254 ASCORBIC ACID 



not affect the validity of the procedure of determining human nutritional 

 status in vitamin C by counting the number of days of test dosing needed 

 to bring about a state of saturation, for the amount of ascorbic acid excreted 

 on attaining saturation is out of all proportion higher than the non-specific 

 "blank correction" before saturation.'"^'' 



(2) Tin, Iron, and Sulfur Dioxide. Tin, although reacting with the dye, 

 is normally present in insufficient quantities to cause interference, but 

 sulfur dioxide and ferrous iron, both of which reduce 2 , 6-dichlorophenol- 

 indophenol, are relatively more common causes of difficulty, the former 

 by reason of its use as a preservative of foodstuffs and the latter because 

 of the possibility of its presence in stored canned fruits and vegetables and 

 in pharmaceutical products. To eliminate the effect of sulfur dioxide, re- 

 moval of the gas by means of a current of nitrogen and exhaustion in 

 vacuo has been used.''° Hydrogen peroxide has been suggested as a means 

 of oxidation of the sulfur dioxide before titration, but low values for ascorbic 

 acid have been found when this method of assay was used in the presence 

 of copper and ferrous ions.'^' Mapson proposed the use of acetone for in- 

 hibiting the effect of sulfur dioxide,''" and the reliability of this procedure 

 was confirmed by Loeffler and Ponting."^^ HueUn found that the titration 

 lacked sharpness at the pH specified (1.5) and preferred 2.5."^ 



The interference of ferrous ions may be prevented by the use of suitable 

 extractants for the sample (see p. 249), by passing an oxalic acid extract 

 through a column of an ion exchange resin, "^'^ or by treatment with hy- 

 drogen peroxide or formaldehyde before titration of the metaphosphoric 

 acid or oxalic acid extract. Chapman et al.^^^ have found the method of 

 Brown and Adam^^ to be the most suitable of several procedures tested for 

 the routine estimation of pharmaceutical products containing ferrous iron, 

 but when copper is also present they recommend the method of Roe et al.^^ 



(3) Redudones and Allied Substances. Cocoa,"^ kaffir beer,"^ yeast,^* 

 malt extracts,"^ molasses,"^ scorched and some stored dehydrated vege- 

 tables,"'' honey,^" and walnuts"^ have all been found to reduce the dye, 

 under normal conditions of test, to a degree which is not commensurate 



lo^" L. J. Harris, Lancet I, 642, 644 (1942); I, 515 (1943). 



110 L. W. Mapson, Biochcm. J. 36, 196 (1942). 



"' L. F. Levy, Nature 152, 599 (1943); Biochem. J. 37, 713 (1943). 



"2 F. E. Huelin, Australian Chem. Inst. J. & Proc. 14, 498 (1947). 



112a T. L. Parkinson, J. Sci. Food Agr. 3, 555 (1952). 



1" D. G. Chapman, O. Rochon, and J. A. Campbell, Anal. Chem. 23, 1113 (1951). 



"« F. Wokes, J. G. Organ, J. Duncan, and F. C. Jacoby, Nature 152, 14 (1943). 



1" F. W.. Fox and W. Stone, Nature 140, 234 (1937). 



"«L. J. Harris, Nature 132, 27 (1933). 



1" L. W. Mapson, J. Soc. Chem. Ind. {London) 62, 223 (1943). 



ii» R. Melville, F. Wokes, and J. G. Organ, Nature 152, 447 (1943). 



