Basis and optimization of the indooxine method to determine oximes

Summary

Consideration is given to a spectrophotometric method for determining oximes that involves acid hydrolysis followed by reaction of liberated hydroxylamine with 8-quinolinol to form a dye, indooxine (5-[(8-hydroxy-5-quinolyl)imino]-8-(5H)-quinolone). The various steps of the method, applied to pyruvic oxime (2-oximinopropanoic acid) as representative of α-oximino acids, were investigated and optimized, leading to improvements in reproducibility and sensitivity over the existing procedure. Incomplete liberation of hydroxylamine from oximes is due largely to incomplete hydrolysis at equilibrium in acid solution. An additional cause with α-oximino acids, as opposed to simple oximes, is decarboxylative dehydration to form the corresponding nitrile. Hydrolysis is more complete and nitrile formation less favored the lower the pH, at least in the pH range 0 to 2.5. The importance of calibration curves and the control of pH, time of hydrolysis and temperature in quantitative application of the method is emphasized. The method can be used quantitatively for determination of a single oxime but only semiquantitatively or qualitatively for determination of total oximes in an unknown mixture of oximes.