A NEW CLASS OF SAI.TS. 
487 
holic solution of tartaric acid, the quantity of the latter being just sufficient to form 
bitartrate of potash with the potassium ; but as the acid dissolves some of the latter 
salt, this process does not yield a pure product. 
A dark red-coloured solution, strongly acid, is obtained by these methods. Ether 
does not precipitate the acid as it does ferrocyanic acid. Soon however the solution 
begins to form hydrocyanic acid, and either to deposit oxide of iron or to hold iron 
in solution, which maybe detected by a prusside. When this change has taken place, 
evaporation in vacuo over sulphuric acid yields crystals of the acid, which is how- 
ever found to contain a small quantity of an impurity, probably of a cyanide of iron, 
which cannot be separated by crystallization, or any other of the numerous methods 
tried. The amount of this impurity is from 2 to 3 per cent. This crystalline acid 
belongs to the oblique system, and its crystals are described and measured in a 
further part of this paper, together with its analyses. It possesses all the properties 
of nitroprussic acid, and only differs by containing this small quantity of impurity. 
The perfectly pure acid in crystals has not been obtained, notwithstanding very many 
efforts to obtain this desirable result. Fide page 499. 
Nitr'oprusside of Sodium. 
11. This salt is the most readily procured, in a crystallized state, of all the nitro- 
prussides ; it may be obtained by decomposing the nitroprussides of copper or iron 
by means of soda, filtering from the oxides of these metals and evaporating the solu- 
tion by a gentle heat. When prepared from the iron salt, it is apt to contain a little 
iron in excess. 
Nitroprusside of sodium is however most easily prepared in the following manner: — 
1 equiv. of yellow prusside of potassium is digested with 5 equivs. nitric acid, as de- 
scribed in page 481, until the solution precipitates salts of protoxide of iron of a 
slate colour. It is now neutralized with carbonate of soda, both solutions being- 
employed cold. The neutralized liquid is now boiled, and the green precipitate is 
separated by filtration from the dark red-coloured solution. This is now evaporated 
down and again filtered from a brown precipitate which falls during evaporation. 
The nitrates of soda and potash are allowed to crystallize out. 
The dark red solution is now evaporated on the sand-bath, and during evaporation 
prismatic crystals separate from the hot solution. These are removed, dissolved in 
water, and again crystallized by allowing the solution to cool. The reason of taking 
the crystals from the hot solution in the first instance is to obtain them uncontami- 
nated with the nitrates, which are more soluble in hot water than this nitroprus- 
side. By this process any quantity of the nitroprusside of sodium may be obtained 
in fine large ruby-coloured crystals. 
Properties . — This salt crystallizes in fine ruby-coloured prisms, which have been 
measured by Prof. Miller. 
3 R 2 
