1919] 



DUGGAR AND DODGE — COLORIMETER IN H ION DETERMINATION 65 



indicator. The disadvantages of this instrument are prac- 

 tically the same as those mentioned above, but it has obvious 

 advantages over the usual test-tube comparison. 



In some studies on the nutrition of the fungi wherein a 

 variety of plant decoctions was employed the writers ex- 

 perienced the usual difficulties in rapidly and accurately em- 

 ploying the indicator method for determining the active 

 acidity of these media. The plant decoctions were made in 

 accordance with our usual method, which consists in slicing 

 the product, or cutting it into short lengths, adding the 

 requisite amount of water, and autoclaving at 10-15 pounds 

 pressure for one hour. The effect of this autoclaving for ex- 

 traction, together with another interval of 15-30 minutes for 

 sterilization, after filtering into flasks, is to yield a decoction 

 which is often highly colored. Solutions prepared from 

 rhubarb, celery, carrots, prunes, apples, mangolds, and sweet 

 potatoes gave, as might be expected, more pronounced color 

 than those made from sugar beets, potatoes, and green beans. 

 In any case, after repeated sterilizations the deepened color 

 became a source of considerable annoyance. It should be 

 stated that this work was begun prior to 1917, so that we 

 were not at first in possession of the newer indicators. 



In any case it seemed wise to investigate the possibility of 

 employing the colorimeter in such work. At first no refer- 

 ence could be found in the literature to the use of the colori- 

 meter in that way. Nevertheless, Veley ('06), Tizard (10), 

 Walpole, and perhaps others had apparently, with no great 

 amount of consistency, employed the colorimeter in the de- 

 termination of the constants of indicators and in other re- 

 lated work. Prideaux ( '17) expresses regret that all neces- 

 sary conditions — referring especially to the concentration of 

 the indicators and to whether or not a colorimeter was used 

 — have not been carefully specified for each indicator con- 

 stant, so that it might be employed in the colorimetric de- 

 termination of the H ion with greater confidence. It would 

 appear, however, that he has employed the colorimeter 

 directly in the determination of H ion concentration, because 

 of the following statement : * ' The accuracy of a colour com- 



