Estimation of the solubility product of hydrocalumite–hydroxide, a layered double hydroxide with the formula of [Ca2Al(OH)6]OH·nH2O

• From NaOH/NaAl(OH)4/Ca(OH)2/H2O mixtures, Ca(OH)2 precipitates at low [Al(III)]T-s.
• At a given Al(III)-concentration, [Ca2Al(OH)6]OH⁎nH2O appears in the system.
• From this, the solubility product of hydrocalumite–hydroxide was determined.
• An upper limit for the formation constant of CaAl(OH)4+ was also computed.

From aqueous NaOH/Ca(OH)2/NaAl(OH)4 mixtures, after allowing short reaction times we observed the precipitation of Ca(OH)2(s) at lower, and a mixture of Ca(OH)2(s) and a layered double hydroxide, hydrocalumite (HC) at higher aluminate concentrations. From the maximum aluminate concentration, at which the equilibrium solid phase is still portlandite (i.e., further increase in the aluminate concentration results in HC appearing in the precipitate beside the portlandite), the concentration based solubility products of two polymorphs of HC with the formula of [Ca2Al(OH)6]OH·nH2O (differing in n) has been estimated and was found to be log LHC=−11.4 at 25 °C and −12.1 at 75 °C, respectively (where LHC=[Ca2+]2[Al(OH)4−][OH−]3) and at constant ionic strength (I=1 M NaCl). To the best of our knowledge, this is the first published estimate for the solubility product of hydrocalumite. Additionally, from the composition obtained for NaOH/Ca(OH)2/NaAl(OH)4 mother liquors in equilibrium with Ca(OH)2(s), attempts were made to extract the formation constant of the ion pair CaAl(OH)4+. It was found, that the effects caused by the supposed formation of this solution species are too small to be reliably determined, which allowed an upper estimate for its formation constant, K, to be suggested in the temperature range of 25–75 °C (K<200 and 40 M−1 at 25 and 75 °C, respectively).

Graphical AbstractThe solubility products of two polymorphs of hydrocalumite–hydroxide, [Ca2Al(OH)6]OH.nH2O (differing in n) has been estimated in the temperature range of 25–75 °C and an upper limit for the formation constant of the ion pair CaAl(OH)4+ is also given.Figure optionsDownload as PowerPoint slide