8 THE LIVING PLANT 



is effected by the presence of certain salts such as the 

 phosphates of the alkali metals or sodium bicarbonate, etc. 

 These salts exert what is known as a Buffer action in counter- 

 acting any considerable increase in P H on the introduction into 

 the solution of a small quantity of acid. This principle may 

 be illustrated as follows. If a single drop of dilute hydrochloric 

 acid is added to a quantity of distilled water, the P H of this 

 water, which should be 7*07, may be very considerably altered, 

 and the same would apply if instead of pure water, a dilute 

 solution of sodium chloride had been used. If, however, the 

 water had contained, in the place of the sodium chloride, an 

 equivalent amount of sodium phosphate, the effect of the 

 addition of the hydrochloric acid would merely have been to 

 displace a corresponding amount of feebly ionized phosphoric 

 acid whereby the P H would have been hardly altered at all. 

 This may be expressed by saying that sodium chloride has no 

 buffer action whereas sodium phosphate and the salts of other 

 feeble acids, such as boric, citric and amino acids, have strong 

 buffer action. 



The blood, as a typical physiological fluid, is provided with 

 a complex system of sodium phosphate and bicarbonate which 

 has a most efficient buffer action preventing the fluid from 

 having its P H appreciably altered in the event of the sudden 

 abnormal development of acid. 



Acting upon this principle, standard solutions of known P H 

 are best made from suitable concentrations of salts of known 

 marked buffer action ; such solutions may be kept without 

 fear of alteration through contamination with atmospheric car- 

 bon dioxide or alkali from the glass bottle, whereas solutions 

 made from salts with little or no buffer action would rapidly 

 alter and be useless. 



In practise it is found convenient to keep a number of such 

 standard buffer solutions of known P H for the purpose of 

 determining the P H of a given liquid by comparison of the 

 colours given with the same indicator. For this purpose a 

 small quantity of the liquid under examination is treated with 

 a few drops of the appropriate indicator and its colour is 

 matched against that buffer solution which gives the closest 

 approximation to its own with the same indicator. It should 

 be noted that the indicators employed in this work are sensitive 



