A NEW CLASS OF SALTS. 
481 
preparation of the nitroprusside. Nitric acid of commerce is diluted with its own 
bulk of water, and the quantity of it necessary to neutralize 53‘3 grs. of carbonate 
of soda (1 equiv.) is ascertained by the alkalimeter. This quantity denotes 1 equiv. 
of acid. 
Ferrocyanide of potassium is now reduced to powder and is placed in a convenient 
vessel, and for every 422 grs. of the salt used (that is for 1 equiv. Fcg Cyg 4K+6HO) 
5 equivs. of the acid are employed. This quantity of acid is found to produce an 
economical result, but it is very remarkable that one-fifth of the quantity, or 1 equiv., is 
sufficient to convert a large portion of the prusside into nitroprusside. This is the more 
remarkable, because there are four available equivalents of potassium, and it was to be 
expected that nitrate of potash would be produced. This however is not the case, 
1 equiv. of nitric acid effecting oxidation to a considerable extent on a double equivalent 
of yellow prusside. The five equivalents of acid mentioned above are at once poured 
on the prusside, as the cooling effect of the whole reduces the violence of the action. 
The mixture assumes a milky appearance, but soon the salt dissolves with a brownish- 
red colour like coffee, the mixture of gases already described being freely evolved. 
When the solution is complete, it is found to contain ferridcyanide of potassium mixed 
with a nitroprusside and nitrate of potash. It is now removed into a bolt-head and 
digested in the water-bath. It continues to evolve gas, and after a time it no longer 
yields prussian blue with sulphate of iron, but forms a dark green or a slate-coloured 
precipitate. The solution is now removed from the water-bath and is allowed to cool, 
during which abundance of nitrate of potash crystallizes out, and always more or 
less of a peculiar white substance. The dark coffee-coloured mother-liquor is now- 
neutralized with carbonate of soda or carbonate of potash, according as salts of 
sodium or potassium are desired. The neutralized solution shows the presence of iron 
existing as a base, for prussian blue is precipitated on the addition of a prusside. The 
neutral solution is now boiled, and it deposits generally a green precipitate, though 
occasionally one of a brown colour ; and the filtrate is found to be of a dark ruby-red, 
containing only nitroprusside of the base employed and a nitrate. The latter is 
separated by crystallization in the manner pointed out under the respective salts. 
Nitroprusside of sodium being most easily prepared, is recommended as the product 
of the process here given. 
Some practical difficulties may be mentioned so as to prevent disappointment in 
the preparation. A carbonate of and not the caustic alkali should be employed in 
the neutralization. When the latter is used, the solution of nitroprusside is apt to 
be mixed with ferrocyanide. When this takes place an addition of acid serves to 
remove the impurity, as some of the precipitated oxide of iron is dissolved, and form- 
ing prussian blue with the ferrocyanide, removes it from the solution. This impurity 
may also be removed by the addition of nitrate of lead, which precipitates the prus- 
side but not the nitroprusside ; or it may be taken away by the gradual addition of 
