105 



The dissociation constants of tlie successive hydrogens of Orthophos- 

 phoric Acid will be designated Kj, K., and K,,. They are defined by the 

 Mass Action equations : 



Ki = H X H, p6, 

 H3PO, 



K, = H X HPOt 

 H2PO5 



K3 = H X POf 

 HPOt 



They will be considered in inverse order. 



Ks may be determined by substituting the value of h, obtained by the 

 partition method, in the hydrolysis c<iuation, 



h = .95. 



Kw = 8 X 10"' 5 (mols. per litre). 



Kb = 1.72X10-^ 



(.95)2 _ 50X10"'^ 

 (1. — .95)^~(K3) (1.72X10 ') whence, 

 K3 = 6.48 X 10-'^ (mols. per litre). 



Kt was also determined by an utterly independent method based upon 

 the measurement of the increase of electrical conductivity produced on 

 adding to solutions of Na.fJPOi, varying amounts of ammonia. Time will 

 not permit a description of the method and calculations which are some- 

 what complicated, but the values obtained at different concentrations 

 agreed remarkably well with the above value. 



In like manner K. may be calculated from the hydrolysis of NaNH4 

 HPO,. The value obtained by substitution in the above equation is K,^ 

 3.9 X 10 — '. but this calculation fails to take into account the influence of 

 the unionized substances in the solution. , 



The correction involves merely the application of the Mass Action Law, 

 and the principle that, in u mixture of salts with a common ion each salt 

 has the same degree of ionization as if it were present alone at a concen- 

 tration e<]ual to the sum of the concentration of the two salts. However, 

 the algebra involved is not particularly entertaining, and it will perhaps be 

 sufficient to give the mean corrected- value of K.===:2.09 x 10 — ". It is then 

 seen that the correction is large. The value of K3, when corrected for the 

 influence of unionized substances becomes Kj=5.5d x 10 — •". 



I'he hydrolysis of the salt NH4H.,P04 is too small to be measured by the 



