370 Hydrogen Ions in Biological Fluids [April 



would, therefore, indicate a too alkaline (or too acid) reaction of 

 the fluid. The liquid is now renewed without changing the gas- 

 mixture around the electrode, and shaking is repeated. It is easily 

 Seen that electromotive constancy may now be obtained without 

 any, or at least without any appreciable, alteration of the tension of 

 the volatile acid (or base), i. e., without alteration of the original 

 hydrogen-ion concentration of the liquid. 



This procedure may be repeated, if necessary, until the renewal 

 of liquid no longer causes any alteration in the measured potential. 

 For blood, urine, and probably the majority of biological fluids, a 

 Single renewal is sufficient. Sea-water and similar Solutions, which 

 are poor in "reaction regulators "^^ (Henderson^'^), are eo ipso far 

 more susceptible to the change in carbonic-acid tension resulting 

 from the method of measurement and would require three to four 

 or, according to circumstances, even a larger number of renewals 

 of the liquid before constancy is reached. In such cases it is easier, 

 and more correct, to extrapolate graphically from the first three 

 measurements (i. e., after two renewals of the liquid) in order to 

 get the final value. 



The procedure described here permits one inconsiderable error, 

 for which a correction may be made, if necessary. When diffusion 

 equilibrium between the hydrogen and the liquid has been obtained, 

 none of the components are, strictly speaking, any longer saturated 

 with moist hydrogen at the existing barometric pressure but at a 

 somewhat lower pressure. The potential changes, however, in ac- 

 cord with the logarithm of the hydrogen pressure, so that, e. g., a 

 fall in the hydrogen pressure from 760 to 700 (and a greater fall is 

 practically inconceivable) would drop the measured potential to a 

 value ca. i milli-volt too low. An error like this lies very near the 

 limit of error of the whole method but may be eliminated, as already 

 mentioned, by analysis of the hydrogen mixture and by calculation. 



Solutions which are poor in "reaction regulators," but whose 

 hydrogen-ion concentration (owing to the volatile acid or base they 

 contain) must necessarily be measured in the above-mentioned way 

 (if it cannot be measured colorimetrically), have been found to 



'" Compounds which, by their presence, diminish the effect on the hydrogen- 

 ion concentration of changes in the Proportion of acid or base. 

 " Henderson : Ergebnisse der Physiologie, 8, 1909. 



