Voltammetry of Tetra-alkyl Ammonium Salts at Platinum in Sulphur Dioxide Solution: Oxidation of the Halides*

The polarographic or voltammetric behaviour of a selected group of tetra-alkyl ammonium halides and related compounds in liquid sulphur dioxide was investigated, using a stationary disk platinum indicating micro-electrode and a silver/silver-bromide reference electrode withcompensation for the IR drop of the solution. The electrochemical oxidation-reduction patterns are characteristic of the natures of the cation and anion composing the compound. Generally, good reproducible polarogmms are obtained. The anodic patterns indicate that the oxidation of each halogen ion can occur in multiple stages, although the bromides generally show only a single anodic wave. The order of ease of electrolytic oxidation in sulphur dioxide is iodide (easiest), bromide and chloride. Sulphur dioxide solutions of the tetra-alkyl ammonium salts show a characteristic cathodic discharge potential in the range of -0.3 to -0.6 V us Ag/AgBr. If the sulphur dioxide is insuthciently purified, two cathodic waves appear, which are apparently due to a non-condensable gas other than oxygen, sulphur trioxide or water. R&sunr&Au moyen dune micro-&&ode a disque fixe de Pt et dune electrode de reference AgAgBr, compensant par ailleurs les chutes ohmiques des solutions, Ton etude les comportements voltam&rique et polarographique de tout un groupe d’halogtnures de tttra-alkyl ammonium et certains d&rives, dans SO, liquide comme solvent. Les caracteres 6lectrochimiques de l’oxydation et de la reduction sent specifiques des natures du cation et defanion du compose. sont g&ralement reproductibles. Les polarogrammes Les branches anodiques indiquent que l’oxydation d’un ion halogent s’etTectue en plusieurs stades; In bromures, toutefois, ne manifestent qu’une vague anodique. L’ordre de facilite d&o&ante de l’oxydation electrolytique darts SOI liquide est I-, Br-, Cl-. Une tension caracteristique de d&charge cathodique se manifeste pour chaque sel, entre -0,3 et -0,6 V (es Ag-AgBr). Si le solvant (SOI liq.) n’est pas su!Bsamment pur, deux vagues cathodiques apparaissent, probablement B cause de la liberation d’un gaz autre que 0,. SOI, H,O. Zipunmenfassung-Es wurde das polarographische oder voltametrische Verhalten einer ausgewahlten Gruppe von Tetra-Alkyl-Ammoniumhalogeniden und verwandter Verbindungen in Rilssigem Schwefeldioxyd untersucht unter Verwendung einer stationi-iren Pt-Scheiben-Indikator-Mikroelektrode und einer Silber/Silber-Bromid-Referenzelektrode, wobei fiir den Spannun&sabfall in der Lzjsung korrigiert wurde. Das Bild der Oxydation/Reduktionskurve ist charakteristisch fur die Natur des Kations und des Anions, aus dem sich die Verbindung zusammensetzt. Im allgemeinen wurden gut reproduzierbare Polarogramme erhalten. Die anodischen Kurven zergen, dass die Oxydation jedes Halogenions in mehreren Stufen statttinden kann, obschon die Bromide im allgemeinen nur eine einzige anodische Welle zeigen. Die Reihenfolge der Leichtigkeit der elektrolytischen Oxydation in Schwefeloxyd ist Jodid (am leichtesten), Bromid und Chlorid. Liisungen der Tetra-Alkyl-Ammonium-Salze in Schwefeldioxyd zeigen charakteristische kathodlsche Abscheidungspotentiale in der Gegend von 0,3-0,6 V gegen Ag/AgBr. Wenn das Schwefeldioxyd ungentigend gereinigt ist, treten zwei kathodische Wellen auf, welche augenscheinlich einem nicht kondensierbarem Gas, jedoch nicht als Sauerstoff, Schwefeltrioxyd oder Wasser, zuzuschreiben sind. INTRODUCTION ALTHOUGH the feasibility of voltammetry and polarography in the non-protonic solvent, liquid sulphur dioxide, has been demonstrated,lp* the fundamental experimental requirements for successful voltammetry of (a) a reliably responsive indicating l Manuscript received 18 June 1964. 