224 PRINCIPLES OF GENERAL PHYSIOLOGY 



hydrogen or hydroxyl ions. A weak acid or a low degree of acidity is such because 

 there are relatively few hydrogen ions present. 



The different degrees of dissociation enable us to express the strength of acids 

 or bases, with the exception of those which do not obey the law of mass action, 

 in numerical quantities, known as their "dissociation constants" or "affinity 

 constants." 



To understand the meaning of these, a brief account of the law of mass action 

 is introduced. This law states that the rate of any reaction is proportional to tin- 

 masses of the reacting substances. The meaning of " velocity constant " and of 

 "equilibrium constant," as the ratio of the two velocity constants of the two 

 opposite reactions in a reversible system, is explained. 



The " dissociation constant " is the equilibrium constant of the reversible 

 reaction of electrolytic dissociation. Since it presupposes that the law of 111.1-^ 

 action is followed, it can only be given in the case of weak electrolytes. 



Instances of the activity of hydrogen and hydroxyl ions in cell processes art- 

 given ; such are the action of enzymes, the character of the heart beat, and so on. 



Hence accurate methods of determining the hydrogen ion concentration arc 

 indispensable. The methods of the use of indicators, the gas electrode and the 

 hydrolysis of esters or cane-sugar are described. 



In connection with the hydrogen electrode, the theory of electrode potentials is 

 discussed and the precautions necessary in the use of the method with blood .un- 

 pointed out. 



In the use of the method of hydrolysis of esters, etc., the peculiar effect 

 of neutral salts in increasing the hydrolytic action of a given concentration of 

 strong acid has to be taken into account. 



The powerful effect of changes in hydrogen ion concentration on physiological 

 processes requires the existence of mechanisms for the prevention of any consider- 

 able changes of this kind. 



There are two chief chemical systems in which the reactions occurring on the 

 addition of acid or alkali are of such a nature as to require the addition of com- 

 paratively large amounts of acid or alkali in order to produce any marked change 

 in the hydrogen ion concentration. These systems are the bicarbonate-carbon- 

 dioxide system and that of the acid and alkaline phosphates. The former is the 

 more widely occurring one, although the phosphate system is also of important- in 

 protoplasmic reactions. The proteins also play a subordinate part, owing to 

 their amphoteric nature, but chiefly on account of their comparatively high 

 concentration. 



In the reactions referred to in the previous paragraph, the phenomena known 

 as " hydrolytic dissociation " play an important part. This process is shown to 

 occur by the presence of free acid and free base in solutions in water of salts of 

 weak acids or bases. It is due to two facts'; the first is that water itself is a 

 very weak electrolyte, being to a minute extent electrolytically dissociated into 

 hydrogen and hydroxyl ions ; the second is the slight electrolytic dissociation 

 of weak acids and weak bases. By interaction of the four ions thus present, 

 there is an excess of hydroxyl ions when the base is the stronger and of hydrogen 

 ions when the acid is the stronger. A very small degree of hydrolysis of the 

 salts of many organic acids with strong bases is frequently to be met with, even in 

 cases where the acid would be expected to be a weak one. 



In the bicarbonate system, the escape of carbon dioxide as gas, when the 

 hydrogen ion concentration of the system rises, is an important factor in the 

 maintenance of neutrality. Numerical results are given in the text, showing 

 the efficiency of the system at hydrogen ion concentrations not very far above 

 or below that of neutrality. 



Carbon dioxide possesses powers of neutralising alkali of a degree not shared 





