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Solubility and Ion Activity Products of Calcium Phosphate Minerals

USDA-ARS;

D. M. Miller

Dep. of Agronomy, Univ. of Arkansas, Fayetteville, AR 72701.

^* Corresponding author (philipm{at}comp.uark.edu).

ABSTRACT

Most phosphate mineral solubility data in use were measured more than 20 yr ago when instrumentation and methods were generally less sophisticated. The objective of this study was to measure calcium phosphate mineral solubilities using modern analytical equipment and techniques. Natural and synthetic Ca phosphate minerals (<50 µm) were equilibrated in water and dilute HCl for 6 mo with continuous agitation. X-ray diffraction (XRD) was used to identify compositions before and after equilibration. Filtered (<0.2 µm) solution aliquots were analyzed for pH and chemistry. Chemical compositions were determined using inductively-coupled plasma spectroscopy (ICP). Aqueous carbonates were determined from alkalinity titrations after phosphate alkalinity corrections. The chemical speciation model "Soilchem" was used to calculate free ion concentrations and ionic strengths. Ion activity products (IAPs) were determined from free ion concentrations and single ion activity coefficients. Experimental log IAPs of monetite (CaHPO₄) were consistent (–6.60) with published values for brushite (CaHPO₄·2H₂O) suggesting that the IAPs of these minerals are identical. No change in mineralogy during equilibration was revealed by XRD. Experimental log IAPs of synthetic whitlockite (–30.74, β-Ca₃(PO₄)₂) indicated somewhat lower solubility than published values. Log IAPs of synthetic hydroxyapatite (–56.02, Ca₅(PO₄)₃OH) were within the range of published values. Natural samples of whitlockite and fluorapatite did not attain equilibrium due to extraneous siderite (FeCO₃) and silica phases within the samples. A long equilibration time, modern analytical equipment, and a chemical speciation program can yield more reliable solubility data for phosphates and other minerals.

Contribution from USDA-ARS.

Received for publication April 28, 1998.

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