MEASUREMENT OF H-ION CONCENTRATION 401 



them, or possibly change the composition of the culture medium 

 ("standard methods" notwithstanding), or resort to even graver 

 expedients, but the last expedient indeed should be the measure- 

 ment of pH for the given purpose. Measurement of pH and 

 titration furnish two distinct methods of attack, each with its 

 own object and interpretation. The principles involved have 

 been carefully discussed by Clark and Lubs (1917). 



As to a statement to the effect that the electrometric 

 method is more accurate than the colorimetric, but that the appa- 

 ratus which it requu^es is bej^ond the possibilities of the average 

 bacteriological laboratory; the writer can subscribe to neither part 

 in the unqualified form, but would refer again to the article of Clark 

 and Lubs (1917) for a discussion of the first part, and to the recent 

 book of Clark (1920) for the second. The writings of Clark 

 and Lubs also contain full discussions of other principal topics, 

 such as titration of culture media, effect of bacterial growth 

 and of sterihzation upon the indicators, etc. 



It is pleasing to note that Medaha was able to preserve his 

 color standards. The standards prepared by me were not perma- 

 nent, and the main difference seems to be in the means taken by 

 Medalia to avoid microbial decomposition, this point having been 

 neglected by me. 



It seems well to describe in this article, otherwise not very 

 constructive, an instrument for further study of the indicator 

 constants and behavior, which was devised too late to be of 

 service in the work published (Gillespie, 1920). The necessary 

 improvements in method, for work substantially better than that 

 aheady published, must include temperature control of the buffer 

 solutions in which the indicator is placed, and more precise meas- 

 urements of the percentage transformation. The apparatus 

 shown schematically in figure 1 can easily be made to satisfy 

 both requirements. The writer has not seen it described. It is a 

 colorimeter for two-colored indicators, and by an obvious modifi- 

 cation it can be used to determine, if desirable, both the percent- 

 ages of the two colors present and the total concentration. A 

 simple apparatus was improvised^ with which the percentages 



^ In the Laboratories of Soil Fertility, Bureau of Plant Industry, Washington, 

 D. C. 



