250 ASCORBIC ACID 



dye with potassium iodide to produce free iodine, which is then titrated 

 with sodium thiosulfate solution, ''^■^^ 



a. Comparison between Results from Indophcnol Titration and Biological 

 Tests 



The adoption of 2 , 6-dichlorophenohndophenol as the basic indicator 

 for the chemical assay of ascorbic acid has undoubtedly been justified in 

 the case of fresh fruit and vegetables and the majority of products prepared 

 from them, and the accuracy has been confirmed by comparison of results 

 against biological test.«- 24. es-e;. 69. 76-83. ssa 



However, it was recognized during the development of this chemical 

 method that, in special cases, small amounts of the biologically active 

 reversibly oxidized form of ascorbic acid, dehydroascorbic acid, might be 

 present and would not be estimated by the indophenol method as normally 

 applied. In addition, it was appreciated that, in some exceptional instances, 

 the samples might also contain substances, not ascorbic acid, which might 

 reduce the dye. Although in general, therefore, the 2,6-dichlorophenolindo- 

 phenol method can be used with confidence for the estimation of ascorbic 

 acid, suitable modifications have to be made in these exceptional cases 

 where either dehydroascorbic acid or interfering substances are present. 

 These possible complications have been studied in some considerable de- 

 tail by many workers, and various alternative modifications have been 

 proposed. 



b. Dehijdroascorbic Acid 



In canned and fresh fruits and vegetables, dehydroascorbic acid is 

 usually present only in very small amounts or else in an unstable con- 

 dition.^- ^^^'^'^ Under some conditions, however, it would appear that signifi- 

 cant quantities of dehydroascorbic acid may be produced.^^*- ^^' ^^ 



" R. E. Buck and W. S. Ritchie, Ind. Eng. Chem. Anal. Ed. 10, 26 (1938). 



'* H. Dick, Ind. Eng. Chem. Anal. Ed. 11, 293 (1941). 



" M. H. Menaker and N. B. Guerrant, Ind. Eng. Chem. Anal. Ed. 10, 25 (1938). 



'6 L. J. Harris and S. N. Ray, Biochem. J. 27, 580 (1933). 



" H. Lund, B. Spur, and L. S. Fridericia, Biochem J. 28, 1825 (1934). 



'8 L. F. Levy and F. W. Fox, Biochem. J. 29, 884 (1935). 



78 A. E. Kellie and S. S. Zilva, Biochem. J. 30, 1216 (1936). 



80 M. Olliver, J. Soc. Chem. Ind. 55, 153T (1936). 



81 E. Mathiesen and V. Aschehoug, Arch. Math. Naturvidenskab 41, No. 8 (1937). 



82 L. J. Harris and L. W. Mapson, Brit. J. Nutrition 1, 7 (1947). 



83 L. J. Harris, L. W. Mapson, K. E. Kodicek, T. Moore, and V. H. Booth, Proc. 

 11th Intern. Congr. Pure and Appl. Chem., London (1947). 



831 E. M. Hewston, M. Fisher and E. Orent-Keiles, U. S. Dept. Agr., Tech. Bull. 



1023, 30 (1951). 

 s^W. Stone, Biochem. J. 31, 508 (1937). 



