3898. To Concentrate Acetic Acid

3898.     To Concentrate Acetic Acid. Acid containing 20 per cent, of water may be deprived of a good deal of its superfluous water by standing over dry sulphate of soda. It may then be used either with or without distillation. Acetic acid of ordinary strength may be concentrated to any degree of rectification once or oftener from dry acetate of po-tassa or soda, rejecting the first and last portions that come over. The same acetate may be used repeatedly. The heat employed must not exceed 500o to 570° Fahr. Pure hydrated acetic acid liquefies above 62° Fahr.; at 50° to 55° it crystallizes in brilliant, colorless, transparent needles and plates; at 40° it is a crystalline solid. Free acetic acid reddens litmus paper, and may bo recognized by its odor and volatility.

3899. Tests for the Purity of Acetic Acid

3899.     Tests for the Purity of Acetic Acid. By heat it escapes entirely in vapor. Either nitrate of silver or chloride of barium being added to it, will produce no precipitate. "When a thin plate of silver is digested in it, and hydrochloric acid subsequently dropped in, no precipitate is formed. Its color is unchanged by the addition of hydrosulphuric acid, or ammonia, or by ferrocyanide of potassium added after the ammonia. The presence of sulphuric acid is indicated by a white precipitate being formed on the addition of a little peroxide of lead.

3900. Oxalic Acid

3900.      Oxalic Acid. This consists of colorless crystals, possessing considerable volatility, and a strong, sour taste; when exposed to a very dry atmosphere they effloresce slightly. Oxalic acid sublimes at 180° Fahr., and melts at 280°; is soluble in about nine times its weight of cold, and in its own weight of boiling water; soluble also, but in a less degree, in alcohol. It has a strong affinity for lime, and is therefore a good test for its presence, by yielding a precipitate insoluble in excess of the acid. With the bases, oxalic acid forms Oxalates.

3901. To Obtain Oxalic Acid

3901.    To Obtain Oxalic Acid. Lie-big proposes: Nitric acid (specific gravity 1.42), 5 parts; water, 10 parts; mix, add sugar, or preferably potato starch, 1 part, and digest by a gentle heat as long as gaseous products are evolved; evaporate and crystal-lize, dry the crystals, redissolve in the smallest possible quantity of boiling water, and crystallize; 12 parts of potato starch yield 5 of acid. The mother water, treated with more nitric acid, and again warmed, will yield a second crop of crystals; and this should bo repeated till the solution is exhausted.

Schlesinger gives the following method: Sugar 4 parts (dried at 257° Fahr.); nitric acid (specific gravity 1.38) 33 parts; the mixture, as soon as the evolution of gas ceases, is to be boiled down to one-sixth its original volume, and allowed to crystallize. The whole process may be executed in 2 hours, and yields of beautifully crystallized oxalic acid from 56 to 60 per cent, of the sugar employed.

On the large scale, the first part of the process is usually conducted in salt-glazed stoneware pipkins, about two-thirds filled and set in a water-bath; but on the small scale a glass retort or capsule may be used. The evaporation should be preferably conducted by steam. The evolved nitrous vapors are usually allowed to escape, but if conveyed into a chamber filled with cold damp air, and containing a little water, they will absorb oxygen, and be recondensed into fuming nitric acid. In England an equivalent proportion of molasses is usually substituted for sugar. Another process consists in first converting potato fecula into grape sugar with sulphuric acid, and then decomposing the sugar thus obtained by nitric acid, in the usual way. Dr. Ure recommends the use of a little sulphuric acid along with the nitric acid, which, ho says, contributes to increase the product; 15 pounds of sugar yielding fully 17 pounds of crystallized oxalic acid.