298 PROTOPLASM 



boric acid is 0.01 per cent, and acetic acid 0.4 per cent disso- 

 ciated, while the strong nitric acid is 82 per cent dissociated. The 

 salt, potassium chloride, is 86 per cent dissociated when at 0.1 

 molar concentration, which means that at a concentration of 

 0.1 ilf (about 0.7 per cent) 86 out of every 100 molecules of 

 potassium chloride are dissociated into their two ions K+ and Cl~, 

 while 14 molecules remain intact. The degree of dissociation, if 

 the theory is correct, is dependent upon concentration. At dilute 

 concentration, salts are more highly ionized than when concen- 

 trated. Infinite dilution yields complete dissociation. 



Arrhenius advanced the dissociation theory of electrolytes in 

 1883. Twenty-one years later, the American physical chemist 

 A. A. Noyes suggested a theory of complete dissociation. Noyes 

 was interested in the optical activity and color of electrolytic 

 solutions which are independent of concentration and therefore 

 of the degree of dissociation. This led him to the conclusion 

 that the salts studied were completely ionized up to a concentra- 

 tion of ilf/30 (a 0.2 per cent solution if the salt is sodium chloride). 

 If the dissociation is complete, then why the anomalous osmotic 

 behavior; that is, why, if there are two ions for every molecule, 

 is not the osmotic pressure twice as great? Similar anomalous 

 behavior exists in the electrical conductivity of solutions. Salts 

 in solution conduct less current than they should if every molecule 

 is dissociated into two ions. In postulating complete dissoci- 

 ation, Noyes must in some way explain the anomalous behav- 

 ior, in osmotic pressure, electrical conductivity, freezing point, 

 etc. He did it in terms of migration velocity — "the decrease in 

 conductivity is due merely to a change in migration velocity." 

 What this means we shall see in a moment. G. N. Lewis went 

 further by saying that the experimental facts suggest com- 

 plete dissociation up to a concentration of normal or half normal 

 (normal sodium chloride is nearly 6 per cent). Then came 

 the work of the Dane Niels Bjerrum, who, with others, has 

 practically convinced the chemical world that many salts, even 

 in high concentration, are fully dissociated into their ions and 

 that all strong electrolytes if not fully dissociated are more so 

 than was formerly thought to be true. The contribution of 

 Bjerrum is so important and so simply and clearly expressed in 

 his original paper that we can do no better than to let him tell 

 what happens in his own words. 



