91 
of Edinburgh , Session 1866-67. 
of the solution of tartaric acid, and a solution of one equivalent of 
chloride of sodium. These cause a further precipitation of potassic 
bitartrate. A slight effervescence now ensues ; and from the in- 
soluble precipitate giving points of escape for the nitrous acid, a 
good deal would be lost. The clear solution is, therefore, rapidly 
decanted from the precipitate, and the latter is thrown on a filter 
to drain. The mixture is now heated up as quickly as possible 
to the boiling point, and is briskly boiled.* Portions of it, taken 
out at intervals upon bibulous paper, must be tested for nitrite 
with a solution of starch and potassium iodide. When the nitrite 
appears to be vanishing, the remaining half of the nitrite solution 
must be added by degrees, so as always to keep nitrite in solution ; 
and it must remain distinctly acid with the original tartaric acid 
added. In about an hour the solution, which had become at first 
coffee-coloured, has acquired a fine clear red ; but it must be boiled 
for some time longer. The reasons for the precautions in the 
manner of adding the nitrite, and preventing any large excess 
either of it or of acid, are the following: — Red prusside of potas- 
sium heated with sodic nitrite, becomes yellow prusside of sodium 
and potassium, which would remain unconverted. Again, if the 
tartaric acid has decomposed all the nitrite, and remains in the 
boiling solution, it decomposes the ferridcyanide, and forms Prus- 
sian blue. But if the above process be followed out, after one or 
two hours’ boiling, the whole of the ferridcyanide is converted into 
nitroprusside. The progress is readily ascertained by placing from 
time to time a drop of the solution on bibulous paper, and touching 
it with a solution of the ferrous sulphate. The last traces of fer- 
ridcyanide are, however, the most difficult to obliterate. The solu- 
tion is now evaporated and crystallized. Two abundant crops of 
nearly pure sodic nitroprusside may thus be obtained. The third 
crop is generally mixed with potassic chloride, and sodic and 
potassic tartrate, and has to be picked out from them. 
Unless the precautions which have been described have been 
strictly attended to, a small quantity of sodic ferrocyanide will be 
found in the mother liquor. 
* If the solution, on mixture, instead of being boiled, as recommended, be 
kept cool, a large quantity of nitroprusside is still formed, and a considerable 
amount of oxamide is deposited. 
