164 THE IONIC THEORY 



charge. Distilled water will not convey an electric current, 

 but if an acid or a salt is added to it the current will flow, 

 from the positive to the negative pole. This current is 

 conveyed by the ions, the positively charged metallic or 

 basic ones being attracted to the negative pole, and vice 

 versa. In the case of sodium chloride, the sodium ions are 

 attracted to, and liberated at the negative electrode and at 

 once react with the water forming NaOH sodium hydroxide, 

 making the water alkaline in reaction, and setting free bubbles 

 of hydrogen gas. If copper sulphate is used copper is de- 

 posited on the negative electrode, whilst the S0 4 ion reacts 

 with water to form sulphuric acid at the other. These 

 substances of which sodium chloride and copper sulphate are 

 examples, are called electrolytes because they promote the 

 electrolysis of water when a current is passed through their 

 solution. On the other hand, cane sugar added to water has 

 no such property, for it does not undergo dissociation. 

 Alcohol and chloroform are similarly devoid of action as 

 electrolytes because they are not ionised. 



The importance of this theory is that it is the ions of a salt 

 and not the molecules as a whole which act on the cells and 

 tissues of the body. Thus KOH and NaOH both dissociate 

 in solution and both possess strong caustic action. This 

 action is not due to the potassium, K, or the sodium, 

 Na, but to the OH group in each. Alcohol, on the other 

 hand, C 2 H 5 OH, does not dissociate and therefore has 

 no free OH group and no caustic action. Again, whilst 

 HCN or KCN are both extremely toxic by dissociation 

 with liberation of the CN ion, potassium ferrocyanide is 

 but slightly toxic since no free CN ion is found in its 

 solution. 



The rate of absorption of different acid or basic ions is 

 greatly different and this has a profound effect on the action 

 of the drug containing them. Thus the intestinal wall resists 

 the passage of magnesium, Mg, ions, whilst sodium, Na, and 

 potassium, K, are readily absorbed. Again, whilst Cl and 

 N0 3 ions are readily allowed to pass through, the S0 4 group 

 is not absorbed to any extent. Thus we can premise at once 

 that sulphates will be more effectual saline purgatives than 

 chlorides, and that magnesium sulphate will be more efficient 



