ESTIMATION OF STARCH 105 



5. Starch is precipitated from its aqueous solution by alcohol 

 or by basic lead acetate (cf. Inulin and Dextrin). 



6. Potash causes the grains to swell and finally to dissolve. 



7. Boil a little starch paste solution with a few drops of 

 dilute sulphuric acid in a test tube, and from time to time 

 remove a little of the solution, cool it and test with iodine 

 solution ; when the starch has been converted into dextrin the 

 blue colour at first formed will give way to a plum colour. 

 If boiled too long only dextrose will remain which gives no 

 colour with iodine. The solution will, however, after making 

 alkaline, reduce Fehling's solution. 



8. Cautiously heat a little starch on a porcelain basin 

 until it has acquired a light fawn colour. Cool and extract 

 with cold water, and filter ; the dextrin produced being soluble 

 in cold water is thus separated from the starch. On adding 

 iodine to the solution a plum colour is produced. 



Estimation of Starch. 



A colorimetric method of estimating starch, depending on 

 the depth of the blue colour produced with iodine, has been 

 described by Dennstedt and Voigtlander.* 



The following method depending on the hydrolysis of 

 starch by hydrochloric acid and the subsequent estimation of 

 the glucose produced, is only reliable if there are no pentosanes 

 or other substances present which on hydrolysis would yield 

 reducing sugars. 



About 3 grams of the substance in as fine a state of divi- 

 sion as possible are covered with 50 c.c. of cold water and 

 shaken at frequent intervals ; after an hour the insoluble 

 portion is filtered off and washed with water until the total 

 filtrate measures 250 c.c. ; the addition of a little alumina shaken 

 up with water will frequently facilitate clear filtration. The 

 soluble carbohydrates contained in the filtrate may if de- 

 sired be determined both before and after inversion. 



The residue remaining on the filter paper is then transferred 

 to a flask with a 250 c.c. graduation mark and heated for two 

 and a half hours under a reflux condenser with 200 c.c. of water 

 and 20 c.c. of hydrochloric acid (sp. gr. 1*125). After cooling, 



* Dennstedt and Voigtlander : " Ber. deut. chem. Gesells.," 1895, 28, R., 

 1025. 



