3(38 Hydro gen Ions in Biological Fluids [April 



not altered. Only two methods of the latter kind are in practical 

 employment, namely, the colorimetric and the electrometric. 



The colorimetric method is based on the above-mentioned fact, 

 that a series of indicators shows certain color nuances with known 

 hydrogen-ion concentrations, which miist be determined electro- 

 metrically, so that the electrometric determination of the hydro- 

 gen-ion concentration miist at any rate be considered as the funda- 

 mental method. The colorimetric method has been indicated by 

 Friedenthal and Salm ;^ its field has been considerably widened and 

 its trustworthiness assured by the thorough investigations and im- 

 provements of S. P. L. Sörensen and his collaborators. I shall not 

 discuss the technical details of the method but merely refer to Sören- 

 sen's latest smnmary of his work.^ 



We owe the electrometric method orginally to Nernst.^ It was 

 first applied to biological fluids by Bugarsky and Liebermann, ^ and 

 by Höber.^ It is based on the fact that a hydrogen-saturated metal 

 electrode in a hydrogen-saturated liquid gives rise to a difference of 

 Potential between the electrode and the liquid, which is dependent 

 on the hydrogen-ion concentration according to known laws. The 

 determination of this difference of potential thus makes it possible 

 to determine the hydrogen-ion concentration of the liquid. 



The experimental method generally employed to measure the 

 difference of potential between the hydrogen-saturated electrode 

 and the hydrogen-saturated liquid has been so often described in its 

 main features, most recently by Sörensen*^ in the above-cited work, 

 that it needs no attention here. The present paper deals with the 

 difficulty of obtaining the condition presupposed by the method, 

 vis., Saturation of the electrode and liquid zvith hydrogen, without 

 any alteration in the hydrogen-ion concentration of the liquid. 



Biological fluids, as is well known, usually contain volatile acids 

 (or bases) which determine, in great part, their hydrogen-ion con- 

 centration, so that the normal electrometric method, by which liquid 

 and electrode are saturated with a current of hydrogen bubbled 



^ Friedenthal and Salm: Zeitschr. f. Elektroch., lo, 1904; 12, 1906; 13, 1907. 



^Sörensen: Ergebnisse der Physiologie, 12, 1912. 



^ Nernst : Zeitschr. f. physikal. Chemie, 4, 1889. 



* Bugarsky and Liebermann : Pflüger's Arch., 72, 1898. 



° Höber: Pflüger's Arch., 81, 1900. 



' Sörensen : Loc. cit. 



