ESTIMATION OF STARCH 121 



cedure may be followed. A section of the material to be 

 examined is cut, and is first treated with a fairly dilute solu- 

 tion of iodine in potassium iodide, the excess of the reagent is 

 then removed, and the section is irrigated with a concentrated 

 aqueous solution of chloral hydrate. This causes the starch 

 grains to swell, and at the same time the other cell contents 

 are dissolved, as are also the starch grains in time. 



3. For microscopic work, the action of dilute aqueous 

 solutions of gentian violet and of safranin is sometimes useful, 

 as they stain the colloidal parts more deeply. 



4. Starch grains are insoluble in cold water, but in hot 

 water they swell up and form an opalescent solution which, if 

 strong enough, will on cooling eventually form a paste. 



5. Starch is precipitated from its aqueous solution by 

 alcohol* or by basic lead acetate (cf. Inulin and Dextrjn). 



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.f 



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 



* Due, according to Harrison (loc. cit.), to the presence of inorganic salts, 

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

 1025, 



