202 SCIENCE PROGRESS 



present, but in their strength as acids. This " strength "of 

 acidity which may be contrasted, as a kind of intensity factor, 

 with the quantity of acid present and measured by titration, is 

 expressed by the hydrogen ion or, more briefly, the hydrion 

 concentration, and is the important factor from the physiological 

 point of view. Methods of great delicacy have recently been 

 placed at the service of physiologists (see Clark, Determina- 

 tion of Hydrogen Ions, Baltimore, 1920) for the measurement 

 of hydrogen ion concentration in biological fluids, and this 

 important physiological factor can now receive adequate 

 numerical expression. 



A neutral aqueous solution may be defined in terms of this 

 factor as one in which the hydrion concentration equals the 

 concentration of hydroxyl ions, as both these ions are produced 

 in equal numbers from water by the reversible reaction 



H2O ^ H + + OH - 



The product of the concentrations of H + and OH ~ ions is a 

 constant at constant temperature, so that at constant tempera- 

 ture increasing acidity is measured by an increasing prepon- 

 derance of H+ ions over the OH ~ ions. 



At neutrality, the concentration of hydrogen ions expressed 

 in terms of the chemists' " normal " solution (a solution con- 

 taining one gram of hydrogen ions per litre under standard 

 conditions) is approximately i X lo'^N. 



With increasing acidity this concentration increases rapidly, 

 so that the minus index to the ten falls rapidly ; but these 

 numbers remain cumbrous to express and practically impossible 

 to plot directly so as to show their relation to other changing 

 factors. The custom has therefore grown up of plotting instead 

 the logarithm of the reciprocal of the number expressing hydrion 

 concentration, this number being preceded by the symbol pH. 

 Thus, for true neutrality the pH value would be obtained as 

 follows. Log of reciprocal of hydrion concentration 



^ ^^^ I x\o-^ = Log 10' = 7 = the pH/. 



When hydrion concentration, acidity, or alkalinity are there- 

 fore represented by figures preceded by the symbols pH, it 

 must be remembered that increasing hydrion concentration or 

 increasing acidity are represented by smaller figures, decreasing 

 hydrion concentration or increasing alkalinity by larger figures ; 

 also that the change from pH6 to pH/ does not indicate a small 

 arithmetic decrease in the number of hydrogen ions present, 

 but means that there are one-tenth as many hydrogen ions 

 present. 



The first essential point in Loeb's work is that all his studies 



