ISOTOPIC EXCHANGE OF PHOSPHATE ON SOME PHOSPHATIC COMPOUNDS

The kinetics of isotopic exchange of phosphate on hydroxyapatite, amorphous aluminium phosphate, phosphate-treated aluminium oxide and dicalcium phosphate dihydrate were studied in aqueous suspensions as a function of solid/solution ratio and amount of added isotope. The tracer exchange reaction was independent of time for the dicalcium phosphate due to a very small amount of exchangeable phosphate found in the solid and to the high solubility of the material. For the other phosphates, a power function and/or a function composed of a series of exponential terms were found suitable for representing the tracer kinetics. The theoretical background for both functions is shown to be approximately the same. Both describe tracer kinetics for a system containing more than one solid phase. The amounts of exchangeable P were found to depend strongly on the solid/solution ratio but not on the amount of radioactivity added. The dependence of the tracer kinetics on the solid/solution ratio is due to the ageing processes that give rise to the formation of surface layers having physical and chemical properties different from those of the bulk solid.     

APPARENT AND PARTIAL MOLAL VOLUMES OF CATION-SATURATED KAOLINS

Apparent and partial molal volumes of K-kaolin and K₂SO₄, Mg-kaolin and MgSO₄, and Ca-kaolin and CaCl₂ as a function of electrolyte and particle concentration were determined from density measurements at 30°C. The apparent (ø) and partial molal volumes (V) of the kaolin or the salts studied increase with concentration and are larger in salt-clay mixtures. Kaolin saturated with electro-strictive cations has lower volumes than that saturated with less strictive ones.     

POSITIVE ADSORPTION FROM MIXTURES OF THREE ELECTROLYTE SOLUTIONS

The double layer theory was used for estimating the relative amounts of positively adsorbed cations when a mixture of three salts with mono-, di-, and trivalent cations is present in clay electrolyte systems. It is shown that the adsorbed cations are dominated by the trivalent cation when the salts are present at equal concentrations.