REACTIONS TO INDICATORS 65 



log Cjj 5 X 10-6 = log 5 + log 10-6 



= -6990 + ( — 6-0000) 

 = — 5-3 i.e. p^, — 5'3 

 or Ch 5 X 10-6 = 10-699 X 10-6 = 10-5 3^ j;^ 5-3. 



Graph for conversion from one notation to other, Part II., 

 p. 564.) 



Reaction to Indicators. 



It is very important to be able to ascertain with great exact- 

 ness, the true acidity or alkalinity of physiological media. It is 

 not sufficient to state that a certain fluid is acid to litmus, etc. 

 Litnuis, for one thing, is not nearly sensitive enough to indicate 

 the minute changes in reaction which alone are of physiological 

 value. The whole activity, of the mammal, at any rate, is regu- 

 lated by reaction. Alterations in acidity are the causative factor 

 in the regulation of respiration, the activity of muscle, the ex- 

 citability of nerve, and play an important part in regulating 

 secretion and excretion. Physical and chemical means are 

 employed to keep the healthy body within a narrow range of 

 reaction, about the neutral point. Any marked deviation from 

 this is pathological, and is the result of pathological (or experi- 

 mental) conditions. As we shall see later, the neutrality of the 

 organism is an equilibrium, any disturbance of which will produce 

 change, and, moreover, any change in the organism will tend to 

 disturb this equilibrium (Chap. XXXI.). 



Examination of a number of acids shows that when they 

 dissociate in water they disturb the balance existing between the 

 concentrations of H^ and OH- ions. 



For example : 



HCl = H+ + Cl- 



HNO3 = H+ + NO3- 



H2SO4 = H+ + HSO4- 



CH3COOH = H" + CH3COO- 



In each case the acid produces H ' ions. Now, as [H] X [OH] is 

 a constant, the result of this increase of H+ ions must cause a 

 decrease in the concentrations of OH- ions. 



In the same way, examination of the behaviour of alkalies 

 shows also a disturbance of the ratio of [H+] to [OH-]. 

 For example : 



NaOH = Na+ + OR- 

 NH4OH = NH4+ + 0H-. 

 The concentration of 0H~ ions is increased. 



