I9I3] K. A. Hasselbaich 369 



through the fluid, cannot be used. Let us take an extreme case and 

 see what even a slight carbonic-acid tension — according to ordinary 

 ideas — may mean for the hydrogen-ion concentration o£ a liquid. 

 Sea-water and the surrounding atmosphere have the same carbonic 

 acid tension — ca. 0.04/100X760 = 0.3 mm. On driving all the 

 carbonic acid f rom the sea-water we should cause the hydrogen-ion 

 concentration to sink from ca. lo"^ to ca. lO"^ or, using Sörensen's 

 terminology/ the hydrogen-ion exponent, pn-, would rise from 8 

 to 9. If, therefore, in this case we saturated the liquid and elec- 

 trode with a current of hydrogen, quite an erroneous result would be 

 obtained. 



A similar error, though less in amount, would also arise if we 

 had recourse to the auxiliary method applied in such cases at the 

 beginning of this Century, namely, if the hydrogen-saturated elec- 

 trode remained in contact with the liquid and we waited until the 

 Potential became constant, i. e., until equilibrium had been attained 

 in the diffusion between the liquid and the hydrogen atmosphere. 

 For instance, a sample of sea-water (kept in a bottle for nearly a 

 year), whose p-a: was in reality 7.55, showed /'h- = 7.74 on using 

 this method. The error may be reduced if, with Michaelis,^ we take 

 a small quantity of hydrogen and let the electrode only just touch 

 the surface of the liquid. But the most satisfactory method of 

 proceeding seems to me the f ollowing :^ 



A current of pure hydrogen, saturated with moisture, is led 

 through the vessel containing the electrode until the latter has be- 

 come saturated with hydrogen. The experimental liquid, which is 

 stored in such a way that it retains its natural tension of volatile 

 acid (or base), is now led into the vessel in such a quantity that 

 the electrode reaches more or less deeply into the fluid (see below) 

 and the vessel is then closed. By shaking the vessel, the establish- 

 ment of diffusion equilibrium between liquid and hydrogen, and 

 attainment of constancy in the measured potential, are accelerated. 

 This constancy, however, has been obtained by the loss of part of 

 the volatile acid (or base) from the liquid to the hydrogen and 



^Sörensen: Bloch. Zeitschr., 21, 1909. 



^Michaelis: Ibid., 18, 1909; 46, 1912. 



'Hasselbaich: Ibid., 30, p. Z17, 1910; 38, p. 77, iQiS; 49, P- 45o, 1913- 



