18 REMARKS ON THE ACIDS OF THE BRITISH PHARMACOPCEIA. 
Pharmacopoeia, having the composition As 2 0 3 , is represented by most modern 
writers as an anhydride ; and the real acid is described as a body having the 
formula H fj As0 3 , which however cannot be isolated, and is only known in 
ablution, being produced by dissolving the anhydride in water. 
As 2 O a +3 EL. 0—2 H 3 As0 3 . 
This view of the nature of arsenious acid is derived from the composition of 
the arsenites, which are in strict accordance with it. Thus, arsenite of silver is 
AgAs0 3 ; Scheele’s green, Cu"H As0 3 ; arsenite of potash, KH 2 As0 3 , etc. 
The volumetric test, by which the purity of arsenious acid is to be ascertained, 
stands as before. u 4 grains of the acid dissolved in boiling water with 8 grains 
of bicarbonate of soda, discharge the colour of 808 grain-measures of the volu¬ 
metric solution of iodine.” When iodine is added to an aqueous solution of 
arsenious acid, water is decomposed, hydriodic acid and arsenic acid being 
formed. This change may be thus represented :— 
As 2 0 3 + 5H 2 0 + 41 =2(H 3 As0 4 ) + 4 HI. 
Arsenic acid. 
When carbonate of soda is present in the liquid, the change is instantaneous 
and definite. Iodide and arsenate of sodium are formed. The action may be 
thus expressed :— 
As 2 0 3 -f 4 4-I 4 =2 (HAoHAs 0 4 ) T 4 C0 2 * 
Arsenate of 
sodium. 
The bicarbonate of soda ordered is converted into normal carbonate by the 
boiling, and serves to dissolve the arsenious anhydride. The quantity which the 
Pharmacopoeia directs to be taken, however, is insufficient to satisfy the above 
reaction. One atom of the anhydride requires 8 atoms of the bicarbonate 
(HaHC0 3 ); therefore 4 grains require 13 - 5 grains, whereas only 8 grains 
are ordered. But this deficiency does not interfere with the accuracy of the 
result. It simply leads to the formation of a little free acid in the liquid. From 
the equations given, it will be seen that 4 atoms of iodine are capable of oxidizing 
one atom of arsenious anhydride. The volumetric solution contains 1*27 gr. 
(two an atom) of iodine in 100 gr. meas. ; consequently 400 gr. meas. are 
equal to 198 gr. of arsenious acid (As 2 0 3 = 198), which is in the proportion of 
808 meas. to 4 grs. Until all the arsenious acid is oxidized, the brown of the 
iodine solution is destroyed as it enters the liquid. When the colour remains 
permanent, it indicates the termination of the reaction. 
Acidum Benzoicum. —This substance is now defined as “ a crystalline acid 
obtained from benzoin, and prepared by sublimation,” and no process is given 
for its preparation. It is consequently left open to the manufacturer either to 
sublime it, directly from the gum, or to obtain the acid first by precipitation, 
and sublime it afterwards. The first method does not yield so large a product, 
and is not therefore generally adopted. By boiling the gum with lime and 
water, and precipitating the filtered solution with hydrochloric acid, the whole of 
the benzoic acid is easily got. But in this state it is pulverulent and odourless. 
If the precipitated acid be sublimed, however, it is obtained in the character¬ 
istic feathery crystals, possessing the u>ual agreeable aromatic odour. 
Acidum Carbolicum. —This substance is for the first time officinal. One 
synonym is given for it; but in fact it has many. It has been called carbolic 
acid, phenic acid, phenylic alcohol, hydrate of phenyl, phenol, and coal creasote. 
Within late years it has come into commerce in considerable quantity, and in a 
great state of purity. I believe we are mainly indebted to l)r. Crace Calvert 
for its production in this state. It is obtained from that portion of coal-tar 
oil which boils between 300° and 400° Fahr., but its separation is attended with 
considerable difficulty. The oil is mixed with a saturated solution of hydrate 
