Electrochemical studies of quinoline and isoquinoline

Abstract

The electrochemical behaviour of quinoline and isoquinoline in 0.1 N HCl has been studied by (i) the polarographic method, (ii) direct measurement of the differential capacity of the electrical double layer and (iii) qualitative measurements of electrocapillary curves. It has been established polarographically that certain quinoline concentrations give a well-marked adsorption wave; the wave of isoquinoline at the same concentrations is less expressed. From the data obtained, the surface coverage with adsorbed quinoline molecules has been calculated and proved to correspond to perpendicular orientation. Quinoline and isoquinoline have been proved also to cause catalytic hydrogen evolution. The dependence of this effect upon concentration for both quinoline and isoquinoline, and their mutual differences have been established. Measurements of the electrical double-layer differential capacity have confirmed the existence of the above-mentioned perpendicular adsorption range, and, at more positive potentials, the previously established planar orientation range; quinoline and isoquinoline show qualitatively the same behaviour. The quantitative treatment of the quinoline differential capacity curve has been performed. A comparison has been made with frumkin's adsorption isotherm and constant a at potential of −900 mV vs. S.C.E. has been calculated; application of the same isotherm at —400 mV is unsatisfactory. Comparing the results on the basis of the common variable-electrode potential, an analysis has been made of the electrochemical phenomena of the system Hg—quinoline solution in 0.1 N HCl. In the discussion special attention has been paid to adsorption phenomena as well as to the catalytic hydrogen evolution pre-wave. In connection with this, an extension of the mechanism of lowering of the hydrogen overvoltage has been discussed. The established differences in the electrochemical behaviour of quinoline and isoquinoline point to the influence of steric factors.