Calcium hydroxy palmitate: Possible precursor phase in calcium precipitation by palmitate

Calcium(II) precipitates with palmitate (Pal−), a process affecting calcium absorption in the gut, in a first-order reaction, as followed by a calcium electrode in neutral aqueous solution for excess of calcium(II), with a rate constant showing a minimum at physiological ionic strength of 0.32 ± 0.09 d−1 at 25 °C with a stoichiometry of 1.79 ± 0.03, lower than the expected 1:2. During ageing of precipitate, pH increases in the supernatant, while pH decreases for precipitation with excess of palmitate. Increasing pH, as in some aged food products, decreases the solubility, and K sp ′ = [Ca2+][Pal−][OH−] is found constant rather than Ksp = [Ca2+][Pal−]2 in agreement with an initial precipitation of Ca(OH)(Pal), which slowly may convert into Ca(Pal)2 · Ca(OH)(Pal) has a solubility minimum at physiological ionic strength with K sp ′ = (1.4 ± 0.4)10−16 M3. X-ray diffraction showed a d-spacing of 44.15 Å (different from 45.2 Å reported for Ca(Pal)2), and infrared spectroscopy confirmed the presence of a hydroxy group.