105 



The dissociation constants of the successive liydrogens of Ortliopbos- 

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

 Mass Action equations : 



Ki = H X H, PO, 

 H3 PO, 



K2 = H X HPOt 



H,PO:f 



K3 z= H X POf 

 HPO^ 



'i:bey will be considered in inverse order. 



K:! may be determined by snl)stituting the value of h, obtained by the 

 l»artitiou method, in the hydrolysis c<iuation. 



h = .95. 



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

 Kb = 1 72 X 10'. 



(.95)^ _ 50X10^' 



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

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



Ki was also determined by an utterly independent method based upon 

 the measuremeut of the increas^e of electrical conductivity produced on 

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

 not permit a desci'iptiou of the method and calculations which are some- 

 what complicated, but the values obtained at different concentrations 

 agreed remarkabl.\- well with the above value. 



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

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

 3.9 X .10 — '. but this calcuhitiou 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 irruixture 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<iual to the siun of the concentration of the two salts. However, 

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

 sufhcient to give the mean corrected value of K.=2.09 x 10 — '. It is then 

 seen that the correction Is large. The value of Kg, when corrected for the 

 influence of unionized substances becomes K3=5.55 x 10 — ". 



The hydrolysis of the salt NH^HPOj is too small to be measured by the 



