Combinations of isocitrate and citrate enhance calcium salt solubility and supersaturation robustness

Calcium isocitrate tetrahydrate was characterised and found to be less soluble in water than calcium citrate tetrahydrate. The solubilities of both decrease with temperature, but increase with pressure. Calcium binds isocitrate with a constant of Ka = (6.80 ± 0.05) × 103 (ΔH° = −22.1 ± 2.6 kJ mol−1; ΔS° = −6.7 ± 8.7 J mol−1 K−1). The solubility product is Ksp = (8.7 ± 0.08) × 10−17 (ΔH° = 30.1 ± 2.2 kJ mol−1; ΔS° = −206 ± 7 J mol−1 K−1). Calcium isocitrate dissolves in aqueous sodium citrate, spontaneously forming solutions supersaturated in calcium citrate by a factor of at least two for 0.50 mol L−1 sodium citrate from which precipitation initiates after 20 h. Calcium citrate dissolves under similar conditions in sodium isocitrate forming supersaturated solutions in calcium isocitrate without precipitation for months. Slow crystallisation kinetics of calcium isocitrate explains the supersaturation robustness for combinations of calcium citrate and sodium isocitrate inspiring for formulation of whey mineral products.