RELATIONS TO RACEMIC ACID.

437

are formed. This new acid is identical with the paratartaric or racemic acid: it has no longer the power of causing the rotation of a ray of polarized light, and it presents differences in its chemical reactions (1322) from those of both of its components.

Yet, if the acid thus obtained be divided into two equal parts, one half being neutralized with soda, then mixed with the other half of the acid and neutralized with ammonia, the solution on evaporation will yield crystals, each of which has a powerful rotatory action on polarized light; but the crystalline mass will be found to consist of a mixture of the right-handed and of the left-handed ammonio-sodic tartrate, in equal proportions. The crystals of the two salts are recognized by their opposite hemihedral form, and may be separated by hand. If all the righthanded crystals be dissolved separately in water, and decomposed by adding a solution of nitrate of lead, a precipitate consisting of the dextrotartrate of lead will be deposited. From this precipitate the dextrotartaric acid may be separated by means of sulphuric acid, and may be obtained in crystals on evaporation if the lævohemihedral crystals be treated in a similar manner, lævotartaric acid may also be procured.

:

It was by treating the natural ammonio-sodic racemate in this manner, and picking out each crystal separately, that Pasteur was enabled to prove the existence of the two modifications of tartaric acid, and to show that racemic acid consists of a mixture of two acids which exert opposite effects of rotation on polarized light.

Very few of the salts of racemic acid thus spontaneously separate into two different hemihedral varieties of crystalline form; the racemate of cinchonicine (1393) is easily separable into its component crystals, and the racemate of quinicine also exhibits a similar property. These salts may therefore be employed for the purpose of isolating the two modifications of tartaric acid when they occur in combination: at a certain stage of concentration of the racemate of cinchonicine the first crop of crystals consists chiefly of the lævotartrate. When racemate of quinicine is employed, the crystals which are first deposited consist mainly of the dextrotartrate.

(1319) ORDINARY TARTARIC ACID, or Dextroracemic Acid (H,CHO, or or 2 HO,C,HO10=150).-The principal supply of this acid is obtained from the grape; but it is also present in the tamarind, and in the unripe berries of the mountain ash; it is likewise frequently met with in small quantity in other plants. Liebig also found it in the acid mother-liquor obtained in pre

438

DEXTRO-TARTARIC ACID, OR

paring mucic acid from sugar of milk and from gum. Mannite and dulcite, according to Carlet, yield a portion of racemic acid when oxidized by nitric acid. Grape juice contains dextrotartaric acid in the form of acid tartrate, or, as it is sometimes called, supertartrate of potash (hydropotassic tartrate), constituting the crust or tartar which is deposited in the vessel in which the wine is kept.

Preparation. In order to prepare the acid, crude tartar is dissolved in boiling water, and chalk is added so long as effervescence occurs, 4 parts of tartar requiring about I part of chalk; an insoluble calcic tartrate is thus precipitated, and neutral potassic tartrate remains dissolved. Tartaric acid being dibasic, the change which occurs may be thus represented:-Two atoms of hydro-potassic tartrate (bitartrate) react upon one atom of calcic carbonate, and form one atom of neutral calcic tartrate, and one of dibasic neutral potassic tartrate; thus :

:

Potassic tartrate.

The potassic tartrate in solution may be decomposed by the addition of an equivalent quantity of calcic chloride, and the whole of the tartaric acid may thus be separated as calcic tartrate:

The product of the two operations is well washed, and digested with oil of vitriol diluted with 6 or 8 times its weight of water; for every 5 parts of tartar 3 parts of concentrated sulphuric acid are required; the solution is allowed to digest at a gentle heat, calcic sulphate is formed, and tartaric acid set free. When cool, the liquid is filtered, evaporated in leaden vessels to the consistence of syrup, and allowed to crystallize; the formation of crystals is favoured by the presence of an excess of sulphuric acid.

Properties. The acid thus obtained assumes the form of oblique rhombic prisms; when pure they are colourless, transparent, and permanent in the air. Tartaric acid is very soluble in water, in alcohol, and in wood spirit; it has a sharp agreeable acid taste. If heated with the caustic alkalies, water is expelled, and oxalate and acetate of the basyl are formed :—

Its aqueous solution becomes mouldy when long kept, and is slowly converted into acetic acid.

ORDINARY TARTARIC ACID.

439

The crystals of tartaric acid, when gently heated, become as strongly electrical as those of tourmaline. Its solutions, particularly when hot, exert a powerful right-handed rotation upon a ray of polarized light. Powdered tartaric acid furnishes with a large excess of nitric acid (sp.gr. 1 ̊5) a nitro-acid, [ЄH ̧(NO2)1⁄2Ð ̧], soluble in absolute alcohol, from which it may be obtained in crystals by spontaneous evaporation, but it is very unstable. When tartaric is boiled with nitric acid, oxalic acid is formed, together with an acid, the tartronic (H,Є,H,,), homologous with the malic. Tartaric acid exhibits considerable tendency to combine with oxygen. If it be boiled with peroxide of lead it undergoes decomposition, water and carbonate of lead being produced, whilst formiate of lead remains in the solution :

Tartaric acid.

