Fig. 3.

The first sharp peak is produced by the ν1 symmetric stretching of the aqueous HPO₄²⁻ molecule of the Na₂HPO₄ solution. Titration of CaCl₂ immediately induces precipitation of ACP (dark blue line, Fig. 4A), completely consuming all aqueous HPO₄²⁻. Transformation of ACP into OCP begins after ca. 150 min (bright blue line, Fig. 4B) and to HAP after ca. 500 min (ca. 8 h; green line, Fig. 4C) with complete transition achieved after ca. 1000 min (ca. 18 h; orange line, Fig. 4F–G) indicated by a clear peak sharpening.