338 



PRACTICAL PHYSIOLOGY 



CHAPTER XXV 



HYDROGEN ION CONCENTRATION 



For the proper action of the tissue cells a balance is maintained 

 between the acids and bases present in the tissue fluids so that they are 

 kept constantly just on the alkaline side of neutrality. This balance is 

 maintained in spite of the varying production of various acids during the 

 course of metabolism, even when as the result of various stimuli, acids, 

 such as lactic acid, are produced in excess. Acidosis does not mean 

 that the reaction of the blood has become acid. In this maintenance of 

 neutrality sodium carbonates and phosphates play the chief role ; they 

 act as " buffers " in that when they are present in solution the addition 

 of acid has but little effect so far as the reaction of the fluid is concerned. 



The real acidity or alkalinity of a solution cannot be determined by 

 titration but by the hydrogen ion concentration (cH). 



This value varies with the concentration and the degree of dissociation 

 of the acid or alkali tested. Thus a decinormal solution of HC1 as 

 commonly used in titrations is not 0-1 normal, but only about 0-09 

 normal when valued in terms of hydrogen ion concentration. This is 

 due to the fact that dissociation of HC1 into H+and Cl~ is not complete. 

 The same is true of alkaline solutions which may be expressed in terms 

 of diminishing hydrogen ion concentration instead of increasing hydroxyl 

 ion concentration. The neutral point for all standards is taken as 

 that of specially distilled (pure conductivity) water, where [H+] = [OH~ ] 

 and the H ion value is 10 7 . The various values are now usually stated 

 in terms pH, which is the logarithm of the hydrogen ion concentration 

 (cH) with the minus sign omitted, cH of 10 7 = pH of 7. 



Note that as the >H value decreases the hydrogen ion concentration 

 increases, the solution becomes more acid and as it increases the solution 

 becomes more alkaline. 



The approximate pH values of various secretions, etc., are given in 

 the following table : 



pH 

 1-77 



Gastric juice 

 Urine 

 Milk (cow) 



,, (human) 

 Saliva 

 Blood 

 Faeces (variable) 

 Pancreatic juice 



5-5-7 



6-6 



7-0 



6-9 



7-4 



8-0 



8-3 



Two methods have been employed for the determination of the 

 hydrogen ion concentration, (a) the electrometric method and (6) the 

 indicator or colorimetric method. The first of these methods demands 

 the use of very elaborate apparatus, whereas the second, while perhaps 

 not quite so accurate, is more easily carried out. 



Whatever the nature of the process certain chemical bodies called 

 indicators change in colour with variations in the hydrogen ion con- 

 centration. In recent years a large number of indicators have been 

 introduced which permit of the determination of a wide range of reaction 

 (plS. of 1 to 2>H of 14) with ease and rapidity. 



In carrying out the determination an ordinary test tube rack, with a 



