482 
DR. PLAYFAIR ON THE NITROPRUSSIDES, 
sulphate of iron, which removes the ferrocyanide before precipitating the nitroprusside* 
When the quantity last added precipitates the solution of a salmon colour, the impu- 
rity has been removed*. 
Red prusside (ferridcyanide) of potassium may be used in the preparation exactly 
as described for the yellow prusside. 
7 . The following experiments were made in order to ascertain approximatively how 
much nitroprusside was formed by the process now described. 105'5 grs. crystallized 
ferrocyanide of potassium were digested with 1^ equiv. of nitric acid. After diges- 
tion the liquid was neutralized with carbonate of soda and boiled, the resulting green 
precipitate being collected on a weighed filter. The filtrate was precipitated by a salt 
of copper, and the nitroprusside of copper was collected and weighed. 
It was found by various trials that perfectly uniform results could not be obtained, 
the amount and even the composition of the precipitate-^ on boiling varying with the 
conditions of the preparation. The two following experiments may be taken as 
giving mean results : — 
I. 105’5grs. yellow prusside gave 8’275 green precipitate, yielding on incineration 
7*95 grs. peroxide of iron ; the filtrate gave 48‘90 grs. nitroprusside of copper. 
II. 105’5grs. yellow prusside gave 8'32grs. green precipitate, yielding by calcula- 
tion 6'30grs. peroxide of iron ; the filtrate gave 46‘12 grs. nitroprusside of copper. 
Taking the mean of the two experiments, 105'5grs., or one-fourth of the double 
equivalent of yellow prusside, yield 8‘297 grs. green precipitate containing 4'984 grs. 
iron; the filtrate yields 47‘51 grs. nitroprusside of copper. But before drawing de- 
ductions, it is necessary to know the composition of the green precipitate. It con- 
sists of a mixture of prussian blue, nitroprusside and peroxide of iron, this mixture 
not being constant. However, to take a special case as an example, — 
22‘26grs., calcined and treated with nitrate of ammonia, gave 13*62 grs. peroxide 
of iron, or 42*83 per cent, of metallic iron. 
9*49 grs. burned with oxide of copper, gave 4*13 grs. carbonic acid and 0*96 gr. 
water; the carbon is therefore 11*87 per cent., the water 10*11. 
35*02 grs. treated by caustic potash, gave, when neutralized by acetic acid, a red 
* It is perhaps needless to remark, that when the ruhy-red solution free from prussides has been obtained by 
any of the processes above described, it may be used at once for the precipitation of the insoluble nitro- 
prussides. 
t The composition of the precipitates varies considerably. If on neutralizing the acid solution an excess of 
alkali be added, the addition of an acid gives a slaty precipitate, which consists mainly of oxide of iron mixed 
with Prussian blue. Under somewhat similar conditions, I believe, though on this subject I am not certain, 
the precipitate on boiling, instead of being green, is brown, like oxide of iron. On washing and exposure to 
the air it becomes green. 
In an experiment where this precipitate came, 105'5 grs. yellow prusside gave 5'83 grs. of a brown precipi- 
tate, and 50' 66 grs. of nitroprusside of copper. In another experiment with a like quantity, 4*755 grs. of the 
brown precipitate were obtained. 
