Unusual potentiometric titration curves of 2,8-dimercapto-6-hydroxypurine and 4,4-dimethyloxazolidine-2-thione with iodine

Iodimetric volumetric titration with potentiometric detection of the end-point has been performed in order to determine 2,8-dimereapto-6-hydroxypurine and 4,4-dimethyloxazolidine-2-thione in strongly alkaline medium. Determination range found was 100-500 mumol for 2,8-dimercapto-6-hydroxypurine and 50-1000 mumol for 4,4-dimethyloxazolidine-2-thione. The errors and the relative standard deviations were below 1 %. The shapes of the obtained potentiometric titration curves depended on the concentrations of sodium hydroxide and the analytes, the material of the working electrode (platinum, gold) and its surface. Addition of iodine resulted in a significant potential drop. The studied systems did not exhibit Nernstian behaviour.