HYDROGEN-ION CONCENTRATION 67 



thus neutralising each other. Acids and bases dissociate 

 to a much larger extent than pure water, the amount of 

 dissociation depending ordinarily upon their strength. 

 An acid dissociates into positive H-ions and negative 

 ions consisting of the acid radicle. A base dissociates 

 into positive metallic ions and negative hydroxyl (OH) 

 ions. The stronger acids dissociate to a larger extent 

 than the weaker acids and upon this depends the 

 " strength " of the former. Thus, while normal solutions 

 of sulphuric and acetic acids neutralise the same amount 

 of alkali, sulphuric acid will displace acetic acid from its 

 compounds, i.e. sulphuric acid is a " stronger " acid than 

 acetic acid. This is due to the fact that sulphuric acid 

 ionises to a greater extent than does acetic acid. That is, 

 for solutions of equal strength, so far as capacity for 

 neutralising an alkali is concerned, the hydrogen-ion 

 concentration of sulphuric acid is greater than that of 

 acetic acid. Hydrogen-ion concentration may, therefore, 

 be used as an index of acid reaction. 



When an acid is mixed with water containing other 

 substances in solution the matter may be more com- 

 plicated. Organic matter, such as protein, in particular, 

 may prevent the H-ion concentration being so great as 

 it would have been had the same amount of acid been 

 added to water. Substances which thus repress the 

 H-ion concentration are known as " buffers." The 

 H-ion concentration is therefore a more correct index of 

 acidity than the amount of acid added. Acidity means 

 excess of H-ions over OH-ions, and alkalinity the con- 

 verse. Hydrogen-ion concentration is measured electro- 

 lytically by determining the electromotive force set up 

 between the solution and an electrode (such as platinum 

 foil coated with platinum black with hydrogen bub- 

 bling over it) immersed in it, and has been determined 

 for a number of substances. The symbol P H is the 



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