Rufocatechucic acid or rubinic add is a red, amorphous, flocculent precipitate, slowly deposited from a solution of tanningenie acid in an alkaline carbonate. It forms red slightly soluble salts with bases.
Japonic add is a black substance formed on exposing to the air a solution of tanningenic acid in caustic potash. It is insoluble in cold water, and gives black precipitates with most of the metals.
Cafotannic, cafeic, or chloroginic acid I), occurs in coffee berries as a double salt of potash and caffeine, and combined also with lime and magnesia. It may be precipitated by subacetate of lead from an alcoholic infusion to which water has been added to separate resinous matter. The lead precipitate suspended in water and treated with sulphuretted hydrogen, and the filtrate evaporated, yields a semi crystalline mass of caffotannic acid. It is very soluble in water, less so in alcohol, has an astringent taste, and strongly reddens litmus-paper. By heat it gives oxyphenic acid and an odour of burnt coffee ; aud by distillation with peroxide of manganese aud sulphuric acid, yields kinone. It does not act upon ferrous salts, but to the ferric salts a green colour is imparted. It does not precipitate tartar emetic or gelatine, but throws down quinine and cinchonine from solutions. The cafotannates have been but little studied. If the above formula be correct, caffotannic acid is obviously a homologue of gallotannic acid, the difference being eight equivalents of C.11,.
Viridic acid is a product of the oxidation of caffotannic acid in the presence- of ammonia. It is precipitable by subacetate of lead, and gives deep green solutions with the alkalies.
Morintannic acid, from yellow wood or fustic (Merits tinctoria). This acid often forms considerable deposits in the logs.of fustic, and after being purified by crystallisation from water is obtained in minute prisms. It has a sweetish and yet astringent taste ; is soluble in alcohol, ether, and wood spirit, but insoluble in turpentine and the fixed oils. By distillation it yields phenio and oxyphenic acids and much charcoal. The morintannates have not received much attention. The solution of the acid in potash absorbs oxygen from the air, blackens, and morarylic acid is probably formed.
Rufinioric acid is deposited in crystalline grains on exposing to the air a solution of morintannic acid in sulphuric acid ; or on boiling the same acid with dilute hydrochloric acid. When dry it forms a deep red amorphous powder, very soluble in alcohol, less so in water, and only slightly so in ether. It is very soluble in a weak solution of ammonia, and the liquid has a dark purplish colour. In composition it differs but slightly from morintannie acid, but both bodies require farther investigation.
Mork add, or ?florin 2 Aq.), as a lime salt, is deposited on cooling an infusion of fustie. Liberated by oxalic acid in a boiling alcoholic solution and precipitated by water, it occurs as a white crys talline powder that becomes slightly coloured on exposure to the air.
It is almost insoluble in cold water, only slightly soluble in boiling water, and very soluble in alcohol or ether.
Quercitannic acid. This is the variety of tannic acid contained in oak bark. According to Stenhouse it differs considerably from that of oak galls (gallotannic acid). It cannot be made to yield gallic acid, nor does it give pyrogallio acid by dry distillation. In other respects it resembles gallotannic acid.
The tannic acid (boheic acid) contained in black tea (Mu bohca) ap pears to be identical with that of oak bark.
Cinchotannic, kino- or quino-tannic acid, occurs, along with kinic acid, combined with alkaloids in the cinchona barks. It resembles gallo tannic acid in precipitating gelatin, starch, and albumen from their solutions ; with tartar-emetic it gives a grayish-yellow precipitate, and colours persalts of iron green. Its solution absorbs oxygen from the air, especially if it be alkaline, and a deep red coloured body termed red cinchone is formed. By dry distillation the latter body furnishes pyrogallic acid.
Other tannic acids. Astringent principles, termed tannic acids, have been found in plants other than those above described. The following are the names and sources of these acids, but their individuality has yet to be established. Aspertannic acid, from the Asperula otiorata, or lrabitannic acid, from the leaves of the .Madder plant ; Callstansic arid, in the common Heather ; Illtodutannic add, in the Rhododendron leaf; Leditantlic arid, in the wild Rosemary ; aud Pita lassie arid, in the bark of various Pines. Gallerythronic add has been referred to this article on tannic acids; it is, however, a derivative of genie acid, and is the cause of the deep blue coloration Imparted to an aqueous solution of gallio acid, containing bicarbonate of lime, on exposure to the air : acids change it to red, but lime restores its blue colour.
Estimation of tannic acids. The amount of actual tanning material contained in a bark, fie., is often required to be known by the termer. At present the only practical means of doing this, is to add a standard solution of gelatin to the decoction of a small quantity (10 to 50 grains) of the tanning substance until no further precipitate of tanuato of gelatin is thrown down. The solution of gelatin is standerised by a known quantity of pure gallotannio acid; the amount of it required to precipitate the acid in the specimen under examination, is supposed to indicate the tanning value of that sample. It is a question, however, whether gelatin in solution precipitates the whole of the matters that skin does in the taupit ; if it does, the process may be relied on, and it is certainly fair to assume that it does, inasmuch as skin is nearly all gelatinoid matter.
The following table indicates the amount of tannic acid in the chief varieties of tanning material. Not only do different substances differ much in the quantity they contain, but the same plant is much in fluenced by climate, soil, and age.