620 0.500 0.57 6.52 10 -1 N H2SO4+ 10 -1 N KCI 0.520 0,618 0.450 0.50 5.91 10 - 1 N H2SO4+ 10 -1 N KBr 0.610 0,634 0.470 0.57 6.56 10-1 N H2SO 4 + 10-1 N KI 0,780 0.660 0.525 0.65 6.94

and it may be seen that the electrocapillary maximum in the presence of SOl- coincides within 10 mV with the initial discharge potential of the nitro-derivative; therefore, since the anion is weakly adsorbed, its influence may be considered con- stant throughout the entire reduction wave and the log kf,h vs. E plot is a straight line.

The value of the electrocapillary maximum in the presence of C1- is more negative by about 70 mV than the discharge potential of 1-nitropropane; thus the reduction of the nitro-derivative, which starts in the range of maximum adsorption of C1-, is affected by the progressive desorption of the anion at potentials more negative than Ez. This desorption causes a decrease of the diffusion current and the appearance of a broken line in the log kf, h vs . E diagram. The values of kf0,h a n d an, taken from the upper part of the curve are shown in Table 2.

A broken line with the same intersection potential and the same values for the kinetic parameters also appears when an equivalent amount of C1- ions is added to the solution ofH2SO 4 (Fig. i and Table 2). It may be assumed that the presence of the broken line is bound up with the desorption of the C1- ions.

Similar experiments were carried out adding KBr and NaI, respectively, to the solution of sulphuric acid. In both cases the log kf,h vs. E diagram is a straight line and the values of the kinetic parameters are affected by the particular anion used (Table 2). This result, which is an apparent disagreement with that obtained in the presence of CI-, is easily explained by taking the value of E~ in the presence of Br- and I- into account. The initial discharge potential of the nitro-derivative is in fact much more positive than E~ in each case and therefore reduction is not significantly affected by the desorption of the anion.

It may therefore be concluded that in each solution the reaction is controlled by a single step. The presence of different, specifically adsorbed anions influences the reduction kinetics and consequently, its parameters, in accordance with the potential applied to the electrode. If reduction occurs at potentials which are more positive than the E~ value the kinetics are influenced by the anion adsorbed on the electrode. At potentials which are more negative than Ez, reduction is not affected by the ad- sorption of the anion ; if the initial discharge potential coincides with or is very close to the E z value, reduction is affected by the progressive desorption of the anion according to the potential applied. In the latter case the appearance of the broken line in the l o g kf, h vs. E diagram clearly cannot be explained by a multi-step reaction; it is simply due to a decrease in i d brought about by the desorption of the anion. It is obvious therefore that the presence of a broken line in the log kf, h vs . E diagram may be caused

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180 SHORT COMMUNICATIONS

not only by a complexity of the reduction mechanism with the presence of several consecutive rate determining steps but also by factors that influence the experimental potential and current values.

Measurements were carried out at constant dropping time and at T= 25_+0.5°C.

Istituto di Chimica Fisica, Universit~ di Modena, 41100 Modena (Italy)

Giulia Grandi Roberto Andreoli

Giovanna Battistuzzi Gavioli

1 E. W. MILLER, A. P. ARNOLD AND M. J. ASTLE, J. Am. Chem. Sot., 70 (1949) 3971. 2 T. S. LEE AND T. DE VRIES, Univ. Microfilms (Ann Arbor, Mich.), order n. 64-4595; Dissertation Abstr.,

24 (1964) 5026; Chem. Abstr., 61 (1964) 10320c. 3 S. G. MAIRANOVSKII, V. M. BELIKOV, TS. B. KORCHEMNAYA AND S. S. NOVIKOV, Izv. Akad. Nauk SSSR,

Otd. Khim. Nauk, (1962) 522. 4 G. BATTISTUZZl GAVIOLI, G. DAVOLIO, G. GRANDI AND R. ANDREOLI, to be published. 5 J. KOUTECKV, Collection Czech. Chem. Commun., 20 (1955) 981. 6 M. SUZUKI AND P. J. ELVING, Collection Czech. Chem. Commun., 23 (1960) 3202. 7 P. DELAHAY, Double Layer and Electrode Kinetics, Interscience Publishers, New York, 1965.

Received February 16th, 1970; in revised form March 24th, 1970

J. Electroanal. Chem., 27 (1970) 177-180