Casper states in his Handbook (vol. iii., illustrative cases of HCN) that Schauenstein (one of the Prussian official chemists), twenty-six hours after death, found no HCN in the stomach, but a considerable amount of formic acid, the result of its metamorphosis. We know that strong HCN, exposed to light, decomposes into formate of ammonium, which, by distillation with a dilute acid, would give formic acid in the distillate. That such a change should occur so rapidly in a dilute solution, and in the darkness of the body, is improbable. It would be well, however, that formic acid should be looked for in the distillate thus:—
Carefully neutralize a measured portion with pure soda or potash; evaporate on the water-bath to dryness. The alkaline formate will be left in white crystals if present, together with the cyanide, which will not crystallize, but remain as a deliquescent mass. Dissolve in a little water, and divide into three equal portions.
(1.) To the first add silver nitrate in slight excess. Cyanide of silver will precipitate, formate will remain in solution, if not too concentrated. Filter, if possible. On boiling, if any blackening happens from reduction of the silver, formic acid is probably present. Acetic acid does not reduce silver nitrate.
(2.) To the second add dilute neutral ferric chloride (a solution of iron-alum answers admirably). A red-brown colour, removed by a drop of hydrochloric acid, indicates either acetic or formic acid.
(3.) Evaporate the third portion to dryness, and ignite gently in a closed crucible. Formate and acetate will be turned into carbonate, while cyanide will remain unchanged if air be excluded. If then effervescence take place on treating the residue with a little hydrochloric acid, it is a confirmation of the presence of formic or acetic acid. The first test will have revealed which it is.
Many animal substances, when distilled with strong acids, do give acetic and formic acids, but they do not act thus with dilute acids. Yet a stomach will usually yield a little acetic acid from the food having turned sour.
If formic acid be present, it will probably have proceeded from the decomposition of HCN. Then the reduced silver obtained in the first test should be weighed, and calculated into formic acid, and also into hydrocyanic acid (108 parts of silver = 46 parts formic acid, or 27 parts HCN). The result may be stated thus:—
“Hydrocyanic acid actually found,—— grains. Formic acid found,—— grains. If this had proceeded from the decomposition of hydrocyanic acid, it would correspond to an additional amount of—— grains of hydrocyanic acid.”
It is needless to observe that the mere finding of formic acid would be no proof of the administration of HCN, unless strong corroborative evidence were at hand.
On the whole, we must always try, and we may often hope, to find HCN if given, either free, as cyanide, or as sulphocyanide, even after months have elapsed.