APPENDIX 511 



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 Pg 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 Pj, appreciably altered in the event of the sudden 

 abnormal development of acid. 



Acting upon this principle, standard solutions of known P^ 

 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 practice it is found convenient to keep a number of such 

 standard buffer solutions of known Pjj for the purpose of 

 determining the Pjj 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 

 only over a certain range of Pjj, say from P^ 2-8 to Pjj 4-6 for 

 bromphenol blue and from P^ 4-4 to P^ 6-0 for methyl red, 

 and from P^ 6 to P^ 7-6 for bromthymol blue and so on ; 

 hence if no match was obtained with one indicator, the P^ 

 of the solution lies outside that range and another indicator 

 has to be employed until the correct one has been found and 

 the Pg fixed with the greatest possible degree of accuracy. 



