258 ASCORBIC ACID 



quently described a method whereby the apparatus was used with a number 

 of fruit and vegetable extracts.^^ It was found that good agreement was 

 obtained between results from the polarograph on the one hand and the 

 visual titration and photometric methods on the other, although with some 

 materials, e.g., dehydrated potatoes, the colloidal content of the extract 

 interfered. It was recognized that the method failed in the presence of a 

 high content of interfering substances, but since their presence can be 

 detected by inspection of the anodic wave, it was claimed that the method 

 would, in this respect, appear to have advantages over the "straight" 

 titration methods. Various modifications along the lines adopted for the 

 keto steroids have been suggested for improving the accuracy of the method 

 of assay. '^'* At present, however, the use of the polarograph for the estima- 

 tion of ascorbic acid is limited, since, for instance, Mackenzie has noted that 

 changes in viscosity produced by substances in true solution significantly 

 affect the diffusion currents. ^'^^ Polarography seems to have no advantage 

 over the simpler method of chemical titration, not even as regards 

 specificity. 



Spectrophotometric methods have also been suggested, although it has 

 been shown that, beyond specified limits, the intensity of absorption is not 

 directly proportional to the concentration of ascorbic acid.''*® Another dis- 

 advantage is that the well-defined absorption band of ascorbic acid is 

 subject to interference from many other substances in this region. Johnson 

 attempted to overcome this by determining the characteristic absorption 

 spectra before and after destruction of the ascorbic acid with cuprous ions.'*^ 

 The same principle has been adopted by other workers, but using means 

 other than cuprous ions for destruction of the vitamin, e.g., irradiation'^^- '^^ 

 and ascorbic acid oxidase. '^^ The effect of pH in altering the position of the 

 absorption band of ascorbic acid, without necessarily moving those of the 

 interfering substances, has been used as another means of spectrophoto- 

 metric assay, measurement being made at two or more levels of pH.'^'- '^- 



A spectrophotometric method which is based on the color reaction be- 



"2 T. Psterud, Tek. Ukeblad 86, 216 (1939). 



'« D. Cozzi, Ann. chim. appl. 29, 4.34 (1939). 



'" J. E. Page and J. G. Waller, Analyst 71, 65 (1946). 



'" H. A. McKenzie, J. Council Set. I rid. Research 21, 210 (1948). 



i« R. W. Herbert, E. L. Hirst, E. G. V. Percival, R. .J. W. Roynolds, and V. Smith. 



/. Chem. Soc. 1933, 1270. 

 i« S. W. Johnson, Biochem. J. 30, 1430 (1936). 



1" A. Chevallier and Y. Choron, Bull. soc. chim. biol. 19, 511 (1937). 

 '■*" A. Chevallier and Y. Choron, Compt. rend. soc. biol. 124, 453 (1937). 

 "« A. Fujita and T. Sakamoto, Biochem. Z. 297, 10 (19.38). 

 '5' M. Vacher and D. Faucquembergue, Bull. soc. chim. biol. 31, 1419 (1949). 

 1" C. Daglish, Biochem. J. 49, 635 (1951). 



