LUBS AND CLARK: HYDROGEN-ION INDICATORS 481 



On the silver basis (Ag = 107.88) 96,494 



On the iodine basis (I = 126.92) 96,512 



Mean 96,503 



The best round value which can be assigned to this constant 

 appears to be 96,500 coulombs. 



A brief history of the specifications for the voltameter, as well 

 as revised specifications proposed by the Bureau of Standards, 

 will be given in the detailed paper. 3 As no adequate specifica- 

 tions have been adopted since the London Conference of 1908 it 

 is hoped that the carefully drawn specifications whicl^ the Bureau 

 will present may be adopted as a whole or in part, whenever it 

 is possible to reach an international agreement. In any case, 

 for the present, these specifications will be available for the 

 guidance of such investigators as may wish to use the silver 

 voltameter. 



An appendix to the paper will contain an extensive bibli- 

 ography of the subject. 



CHEMISTRY. — A note on the sulphone-phthaleins as indicators 

 for the colorimetric determination of hydrogen-ion concentra- 

 tion. 1 Herbert A. Lubs and William Mansfield Clark, 

 Bureau of Animal Industry. 



In a previous paper 2 from this laboratory there were described 

 several new indicators of the sulphone-phthalein series and an 

 improved method for the preparation of those previously made 

 by others. In .our subsequent work on indicators suitable for 

 the colorimetric determination of hydrogen-ion concentration, 

 we have investigated other compounds of this series and have 

 found another that promises to be particularly useful for this pur- 

 pose; namely, dibrom-o-cresol-sulphone-phthalein. We have also 

 found that slight modifications in the methods of preparation of 

 some of the indicators described in our earlier paper will insure 



3 Shortly to appear as Bur. Stds. Sci. Paper No. 285 (Bull. Bur. Stds.,13: 

 479-514. 1916). 



1 From the research laboratories of the Dairy Division, Bureau of Animal 

 Industry. Published by permission of the Secretary of Agriculture. 



2 Journ. Wash. Acad. Sci., 5: 609. 1915. 



