190 PRINCIPLES OF GENERAL PHYSIOLOGY 



in brackets, it contains only those found by the latter investigator to be unaffected 

 by the presence of moderate amounts of such substances, proteins or neutral salts, 

 as are likely to be present in physiological solutions. I have omitted two of 

 those recommended by Sorensen on account of the difficulty of obtaining them, 

 and have inserted in place of them, where the series would otherwise be incomplete, 

 other indicators in common use, but more sensitive to the disturbing presence of 

 neutral salts and proteins. These are marked by brackets. 



Neutral red is an extremely valuable indicator for many physiological purposes. It 

 changes colour at the neutrality of water, and has obvious changes at points just above and 

 just below this concentration in hydrogen ions. It is practically unaffected by the presence 

 of protein and is innocuous to living protoplasm. 



The cautions to be exercised when neutral salts or proteins are present in any considerable 

 quantity may be found in the paper by Sorensen (1909, pp. T'2-120). Attention may be called 

 to phenol- and thymol-phthaleins as being least affected thereby, and especially to the 

 new indicator, a-naphthol-phthalein, which changes colour between the H' ion concentra- 

 tions of 10" 7 ''-* and 10" 8 ' 68 , i.e., a very little on the alkaline side of neutrality (Sorensen and 

 Palitzsch, 1910). 



The Hydrogen Electrode. This method, although somewhat elaborate in the 

 apparatus required, and demanding careful work if small differences in H* ion 

 concentration are to be measured, is the most direct and the least liable to 

 disturbance by foreign substances. 



In order to understand the principle of it, the reader may be glad of a few 

 words on the theory of electrode potential. 



When a solid is placed in water, it has a certain tendency to send off its 

 molecules into the water so as to form a solution. The intensity of this varies 

 greatly in different cases, and is known as the solution pressure of the substance in 

 question. It occurred to Nernst (1889, pp. 150-151) that the electrical phenomena 

 shown by metals immersed in solutions of their own salts might be treated 

 quantitatively from a similar point of view, on the assumption of the truth of the 

 electrolytic dissociation theory. When a metal, say copper, is immersed in a 

 solution of one of its own salts, say the sulphate, the copper has a tendency to 

 give off Cu" ions into the solution. There are already ions of the same kind in the 

 solution, which, by their osmotic pressure, oppose the passage of similar ions from 

 the metal. The force with which the metal tends to send out ions into the 

 solution is called by Nernst its " electrolytic solution pressure," and may be greater 

 or less than the osmotic pressure qf the metallic ions in the solution. It will be 

 plain that, in the former case, the metal will become negatively charged, owing 

 to its giving off positive charges on the ions which leave it. Its potential will 

 depend on the difference between its electrolytic solution pressure and the 

 osmotic pressure of the ions in the solution. If the latter is the greater, the 

 electrode will have a positive charge, owing to the receipt of positive ions from 

 the solution. It is to be remembered that the ions given off from the metal 

 cannot travel beyond an infinitesimal distance from the oppositely charged mass 

 of metal, owing to electrostatic attraction, as has been pointed out above. 



It is obvious that we cannot make use of any one of these electrodes alone, 

 since we must have metal at both ends of our cell in order to form the circuit 

 for the purpose of measurement. If we form our battery by joining up two 

 electrodes of the same metal in solutions of the same concentration, there will 

 be no electromotive force in the combination, since the two electrode potentials 

 are equal and in opposite direction to one another. If, however, the concentra- 

 tions of the' metallic ion in the two solutions are unequal, the electromotive 

 force of the battery is equal to the difference between that of the two electrodes. 

 This arrangement is known as a "concentration battery." If we know the 

 concentration of one of the solutions, and can measure the electromotive force 

 of the combination, we can obtain the concentration of the other solution by 

 difference, supposing that we know the law which governs the relation between 

 the potential and the concentration of the solution. Now it has been shown by 

 Nernst (1889), originally from thermodynamic considerations, although the 

 assimilation by van't Hoff of solutions to the gas laws would lead to the same 

 result, that this relation is given by a similar expression to that for the work 



