248 ASCORBIC ACID 



when the sample solution, mth or without preliminary treatment, is ti- 

 trated against an indicator. Among the substances recommended for this 

 purpose are 2,6-dichlorophenolindophenol (2 , 6-dichlorobenzenonindo- 

 phenol), iodine, 2,4-dinitrophenylhydrazine, methylene blue,^'"^* phospho- 

 tungstic acid,^^ phosphomolybdic acid,^^'' P'Sulfophenylhydrazine,^^'' ura- 

 nium nitrate, ^^° silicomolybdic acid,^^ potassium ferricyanide," diazotized 

 sulfanilamide,^^ ferridipyridyl,^^ and thionine.^" 



Many of these methods have received but little application, mainly 

 because of their lack of specificity, and attention has been essentially con- 

 centrated on the reagents 2 , 6-dichlorophenolindophenol, 2 , 4-dinitrophenyl- 

 hydrazine, and, to a lesser extent, iodine. The use of 2,4-dinitrophenyl- 

 hydrazine is of special interest in connection \vith the estimation of 

 dehydroascorbic acid and ^\^ll therefore be dealt with under the appropriate 

 section (see p. 251). The reaction of iodine with ascorbic acid has been 

 made the basis of several methods, and apart from direct titration with 

 iodine®^ the use of iodate®^ and similar compounds has been suggested. 

 These methods are simple and easy to use, but iodine is so relatively strong 

 an oxidizing agent that it will also react with substances, other than ascorbic 

 acid, e.g., glutathione, which may be present in the sample extract; there- 

 fore it is mainly of value in testing pure ascorbic acid or pharmaceutical 

 preparations free from interfering substances. 



In 1927 Tillmans published the results of his investigations into the 

 estimation of oxidation-reduction potentials and their application to food 

 chemistry. ^^ An especially interesting observation was the strong reducing 

 power sho\vn by fresh lemon and other fruit juices against 2,6-dichloro- 

 phenolindophenol, and Tillmans used this as a means of distinguishing 



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