§ 355. Tests for Morphine.—(1.) One hundredth of a milligrm. of pure morphine gives a blue colour to a paste of ammonium molybdate in sulphuric acid; 20 mgrms. of ammonium molybdate are rubbed with a glass rod in a porcelain dish, and well mixed with 5 drops of pure strong sulphuric acid and the morphine in a solid form applied; titanic acid and tungstates give similar reactions.
(2.) Morphine possesses strong reducing properties; a little solid morphine dissolved in a solution of ferric chloride gives a Prussian blue precipitate when ferridcyanide solution is added. A number of ptomaines and other substances also respond to this test, so that in itself it is not conclusive.
(3.) Iodic Acid Test.—The substance supposed to be morphine is converted into a soluble salt by adding to acid reaction a few drops of hydrochloric acid, and then evaporating to dryness. The salt thus obtained is dissolved in as little water as possible—this, as in toxicological researches only small quantities are recovered, will probably be but a few drops. A little of the solution is now mixed with a very small quantity of starch paste, and evaporated to dryness at a gentle heat in a porcelain dish. After cooling, a drop of a solution of 1 part of iodic acid in 15 of water is added to the dry residue; and if even the 1⁄20000 of a grain of morphine be present, a blue colour will be developed.
Another way of working the iodic acid test is to add the iodic acid solution to the liquid in which morphine is supposed to be dissolved, and then shake the liquid up with a few drops of carbon disulphide. If morphine be present, the carbon disulphide floats to the top distinctly coloured pink. Other substances, however, also set free iodine from iodic acid, and it has, therefore, been proposed to distinguish morphine from these by the after addition of ammonia. If ammonia is added to the solution, which has been shaken up with carbon disulphide, the pink or red colour of the carbon disulphide is deepened, if morphine was present; on the contrary, if morphine was not present, it is either discharged or much weakened.
Other Reactions.—There are some very interesting reactions besides the two characteristic tests just mentioned. If a saturated solution of chloride of zinc be added to a little solid morphine, and heated over the water-bath for from fifteen minutes to half-an-hour, the liquid develops a beautiful and persistent green colour. This would be an excellent test for morphine were it not for the fact that the colour is produced with only pure morphine. For example, I was unable to get the reaction from morphine in very well-formed crystals precipitated from ordinary laudanum by ammonia, the least trace of resinous or colouring-matter seriously interfering. By the action of nitric acid on morphine, the liquid becomes orange-red, and an acid product of the formula C10H9NO9 is produced, which, when heated in a closed tube with water at 100°, yields trinitrophenol or picric acid. This interesting reaction points very decidedly to the phenolic character of morphine. On adding a drop of sulphuric acid to solid morphine in the cold, the morphine solution becomes of a faint pink; on gently warming and continuing the heat until the acid begins to volatilise, the colour changes through a series of brownish and indefinite hues up to black. On cooling and treating the black spot with water, a green solution is obtained, agreeing in hue with the same green produced by chloride of zinc. Vidali[380] has proposed the following test:—Morphine is dissolved in strong sulphuric acid, and a little arsenate of sodium is added; on gently warming, a passing blue colour develops; on raising the temperature higher, the liquid changes into green, then into blue, and finally again into green. Codeine acts very similarly. The following test originated with Siebold (American Journal of Pharmacy, 1873, p. 544):—The supposed morphine is heated gently with a few drops of concentrated sulphuric acid and a little pure potassic perchlorate. If morphine be present the liquid immediately takes a pronounced brown colour—a reaction said to be peculiar to morphine, and to succeed with 1⁄10 of a mgrm. In order to obtain absolutely pure perchlorate, potassic perchlorate is heated with hydrochloric acid so long as it disengages chlorine; it is then washed with distilled water, dried, and preserved for use. There is also a test known as “Pellagri’s”; it depends on the production of apomorphine. The suspected alkaloid is dissolved in a little strong hydrochloric acid, and then a drop of concentrated sulphuric acid is added, and the mixture heated for a little time from 100° to 120°, until it assumes a purple-black colour. It is now cooled, some hydrochloric acid again added, and the mixture neutralised with sodic carbonate. If morphine be present, on the addition of iodine in hydriodic acid, a cherry-red colour is produced, passing into green. Morphine and codeine are believed alone to give this reaction.
[380] D. Vidali, Bull. Farmaceut., Milano, 1881, p. 197; D. E. Dott, Year Book of Pharmacy, 1882.
The acetate of morphine, and morphine itself, when added to ferric chloride solution, develop a blue colour. When 1 molecule of morphine is dissolved in alcohol, containing 1 molecule of sodium hydroxide, and 2 vols. of methyl iodide are added, and the mixture gently heated, a violent reaction sets in and the main product is codeine methiodide (C17H18NO2OCH,MeI). If only half the quantity of methyl iodide is added, then free codeine is in small quantity produced; if ethyl iodide be substituted for methyl, a new base is formed homologous with codeine—codeine is therefore the methyl ether of morphine. If morphine is heated with iodide of methyl and absolute alcohol in a closed tube for half an hour at 100°, methyl iodide of morphine is obtained in colourless, glittering, quadratic crystals, easily soluble in water (C17H19NO3MeI + H2O); similarly the ethyl iodide compound can be produced.
If morphine is heated for from two to three hours in a closed tube with dilute hydrochloric acid, water is eliminated—