INDICATORS 7 



and consequently at exact neutrality, when the concentrations 

 of the two are exactly equal, C H = icr 7 ' 07 and CQH = io~ 7 ' 07 

 whence it follows that for absolute neutrality, in which the 

 concentration of hydrogen ions is exactly equal to that of the 

 hydroxyl ions P H = 7*07. 



There is no need to determine the OH ion concentration 

 since it is easily found from the difference between 14*14 and 

 the hydrion concentration. 



Thus for P H i the hydroxyl ion concentration would be 

 P OH I3'I4, and for PHIO it would be P OH 4'I4. 



Since for Pn7'O7 there is exact equality between H and 

 OH ions, it follows that on either side of this value one or 

 other will be in excess. Thus values of P H below 7 '07 indicate 

 acid solutions, while values of P H above 7-07 are alkaline. 



The most accurate method of determining PH 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 

 P H and which are sensitive to changes in P H within certain well 

 defined limits. 



Taking for example the commonly used indicators, the range 

 for methyl orange is from P H 3 "I to Pn4'4 red to yellow, 

 litmus PnS'4 to P H 7'8 red to blue, 



phenolphthalein Pn8'3 to P H IO 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 P H 5 may have an alkaline reaction to methyl 

 orange and yet be acid to litmus or phenol phthalein, 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 phenol-phthalein 

 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 P H of the medium shall not be easily disturbed ; this 



