DIFFUSION POTENTIALS 181 



between any two solutions of KCl there arises no potential of any 

 practical importance, as these calculated figures show: 



volts 



Ci :C2 = 1 : 10 . 0004 



1 : 100 . 0008 



1 : 1000 0. 0012 



And even these values must surely be much too high for contact 

 boundaries that are not very sharply defined. Hence the diffusion 

 potential arising between any electrolyte solution and a KCl solution 

 is always smaller than the one with a solution of any other salt in 

 which the mobilities of the cations and anions differ very markedly. 

 Consequently it should be possible to diminish greatly the diffusion 

 potential between any two solutions by interposing between them a 

 KCl solution. This effect of KCl increases with the increasing 

 concentration of its solution, since then the conduction is due more 

 and more to the K- and Cl-ions. 



In most cases then it suffices to interpose a saturated (4J N) KCl 

 solution in order to practically abolish the diffusion potential. 



A wide use is made of this principle in practical concentration 

 chain measurements. 



In most cases then the diffusion potential may be neglected when a 

 KCl solution is used as a liquid bridge. And yet in veiy acid (pH 

 < 3) and in very alkahne solutions the aboKtion of this potential is 

 not quite complete. In such cases Bjerrum's^ empirically derived 

 extrapolation method is apphed. The E.M.F. of the chain is 

 measured once with the interposition of a 1.75 M KCl solution and 

 then it is measured again with a 3.5 M KCl solution. If a difference 

 between these measurements is found, this difference is to be taken 

 into account in order to obtain the approximately true value of the 

 potential difference of the electrodes. For example 



volts 



p.d. with 1.75 M KCl 0.2048 



p.d. with 3.5 M KCl 0.2068 



Then the true value is 0.2088 



This extrapolation must amount to but a few millivolts, if its 

 claim to certainty is to be assumed. Of all physiological fluids such 

 an extrapolation is taken into consideration only in the gastric juice 



9 N. Bjerrum, Zeitschr. f. physikal, Chem. 53, 428 (1905). 



