Dissociation of Compounds in Water. 479 



used was standardized against sodium and potassium car- 

 bonates, both of which had been repeatedly recrystallized. 

 The chlorine was also determined as silver chloride. 



It will be of interest to compare the amount of the disso- 

 ciation of the acids with that of their salts, to ascertain 

 whether the hydrogen of the acid or the replacing metal has 

 the greater tendency to dissociate from the molecules into ions. 



The chlorides of sodium, potassium, and ammonium disso- 

 ciate less than hydrochloric acid. At 0*1 n this difference 

 amounts to about 4'5 per cent. The difference is still greater 

 when the acid is compared with the chlorides of barium, cad- 

 mium, and zinc, which dissociate much less than the univalent 

 alkali chlorides. 



Sulphuric Acid. — The solution was titrated against an 

 alkali which had been titrated against the standardized hydro- 

 chloric acid. The sulphur was also determined as barium 

 sulphate. Like the sulphates, it is quite possible that the acid 

 dissociates first into H and HS0 4 , and with increased dilution 

 HS0 4 dissociates into H and S0 4 . Sulphuric acid dissociates 

 less than the potassium salt, which is the reverse to that 

 observed with hydrochloric acid. The dissociation of the acid 

 is much greater than that of the sulphate of magnesium, but 

 the dissociation of this salt is abnormally small, as will be 

 shown later. 



Nitric Acid. — The solution was compared with alkalies 

 which had been standardized against both hydrochloric and 

 sulphuric acids. Nitric acid dissociates slightly more than its 

 alkali salts ; it is also a little more dissociated than hydro- 

 chloric acid, but the difference is slight. 



Phosphoric Acid. — The phosphoric acid used was prepared 

 by the oxidation of phosphorus with nitric acid. The arsenic 

 present was precipitated by hydrogen sulphide. A qualitative 

 analysis of the preparation showed it to be free from those 

 impurities which would be expected to accompany the phos- 

 phorus, or to be introduced by the method of preparation. 

 The solution was standardized by determining the phosphorus 

 as magnesium pyrophosphate. The acid is of special interest, 

 because the conductivity shows very small dissociation, lb 

 shows, further, that the acid probably dissociates into only 

 two ions, H and H 2 P0 4 , since the greatest conductivity is less 

 than if the dissociation into two ions were complete. The 

 freezing-point results lead to the same conclusions as the 

 conductivity. For the concentrations studied, the decrease 

 in dissociation is over 60 per cent. 



These results show very satisfactorily the rate of chano-e in 

 the dissociation. The rate of change is slow as we approach 



