REMARKS ON THE ACIDS OF THE BRITISH PHARMACOPOEIA. I? 
or a compound which, when in contact with a metallic hydrate, exchanges hy¬ 
drogen for a metal. But the substance C 4 H 6 0 3 contains no exchangeable 
hydrogen ; consequently many chemists do nut regard it as an acid at all, but 
name it simply an “ anhydride.” 
Nitric, phosphoric, and sulphuric acids are defined like acetic acid ; and the 
diluted acids have their strengths expressed in percentages of the anhydrous 
acids. It is very desirable to avoid as far as possible in a Pharmacopoeia the 
use of terms involving theoretical views not universally accepted ; and no doubt 
it would have been advantageous therefore if all reference to u anhydrous 
acids” had been omitted from the present work. 
One of the principal impurities of acetic acid is sulphurous acid. The test 
for this substance, given in the previous edition of the Pharmacopoeia, namely, 
iodate of potash and starch, is now omitted in favour of another. The present 
method is as follows:—The acid is mixed with water, pure hydrochloric acid, 
and a few pieces of granulated zinc, in a small flask. During the effervescence 
a slip of bibulous paper wetted with solution of subacetate of lead is suspended in 
the neck of the flask; when, if sulphurous acid is present, the paper will 
be blackened. The zinc, acting on the hydrochloric acid, liberates hydrogen. 
Nascent hydrogen reacts on sulphurous acid, forming sulphuretted hydrogen, 
which, escaping with the excess of hydrogen, blackens the lead salt. 
H 6 +S0 2 ~2 H 2 0 + H 2 S. 
As hydrochloric acid itself frequently contains sulphurous acid, care should 
be taken to ascertain its purity before using it for this test. 
Acidum Aceticum Glaciale.— This is defined as u Concentrated Acetic 
Acid, corresponding to at least 84 per cent, of anhydrous acid, C 4 H 6 0 3 .” This 
equals 99 per cent, of real acid, HC 2 H 3 0 2 ; consequently the presence of 1 
per cent, of water is permitted. No process is now given for its preparation. 
The method given in the previous edition* was a very bad one. The pro¬ 
duct was never really glacial, and always contained sulphurous acid. A better 
result is obtained by the process given in the last Dublin Pharmacopoeia. Dry 
hydrochloric acid gas is passed into well-dried acetate of lead, in the propor¬ 
tion of two atoms of the former to one of the latter. After all the gas has 
been absorbed, the liberated acetic acid is distilled off. 
Pb (C 2 H 3 0 2 ) 2 + 2 H Cl = Pb Cl 2 +2 (h C,H 3 0 2 ). 
The drying of the acetate of lead for this operation, however, is very difficult and 
troublesome ; and the process is greatly improved by substituting acetate Qf lime, 
which is capable of enduring a much higher temperature without injury. The 
method said to be adopted upon the large scale on the Continent for manufac¬ 
turing glacial acetic acid, is the distillation of binacetate of potash. Acetate 
of potash mixed with aqueous acetic acid forms an acid salt which may be con¬ 
centrated down to a certain point, when it splits up into the strong glacial acid 
and neutral acetate; the latter can consequently be used over and over again. 
Acidum Arseniosum.—A change has been made in the description of this 
substance, and the process for its re-sublimation is omitted. In the present 
work arsenious acid is described “as a heavy white powder, or in sublimed 
masses which usually present a stratified appearance, caused by the existence of 
separate layers, differing from each other in degrees of opacity whereas in 
the previous edition the powder only was mentioned. The lump condition 
ensures to a certain extent the purity of the acid, and many pharmaceutists, 
therefore, prefer it in this state. Moreover, the appearance of the lump is emi¬ 
nently characteristic, while that of the powder is not. Arsenious acid of the 
# 
Distilling anhydrous acetate of soda with strong sulphuric acid. 
Na Co H 3 0 2 + H 2 S C 4 = Na H S C 4 + IIC 2 H 3 0 2 . 
VOL. IX. 
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