

SENSITIVITY OF INDICATORS 9 



only over a certain range of P H , say from Pn2*8 to Pn4'6 for 

 bromphenol blue and from Pn4'4 to PH^'O for methyl red, 

 and from P H 6 to P H 7 '6 for bromthymol blue and so on ; hence 

 if no match was obtained with one indicator, the P H of the 

 solution lies.outside that range and another indicator has to be 

 employed until the correct one has been found and the P H 

 fixed with the greatest possible degree of accuracy. With a 

 little practise it becomes possible to detect differences of O 1 ! in 

 the value of the P H . 



The applications of the above principles to the problems 

 of soil chemistry will be obvious, and it will be realised that 

 some of the soil constituents have marked buffer action. 



