212 THE VITAMINS 



Doubtless this is due in part to the lowering, during the period of 

 experimental heating, of the initial oxidation potential, through the con- 

 sumption or expulsion of the residual dissolved oxygen or the destruc- 

 tion of labile oxidizing substances, or both. 



Effect of Reduced Concentration of Hydrogen Ions {Hydrogen-ion 

 Activity). — In experiments in which the natural acidity of the tomato 

 juice was first neutralized in whole or in part, the juice then boiled for 

 one hour and immediately cooled and reacidified, it was found that at 

 pH 5.1 to 4.9 the destruction during one hour's boiling was increased 

 to 58 per cent (as compared with 50 per cent at natural acidity, pH 4.3). 

 Neutralization of a larger proportion of the natural acidity increased 

 the rate of destruction of the vitamin. When alkali was added to an 

 initial pH of 11, which fell to about pH 9 during the hour of heating 

 (doubtless because of reaction of the alkali with the sugars, proteins, 

 etc., present), the destruction found by feeding of the juice thus 

 treated, but immediately cooled and reacidified, was about 65 per cent. 



Heating at 100° C. for one hour at pH 11 to 9 as described above, 

 followed by standing for one to five days in stoppered but only partially 

 filled bottles in a refrigerator at 10° C. at an alkalinity of only pH 9 

 was found to destroy 90 to 95 per cent of the antiscorbutic vitamin, 

 as compared with 65 per cent when the solution was reacidified after 

 heating ; thus confirming, extending and bringing into more quantitative 

 form the observations of Harden and Zilva (1918d) and of Hess and 

 Unger (1919) upon the susceptibility of this vitamin to alkalinity even 

 at low temperatures. 



Effect of Oxidation and Reduction. — In all of the foregoing experi- 

 ments the heating was performed in cotton-stoppered, narrow-necked 

 flasks from which air was probably very largely displaced by water 

 vapor early in the heating. When samples of the juice at pH 3 and 8.3 

 were heated at 100° C. for one hour with oxygen bubbling through 

 the liquid, practically complete destruction of vitamin C took place in 

 both acid and alkaline preparations. Kenny (1926), continuing the work 

 of La Mer, Campbell and Sherman, showed that when the tomato juice 

 of natural acidity was heated in a most rigorously maintained anaerobic 

 environment, the previously observed rate of destruction was reduced 

 by about two-thirds ; and by strictly parallel experiments with the juices 

 of tomato and cabbage, he showed that the more rapid destruction of 

 the vitamin C of the latter could be correlated with, and is presumably 

 a function of, its higher oxidation potential. At pH 4.2 he found 

 En=^ +0.137 volts for tomato juice and En-= -\- 0.362 volts for cab- 

 bage juice; at pH 6.2, Ea = + 0.045 volts for tomato juice and 



