A DEFINITE H-ION CONCENTRATION 165 



of the substance. Many of these non-electrolytes have 

 another property which is of great importance in connection 

 with reaction. This is the property which they possess of 

 forming addition compounds loith acid or alkali. From 

 this it follows that into solutions containing them ax^id or 

 alkali may he added without affecting the relative H-ion con- 

 centration, i. e., the reaction. 



Substances like weak electrolytes or certain non-electrolytes 

 to whose solutions acid or alkali may be added without 

 affecting the reaction, i. e., the free H ions, are called '' buffer" 

 substances or "buffers." 



The foregoing discussion should make it clear why the 

 method of titration by using phenolphthalein is not neces- 

 sarily accurate, in fact is not apt to be. The method (page 

 160) is to add to a definite volume of the solution to be 

 titrated alkali (usually) until a faint pink color is obtained, 

 then to calculate from the amount of alkali used the quantity 

 of alkali to bring the entire bulk of medium to the same end 

 point. This would he accurate. The error lies in assuming 

 that if a less amount of alkali is added the reaction ivill he 

 proportionately less alkaline or more acid. From the nature of 

 buffers it should be evident that this is not true. Since reac- 

 tion is dependent on free H ions, and since in the presence 

 of buffers considerable acid or alkali may be added without 

 affecting the reaction, calculations based on the assumption 

 of change in reaction in proportion to the acid or alkali 

 added are very apt to be wrong. 



The following series of titrations in a buffered medium 

 illustrates this point nicely. These were made with a 

 potentiometer by Mr. R. Franklin Jukes, instructor in the 

 author's department: 



