Polarography of β-Mercapto-Propionic Acid at the Dropping Mercury Electrode

The polarographic behaviour of β-Mercapto-propionic acid (RSH) at d.m.e. has been investigated in the presence of 0·1M KCl and 0·005% Triton X-100. The effect of pH, concentration of RSH, temperature and drop time on the wave characteristics and the mechanism of the reaction occurring at the surface of the mercury drop have been studied in detail. Variation in the H⁺ ion concentration of the medium (between pH 2·50 to 7·90) had a negligible effect on the well defined reversible anodic wave of RSH; at lower ( < 2·5) and higher(> 7·90) pH values, ill defined waves were obtained. The proportionality of wave height with RSH concentration,\( √heff.,\) and temperature, and a linear plot of\( Ed.e.\) Vs.log\( id-i/i\) with slope 0·061 indicate that the wave produced by RSH at d.m.e. is strictly diffusion controlled and reversible involving one .electron transfer process. The current voltage curves over the pH range 1·30-7·90 have been found to obey the equation 

                                                       RSH +HG \( \rightleftharpoons\) RSH_g+H⁺+e

Mathematical and experimental evidences are obtained to show that the anodic wave of RSH at d.m.e. does not correspond to the formation of di-mercapto-propionic acid (RSSR) but rather to the formation of a mercury compound RSHg in accordance with the above equation. The diffusion coefficient and the temperature coefficient of i_d for temperature range 30-60° at pH= 5·80, have been found to be 4·537 X 10-6 cm²/sec. and 1·04% per degree respectively. The linearity of \('id' \)with RSH concentrations provides a rapid and precise method for the determination of RSH down to 0·20 mM.