5. Scheibler has proposed Phosphotungstic acid as a precipitant. Sodium tungstate is digested with half its weight of phosphoric acid, sp. gr. 1·13: on standing, phosphotungstic acid crystallizes. Its solution is said to give a distinct precipitate with 1/200000 of a grain of strychnia and 1/100000 of quina, and with similar amounts of other alkaloids. From this precipitate the alkaloid is obtained by treating with sufficient milk of lime and shaking with ether-chloroform, &c., as before. He recommends the previous removal of impurities by lead acetate and sulphuretted hydrogen as already described (p. 7) (Fresenius, Zeitschr. f. anal. Chemie, 12, 315).
6. Picric acid, a saturated aqueous solution, gives precipitates in neutral solutions of morphia and atropia. In solutions acidified with sulphuric acid it gives the following:—morphia, and pseudomorphia, no precipitate; aconitia, a precipitate only in concentrated solutions; other alkaloids of opium, a thick precipitate.[7]
7. Animal charcoal, previously purified by hydrochloric acid and thorough washing with water, when digested with neutral or alkaline solutions of alkaloids, not too dilute, absorbs them from the liquid. The charcoal, washed twice or thrice with small quantities of water, is dried at a moderate temperature, and boiled with strong alcohol, which extracts the alkaloid. This process has been used for separating picrotoxin from beer, but has the inconvenience that the alkaloid is liable to gradual oxidation within the pores of the charcoal, and that the separation is never complete. It is this property that has caused charcoal to be recommended as an antidote in poisoning.
8. All alkaloids form with platinic chloride double salts of more or less sparing solubility. These precipitates, washed, dried and weighed, and then burnt, leave metallic platinum, the amount of which yields a clue to the composition of the base. But aconitine and narcotine are only thrown down from concentrated solutions, and a few are not precipitated at all. Hence this test is of only occasional value in toxicological work. The same may be said of auric chloride.
9. Tannin or tannic acid, a moderately strong solution in water, throws down most alkaloids. Coffee and tea, and other tannin-containing infusions, have, therefore, been used as antidotes with dubious success. As a test it is not distinctive.
10. Phospho-antimonic acid (Schultze), prepared by mixing antimony pentachloride with ordinary sodium phosphate and decanting the clear liquid, gives whitish amorphous precipitates with alkaloids.
11. Silico-tungstic acid is prepared by boiling commercial tungstate of soda with fresh gelatinous silica. Filter and allow to crystallize. This gives precipitates with very dilute solutions of alkaloids, but it is also precipitated by ammonium chloride (Godefroy, Arch. d. Pharm., Nov. 1879). Zaubenheimer recommends it as a most delicate test: the precipitate may be decomposed by soda or potash, and the base extracted by ether-chloroform.
12. Auric chloride, palladious chloride, and mercuric chloride have been proposed, but are not of much use. Potassium chromate and sulphocyanide, and sodium nitroprusside give somewhat insoluble precipitates, generally crystalline and of characteristic appearance under the microscope. These tests should be strong, and must be used in small quantity.
Ptomaines or Cadaveric Alkaloids.—Much attention has been attracted lately by the possible interference to toxicological detections owing to the undoubted existence of natural alkaloids in the dead body unpoisoned. Some of these, called by Selmi “Ptomaines” ([Greek: ptôma], a corpse), somewhat simulate strychnia, &c., in their chemical and physiological characters. The observation is not new, as years ago, in the Privy Council’s reports, Thudichum called attention to alkaloids separated by Sonnenschein’s process (phosphomolybdic acid) from the brain, urine, and from decomposed bodies. Various substances of the kind have also been found by other investigators. To these “cadaveric alkaloids” have been attributed the “sausage poisoning,” so frequent in Germany (for cases, see Casper’s Handbook, vol. 3), poisoning by various foods, such as tinned meats, cheese, &c. Some are irritants, others narcotics: different periods and circumstances of putrefaction producing different compounds.
In an Italian criminal prosecution, F. Ciotto, who made the investigation of the corpse, gave it as his opinion that strychnia was probably present. Selmi, for the defence, pointed out differences from strychnia, and considered the compound to be a ptomaine. [Arch. Pharm. (3), 19, 187.] This will show the importance of the subject.