The ether is decanted as completely as possible. Two filter papers are taken and counterpoised—that is to say, they are made precisely the same weight. The filters are placed one inside the other, and the precipitate collected on the inner one; the precipitate is washed with morphinated water—that is to say, water in which morphine has been digested for some days. The filter papers with their contents are washed with benzene and dried, the outer paper put on the pan of the balance carrying the weights, and the inner filter with the precipitate weighed. The precipitate is now digested with a known volume of decinormal acid, and then the excess of acid ascertained by titration with decinormal alkali, using either litmus or methyl orange; each c.c. of decinormal acid is equal to 30·3 mgrms. of morphine.[370]
[370] Pharm. Journal, xix. 45, 82; xxii. 746. Wright and Farr, Chemist and Druggist, 1893, i. 78.
Dott’s Process.—Dott has recently proposed a new process, which he states has given good results. The process is as follows:—10 grammes of powdered opium are digested with 25 c.c. water; 1·8 gramme barium chloride dissolved in about 12 c.c. water is then added, the solution made up to 50 c.c., well mixed, and after a short time filtered. 22 c.c. (representing 5 grammes opium) are mixed with dilute sulphuric acid in quantity just sufficient to precipitate the barium. About 1 c.c. is required, and the solution should be warmed to cause the precipitate to subside, and the solution to filter clear. To this filtered solution a little dilute ammonia, about 0·5 c.c. is added to neutralise the free acid, and the solution concentrated to 6 or 7 c.c., and allowed to cool. 1 c.c. spirit and 1 c.c. ether are then added, and next ammonia in slight excess. The ammonia should be added gradually until there is no further precipitation, and a perceptible odour of ammonia remains after well stirring and breaking down any lumps with the stirring rod. After three hours the precipitate is collected on counterpoised filters and washed. Before filtering, it should be noted that the solution has a faint odour of ammonia: if not, one or two drops of ammonia solution should be added. The dried precipitate is washed with benzene or chloroform, dried, and weighed. It is then titrated with n⁄10 acid, until the morphine is neutralised, as indicated by the solution reddening litmus paper.[371]
[371] Other methods of opium assay have been published: see Mr. A. B. Prescott’s method (Proceedings of Amer. Pharm. Assoc., 1878); Allen (Commercial Org. Analysis, vol. ii. p. 473); E. R. Squibb’s modification of Flückiger’s method (Pharm. Journ. (3), xii. p. 724); a rapid mode of opium assay, MM. Portes and Lanjlois (Journ. de Pharm. et de Chim., Nov. 1881); Year Book of Pharmacy, 1882.
To the above may be added—(1.) Schacht’s Method.—5 to 10 grms. of dry, finely-powdered opium are digested with sufficient distilled water to make a thin pulp. After twenty-four hours the whole is thrown on a weighed filter, and washed until the washings are almost colourless and tasteless. The portion insoluble in water is dried at 100° and weighed; in good opium this should not exceed 40 per cent. The filtrate is evaporated until it is about one-fifth of the weight of the opium taken originally; cooled, filtered, and treated with pure animal charcoal, until the dark brown colour is changed into a brownish-yellow. The liquid is then refiltered, precipitated with a slight excess of ammonia, allowed to stand in an open vessel until all odour of ammonia disappears, and at the same time frequently stirred, in order that the precipitate may not become crystalline—a form which is always more difficult to purify. The precipitate is now collected on a tared filter, washed, dried, and weighed. With an opium containing 10 per cent. of morphine, its weight is usually 14 per cent. A portion of the precipitate is then detached from the filter, weighed, and exhausted, first with ether, and afterwards with boiling alcohol (0·81 specific gravity). Being thus purified from narcotine, and containing a little colouring-matter only, it may now be dried and weighed, and the amount of morphine calculated, on the whole, from the data obtained.
(2.) Fleury has proposed a titration by oxalic acid as follows:—2 grms. of the powdered opium are macerated a few hours with 8 c.c. of aqueous oxalate of ammonia, brought on a filter, and washed with 5 c.c. of water. To the filtrate an equal volume of 80 per cent. alcohol and ammonia to alkaline reaction is added; and, after standing twenty-four hours in a closed flask, it is filtered, and the flask rinsed out with some c.c. of 40 per cent. alcohol. The filter, with its contents, after drying, is placed in the same flask (which should not be cleansed), a few drops of alcoholic logwood solution are added, with an excess of oxalic acid solution of known strength, the whole being made up to 100 c.c. This is divided into two parts, and the excess of acid titrated back with diluted soda-lye. If the oxalic acid solution is of the strength of 4·42 grms. to the litre, every c.c. of the oxalic acid solution which has become bound up with morphine, corresponds to 0·02 grm. of morphine.