LIFE AND ENERGY 27 



hydrogen ion which is common to all acids, and is responsible for 

 their characteristic acidic properties, such as taste, and so on, we 

 always speak of the " hydrogen-ion concentration"' We shall find 

 later that physiological phenomena are extremely sensitive to the 

 precise value of this property of the medium in which they take 

 place, and that there are means taken to maintain it at its most 

 appropriate value. 



Similar considerations may be applied to the case of alkaline 

 solutions, and their alkalinity may be expressed in terms of con- 

 centration of OH ions. But, since the product of the H and 

 OH ionic concentrations in all solutions is the same (P., p. 197), 

 it is best, for the sake of uniformity, to give the " reaction " of all 

 solutions in terms of H-ion concentration, from which the OH-ion 

 concentration can be easily calculated. The reaction of distilled 

 water being taken as the point of neutrality, those solutions whose 

 H-ion concentration is greater than this are acid, those below it 

 are alkaline (P., p. 184). 



Indicators. The question next arises as to how this hydrogen- 

 ion concentration is to be estimated. The most direct method is 

 by the use of the hydrogen electrode, in which a battery is fitted up 

 whose electrodes consist of hydrogen. The electro-motive force in 

 such a case is proportional to the concentration in H ions of the 

 solutions in contact with the electrodes, and can be measured in 

 the usual way (p., p. 190). But although this method is the most 

 accurate in cases where it can be used, in the physiological solutions 

 of most interest to us its application requires somewhat complicated 

 procedures if correct values are to be obtained. A case in point is 

 that of the blood. For this reason the more indirect methods are, 

 as a rule, more useful. Of these methods, the use of what are 

 known as " indicators " is the simplest. There are many coloured 

 chemical compounds which have a different colour, according to the 

 H-ion concentration of their solutions. There is some dispute as 

 to how this change of colour is related to the chemical changes in 

 the indicator, but this does not concern us here. In general, the 

 range is small, so that above a certain concentration there is no 

 further change in colour, nor is there below a certain concentration. 

 The particular concentration in hydrogen-ions, at which the more 

 or less sudden change in colour takes place, is not the same with 

 the different indicators (E., p. 175); so that it is posssible, by 

 taking an appropriate series, to obtain the H-ion concentration of 

 a given solution with some degree of accuracy (p., p. 189). In 

 most cases of interest to us, the H-ion concentration is not far 

 distant from that of distilled water, and in such cases the dye 

 known as " neutral red " is very useful, since it shows a series of 

 changes, from crimson through red and orange to yellow, in this 



