REACTION 59 



In each case the acid produces H+ ions. Now, as [H] x[OH] is 

 a constant, the result of this increase of H + ions must cause a 

 decrease in the concentrations of OH" ions. 



In the same way, examination of the behaviour of alkalies 

 shows also a disturbance of the ratio of [H + ] to [OH~]. 

 For example : 



NaOH =Na++OH~ 

 NH 3 OH=NH 3 ++OH-. 



The concentration of OH~ ions is increased. 



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. KHSO 4 =K+ +S0 4 - 



NaHPO 4 =2Na+ +PO 4 ~ 

 COOH 



I 

 COONH 4 =NH 4 + +(COO) 2 



(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(OH) 2 Cl =Fe+++ +C1- +20H-, 



CH 2 -NH-OH-COOK=K++CH 2 NH 3 COO-+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, K n 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 concen- 

 tration of hydrogen ions is 1 xlO~ 7 , i.e. p^^l. If the concen- 

 tration is greater than this, e.g. 1 xlO" 5 , or p H =5, then the 



