B. Mercuric Cyanide, Hg(CN)2. Of all metals mercury has most affinity for HCN, mercuric oxide decomposing other cyanides, even Prussian blue, and dissolving readily, as we have seen, in free HCN, or in alkaline cyanides. Hence if a compound of mercury have been given medicinally, the prussic acid will be found in the stomach as mercuric cyanide, which is easily soluble in water, neutral to test paper, quite inodorous, and extremely poisonous. It is not officially recognised in any Pharmacopœia, except the French; has been occasionally used in medicine instead of mercuric chloride, which it resembles in action, but has the advantage of not being incompatible with alkalies and organic matters (Royle’s Mat. Med., 6th ed.). It crystallizes in anhydrous four-sided obliquely-truncated white opaque prisms, with a disagreeable metallic taste, is permanent in the air, easily soluble in water, less in alcohol. It fails to respond to the silver nitrate (partially) or Prussian blue tests, and gives the sulphur test with difficulty. It is decomposed by distillation with hydrochloric acid, but only ⅔rds of the HCN pass over into the distillate, unless ammonium chloride be added (Roscoe and Schorlemmer’s Chemistry). Whenever HCN is looked for, it is safer to examine also for mercury, and, if found, to add a little hydrochloric acid and sulphuretted hydrogen to the original liquid, thereby precipitating mercuric sulphide (black) and liberating the HCN, which may be distilled off. If, however, excess of sulphuretted hydrogen has been inadvertently added, it would blacken silver nitrate, and hence the silver test would not be available, unless the solution was previously shaken with lead carbonate to remove the sulphide. But it would not affect the Prussian blue or sulphur tests, as sulphide of iron is soluble in hydrochloric acid. Mercuric cyanide also gives off all its HCN when distilled with iron filings or zinc dust, sulphuric acid, and water. This seems a better method.
Mercuric cyanide is said to be an irritant poison, and to be similar in its action to corrosive sublimate. Combination with mercury seems to mask the physiological action of HCN, just as it does its chemical action. The medicinal dose is 1/16th grain gradually increased to ½ grain, in pills or solution (Royle). 10 grains have proved fatal. By heat, when dry, it is broken up like silver cyanide into mercury and cyanogen.
C. Cyanides of the Heavy Metals, as zinc, lead, copper, &c. Silver cyanide has already been described. These are insoluble in water, inodorous, and probably, while intact, not poisonous. But they are decomposed by mineral acids, and, as the gastric juice is acid, they would more or less readily yield free HCN, with its usual odour and effects. The influence of the metal has also to be considered.
D. Double Cyanides, derived from iron, cobalt, manganese, chromium, platinum, &c., are inodorous. Those of the alkalies and alkaline earths are alone soluble. The only common ones are ferro-and ferricyanide of potassium, the so-called yellow and red prussiates of potash. They are said to be merely purgative, not poisonous, but, from the comparative facility with which they yield HCN by acids, they cannot be considered safe. Soluble ferrocyanides give, with pure ferrous sulphate, a white precipitate turning blue in air; with ferric chloride a precipitate of Prussian blue; with cupric sulphate a maroon precipitate. Ferricyanide solutions give with ferrous salts a deep blue precipitate; with ferric salts a dark-brown coloration. These reactions would be applied to a filtered portion of the stomach contents. Prussian blue is ferric ferrocyanide mainly, but varies in composition: it is supposed to be inert.
Almen states (Chem. Centr. 1872, 439) that potassium ferrocyanide in solution decomposes at ordinary temperatures, especially if a little free acid be present, HCN being formed. Prussian blue only decomposes when warmed to 40° or 50° C. (therefore not in the body, C. G. S.), “hence the presence of HCN, if accompanied by ferrocyanide, is not a proof of poisoning.” But ferrocyanide is not in any Pharmacopœia, and is not administered medicinally. Yet, to answer a possible question, a known fraction of the original substance might be extracted with water, and tested as above. The same observations apply to ferricyanide.
When ferro-or ferricyanides are distilled with moderately strong sulphuric acid, a portion of the contained HCN passes over; in fact, this is the common process for preparing prussic acid. The iron remains behind in the retort, in combination with potassium and the rest of the cyanogen. If ferric hydrate —“ferri peroxidum humidum”), or ferrous sulphate and potash, have been administered as antidotes to HCN, Prussian blue might be formed in the stomach. It would then show a blue colour, either by itself or on addition of an acid, and blue particles under the microscope, if in sufficient quantity. In this case the HCN left in the stomach would have been rendered innocuous, and the prussic acid which had actually caused the death would be found free in the blood, &c. The stomach contents might then show no HCN, either by odour or distillation, as Prussian blue is inodorous, and not easily decomposed by dilute acids. With alkalies it turns brown, giving ferric hydrate and an alkaline ferrocyanide.
Ludwig and Maushner (Chem. Centr. 1881, 43), in a case of poisoning, discovered a quantity of potassium ferrocyanide in the body. This was removed by slightly acidulating and carefully precipitating by ferric chloride. The filtrate, distilled with tartaric acid, yielded much HCN. The sample of cyanide of potassium, which had probably caused death, was afterwards found to contain a large proportion of ferrocyanide.
E. Sulphoncyanides (Thiocyanates). Those of the alkalies and alkaline earths are soluble and colourless; ferric sulphocyanide is soluble, and intense blood-red (sulphur test); other sulphocyanides are mostly insoluble. They are all inodorous, poisonous in moderate quantities, and are not officinal in any Pharmacopœia. Distilled with acids they break up, HCN being found in the distillate. It has been mentioned already that ammonium sulphide, produced by putrefaction, may combine with any HCN present to form ammonium sulphocyanide; therefore, if the matters to be examined are alkaline, and putrefaction has commenced, Allen (Commerc. Org. Anal., 1879, art. HCN) recommends us to digest with alcohol, filter, evaporate to dryness on a water bath, redissolve in a little water, filter again, and test the filtrate with ferric chloride after just acidulating with hydrochloric acid: the well-known blood-red colour will result (see “Sulphur Test”). But the ordinary distillation with tartaric or sulphuric acid would in this case also detect the HCN, though the whole might not pass into the distillate.
Sulphocyanide of mercury is the toy called “Pharaoh’s Serpent.” A case of poisoning by it is recorded.
It is important to notice that traces of sulphocyanide are naturally present in the saliva. If this salt be found, the question will occur, how much could be accounted for by the saliva? Carpenter (Princ. of Human Physiol.) quotes Harley to the effect that the average daily secretion from the salivary glands is 1 or 2 pounds: other observers have stated that it varies greatly. The secretion itself is said to contain, in 1000 parts, one part (Frehrichs), or 0·6 part (Jacubowitsch), or even 0·3 part (Oehl), of potassium sulphocyanide; that is, 4·2 to 7 grains per pound, equivalent to from 1 to 2 grains of HCN, or 2 to 4 grains if 2 pounds of saliva were secreted. This would be a serious matter but for the fact that, whether from decomposition by the gastric juice or otherwise, or from its passing out of the stomach as it passes in, it is certain that no such quantity is ever found naturally in the stomach, not more than a minute trace being ever given by the processes, unless hydrocyanic acid, in one of its forms, has actually been administered.