2 41

6

Formiate of lead.

2

2

Carbonate of lead.

3

H,€ H Ꮎ + 3 PbᎾ, = Pb 2 CHO, + 2 PbCO + 2 H,O. The salts of silver, of gold, and of platinum, are also reduced when boiled with tartaric acid.

Tartaric acid is extensively used by the calico-printer and dyer for the removal of certain mordants from particular portions of the cloth. It is also used in the preparation of effervescing draughts with the acid-carbonates (bicarbonates) of the alkali metals.

An interesting relation exists between tartaric and succinic acids, in consequence of which tartaric may be reduced to succinic acid by saturating its concentrated aqueous solution with hydriodic acid, sealing it up in a strong glass tube and heating it, for six or eight hours, to a temperature not exceeding 248° (120 C.). (Schmitt, Liebig's Annal. cxiv. 109):—

H,CHO + 4 HI = HỌC,H,O + 2 + 2 HO ;

4 6

Ꮎ I, 2 Ꮎ ;

2

and succinic acid may, in turn, be reconverted into tartaric acid, by changing it first into dibromosuccinic acid, and then decomposing this compound with oxide of silver (Perkin and Duppa; 1303). Dessaignes, by treating tartaric acid with a mixture of phosphorus and iodine, also procured succinic acid, and at the same time obtained malic acid in the mother-liquor, the tartaric acid losing first I atom of oxygen, thus furnishing malic acid, and this acid in turn losing another atom of oxygen, thus yielding the succinic:

(1320) Tartrates.-The tartaric is a dibasic acid,* and it consequently has a strong tendency to form double salts, three varieties of which may be distinguished :

1. Salts formed from protoxides only; such as

Rochelle salt

Cream of tartar

Potassic cupric tartrate

4

6'

KNa€ H Ꮎ, 4 H, Ꮎ кнен 4 6

K,Єu 2 €,H ̧ ̧.

2. Salts formed from both protoxides and sesquioxides, but in which the sesquioxide is combined with the same proportion of acid as the protoxide; such as

The salts of this third class, with the exception of the compound with arsenious acid, are remarkable for the property which they possess, when apparently anhydrous, of losing an additional atom of water, so as to present a composition which may be regarded as exhibiting a certain resemblance to that of tartarie anhydride (Gerhardt): Sb→ (or its equivalent representative, BO) being able to occupy the position of 1 atom of hydrogen in the compound, for example:

It is, however, probably formed on the tetratomic type, the atomicity and basicity being different, as in the case of the glycolic group, which would account for the anomalous effect of heat in expelling an atom of water from certain of the anhydrous tartrates, as will be mentioned immediately; H2H, being probably in the type, or 20, the normal tartrates, such as potas

H,K,
e.

sic tartrate, being HK Tartaric acid might then bear to erythrite (1243) the same relation that glycolic acid bears to glycol, in which two atoms of hydrogen in the radicle are displaced by one atom of oxygen; in erythrite the exchange would be of H for ; this reaction has not, however, as yet been actually effected:

4

4

8 4

441

Potassic Tartrate (K,,H,,,=226) crystallizes readily; it is somewhat deliquescent and very soluble. The hydropotassic tartrate, or bitartrate of potash, (KHC,H,, or KO,HO,C,H2O10= 188) is the ordinary source of the compounds of tartaric acid: it constitutes the crude tartar or argol of commerce, and is gradually deposited from all wines, forming a crust upon the inside of casks in which the wine is stored. This salt is very sparingly soluble in cold water, of which it requires about 180 parts for solution; boiling water dissolves about one-sixth of its weight of it, and deposits the salt in oblique rhombic prisms, which when pure are of snowy whiteness, forming common cream of tartar. It produces a gritty sensation under the teeth, and has a sour taste. When heated to redness in covered vessels the acid is decomposed, and a charred mass remains, consisting of potassic carbonate and unconsumed carbon; this product is often used in the laboratory under the name of black flux. If the salt be calcined with twice its weight of nitre, white flux is obtained in this operation the carbon of the organic acid is completely burned, and potassic carbonate is left.

4

4 4

;

be

Sodio-potassic Tartrate (KNa¤ ̧н ̧Ð ̧, 4 H2, or KO,NaO, CH2O10, 8 HO=210+72).—The atom of basic hydrogen may displaced from cream of tartar by sodium, and then a double salt, often called Rochelle salt, is produced: it forms large beautiful crystals which have the appearance of six-sided prisms, one-half of which only is developed. Hydrosodic tartrate (NaH¤ ̧H ̧ ̧, H2O) is a soluble salt, and is sometimes employed to separate potash from soda in concentrated solutions. The tartrates of calcium (Єa¤ ̧H ̧Ð ̧, 4 H ̧Ð), barium, and magnesium, are sparingly soluble. Iron forms with potassium and ammonium double tartrates which are used in medicine: the potassium salt when dried at 2120 is anhydrous: the ammoniacal salt has the formula (H2N,Fee,,H ̧Ð ̧, 2 H ̧Ð).

4

6'

Potassic Borotartrate; Soluble Tartar (K,B,C,H; at