CHAPTERS FOR STUDENTS. 867 
Roman type will not, as a rule, be alluded to. Literal quotations from the 
Pharmacopoeia will appear in brackets, thus [§ ]. 
Acetum. Vinegar. —An impure dilute acetic acid prepared from malt. 
Malt is barley, of which the starch has been changed into glucose (grape-sugar) 
and dextrine, by allowing it to germinate and then drying it. An infusion of 
malt allowed to ferment gives alcohol and carbonic acid gas ; the latter escapes, 
whilst the former, exposed to atmospheric oxygen, changes slowly into acetic 
acid. British vinegar is thus made, and is employed in the Pharmacopoeia. 
On the Continent, vinegar is made by the acetification of certain kinds of wine. 
In whatever way it is made, the acetic acid of vinegar results from the substitu¬ 
tion of an atom of oxygen in place of two atoms of hydrogen in alcohol:— 
c 2 h 6 o + o 2 = c„h 4 o 2 + h 2 o 
Alcohol. Oxygen. Acetic Acid. Water, 
The process is accelerated by the growth of a spongy fungoid organism, 
which appears to act by first absorbing the oxygen from the air and then giving 
it up to the alcohol. Pure spirit of wine is not converted into acetic acid by 
exposure to the air. 
The B.P. vinegar contains 4*6 per cent, of acetic anhydride, C 4 II 6 0 3 , cor¬ 
responding to 5*4 per cent, of acetic acid, C 2 H 4 0 2 . Ten minims of the solution 
of chloride of barium are sufficient to remove from a fluid ounce the whole of the 
ToVoth of sulphuric acid allowed by law. If, on the further addition of the test, 
more sulphate of baryta were thrown down, it would indicate adulteration. 
Sulphuric acid is supposed, though doubtfully, to prevent “mothering” of the 
vinegar. Sulphuretted hydrogen causes no precipitate in pure vinegar, show¬ 
ing the absence of metals, such as copper and lead. 
Acidum Aceticum. Acetic Acid. —The crude, pyroligneous acid, is made by 
destructive distillation of wood in iron retorts. Tar, a watery liquid, and gas 
are the three products of the operation. The watery liquid drawn off and dis¬ 
tilled gives at first wood naphtha (impure methylic alcohol) in the receiver ; 
afterwards, acetic acid contaminated with tar passes over and is collected sepa¬ 
rately. 
The purified acetic acid of the Pharmacopoeia is made by neutralizing the 
impure acid with chalk or lime, decomposing the clear solution with carbonate 
of soda, which gives, by double decomposition, acetate of soda, and purifying 
this by heating it in the dry state and crystallizing from water. Pure acetate 
of soda, distilled with sulphuric acid, gives acetic acid in the receiver, sulphate 
of soda or a mixture of acid and neutral sulphate is the residue left in the 
retort:— 
Na C 2 H, 0 2 + II 2 S0 4 = HC 2 H 3 0 2 + Na HS0 4 . 
One molecule of sodic acetate and one of sulphuric acid, give one molecule of 
acetic acid, and one of the acid double sulphate. 
18'2 grams of the acetic acid, B.P., require, for neutralization, 100 cubic 
centimetres of the volumetric solution of soda. This indicates 33 per cent, of 
real acetic acid, 0 2 H 4 0 2 . 
The calculation is made thus,—the vol. sol. of soda is made of such strength 
that 1000 cubic centimetres of it contain one molecule, or 40 grams of Na HO. 
This quantity will, therefore, neutralize one molecule or 60 grams of acetic 
acid:— 
Na . . . 23 C 2 =12x2 = 24 
II ... 1 H 4 = 1 x 4 = 4 
O ... 16 0 2 = 16 x 2 = 32 
C 2 II 4 0 2 = 60 
NallO = 40 
