66 ION I SAT ION 



In water the concentrations of H and OH are equal. These 

 facts lead to the following definitions : 



(a) Any substance which when dissolved in water yields H^ 

 as one of the direct products of its ionisation is an acid. 



(b) Any substance which when dissolved in water yields OH 

 as one of the direct products of its ionisation is a base. 



(c) Any substance which on ionisation yields at least one 

 positive ion other than H+ and at least one negative ion other 

 than OH" is a salt. 



(d) If, in addition to the positive and negative ions mentioned 

 in (c), the salt yields an H+ ion, it is called an acid salt. 



E.g. KHSO4 = K+ + SO4- - + H+ 



Na2HP04 = 2Na+ + PO4 + H+ 



COOH 



COONH4 = NH4+ + (C00)2" - + H+ 



{e) If, in addition to the positive and negative ions mentioned 

 in (c), the salt yields an OH-ion, it is called a basic salt. 



E.g. Fe(0H)2Cl = Fe+ + + + CI" + 20H-, 



CH2 • NH • OH • COOK = K+ + CHgNHCOO + OH". 



(/) Substances which produce both H* and OH' ions on disso- 

 ciation are called amphoteric electrolytes or ampholytes. They 

 must evidently have two ionisation constants, Kj^ and -K^oh- 

 It is obvious that acidity depends on the preponderance of hydrogen 

 ions over hydroxyl ions, and conversely, alkalinity is due to the 

 presence of hydroxyl ions in excess of hydrogen ions. Neutrality 

 is an equilibrium between H* and OH'. 



This neutral point occurs in water at 23° C. when the concentra- 

 tion of hydrogen ions is 1 X 10~', i.e.pjj = 7. If the concentration 

 is greater than this, e.g. 1 X 10" 5, or p^^ = 5, then the concentra- 

 tion of OH' must be correspondingly decreased according to the 

 equation, 



[H+] [0H-] = 10-14 



or [0H-] = 7.PJXT = 10-14 (-5' = 10-9. 



A jjjj of 5 will be accompanied by a p^J^ of 9. This will be an 

 acid solution. 



Conversely, if the concentration of hydroxyl ions is increased, 

 there is a corresponding decrease in hydrogen ions. E.g. 



if i^oH = 3, then (H+) = ^' = 10-^ = p„ of 1], 

 an alkaline solution. 



