H. J. H. Fenton says:in a recent number of the Chemical News,' that he has lately noticed the following reaction, which may, so far as he can judge at present, be proposed as a test for tartaric acid. To a very dilute solution of ferrous sulphate or chloride, a small quantity of a solution of tartaric acid or a tartrate is added, followed by a few drops of chlorine water or hydric peroxide, and lastly, excess of caustic potash or soda, when a fine violet colour is obtained. Fenton tried the same experiment with citric, succinic, malic, oxalic, and acetic acids, and with sugar, but with out getting a similar result. If a. ferric salt he used instead of a ferrous salt, the colour is not produced. The violet compound formed seems to be potassic or sodic ferrate. It is destroyed at once by sulphurous acid, and is slowly discharged by boiling. Success has not attended attempts to obtain the higher oxides of manganese and chromium in the same way.
Janette and Pontieves have also paid much attention to this subject, and to them the fol lowing observations aro due.
Of all acids, tartaric and mail° acids alone possess the known property of rendering iron, alumina, manganese, rte., soluble in alkaline liquids. Peroxide of iron in acid solution, containing neither tarlitric nor mane acids, is precipitated as soon as the liquid is neutralized by ammonia. If, on the contrary, iron and tartaric acid be mixed in determinate proportions, or if the tartaric acid be in excess, there will be produced, after saturation with ammonia, a tartaro-ferric ammoniacal composi tion of a fine red colour, soluble in acid or alkaline liquids, provided they do not contain any of the alkaline-earthy metals. The study of this phenomenon has led to a method of estimating either tartaric or malio acids with a standard solution of iron and aluminium, or of these metals by a standard solution of crystallized acid. A given weight of pure iron is dissolved in nitric acid, which is then diluted with distilled water to form a standard liquor containing or of iron. If to a solution of 100 milligrammes of iron 45.5 milligrammes, or any larger quantity of tartaric acid be added, and also 1 or 2 cc. of common ammonia to render the liquid decidedly alka line, the product will he, after vigorous stirring, a red liquid, at first thick, but which, when left to itself, becomes and remains limpid. If, on the contrary, to 100 milligrammea of iron be added 45 milligrammes or more of tartaric acid, and then an excess of ammonia, &c., the liquid, thick at first, deposits the characteristic precipitato of peroxide of iron. The soluble compound produced by a proportion of tartaric acid equal to or exceeding to 100 is permanent in the presence of acids, alkalies, and alkaline carbonates, provided they be oxempt from lime, and also in the presence of ammoniacal salts, alcohol, ether, &c. If the compound be heated to ebullition the iron is almost
entirely precipitated ; this may also be done by adding to the liquid, some hours afterwards, ordinary water containing calcareous salts. Io practice grammes of the substance to be aasayed aro dissolved in acidulated water, which is then diluted with common water to form a dt terminato volume, such as, for instance, 100 cc. ; 10 cc. are deducted, and according as the matter contains 1, 2, 3, n hundredths of tartaric acid, 1, 2, 3 .... n milligrammee of iron may be added, which will remain undissolved. Thus, in a simple manner, by two trials, two different results aro ob tained, viz.:— With n milligrammes of iron limpid solution, „ n I „ .. precipitate, n is the number of hundredths of tartaric acid contained in the substance.
The estimation of tartaric acid in crystallized bitartratea and neutral tartrates gives to nearly the proportion of tartario acid indicated by the formula.
Tartaric acid crystallizes with two molecules of water in large, colourless, transparent, oblique prisms of highly sour yet agreeable taste, which melt at 170° (338° F.). They dissolve easily in water, which solution turns mouldy by keeping, then yielding a minute proportion of acetic acid. A remarkable feature of the acid conaists in its turning the plane of polarization of a ray of light towards the right, the degree of the angle being exactly in proportion to the acid through which the ray passes.
Sulphuric acid gently heated with tartaric acid converts it into tartralic acid, and tartrelio acid but at a strong heat carbonic oxide and sulphurous acid only ere produced. By the action of nitric acid, acetic, oxalic, and saccharine acids are formed.
Simple heat is capable of exerting remarkable influence on this acid. At a temperature from 170° to 180° (338° to 356° F.) it becomes first metatartaric acid, an isometric acid with itself, but differing from it in forming a gummy transparent mass, which ultimately yields opaque crystals. If the heat be continued for a short time a certain amount of water is disengaged, and the malting acid is known as tartralio, isotartaric or bitartaric. It is very soluble and quite uncryetallizable, as well as the salts which it forms, which latter are all soluble in water.
By maintaining the temperature for a still longer time a further proportion of water escapes, and the monobasic, tartrelic, soluble anhydrous or dehydrated tartaric acid is obtained. No more water will be liberated by continuing the heat beyond this point, but the anhydrous acid is rendered insoluble and neutral. This, when heated suddenly and very strongly, resolves itself into carbonic oxide, carbonic acid and acetic acid.