62 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



[Vol. 6 



arise; and (3) it requires no thermal bath or other supple- 

 mentary apparatus except perhaps a colorimeter in the cases 

 to be discussed later. So long as the test solutions or media 

 employed are colorless, or practically so, the indicator 

 method presents now no difficulties which are not readily 

 precluded by a little experience. 



Kapid advances, however, have been made during the past 

 few years in the perfection of buffered or standard solutions 

 of carefully determined hydrogen ion concentration with 

 which to compare the fluids studied. The contribution made 

 by Clark and Lubs (17) in respect to standard solutions is 

 of almost equal importance to the excellent choice of indi- 

 cators presented by them. In view of the availability of the 

 work of Clark and Lubs and the detailed discussion by them 

 it is unnecessary to refer to the preparation of such standard 

 solutions further than to emphasize the necessity for all the 

 refinements prescribed. In the work here reported, as well 

 as in other studies now in progress, use has also been made 

 of the standard solutions of Sorensen ('OO-'IO). 1 We have 

 found, however, that the citrate and glycocoll mixtures 

 undergo rapid deterioration, while the thallate, phosphate, 

 and borate mixtures are much more stable. All solutions, 

 whether the prepared standards or the stock solutions from 

 which these are made, should be kept in well-seasoned glass- 

 ware, and, so far as possible, the same container should be 

 employed for a particular ionic concentration. Moreover, 

 since the introduction of the thymol, cresol, phenol, and cer- 

 tain benzene products, it is no longer necessary to choose a 

 doubtful indicator from the extensive charts of the earlier in- 

 vestigators, such as those of Salm ('06). 



The newer indicators exhibit, for the most part, brilliant 

 color changes throughout the range of P H values usually re- 

 quired; although, as subsequently emphasized, particular 

 care is required in the case of colored test fluids both in re- 

 spect to the choice of the indicator and in checking the ac- 



1 Reference is here made to Sorensen's paper in the Carlsberg Compt. rend, 

 des Trav. rather than to the other source of this material — Biochem. Zeitschr., 

 1913 — in view of the fact that in the former only is a correction made (at the 

 end of the paper) for an error in stating the amount of the phosphate employed. 



