240
ELEMENTS OF MATERIA MF.DICA.
therewith white chloride of silver insoluble in boiling nitric acid,whereas the white cyanide of silver is soluble in nitric acid at a boilingtemperature. I would observe, that the presence of either of these acidsis no further objectionable, than that it creates a difficulty in the deter-mination of the strength of the hydrocyanic acid: while, on the otherhand, it confers the advantage of rendering the hydrocyanic acid muchless liable to decompose. The acid prepared from ferrocyanide of po-tassium will keep for years (Dr. Christison has had some unchanged fortwo years and a half, though it was exposed to day-light), owing, it issupposed, to the presence of some sulphuric acid. Mr. Barry adds alittle hydrochloric acid to all his medicinal hydrocyanic acid, in order topreserve it. As air and light hasten, though they are not essential to,the decomposition of the acid, they should be carefully excluded.
Characteristics. —The following are the best tests for hydrocyanic acid.
1. The odour. —The peculiar odour of hydrocyanic acid is well known.It must not be confounded with the odour of the volatile oil of bitteralmonds. Orfila says, that this is the most delicate characteristic of theacid, since it is very marked when the liquid tests give very slight indi-cations only. But I have not found this to be invariably the case: itdepends much on the nature of the mixture containing the acid.
2. Formation of Prussian blue (ferrosesquicyanide of iron ).—Addsufficient caustic potash to the suspected acid to saturate it; then a solu-tion of some proto- and per-salt of iron : the common sulphate of ironof the shops, or the tincture of the chloride, answers very well, sinceboth of these preparations usually contain the two ( prot - and per-) saltsof iron. A precipitate is thus obtained, which is liable to considerablevariation in its colour, depending on the quantity of potash and the qua-lity of the ferruginous salt employed; it may -be yellowish brown, orgreenish, or bluish. Then add dilute sulphuric or hydrochloric acid,when Prussian blue (ferrosesquicyanide of iron) will immediately make itsappearance, if hydrocyanic acid were present.
The formation of Prussian blue is thus accounted for. When potash isadded to hydrocyanic acid, water and cyanide of potassium are gene-rated. By the reaction of this salt on a proto-salt of iron the proto-cyanide of iron is produced, while with a per-salt of iron it forms sesqui-cyanide of iron. The two ferruginous cyanides, by their union, constitutethe ferrosesquicyanide or Prussian blue .
The following diagram explains the reaction of nine equivalents ofcyanide of potassium on three equivalents of the protosulphate, and fourequivalents of the sesquisulphate of iron.
INGREDIENTS REACTING.
9 eq. CyanidePotasm. 594
3 eq. Protosul-phateIron228
4eq.Sesquisul-
pliatelron400
1
!
{
9 eq. Potassium 360-3 eq. Cyanogen 78.
6 eq. Cyanogen 156,
3 eq. Sulphc. Ad.120^ ;:3eq. Oxygen.. 24'''3 eq. Iron .... 84— .6 eq. Sulph<\ Ad. 240(jT
6 eq. Oxygen.. 48*
4 eq. Iron-112-
7 9 eq. Potash .... 432-
3 eq. Protocyan. Iron 162
^ 4 eq. Sesquicyan. Iron 268
RESULTS.
9 eq. Sulphate Potash.... 792
3. Nitrate of silver. —This is by far the most delicate test of thepresence of hydrocyanic acid. It causes a white precipitate of cyanideof silver, which is soluble in boiling nitric acid. By this lattercharacter cyanide is distinguished from chloride of silver. If carefully