106 



partition method, for the ionization of the first hydrogen of Orthophos- 

 phoric Acid is fairly large. It does not accurately obey the Mass Action 

 Law : hence K^ has no defi . ite meaning. However, the degree of disso- 

 ciation was determined from the values of the electrical conductivity of the 

 acid and its salts, and other known data, and the following values were ob- 

 tained, at 18°C : 



Cone. Mols. per Litre. Dej/ree of Ionization. 

 0.1 0.286 



0.05 .364 



0.01 .602 



0.002 .839 



0.0002 .965 



Ostwald's Dilution Law requires that 



Cr2 _ 

 l—r~ 



Ki 



wherein C represents the concentration and r the degree of ionization. 

 Substituting the values of r we obtain, for the values of Ki at the different 

 concentrations : 



Coneentration. K. 



0.1 0.0114 



.05 .0104 



.01 .0091 



.002 .0087 



.0002 .0053 



and it is seen that the deviation from the law is marked. 



A comparison of the ionization constants of phosphoric acid with those 

 of some other acids is interesting. 



KXlQi" 

 Acetic Acid, C2H3O2 — H 180,000. 



Carbonic. HOO3 — H 3,040. 



Hydrosulphuric, HS — H 670. 



Boric, H.BOg — H 17. 



»Phenol, C.HjO — H 1.3 



Phosphoric Acid, K, = H.PO^ — H 100,000,000. (Approx.). 



" K2 = HPO'^4— H 2,090. 



" K, =PCM— H 0.00555 



♦These values are taken from Walker, "Zeit Phys. Chein." S2, 137, 1900. 



