32 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



hydrogen electrodes, of which one contains a solution of known H-ion 

 concentration, and the other the solution in which this is unknown; 

 and a cell of which one electrode is a standard calomel electrode and 

 the other, a hydrogen electrode containing the unknown solution. 



The exact arrangement of the apparatus in which the calomel elec- 

 trode is used will be seen in the accompanying sketch. The hydrogen 

 electrode, it will be noticed, is a very small V-shaped tube, in which is 

 suspended a platinum wire coated with spongy platinum and dipping 

 into a solution which nearly fills the tube. The space above the solution 

 is filled with pure hydrogen. This and the calomel electrode are con- 

 nected with suitable electric measuring instruments, and the circuit is 

 completed by connecting the two electrodes by means of an intermediate 

 vessel containing a saturated solution of potassium chloride. This con- 

 necting solution is used because it has been found that the electric cur- 

 rents set up at the contact between different solutions are so small that 

 they can be disregarded.* 



As outlined above, the hydrogen electrode is that which is used to 

 determine the H-ion concentration of blood, the particular point about 

 it, in comparison with the apparatus used for simpler solutions, being 

 that the hydrogen is not changed in the course of the experiment. This 

 precaution is to prevent the carbon dioxide of the blood from being 

 " washed out'* of it by a frequently changing atmosphere of hydrogen. 

 Many inaccuracies in the earlier results obtained by this method were 

 due to the removal of carbon dioxide, which, as we shall see later, is 

 one of the chief acids contributing to the H-ion concentration of blood. 



The Indicator Method 



As pointed out in a previous chapter (page 22), the method of titra- 

 tion for acidity or alkalinity in which a standard solution of alkali or 

 acid is added until a certain change in the color of a suitable indicator 

 is detected, does not afford any information regarding the H-ion con- 

 centration actually present in the solution. It tells us the total con- 

 centration of available acid or base, both dissociated and undissociated. 

 By modification of the method of procedure, however, we may also use 

 indicators for determining the H-ion concentration. The principle of 

 this method depends on the fact that there are certain dyes which 

 change quite distinctly in tint with very slight changes in the H-ion 

 concentration, so that if we use dyes which possess this property at a 

 point near that of neutrality (i. e., between P H 6.5 and P H 8), we can es- 



*A description of the technic for measuring the electric potential developed by the cell would 

 be out of place here. Suffice to say that the strength of the current is compared with that of a 

 current of known strength furnished by a normal cell, the comparison being made by a bridge wire 

 F, a capillary electrometer H being employed to detect the direction and degree of current. 



