Quantitative Colorimetric Estimation of Morphine in Biological Fluids by the Iodoxybenzoate Method. ∗

Abstract

Summary

The qualitative iodoxybenzoate test of Leake has been found suitable for quantitative adaptation to the estimation of morphine. Methods for determining morphine in biological fluids and a rapid method of accurately checking the morphine content of morphine hydrochloride are suggested.

Development of a satisfactorily accurate quantitative application of the usual color reactions for morphine has been retarded because of their general lack of (a) proper selectivity for this single opium alkaloid, (b) permanence of hue produced or (c) sufficient sensitivity. In addition such reactions are in most instances dependent to an undesirable degree on the temperature and pH of the test solution and the presence of commonly occurring contaminants. This situation is not surprising since most of these reactions are supposedly due to the formation of complex unstable chromogenic substances. A more ideal quantitative test should be the stoichiometric conversion of morphine to a simple colored form.

Leake 1 suggested that such an effect could be obtained for many substances by reacting their free phenolic groups with an aqueous solution of ammonium iodoxybenzoate, the oxidative powers of which had been studied in another regard by Loevenhart 2 and others. Morphine rapidly develops the same color when treated with this agent as is produced by slow spontaneous oxidation of morphine solutions in air, and it was recommended 1 that this reaction be used not only as a qualitative test to differentiate morphine from codeine, heroin, dionin and apomorphine, but also as the basis of a quantitative colorimetric estimation. We have found that when 25 to 50 mg. of powdered ammonium iodoxybenzoate are added to 5 to 10 cc. of 0.005 M to 0.02 M aqueous solutions of morphine salts sufficient agreement with Beer's Law holds so that colorimeter readings yield results accurate to ±3% if the comparison with a standard is made after 30 minutes are allowed for full development of the color. Spectroscopic examination of color absorption by the treated solutions reveals that in concentrations higher than 0.03 M extinction extends further toward the yellow band than with the lower concentrations, which is represented grossly by a change in shade from orange to garnet and considerable deviation in colorimetric measurements unless a suitable filter is used.