64 
REMARKS ON THE ACIDS OF THE BRITISH FI!ARMACOP(EIA. 
Acidum Phosphoricum Dilutum. —The description and process for this 
acid are unaltered, excepting that we are now told to conduct the last part of 
the evaporation in a platinum vessel, instead of finishing it in the porcelain 
dish as before. Concentrated phosphoric acid rapidly attacks the glaze of 
earthenware, and becomes contaminated ; hence the object of this change. 
The process consists in heating together phosphorus and diluted nitric acid 
until the former has disappeared. The solution is then evaporated down 
until all the excess of nitric acid has been expelled, and the residue is dis¬ 
solved in water. At one period of the evaporation considerable effervescence 
takes place, and nitrous gases are evolved. The first reaction whicluoccurs 
is to produce phosphorous acid. 
p+hno 3 +h 2 o=h 3 po 3 +no. 
Phosphorous 
acid. 
Subsequently, and generally when the nitric acid becomes concentrated by 
the evaporation, this phosphorous acid is converted into phosphoric acid. 
3H 3 P0 3 +2HN0 3 =3H 3 P0 4 +2N0+H 2 0. 
It is sometimes necessary to carry the evaporation so far that the ortliophos- 
phoric acid, H 3 P0 4 , is resolved into the glacial acid, HP0 3 . 
h 3 po 4 -h 2 o=hpo 3 . 
When this is redissolved it does not immediately assimilate the atom of water 
unless the solution is boiled. 
The distillatory apparatus indicated is not the most convenient for effecting 
the oxidation of the phosphorus in ; unless constant attention be given *to it 
the nitric acid is apt to concentrate in the retort, and accidents sometimes 
result from the violence of the action which ensues. 
The operation is more safely, more rapidly, and quite as economically con¬ 
ducted in a large flask with a long neck, in the mouth of which a funnel is 
placed,—a second funnel, a little smaller in diameter, being inverted in the 
mouth of the first, so as to form a chamber above the flask. A gentle sim¬ 
mering heat is applied; the nitric acid which volatilizes, condenses in the 
funnels ; the nitric oxide resulting from the action meets with air in the 
funnels, is converted into peroxide of nitrogen, which is absorbed by the con¬ 
densing liquid, carried back to the flask, and again contributes to the oxida¬ 
tion of the phosphorus. Indeed, the presence of these nitrous gases in the 
liquid contributes very much to the rapidity of the process, and to have them 
constantly carried back is therefore a great advantage. Moreover, the ope¬ 
ration goes on thus automatically, and no risk is incurred of an uncontrollable 
action setting up. 
This acid contains 13*8 per cent, of H 3 P0 4 , or 10 per cent, of P 2 0 5 . Its 
strength is arranged like that of the other diluted mineral acids, so that six 
fluid drachms shall saturate one atom in grains of an alkali. Although phos¬ 
phoric acid is a tribasic acid, the most neutral phosphate of an alkali contains 
only two atoms of metal, i. e. Na 2 HP0 4 . Consequently, the six drachms 
contain half a molecule in grains of H 3 P0 4 , instead of one-third. 
Acidum Sulphuricum. —The sulphuric acid of commerce is now the acid 
of the British Pharmacopoeia. The process of distillation has been aban¬ 
doned ; the specific gravity is reduced to that of the acid commonly manu¬ 
factured, and excessive purity is no longer demanded. The density of the 
officinal acid is now 1*843, and it is stated to contain 96*8 percent, of H 2 S0 4 , 
corresponding to 79 per cent, of S 0 3 . 50*6 grains by weight contain half 
a molecule in grains of H 2 S0 4 , and therefore saturate 1000 grain-measures 
of volumetric solution of soda. 
Acidum SulphuroSum. —This acid is of the same strength *and is pre¬ 
pared in the same manner as before. Charcoal and sulphuric acid are heated 
together in a flask, and the evolved gas is passed into cold water. 
