June 26, 1916 Mineral Phosphates and Plant Nutrition 509 



Equations A and B make it evident that the hydrogen ion concentra- 

 tion for the various acids will determine the course of the reactions ren- 

 dering the rock phosphate available. The hydrogen ion concentration 

 is made up of two factors — namely, the concentration and the strength 

 of the acid. Obviously under the conditions of these experiments satu- 

 rated solutions of rock phosphate and carbonic acid are employed. The 

 relative insolubility of the rock phosphate tends to decrease greatly the 

 concentration of the H+ from either 6, 7, or 8. The relatively greater 

 solubility of the calcium bicarbonate, since it furnishes HCO3, would 

 also tend to decrease the H+ concentration from carbonic acid, but this 

 factor of common ion effect is of far less importance upon the concen- 

 tration of the H+ from H.COg than the solubility of the tricalcium phos- 

 phate upon equations 6, 7, and 8, especially since the Ca++ from the 

 Ca(HC03)2 is removed by plants. 



Assuming equivalent or unit concentrations of the substances HjCOg, 



H3PO4, H2PO4, and HPO4 are present— that is, eliminating the factor of 

 concentration of the substances producing the H — the relative strength 

 of these acids is given by their ionization constants, thus: 



(i)« H2C034=^H'-+HC03 Kai«° 3.0X10- 



(6) HP04^P0,+ H+ Ka^8° 3.6X10-13 



^—7 



(7) H2PO,i=^HP04 + H+ Ka^^" 1.95 X IO- 



CS) H3PO,<=^H^4 + H"" Ka^^o 1.1X10-2 



The mass law for monobasic acids (HAc) has the form 

 (Cone H+) (Cone Ac)^ ^.^^^ ^^^ ^^.^^ ^^ equations i, 6, and 

 Cone HAc 

 7 are weak acids (Ka<io-^), the mass law assumes the form 

 Ka=(Conc H+) (Cone Ac) = (Cone H+)2, because the concentration of 

 HAc is practically unity. The concentrations of H+ for these equations 

 at 1 8° Care for: 



(i) V3X io-' = 5.5X 10-^ for C(i>H+ 



(6) V3-6X 10-^^ = 6X10-' for C(^)H+ 



(7) Vi-95X io-' = 4-4X 10-* for 0')H+ 



For the first hydrogen of H3PO4 the above expression can not be 

 used, since the amount H3PO4 compared to its ions is small rather than 



C0C2 



large. Here the mass law must be used in its true form, K = ^_^, where 

 C is equal to the initial concentration of H3PO4 and oc degrees of ioniza- 



o See equations, page 508. 



