ELECTROCHEMICAL OXIDATION OF IODIDE DISSOLVED IN SODIUM-NITRATE-POTASSIUM-NITRATE EUTECTIC MELT ON A PLATINUM ROTATING DISK ELECTRODE*

-The E/I curve related to the electrochemical oxidation of iodide dissolved in molten nitrates on a platinum rotating disk electrode involves two anodic waves. The Srst corresponds to a reversible two-electron process and the second to a reversible one-electron process. A definite ratio between the anodic limiting currents is established suggesting that the second anodic wave is not independent from the first one. The parameters related to the diffusion of the species participating in the reaction are evaluated. R&u&--Les courbes de polarisation concernant l’oxydation 6lectrochimique de l’iodure dissous dans des nitrates fondus sur une 6lectrode 21 disque toumant de platine comprend deux ondes anodiques. La premikre appartient a un proa5ssus r&versible de deux Blectrons et la deuxihme 8 un proc&ssus r6versible d’un seul6lectron. Une relation dkfinie est 6tablie entre les courants limites anodiques, indiquant que la deuxi&me onde anodique n’est pas indkpendante de la premBre. Les parambtres relatifs St la diffusion des esp&ces qui participent g la r&action sent 6valu&.


THE electrochemical kinetics of the iodine/iodide couple on platinum has been principally studied in aqueous solutiond3
and in various organic solvents such as dimethylsulphoxide4-' and acetonitrile.*The electrochemical oxidation of iodide dissolved in nitrate melts was also studied on platinum9 and mercurylo electrodes.These results showed the qualitative occurrence of two anodic waves, the first exhibiting quite a reversible behaviour.The second anodic wave could not be interpreted in a reasonable way because of an appreciable irreproducibility of the results.
In the present work the anodic reaction has been studied on a platinum rotating disk eIectrode covering a wide range of experimental conditions.
The information reported in the present paper confirms the already known interpretation of the first anodic wave and also permits a tentative interpretation of the second wave in terms of the reaction products formed at lower anode potentials.
The second anodic wave corresponds to an electrochemical process that is purely convective-diffusion controlled.

EXPERIMENTAL TEdHNIQUE
The electrolysis cell and platinum rotating disk electrode assemblies were the same as those described in an earlier publication.l1 The electrode pre-treatment of the platinum rotating disk electrode (0.07 cm3 was also indicated in a previous paper.*Its rotation speed was varied between 200 and 3000 rev/min by means of an electronic device that kept the rotation speed constant within O-5 per cent.11 A silver/silver(I) (0.07 M) reference electrode was employed.It was mounted with the usual Luggin-Haber capillary tip arrangement.The counter-electrode was a large-surface platinum wire placed in a separate compartment, the contact being made with a medium-grade f&ted glass disk.
A.R. quality sodium nitrate, potassium nitrate, potassium iodide and iodine (Mallinckrodt and Carlo Erba) were used.The salts were carefully dried in a vacuum oven and kept under a dry atmosphere afterwards.When iodine was in the melt it was previously mixed with solid potassium iodide and the eutectic components.Later the mixture was warmed up sIowly in the electrolysis cell until the melt reached the temperature of the experiments.
Each series of experiments comprised first the electrolysis of the nitrate eutectic to obtain blank Ej1curves.Later potassium iodide was added at different concentrations and the corresponding E/I curves were recorded at different rotation speeds of the working electrode (200, 500, 1000,200O and 3000 revlmin).
In the experiments where iodine was added, the molar 1,/I-ratio was 2, giving an acceptable excess of iodine if tri-iodide ion is formed.
The concentration of the reacting species was varied from 0.5 x 1O-3 to 5 x lo-5 M and was determined by weighing and by usual potentiometric analysis.Densities and viscosities of the melts employed were taken from previous publications.rssBExperiments covered the temperature range from 234 to 314°C.At constant composition and temperature, both limiting currents related to the iodide oxidation increase linearly with the square root of the rotation speed, CO, as shown in Figs. 3 and 4: At a constant rotation speed of the working electrode, both anodic waves also depend linearly on the iodide ion concentration, as depicted in

Kinetic behaviour of the anodic waves
The (2) ( where the E"s are the standard electrode potentials of the reactions and a, refers to the activity of the i species at the electrode surface.

2.3RT
where E' includes the ratio of the activity and diffusion coefficients of the species entering the reaction.Equation (4) has been tested in Fig. 8. Experimental results fit the theoretical line of slope 2*3(RT/2F) reasonably well.This fact, in addition to the linear dependence of where the E's are defined as above.Equations ( 5) and ( 6) are compared to experimental results in Fig. 9 ; it clearly emerges that in spite of the relatively higher errors involved in the second wave data, the results are reasonably well approximated by ( 5).
Scheme I First wave 2 I-= I, + 2 e, (Ia) Second wave I, = I,+ + e, (Ib) (I,+ + I-= % I,), W and Scheme II First wave 2 I-= I, + 2 e, (IIa) (I-+ I, = 13-1, (Ilb) Second wave 2 1, = 31, + 2 e. (II@ Scheme I involves the formation of the Iz+ ion, which in the presence of iodide would be in equilibrium with iodine in the bulk of the melt, probably through an intermediate species such as I,.Scheme II involves the participation of the tri-iodide ion in equilibrium with iodide and iodine at the interfacial region, as is the case for the iodine/iodide electrode in water and in various organic solvents.l-sSpectroscopic studies14 indicate the existence of t&iodide in molten systems, such as chloride melts, but the present experiments are not conclusive as far as a clear participation of t&iodide in the electrochemical reaction occurring in the nitrate melt.Assuming a reversible behaviour of the electrode reactions, the corresponding

FIG. 11 .
FIG. 9. Plot of equation (5); 240°C.0. CI-O-503 mM; 200 rev/min; 0, CI-I.08 mM; IOOOrev/min; A, q-l-08 mM, 2000 rev/min.The straight line corresponds to the theoretical slope (2.3 RTIF).This fact does not necessarily mean that the second anodic wave is actually represented by reaction Ib, but it is by an equilibrium that yields the same kind of expression.Thus, a Nemst equation formally analogous to (5) can also be obtained on the assumption that the second wave involves t&iodide in the equilibrium I,-*