J. Electroanal. Chem., 89 (1978) 431--432 431 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Short communication PROTONATION OF PYRIDINYL RADICALS: AN ELECTROCHEMICAL INVESTIGATION MAHBOOB MOHAMMAD *, S.U. SHEIKH, M. IQBAL, ROSHAN AHMED, M. RAZAQ and A.Y. KHAN Department of Chemistry, Islamabad University, Islamabad (Pakistan) (Received 29th March 1976; in revised form 23rd August 1977) Studies on 1-ethyl-4-carbomethoxy pyridinium (Py+I-) and 1,1-trimethylene bis-4-carbomethoxy pyridinium (1 : 3 : 1) salts and the free radicals therefrom have contributed much towards the understanding of reactions of biological importance [1]. The main information regarding the oxidation-reduction process of the pyridinyl mdical-pyridinium salt, in aqueous phase, have been lacking so much so that it was claimed, that the one electron electrochemical (reversible) reduction of the pyridinium salt in aqueous solution, would not occur [2]. Kosower had reported about the instability of pyridinyl radical in water [3]. Recently Kosower et al. have proposed a mechanism of disappearance of pyri- dinyl radical in water [4]. We report here the result of triangular wave cyclic voltammetric investiga- tion of pyridinium salts [5] on a hanging mercury drop, in alcoholic and aqueous media with different concentration of acids (three electrode configuration). In ethanol, with tetra n-butyl ammonium perchlorate as supporting electro- lyte the 1-ethyt-4-carbomethoxy pyridinium salt (10 -4 M) was found to give a perfect one-electron reversible wave at 2 V]min scan rate (El/2 = --0.90 V. vs. S.C.E.). In aqueous solution even in presence of 10 -5 M HC1 (with 0.1 M lithium chloride as supporting electrolyte) a one-electron reversible reduction of this pyridinium salt (10 -4 M) occurs at a scan rate 2 V/min El/2 (potential correspond- ing to 85% of the peak current [3,6] ) as obtained from cyclic voltammogram is --0.910 -+ 0.010 V. vs. S.C.E. (Ep) cathodic -- (Ep) anodic ~- 70 mV. Ep -- El/2 35 mV and (Ip)cathodic/{Ip)anodic "" I: all confirm that a one electron revers- ible reduction to pyridinyl radical occurs [7]. O n addition of hydrochloric acid (pH 4), the anodic peak disappears and the cathodic peak shiftsanodically (Ep = --0.870 V. VS. S.C.E.). From the shiftof Ep and using the method reported earlier [6,8], for scan rate 2 V min -1, a pseudo- first order k' for the reaction py. + [n ÷1 ~i, (where [H ÷ ] is in large excess) is calculated, which gives a second order rate constant as 1.5 × l0 s M -1 s-1. At pH 3 there is a further anodic shift in Ep (Ep = --0.840) which again gives a second order rate constant 1.5 × 10 -s M -~ s-1. The same result is obtained for a range of [H+], [Py+I-] and scan rate, e.g. [H +] = 10 -1, 10 -2, 3 X 10 -a, etc.; [Py+I-] = 10 -3, 3 X 10 -4, etc. M; voltage scan rates 4 V, 10 V and 100 V min -1.