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Articte IX—The Determination of Hydrogen Ion Concentration 
in Connection with Fresh-Water Biological Studies. By Victor E. 
SHELFORD. 
INTRODUCTION 
Generally speaking, the significance of hydrogen ion concentration . 
and its determination are among the most complicated problems con- 
fronting students of fresh-water ecology. Determinations of carbon 
dioxid, “alkalinity,” and dissolved oxygen have long been among the 
means used for ascertaining the suitability of a water for organisms, as 
well as its suitability for domestic and industrial purposes, but the rela- 
tions of these properties of a water to each other chemically or to organ- 
isms have never been fully investigated. 
There is great confusion as to methods of measuring and express- 
ing so-called “alkalinity.” It is expressed (a) in terms of a carbonate 
of a predominating alkaline metal—for example, as CaCO, ; (6) as bound 
and half-bound CO,; (c) as alkali reserve; and (d) as buffer value. 
It is from time to time confused with CO, tension, total CO,, and the 
normal acid used. It is not possible to straighten out this confusion. 
The investigator can only ascertain the methods used by different writers 
and carefully translate their records and statements into the terms he 
himself has elected to use. 
There is almost as great confusion in the use of terms and methods 
in connection with hydrogen ion concentration. This grows out of the 
fact that the turning points of various standard indicators have been 
referred to as “neutral,” notably in the case of phenolphthalein, the 
turning point of which is decidedly alkaline. There are now improved 
methods of determining hydrogen ion concentration, including true neu- 
trality. These have been used in the work here reported, which has con- 
sisted chiefly of (1) observations on the hydrogen ion concentration over 
fish breeding-grounds in several Illinois localities, July to June, 1919-20; 
(2) studies of the reactions of fishes to hydrogen ion concentration; and 
(3) studies of the effect of different hydrogen ion concentrations on the 
survival of fishes in water of low oxygen-content. This work has been 
supplemented by studies by Fenner Stickney on the resistance and re- 
actions of a dragon-fly nymph to various hydrogen ion concentrations and 
by experiments by Miss Ada Hall on the effect of hydrogen ions in the 
development of toad and whitefish eggs, carried on under the auspices 
of the Department of Zoology and of the Graduate School of the Uni- 
versity of Illinois. The author’s acquaintance with the questions dis- 
cussed has also been extended by a series of determinations of hydrogen 
ions in rivers and lakes in the following drainage systems: Puget Sound, 
Columbia River, Interior Basin, Colorado River, and Arkansas River. 
These determinations will be published elsewhere. 
