ASTRINGENTS.

5°3

When submitted to a temperature of about 210° it is decomposed, yieldingcarbonic and pyrogallic acids; at a higher temperature water, carbonic acid,and gallohumic acid are formed.

Gallic acid is dissolved by concentrated sulphuric acid ; but since the gallicacid is hereby deprived of its water of constitution, it is simultaneously con-verted into rufigallic acid, C 7 H 4 0 4 .

Pyrogallic acid sublimes in white scaly crystals, and is chiefly remarkablefor its rapid absorption of oxygen when in alkaline solution, being therebyconverted into substances belonging to the ulmic series.

Since the goodness of galls and other astringent materials depends on thequantity of tannin they contain, it is of importance to have a method ofrapidly estimating that quantity. M. Fehling has proposed, for this purpose,the application of a standard solution of gelatin containing 10 grms. to thelitre: a proof test is made, in order thereby to estimate with precision theweight of the tannin which is precipitated by 1 c.c. of this liquid. The waresare tested by exhausting 1 grm. with pure distilled water, and from the bulkof gelatin solution employed the quantity of tannin is calculated : the tanninsolution should be slightly acidulated or mixed with a little alum, since thispromotes the rapid precipitation of the tanno-gelatin compound ; instead ofgelatine solution, a solution of tartar emetic or of sulphate of cinchonin canbe employed.

Dr. Gunther, of St. Petersburg, has recently investigated the differentanalytical methods in use for the determination of the quantity of tannic acidpresent in galls, catechu, ratanhia, kino, and other drugs. In an exhaustivetreatise* are described the various methods which have been practically testedby the author with great care; as a general result arrived at by him, we maystate here that none of the different methods answer the purpose completely,that is to say, that a method which yields excellent results with nutgalls isnot equally suitable for sumac, catechu, &c.; for the good reason, that thesematerials contain either other substances along with tannic acid or specificmodifications of the same.

M. Mittenzwei proposes to estimate the tannin in nut-galls by determiningthe volume of oxygen absorbed in the presence of an alkali:—1 grm. of tanninabsorbs 175 c.c. of oxygen ; gallic acid is thus formed, 07 grm. of whichabsorbs also 175 c.c. of oxygen ; since galls contain, as above stated, bothgallic and tannic acids, the total absorption of oxygen by these together is firstdetermined, and that of the gallic acid present is next estimated in a freshportion of the galls after the tannic acid has been removed therefrom by a pieceof skin. For commercial purposes this method is utterly out of the question.

Lowenthal oxidises the tannin by means of permanganate of potash orbleaching lime in standard solution, an' excess of sulphate of indigo beingadded. The tannin and indigo are oxidised simultaneously, and with thedisappearance of the colour the operation is completed. The articles requiredfor the process are a standard solution of pure tannin, containing in 1 c.c.O'Oor grm. of tannin dried at 212 0 F.; a dilute solution of the best extradtof indigo, containing about 30 grms. of the paste per litre ; and a clearsolution of chloride of lime, which must be preserved in a dark place. Thefirst step is to standardise the solutions. To this end 20 c.c. of the indigo-

“ Pharmaceutische Zcitschrift fur Russland,” No. 6 and following, 1870.