A calcination-based method for the indirect determination of octacalcium phosphate phase purity

Octacalcium phosphate (OCP) is a precursor of bone mineral, but its phase purity is difficult to quantify because of strong peak overlap with biomimetic hydroxyapatite (HAp) in X-ray diffraction (XRD). Here, we present an indirect quantification approach based on the bulk molar Ca/P ratio after controlled calcination. Upon thermal treatment, OCP, HAp, and amorphous calcium phosphate decompose into a binary mixture of β-calcium pyrophosphate (β-CPP) and β-tricalcium phosphate (β-TCP), which can be reliably quantified by XRD. From their relative amounts, the bulk Ca/P ratio and an equivalent OCP phase content can be calculated. Because phosphate evaporation during calcination alters the β-CPP/β-TCP equilibrium and biases the derived Ca/P ratio, we demonstrate that stabilizing the furnace atmosphere is essential for reproducible results. Saturating the atmosphere with sacrificial monetite effectively suppresses phosphate loss and preserves the original composition. By varying dwell time between 2 and 24 h at two temperatures, we delimit the processing window within which the derived phase content is independent of the treatment conditions. At 1000 °C, this approach yields stable and reproducible OCP phase contents with negligible compositional drift over dwell times up to 24 h. The method is simple to implement and provides a robust tool for assessing OCP phase purity in both research and quality control settings. STATEMENT OF SIGNIFICANCE: Octacalcium phosphate (OCP) is an important precursor of bone mineral, yet its phase purity is difficult to quantify due to strong X-ray diffraction peak overlap with biomimetic hydroxyapatite. In this study, we present a simple and reproducible indirect method to assess OCP phase purity based on the bulk Ca/P ratio after controlled calcination and quantitative phase analysis of the resulting high-temperature phases. We demonstrate that phosphate evaporation during calcination can significantly bias the calculated Ca/P ratio and show that stabilizing the furnace atmosphere using sacrificial monetite effectively suppresses this effect. At 1000 °C, the method yields stable and reproducible OCP phase purity values over extended dwell times, making it suitable for both research and quality control applications.