94 



SUBSTANCES SOLUBLE IN DILUTE ACID. 



ment with various solvents, a method that I published in 186 1 1 

 may be adopted by which the substances that accompany the 

 starch are removed. The powdered material is mixed with 

 30 parts of a 4 per cent, solution of caustic potash in alcohol, and 

 heated to 100 for a day or two in a well-closed flask. After 

 filtering and washing with spirit till free from alkali, the substance 

 on the filter is exhausted with water; and to effect this it is 

 advisable to transfer it to a beaker. The residue insoluble in cold 

 water is boiled with dilute hydrochloric acid, and treated as 

 directed in 113. The caustic potash acts upon the foreign sub- 

 stances which interfere with the direct estimation of the starch, 

 rendering them soluble partly in alcohol, partly in water, whilst 

 the starch itself is not attacked. (See 243.) 



1 .Tourn. f. Landwirthsch (May, 1862), and Pharm. Zeitschr. f. Russland, 

 i. 41. For the estimation of starch as glucose after the action of dilute 

 sulphuric acid, see Musculus, Chem. Centralbl. 1860, p. 602 (Am. Journ. 

 Pharm. xxxii. 433) ; and Philipp, Zeitschr. f. anal. Chem. N. F. iii. 400. Sachsse 

 (Zeitschr. f. anal. Chem. xvii.231, 1878; Year-book Pharm. 1878, 97), has shown 

 that the inversion is better effected by hydrochloric acid 1 per cent, of the 

 weight of the liquid. Both Sachsse and 'Niigeli found that the analyses of 

 starch were more accurately expressed by the formula 6C 6 H 10 5 + H 2 O, than 

 by that usually adopted, viz., C 6 H 10 5 . 



