Tho five crops of aceto-chloride are next mixed together, carefully washed with water, and allowed to drain. They are then dissolved in about twice their weight of water, and to the filtered solution a further quantity of about 10 per cent. of chloride of calcium is added. Tho reinforced solution is evaporated to 30° Beaum6, and finally crystallized in precisely the same manner as before to produce four crops. The last mothers from this second operation may be added to the previously obtained impure acetate solution for treatment with sulphuric or hydrochloric acid.
This process may be modified in one or two ways, by the use of distilled acetate, or by the use of pyroligneous acid. in the latter case the acid must be neutralized with lime and tested for brown acetate, a solution containing I part of the salt in 10 parts, requiring the proportions stated above. As brown acetate of lime is a very uncertain article, of constantly varying constitution, it is advisable to test the hot mixture during evaporation, by setting a sample aside to cool from time to time, after 30° has been attained, to see if a good crop of crystals forms. The standards given above are only indications of the strengths required by an average sample of good acetate. If distilled acetate of lime be used instead of brown, the proportions will be 8 parts of the lime salt to 6 parts of chloride of calcium.
It is very advisable in all cases to get rid of a certain amount of the impurities, al ways present, by roasting the acetate before dissolving. For this purpose any unused wood cylinder, say 7 ft. long by 4 ft. in diameter, may he used, a grating being placed near the bottom, which should touch the sides of the retort at as few points as possible. The acetate is placed in shallow sheet iron trays, similar to those used in the distillation with sulphuric acid, about 2 in. deep, and arranged one above another on the grating until the retort is filled. The trays are separated by rods of iron laid the lengthways of the retort. A gentle heat is then applied from fire under neath until the exit pipe from the retort begins to get cool, showing that all the water and volatile impurities have been driven off. The fire is then drawn and the whole allowed to cool down. This operation takes about forty-eight hours for completion.
Instead of roasting the aoetate, the aceto-chloride may be treated in a similar manner, or the mixture of acetate of lime and chloride may be filtered through animal charcoal, or any similar substance capable of separating out the ompyreumatic bodies. The process described, however,
is that usually employed.
To obtain the acetic acid from the crystals of accto-chloride, they are distilled in the usual manner with hydrochloric or sulphuric acid in any suitable still and condensing arrangement. Tho best proportions are 112 parts of aceto-ehloride to 24 parts of sulphuric acid of it specific gravity of 1' 84—preferably diluted with twice its volume of water—or 100 parts of the lime salt to 50 parts of hydrochloric acid. Whichever acid is used, the distillation is more readily effected when the solution in the retort is well diluted with water. This has already been pointed out iu describing the ordinary process of obtaining " second " acid by distillation.
Tho acetic acid produced in this way is purified by re-distillation with an alkali in the propor tions of 1 lb. of alkali to 10 gallons of acid.
As yet only a very small quantity of the acid that comes into the market is obtained by Mr. Coudy's process, there being a prejudice against it on the part of consumers, which seems to be entirely unfounded.
The production of acetic acid from the other acetates, of lead, silver, potash, &c., is not of suffi cient importance to require notice.
Besides the three chief methods described—acetous fermentation, wood distillation, and the distillation of the acetates —acetic acid is occasionally obtained by the distillation of vinegar, and, for laboratory purposes, by the direct oxidation of alcohol through the medium of spongy platinum. This substance possesses the property of absorbing within its pores several hundred times its own weight of oxygen, and the alcohol, presented in a state of vapour, undergoes combustion and is converted into acetic acid. The operation may be conducted on a small scale by means of the apparatus shown in Fig. 26. Air is admitted between the rim of the bell jar and the dish A in which it is supported ; the platinum black is placed in a small porcelain saucer B and alcohol is dropped upon it through the funnel E, which terminates in a fine point. The acetic, acid condenses and collects in the dish.