On a larger scale, a series of shallow earthenware or porcelain vessels arc arranged on shelves fixed about 12 inches apart iu a glass case, or a wooden box with a glass cover, to admit the heat of the sun. In each dish is placed a small tripod about 1j to 2 in. high, bearing a watch glass, the bottom of which is well covered with spongy platinum The lower porcelain dishes are conveniently filled with alcohol, and the temperature of the case raised by any suitable means to about 32° (90° F.). The spirit is converted into a state of vapour, which, coming in contact with the air held in the pores of the platinum, is oxidized into acetic acid, and falls back into the dishes or collects in a receiver arranged at the bottom of the case. To convert the whole of the alcohol into acetic acid, or to keep up a contiuuoua production, it is of cuurse necessary to renew the exhausted air of the case from time to time. The apparatus is shown in Fig. 27.
Formerly, and especially on the Continent, where the duty on alcoholic liquids is low, this process of direct oxidation was largely carried on. It has, how ever, been found that a considerable loss of alcohol takes place through volatilization.
A considerable.number of waste and bye-products exist which contain considerable quantities of acetic acid, and in the treatment of which a good deal might, and will, be done.. The obstacles usually encountered are the large quantity of material to be treated, the difficulty of separating the impuritiea, and the comparatively small value of the acid obtained. The waste liquors from the manufacture of various indiarnhber goods may be cited as an example. Theee liquors contain considerable quantities of acetate of lime, but lnixed intimately with hyposulphite of lime (and lead), and contaminate the product if simple distillation with sulphuric or hydrochloric acid he attempted. It has been proposed to employ chlorine to convert all the sulphur acids and salts present iuto sulphate of lead, which can be filtered off or allowed to aubside.
Special details relating to the determination of the strength of acetic acid are interesting rather to the chemist than the manufacturer. As, however, it is often necessary for the latter to be in possession of some ready means of ascertaining the value of his product, or purchase, it may be atated that three methods of testing may be employed :—(1) neutralization with pure carbonate of soda or potash, and determination of the quantity required to saturate the acid ; (2) the specific gravity after neutralization with hydrate of lime ; (3) the simple specific gravity by acetometer. It has already been shown (see Mohr's table) that the last-named method is very unreliable, and can only be used as a rough test or within certain limits. It must he borne in mind that the test
by acetometer is interfered with by the varying amounts of foreign and organic bodies, always present except in the case of a purified sample, which increase the density of the liquid ; and, furthermore, that the results vary for different temperatures. The following table, drawn up by Oudemanns, shows this difference between 15° and 40°, in liquids containing from 1 to 100 per cent. of acetic acid.
The first method—neutralization with the carbonate of an alkali—is usually adopted for all careful testings. A " standard," or " test," solution is prepared by dissolving, say, 530 grains of pure carbonate of soda in 10,000 grains of distilled \NAM This standard solution may, of course, consist of any quantity, provided that the right pro portions are carefully registered ; it should be kept well stoppered. A known weight of the sample of acetic acid is weighed off into a flask and a little litmus solution dropped in. The standard solution is then added until the solution just turns blue. It is then well boiled to drive off the carbonic acid which would redden the solution, and if the blue colour has disappeared a little more of the standard solution is added to the boiling mixture until a per manently purple hue is induced, showing complete neutralization of the acid. A simple rule-of-three calculation then gives the amount of acetic acid present iu the sample, since every JOG parts of the pure dry carbonate of soda put into the standard solution are equivalent to, or indicate, 120 parts of pure acetic acid. The standard solution must be very delicately used as the point of neutralization is approached, that there may not be an excess of alkali added. The apparatus best adapted for the process is shown in Fig. 28, Mohr's burette.
Acetic acid, especially tho " second " acid, obtained from the acetate of lime, is liable to contamination with considerable quantities of sulphuric or hydrochloric acid if the process has not been carefully conducted. Positive adulteration with these acids, too, is frequently resorted to by vendors with the idea that the acetic acids keep better (a mistaken notion), or to increase unduly the amount of acidity. A rough test of any sample may be readily made by boiling it with a little potato-starch for about ten minutes, allowing it to cool and adding a few drops of iodide of potassium. If the acetic acid he pure, the blue colour of iodide of starch will immediately make its appear ance, but if either sulphuric or hydrochloric acid be present the starch is converted upon boiling into dextrin, and no blue colour becomes visible.