Solubility of dicalcium phosphate dihydrate by solid titration.

It was already known that calcium phosphate phases exhibit distinct solubility behaviors under physiological conditions. However, the stability of dicalcium phosphate dihydrate (DCPD) in specific ionic environments remains unclear. Previous research has shown that hydroxyapatite (HAp) is the only stable phase in 100 mmol x l(-1) KCl at 37 degrees C. This gap motivated further investigation into DCPD's behavior under similar conditions. The assumption that DCPD is stable at low pH has not been fully validated. No prior work had resolved whether DCPD remains stable or transforms under controlled solubility testing. This uncertainty drives the need for precise experimental evaluation. The study addresses a key question in calcium phosphate mineral stability. Understanding these dynamics is essential for dental and biomaterials research.

The aim of this study was to assess the solubility of DCPD in 100 mmol x l(-1) KCl at 37 degrees C across a pH range of 3.2 to 11.6. The researchers sought to determine whether DCPD remains stable or transforms into another phase under these conditions. The study focused on the equilibrium constitution of the precipitate. They used solid titration to measure solubility directly. The motivation was to clarify the stability of DCPD compared to HAp. Prior assumptions about DCPD's stability at low pH needed experimental validation. The researchers also examined the influence of seeding on phase transformation. The findings could impact interpretations of calcium phosphate solubility in biological systems.

The researchers used solid titration to determine the apparent solubility of DCPD in 100 mmol x l(-1) KCl at 37.0 +/- 0.1 degrees C. The pH range tested was from 3.2 to 11.6. X-ray diffraction was used to analyze the constitution of the precipitate. Scanning and transmission electron microscopy provided information on particle morphology. Energy-dispersive X-ray analysis was used to calculate the Ca/P ratio of the precipitate. The titration curve for DCPD was compared with previously reported data. The study also included seeding experiments with HAp to observe phase transformation. These methods allowed for a detailed characterization of the precipitate phases.

The titration curve for DCPD was found to be significantly lower than previously reported values. DCPD was the only identified phase at equilibrium at pH 3.60 and 4.50. At pH 4.47, 3.60, and 3.30, HAp formed after seeding with 1 mg HAp. X-ray diffraction confirmed the presence of DCPD at pH 3.60 and 4.50. The Ca/P ratio of the precipitate supported the identification of HAp after seeding. The observed titration curve for DCPD corresponds to a metastable equilibrium. HAp was found to be more stable than DCPD, especially below pH 4.2. These results challenge the general belief that DCPD is more stable at low pH.

The study concludes that the titration curve for DCPD represents a metastable equilibrium. HAp is more stable than DCPD, particularly at pH values below 4.2. The findings contradict the widely held belief that DCPD is stable at low pH. The researchers propose that solubility results for calcium phosphate phases need reconsideration. The formation of HAp after seeding suggests a transformation pathway. The implications for calcium phosphate studies are significant. The nature of carious lesions may require reevaluation based on these findings. The study highlights the importance of experimental validation in mineral solubility assessments.