Solubility and Solubility Product of Dicalcium Phosphate Dihydrate in Aqueous Solutions and Soil Solutions

Abstract

AbstractThe solubility product (Ca2+)(HPO42‐) of CaHPO4 · 2H2O was determined in several aqueous solutions containing ions common to soil solutions. Extrapolation of analytical Ca and P concentrations to zero ionic strength yielded pKSD values ranging from 6.54 to 6.59 and averaging 6.57 for 10 aqueous salt solutions. A method of successive approximation that computed ionic activity was used to determine the pKSD for three concentrations of 15 different aqueous salt solutions saturated with CaHPO4 · 2H2O. Calculated pKsp values ranged from 6.44 to 6.63; the average was 6.55. Eight soil samples that varied widely in pH and soil‐test P were treated with CaHPO4 · 2H2O at rates of 600 to 3,000 ppm P. Soil solutions were displaced, analyzed, and (Ca2+)(HPO42‐) was calculated. Eight of the 20 samples of soil were considered to be saturated with CaHPO4 · 2H2O because their (Ca2+)(HPO42‐) values were approximately the same as that for similar aqueous salt solutions. It was concluded that the presence of excess CaHPO4 · 2H2O in soil could be inferred by ion‐activity products of displaced soil solutions.