[Pboc. Eot. Soc. Victoria, 33 (N.S.), 1921.] 

 Art. XV. — Tlie Estimation of Acidity. 



By J. M. LEWIS, P.D.Sc., Melb. 



{Communicated by Professor W. A. Osborne, M.B., B.Ch., D.Sc.) 



[Read 9th December, 1920.] 



Three methods are in practice available for the determina- 

 tion of the acidity or basicity of solutions, i.e., their hydrogen ion 

 concentration : — 



(1) The use of a " ladder " of indicators. 



(2) The hydrolysis of esters. 



(3) The electrometric method. 



(1) The employment of a "ladder" of " Indicators" which 

 give definite color changes at different hydro- 

 gen ion concentration. 



These properties, which are possessed by many of the aniline 

 dyes, have been employed by Salm. and Sorensen in arranging 

 a scheme which makes use of about fourteen of these substances, 

 the authors mentioned having placed them in a table, showing 

 the indicators available, and the concentrations which their 

 colour changes cover. This table, which has been altered and 

 improved by Bayliss, will be found in his " Principles of General 

 Physiology," (1), and the student is recommended to examine 

 it. On inspection it will be seen that as many as four changes 

 .•of hydrogen ion concentration may be made apparent with one 

 indicator, and these are so arranged that there is a definite grada- 

 tion in the series, from high, i.e. acid, concentrations, through 

 neutrality, to the low concentrations of alkaline solutions, which, 

 in spite of the seeming paradox, possess a demonstrable hydrogen 

 ion concentration. 



By means of these indicators, concentrations ranging from 

 Nx2 to NxlO 12 , may be measured and some idea of the prin- 

 ciples involved in the arrangement of the " ladder " may be 

 gained when it is pointed out that, whereas crystal violet in solu- 

 tion becomes golden yellow at a hydrogen ion concentration of 



