182 HYDROGEN ION CONCENTRATION 



because of its high acidity. It must scarcely ever exceed 1 to 2 

 niiHivolts. 



The question is of greater importance in deahng with concentration 

 chains in which strong acids and bases are involved, as for example 

 in the determination of the dissociation constant of water or in the 

 standardization of the normal hydrogen electrode. Here the re- 

 quired extrapolation values are frequently so great that pure acid 

 solutions are never used, but instead we employ acid solutions contain- 

 ing KCl, in order to decrease the diffusion potential. But since this 

 affects the activity of the H-ions, which can still be only approxi- 

 mately calculated, therefore, a practical limitation is hereby imposed 

 upon the attainable accuracy of all pH determinations. It may be 

 said that such measurements are inaccurate to the extent of at least 

 0.5 milHvolts, which corresponds to a probable error in pH of about 

 0.01. 



52. The physiological significance of diffusion potentials 



It is evident that electrode potentials do not occur in the living 

 organisms, for there are no free metals present. But the problem 

 arises as to whether or not the potential differences observed in the 

 organism could be explained as being diffusion potentials. But it is 

 out of question that diffusion potentials have any considerable share 

 in these tissue potentials. For the potentials that have been observed 

 between injured and uninjured parts are of the order of magnitude of 

 0.02 to 0.05 volts and even above. Such high diffusion potentials 

 can only develop under extreme conditions such as never occur in 

 the living organism. In the almost always approximately neutral 

 and NaCl-containing tissue fluids not even a diffusion potential of 

 0.01 volt can ever arise as will be readily seen from the discussion 

 given above. 



