ESTIMATION OF TANNINS AND GLUCOSIDES 151 



III. EXTRACTION WITH DILUTE ACID, OR DIGESTION WITH 

 MALT. The material from II. should be washed well with water 

 and treated with i per-cent. sulphuric acid at a temperature of 

 100 C 



In this extract are the products of the hydrolysis of starch, 

 disaccharids and other carbohydrates. The extraction should be 

 continued until the solution fails to give any reaction with iodine. 

 Reducing sugars with possibly a small quantity of dextrin should 

 be the final product. If erythrodextrin is present it will be indi- 

 cated by a red coloration upon the addition of iodine, but achro- 

 odextrin, which is a later stage in the hydrolysis of starch, gives 

 no color with that reagent. 



Instead of boiling with sulphuric acid, the same results may be 

 obtained by digesting the residue with a solution of malt diastase, 

 at a temperature not to exceed 60 C. 



The malt extract may be made by extracting 50 grams of pul- 

 verized malt, which has been dried at a low temperature, with 300 

 cc. of water. 100 cc. of this solution should be used with every 

 10 grams of the material to be digested. As in the case of the 

 extraction with acid, the same tests should be applied to deter- 

 mine the end of the digestion. 



222. Estimation of Tannins and Glucosides. Tannins are 

 slightly acid, amorphous, colorless substances, soluble in alcohol, 

 ether, insoluble in benzine, benzol, chloroform, and oils, giving 

 blue or green precipitates with salts of iron. All are precipitated 

 by gelatin or albumin. Tannic acid is the principal constituent 

 of tannins and it occurs almost pure in nut-galls. 



The alcoholic extract (II.) should be evaporated and the residue 

 dissolved in water and filtered. The reaction to litmus should be 

 tested and if the solution is acid should be carefully neutralized 

 with dilute sodium carbonate. 



Test portions of the neutral solution with the following sub- 

 stances : 



i . Solution of gelatine. Tannins indicated by dirty white pre- 

 cipitate. 



