THE COLORIMETRIC ESTIMATION OF MORPHINE IN CAMPHORATED TINCTURE OF OPIUM

THE official assay for morphine in camphorated tincture of opium utilises the measurement of the yellowish-brown colour produced by the reaction of morphine with a nitrite in acid solution, followed by the addition of excess of alkali. This colour reaction was first reported by Radulescul, and later used as a quantitative method by Fabiny? while van Arke13 made the production of the colour more reliable by controlling the acidity and time of reaction, but claimed no great accuracy. Allport and Jones4 evolved a rapid estimation using a tintometer and claimed an accuracy of 5 10 per cent. Adamson and Handisyde5 investigating the method by use of a Spekker absorptiometer reported that the length of time for the reaction in acid solution had a marked effect upon the depth of colour produced. The same authors6 later critically reviewed the conditions for the production of the colour as applied to the official method, and recommended a time for reaction of exactly 15 minutes, in a specified volume of 0.1 N hydrochloric acid. Under these conditions they gave the method an accuracy of & 2 per cent. Stephens' has given further information on the reaction, particularly with regard to the effect of temperature. As to the specificity of the reaction when applied to the estimation of morphine, Radulescu8 examined some 150 vegetable extracts, but found only one of doubtful origin which gave the same reaction. Adamson and Handisyde5 reported that narcotine, papaverine and codeine do not give a reaction, but the same authors6 stated that it is given generally by other phenols and substances containing a phenolic group. It is interesting to note in this connection that NichollsB found that Mannich's method for the estimation of morphine, which utilises the precipitation of morphine as its dinitrophenyl ester, was not specific when applied to opium and its products owing to excessive interference from other phenolic alkaloids present. The official method now in use consists essentially of the evaporation of a portion of the tincture, extraction of the residue with lime water, followed by extraction of the lime water extract with ether. After reduction of the pH of the aqueous solution with ammonium sulphate the morphine is extracted with ethanol and chloroform, the mixed solvents are removed, and the residue dissolved in acid. The acid solution is diluted to a definite volume to render it 0.1 N and a portion is used for the development of the colour with nitrite and ammonia, which is then measured by comparison with a standard morphine-nitrite reagent. It has been noticed in this laboratory, that the official assay of