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Home › Acids › To Obtain Oxalic Acid
To Obtain Oxalic Acid
Liebig 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 crystallize, 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 be 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, he says, contributes to increase the product; 15 pounds of sugar yielding fully 17 pounds of crystallized oxalic acid.