STATE OF ILLINOIS 
DEPARTMENT OF REGISTRATION AND EDUCATION 
DIVISION OF THE 
NATURAL HISTORY SURVEY 
STEPHEN A. FORBES, Chief 
Vol. XIV. BULLETIN Article IX. 
The Determination of Hydrogen Ion 
Concentration in Connection 
With Fresh-Water Bio- 
logical Studies 
BY 
VICTOR E, SHELFORD 
PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS 
URBANA, ILLINOIS 
February, 1923 
