lOO 



Journal of Agricultural Research 



Vol. XV, No. 2 



much above that at the start. This value remains constant at the lower 

 temperature and is directly proportional to the lowering of the freezing 

 point. On thawing there is then a decrease in the hydrogen-ion concen- 

 tration to a value less than the original, and then a return to the original 

 value, or a value somewhat lower than the original. This change is almost 



paralleled by the in- 



creased acidity of a di- 

 lute solution of acid 

 calcium phosphate, 

 Ca(H2P04)2, on freez; 

 ing. The graph for the 

 plant juice differs from 

 the latter, however, in 

 the return to the origi- 

 nal value at the start 

 of the curve and in the 

 depression past the 

 original value on warm- 

 ing. This lag in the 

 curve for the plant juice 

 may be caused by re- 

 moval of part of the 

 hydrogen ions by pro- 

 teins on freezing and 

 the breaking of the 

 combination on thaw- 

 ing out. If the juice is 

 kept in the frozen con- 

 dition for a longer time 

 the final acidity is less 

 than the original, evi- 

 dently on account of 

 some permanent com- 

 bination of the excess 

 hydrogen ion. This 

 irreversible combina- 

 tion is indicated also 

 by the color changes 

 in coleus leaves. 



There are many objections to this method of measuring the increase of 

 hydrogen-ion concentration on freezing, such as the melting of the ice 

 by the stream of hydrogen, and the lack of uniformity in distribution of 

 the unfrozen juice. To be free from these hindrances the following pro- 

 cedure was adopted: 







-0.5 -LO 



-2.0 -25 



Fig. 2. — Graph showing increase in acidity with increasing depression 

 of the freezing point on concentration of cabbage juice, i and 2, 

 midrib juice: 3 and 4, iuice from leaf minus midrib. 