309 310 M. SAKUMA and J. ELVING electrode, (b) a totally non-polarizable reference electrode, and (c) the availability of soluble electrolytes with relatively high decomposition potentials to serve as background electrolytes and provide solutions with appreciable conductivity, have been only partially satisfied. Pret;ious work The dropping mercury electrode (dme) is inferior as an indicating electrode for voltammetry in liquid sulphur dioxide to the cylindrical platinum electrode (cpe), because of its poor reproducibility, which is ascribed to reaction of the mercury with solutes.‘” The cpe usually exhibits a current peak, which seriously limits its usefulness. Peak potentials and half-height potentials for the reduction of cations are reproducible, but the corresponding currents are somewhat variable. “Renewal” of the cpe by audio-frequency alternating current is necessary. The behaviour of the cylindrical gold electrode, which was used for a few voltammetric studies, is generally similar to that of the cpe with its reproducibility being not quite as good.3 Cruse4 reported the potential of silver/silver-halide electrodes, as indicated by potentiometric measurement, to be quite stable in liquid sulphur dioxide. These electrodes have also been found to be reliable reference electrodes for voltammetry.14*6 No solutes fully satisfactory as background electrolytes in liquid sulphur dioxide have yet been found, although voltammetry of a series of compounds at the cpe3v6 allowed them to be divided into two groups, viz those showing high resistance and those showing low resistance. Consequently, there still exists as characteristic features of voltammetry in liquid sulphur dioxide the absence of a background electrolyte and a high solution resistance. As depolarizers, the behaviour of triphenylchloromethane at the cpe was studied in detail and the reduction of triphenylmethyl free radical discussed.” Other compounds previously studied at the cpe include some quaternary ammonium halides, some alkali metal halides and thiocyanates, and several other inorganic and organic compounds, 1-315 these compounds generally exhibit only peak or discharge currents, except that two variable cathodic waves were exhibited in some runs on the tetraalkyl ammonium bromides. Objective of present study The primary purposes of the present work were (a) to develop a more useful and reliable technique for voltammetry in liquid sulphur dioxide with the platinum electrode and (b) to study systematically a large group of related compounds. The electrode system was modified to improve the reproducibility of polarograms by depressing the polarization of the reference electrode. In addition, a means was introduced of compensating automatically for the IR drop due to the high solution resistance by means of a three-electrode system. The modified electrode system involving a disk indicating platinum microelectrode in place of the cylindrical platinum electrode was therrapplied to the investigation of a large series of tetra-alkyl ammonium halides, in which the identities of both quatemary ion and halide were varied, and of a large number of related compounds. Voltammetry of tetra-alkyl ammonium salts at Pt in SO, solution 311 Chemicals EXPERIMENTAL Anhydrous grade sulphur dioxide (Matheson; stated purity: 99.98 per cent) was treated to remove H,O, SO, and non-condensable gases essentially as previously described? The following were Eastman Kodak chemicals : tetra-methylammonium chloride and bromide (recrystallized from ethanol); tetra-ethylammonium chloride, bromide (recrystallized from ethanol), iodide (recrystallized from acetone-ethyl acetate) and perchlorate; tetra-n-butylammonium bromide (recrystallized from ethyl acetate), iodide (recrystallized from acetone-ethyl acetate), nitrate and picrate; and l-ethylpyridinium bromide. Coleman & Bell n-Bu4NBr was also used. Reagent grade potassium chloride (J. T. Baker), potassium bromide (Allied Chemical), potassium iodide (Merck), potassium chlorate, lithium perchlorate (G. Frederick Smith Chemical), iodine (resublimed), bromine (Mallinckrodt Chemical), and tri-n-butyl amine were also used. Apparatus The disk platinum electrode cell (Fig. I) was constructed from a 1004 Pyrex graduated cylinder by closing the lower end with a flat bottom and sealing a 55/50 Glass Tube____ ,Cu Wire