“…Reduction methods to heteropoly blues suffer problems of longer reaction time, deviations from Beer's law, unstable reagents and products, and high blank absorbances. Alternatively, another procedure measures the 12-MPA formed and then the combined 1ZMPA and 12-MSA concentrations in a less acidic solution (24). However, since the 12-MSA, 12-MPA, and other heteropolymolybdate concentrations vary with acidity, this procedure can also be prone to error.…”