REACTION OP BODY FLUIDS. 31 



processes proceed, that depend on the concentration of H-ions. 

 The conversion of cane sugar into invert sugar is a good process 

 to employ for measuring the speed of reaction. 



But even this refinement in technique does not enable us to 

 measure the H-ion concentration for now we must use this ex- 

 pression when speaking of acidity or alkalinity of such impor- 

 tant fluids as blood and saliva, in which there is an extremely 

 low H-ion concentration. If either of these fluids be placed on 

 litmus papers, the red litmus turns blue, but all that this signifies 

 is that the litmus is a stronger acid than those present in blood 

 or saliva, so that it decomposes the bases with which they were 

 combined and changes the color. If we employ phenolphthalein, 

 which is a much feebler acid, then blood serum reacts neutral and 

 saliva often acid. 



There are two methods open to us for measuring the H-ion 

 concentration in such cases : 



1. The Hydrogen Electrode. Place the fluid (e. g., blood 

 serum or saliva) in a small closed vessel filled with hydrogen and 

 with a platinum electrode dipping into it. Connect this hydro- 

 gen electrode with a standard calomel electrode by wires in the 

 course of which are suitably arranged electrical instruments for 

 the measurement of electromotive force. From the difference in 

 the electromotive force which is found to exist between the hydro- 

 gen and the calomel electrodes, we can calculate the H-ion con- 

 centration. This method is being employed for measuring the 

 reaction of saliva in relationship to its influence on caries of the 

 teeth. 



2. The Use of Standardized Indicators. It has been found 

 that different indicators change color at different H-ion concen- 

 trations. By taking solutions with variable known proportions 

 of acid and alkaline salts such as NaH,P0 4 and Na 2 HP0 4 or 

 NaHCO 3 and measuring their actual acidity in terms of the 

 H-ion concentration by the electrical method and then observ- 

 ing their behavior with different indicators, it has been possible 

 to evaluate the different indicators in terms of the H-ion concen- 

 tration at which they change color. Expressing the results as the 

 fraction of a normal solution of H-ion at which this change 



