REMARKS ON THE ACIDS OF THE BRITISH PHARMACOPOEIA. 63 
sulphuric acid, diluted with fourteen times its bulk of water. Three atoms 
of the acid to one of the salt are employed. The change which occurs may 
be represented as follows :— 
2(K 4 FeCy 6 )+6H 2 S0 4 =K 2 Fe 2 Cy 6 +6KHS0 4 +6HCy. 
Ferrocyanide of potassium. Yellow salt. 
The so-called “yellow salt” which remains in the retort after the distilla¬ 
tion is really a ferrocyanide of iron and potassium. It may be correctly 
written Fe"K 2 FeCy 6 . It is, in fact, ferrocyanide of potassium in which two 
atoms of potassium are replaced by one diatomic atom of iron. The volume¬ 
tric method given for estimating the strength of hydrocyanic acid is a very 
convenient one. 270 grains of the acid, rendered alkaline by solution of 
soda, require 1000 grain-measures of the volumetric solution of nitrate of 
silver before a permanent precipitate begins to form. Nitrate of silver added 
to cyanide of sodium, first forms a definite double salt which is soluble : 
2NaCy+AgN0 3 =NaAgCy 2 +NaN0 3 . 
When the cyanide of sodium has been all converted into this double salt, then 
the latter is decomposed by a further addition of nitrate of silver, and a per¬ 
manent precipitate produced. 
In the above reaction one atom of nitrate of silver (170) equals two of 
cyanogen, and consequently two of HCy (27x2 = 54). 1000 grain-measures 
of the volumetric solution contain 17 0 grains of nitrate of silver, and are 
therefore equal to 5'4 grains of HCy, the quantity contained in 270 grains 
of the diluted acid, corresponding to 2 per cent. 
Acidum Nitricum. —The strength of this acid has been changed to that 
of the last London Pharmacopoeia. It has a sp. gr. 142, and contains 70 per 
cent, of HN0 3 . The acid and water in it are present in atomic proportions, 
namely, 2HNC 3 , 3H 2 0, and it is sometimes represented therefore as a defi¬ 
nite sesquihydrate. At the ordinary atmospheric pressure it distils un¬ 
changed, having a constant boiling-point of 250° F. No process is given for 
its preparation. A stronger acid undergoes rapid decomposition by keeping, 
but this is very stable. On this account, and also by reason of its very defi¬ 
nite composition, it is by far the best suited for pharmaceutical purposes. 
90 grains contain one atom in grains of HN0 3 . and therefore neutralize 
1000 grain-measures of the volumetric solution of soda. 
Acidum Nitro-hydrochloricum Dilutum. —An alteration has been made 
in the directions for preparing this acid, which may be of more importance 
than it at first sight appears. We are now instructed to mix the strong acids 
together, and allow them to remain for twenty-four hours before adding the 
water. The following reaction, which occurs between these acids in the con¬ 
centrated state,is thus allowed to take place to the full extent before dilution:— 
HN0 3 +3HC1 = NOC1 2 +2HoO + C1. 
Chloronitric gas. 
In the previous Pharmacopoeia, we were directed to add the acids separately 
and successively to the water. In that diluted state they are capable of re¬ 
maining for a very long time without decomposing each other ; consequently, 
it is probable that the product rarely had the composition intended. 
According to the present method, after the expiration of the twenty-four 
hours, the bottle in which the strong acids have been mixed is filled in the 
upper part with a yellow gas ; hence we are told to add the water in successive 
portions, shaking after each addition, the object being to absorb this gas. 
In warm weather the action between the acids is apt to be somewhat ener¬ 
getic, and a larger quantity of gas is then evolved and escapes. Some vari¬ 
ation in the strength of the product may arise, therefore, unless in summer 
time an undue elevation of temperature be guarded against by immersing 
the bottle, which should be large and loosely stoppered, in cold water. 
