94 SUBSTANCES SOLUBLE IN DILUTE ACID. 



meut with various solvents, a method that I published, in 1861^ 

 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 "nnith 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.) 



^ .Toiirn. f. Landwii'tlisch (May, 1862), and Pliarm: Zeitschr. f. Eussland, 

 i. 41. For the estimation of starch as gkicose 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 Nageli found that the analyses of 

 starch were more accurately expressed by the formula QC^iff)^ + HoO, than 

 by that usually adopted, viz., G^^^fi^. 



