510 APPENDIX 



or other will be in excess. Thus values of P^ below 7-07 

 indicate acid solutions, while values of Pg above 7-07 are 

 alkaline. 



The most accurate method of determining Pjj is the 

 electrical method depending upon conductivity determinations. 

 For practical purposes, however, a colorimetric method has 

 been devised depending upon the fact that a series of indicators 

 have been found whose colours depend upon the prevailing 

 Pjj and which are sensitive to changes in Pjj within certain well- 

 defined limits. 



Taking for example the commonly used indicators, the range 

 for methyl orange is from Pjj 3-1 to Pjj 4-4 red to yellow, 

 litmus Pjj 5-4 to Pjj 7-8 red to blue, 



phenolphthalein Pjj 8-3 to Pjj 10 colourless to red. 



It will be seen from this table that owing to the fact that 

 these indicators each have their clearly defined range of 

 sensitiveness, it follows that one and the same liquid, such as 

 urine, with a Pjj 5 may have an alkaline reaction to methyl 

 orange and yet be acid to litmus or phenolphthalein, and for 

 the same reason a solution which is neutral to litmus may still 

 be acid to phenolphthalein. This is well illustrated by the 

 fact that many media which require to be neutralized previous 

 to use require more alkali for neutralization if phenolphthalein 

 is used as indicator than if litmus be employed. 



Within recent years the importance of hydrogen ion 

 concentration to the well-being and growth of plants has been 

 more and more recognized. 



In most living organisms provision is made for securing 

 that the Pj, of the medium shall not be easily disturbed ; this 

 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 Pjj 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 Pjj 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 



