11 any doubt whatever exists as to the precipitate produced by potassium acetate being really the acid tartrate, its insolubility in cold water will readily settle the question, but positive proof is easily obtained by the silver test, which is extremely delicate when carefully applied, but remarkably liable to failure if the proper conditions be not carefully observed. The following plan of operation gives very good results :—A small quantity of the precipitate of acid potassium tartrate is washed with a little cold water, and then treated with a slight excess of ammonia. The resultant solution is boiled till neutral, allowed to cool, and then precipitated with excess of argentic nitrate. To the liquid containing the precipitate, dilute ammonia is added till the precipitate has almost disappeared, when the solution is filtered. On heating the filtrate nearly to boiling, for a few minutes, a brilliant mirror of metallic silver is produced on the sides of the tube. Citric acid does not reduce silver under similar circumstances, except on continued boiling.
After the precipitation of the tartaric acid in a solution by addition of potassium acetate, the citric acid may readily be detected in the filtrate (after evaporating off the alcohol) by applying the ordinary tests.
Wigner has suggested that the power possessed by solutions of tartaric acid of altering the plane of polarization of a transmitted beam of light would enable an accurate estimation of that acid to be made in the presence of citric acid, which is inactive. The method would evidently give a practised observer very good results in cases in which the adulterant was all dextro-tartaric acid, but would fail if the sample contained racemic /xvo-tartaric or inactive tartaric acid. This objection does not apply to the estimation as a potassium salt.
Oxalic acid is said to be sometimes employed as an adulterant of citric acid. This dangerous admixture would, of course, be readily detected by treating the aqueous solution of the sample with excess of ammonia, acidifying with acetic acid, filtering from any precipitated acid-ammonium, and testing the filtrate with calcium sulphate.
A method of separating tartaric from citric acid proposed by Barbet is as follows. Some fragments of the substance to be examined are spread over a thin layer of weak solution of caustic potash on a glass plate. After a few seconds, the crystals of tartaric acid become white and then opaque, and finally of almost microscopic size, while the crystals of tartaric acid remain trans parent, and partly dissolve in the alkaline solution. This difference is so strongly marked, that it is even possible to estimate the amount of each which is present. The same method may even be employed with a powder of the acids, when, however, a microscope must be used for the observa tion.
Another plan has been proposed by Dr. Hager. First, a mixture is made consisting of 4 gr, m.
of fused caustic potash, 60 cc. of water, Etna 30 cc. of alcohol at 90 per cent. This liquid is poured into a glass basin placed on a piece of black paper so as to form a layer of some 6 mm. high ; next, crystals of the acid to be tested are placed in this fluid so that they do not touch each other and are some 3 to 5 centimetres apart. After having been left quietly standing for about three hours, the crystals of citric acid will be fouud either entirely, or at least nearly, dissolved, there being left only a whitish speck where they had lain ; but the crystals of tartaric acid, if any, where present, will have been left undissolved, and covered as well as surrounded with a whitish crystalline mass.
With a view to obtaining a trustworthy and easily executed quantitative test for tartaric acid, Dr. Martenson, First Assistant in the chemical laboratory of the Pharmaceutical Institute of Dorpat, in Russia, has made many experiments. First, he ascertained by a number of tests that tartrate of lime is not nearly so soluble iu water as is commonly stated in books, but that one part of this salt requires 2388.26 parts of water at 18° (61° F.) for complete solution, and afterwards dis covered the almost absolute insolubility of the tartrate of lime in alcohol of 85 per cent. strength. In order to estimate the tartaric acid in tartrate of potash, for instance, the salt is dried at 100° and dissolved in a small quantity of distilled water ; next, pure chloride of calcium solution is added, with the precaution to avoid excess of that compound ; then a few drops of pure lime water are introduced, and the porcelain crucible in which this operation has been performed is left standing for some hours. A crystalline precipitate is thus obtained, which is collected on a filter previously dried at 100° and weighed. The supernatant fluid is first poured upon the filter, then the pre cipitate is collected and washed with strong alcohol ; the precipitate and filter are thoroughly dried at 100°, and the precipitate is weighed as + It is of great importance that the porcelain basin which is used be perfectly free from cracks in the glaze, for the precipitate would have a very strong tendency to adhere to such portions of the crucible. When either hydrochloric or nitric acids are present along with the tartaric, the fluid must first be nearly neutralized with pure carbonate of lime, and warmed to expel carbonic acid, while the last traces of acid are removed with lime water. The presence of either chloride of ammonium or of calcium in excess interferes with the correctness of the results, and makes it necessary to add alcohol to the liquor to be treated. When proper care is manifested, the results are said to be perfectly accurate.