104 METHODS OF ANALYSIS, 



(b) METHYL- VIOLET METHOD. 



Add to 5 cc of vinegar 5 or 10 cc of water; after mixing well, add 4 or 5 

 drops of an aqueous solution of methyl-violet (one part of methyl-violet 2B in 

 10,000 parts of water). The occurrence of a blue or green color indicates the 



presence of a free mineral acid. 



i 



16. Determination of Free Mineral Acids, 

 (a) HILGER'S METHOD. 



Exactly neutralize 20 cc with half-normal alkali, using sensitive neutral 

 litmus paper as indicator. Evaporate to one-tenth of its volume in a porcelain 

 dish, add a few drops of methyl-violet (see sec. 15, preceding), dilute with! 

 3 or 4 cc of water if necessary to secure a clear solution, bring to boiling, and 

 titrate with half-normal sulphuric acid until a green or blue color begins to 

 appear. The difference, ^in cubic centimeters, between the half-normal alkali 

 and the acid added, multiplied by the factor 0.1225, expresses the percentage of 

 mineral acid present in terms of sulphuric acid (H 2 SO 4 ). 



(b) HEH NEB'S METHOD. 



To a weighed quantity of the sample add excess of tenth-normal alkali, 

 evaporate to dryness, incinerate, and titrate the ash with tenth-normal acid, 

 using methyl-orange as indicator. The difference between the number of cubic 

 centimeters of alkali added in the first place and the number of cubic centi- 

 meters of acid needed to titrate the ash represents the free mineral acid present I 



17. Reducing Sugar After Inversion. 



Determine after inversion as directed under "XIII. Wine," page 87, except! 

 that it is not necessary to evaporate to remove alcohol. 



18. Polarization. 



Polarize in a 400 mm tube, preparing solution as directed for reducing 

 sugars, under " VI. General Methods," page 40. 



19. Detection of Dextrin. 



Dextrin is often found in glucose vinegar, and is precipitated from the con-J 

 centrated vinegar upon addition of three or four volumes of strong alcohol J 

 the precipitate may be identified by its physical properties and the color reac- 

 tion with iodiu solution. 



20. Detection of Coloring Matters. 



The principal coloring matter used for tinting imitation vinegars in AmerU 

 is caramel. To detect this use Another's method as given under " XV. Dh 

 tilled Liquors," page 101. 



A further test for caramel may be made by boiling the aqueous solution of a 

 portion of the precipitate obtained by Amthor's method, with Fehling's solution ; 

 caramel has a reducing action. 



In the case of wine vinegars, a test for foreign red colors may be made accord- 

 ing to the methods given under " XXVIII. Coloring Matter," page 190. 



