214 GLUCOSES, SACCHAROSES, ETC. 



For the complete inversion of 1 to 1 -3 grams of carbohydrate 

 dissolved in 35 cc. of water, 5 to 6 drops of hydrochloric acid 

 (33 per cent.) were found sufficient; in the case of levulin, two to 

 two and a half hours' boiling is required, whilst triticin is com- 

 pletely converted in twenty-five to thirty minutes (Reidemeister). 



To obtain accurate results the acid must be as dilute as possible, 

 and the action stopped as soon as the inversion is complete. Here, 

 as in many other instances, hydrochloric acid seems to be prefer- 

 able to sulphuric ; considerable quantities of sugar (20 to 30 per 

 cent.) may otherwise be lost by secondary decompositions, and in 

 consequence the estimation of the carbohydrate be attended with 

 results inaccurate in the extreme. 



For the estimation of glucose by titration, see 83, 84. 



For dextrin, see also 200, 201, 202. 



GLUCOSES, SACCHAROSES, ETC. 



200. Detection of Grape-sugar. To detect grape-sugar ( 70, 

 83 to 88), Mulder 1 makes use of its reducing action on indigo. 

 The solution to be examined is rendered faintly blue with sulphin- 

 digotic acid ; carbonate of soda is then added until the reaction is 

 alkaline, and the whole boiled for a few seconds. If grape-sugar 

 is present, the colour passes from blue to violet, and then dis- 

 appears ; the liquid should not, however, be shaken, as the action 

 of the air results in the rapid return of the blue colour. 2 



Tincture of litmus, substituted by Vogel 3 for indigo, is less 

 sensitive. 



Braun 4 directs attention to the reaction between glucose and 

 picric acid, and recommends this reagent for distinguishing 

 between grape- and cane-sugar. In the presence of caustic soda 

 picric acid is converted by boiling with grape-sugar into blood-red 

 picramic acid. Fruit- and milk-sugar exert a similar action, which 

 is not shared by mannite or cane-sugar. 



For the action of grape-sugar on ferridcyanide of potassium, 

 see Gentele 5 and Lenssen. 6 The latter also discusses the reduc- 



1 Chem. Centralblatt, 176, 1861 (Pharm. Journ. and Trans. [1], xviii. 421). 



2 Compare Neubauer, Zeitschr. f. anal. Chem. i. 378, 1862. 



3 N. Kepert. f. Pharm. xi. 62, 1862. 



4 Zeitschr. f. anal. Chem. iv. 185, 1865. 



5 Chem. Centralblatt, 91, 1861 ; Stahlschmidt, Ber. d. d. chem. Ges. 141, 

 1861 (Amer. Journ. Pharm. xxxii. 81). 



6 Zeitschr. f. anal. Chem. ix. 453, 1870. 



