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

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 [Ca 2 Al(OH) 6 ]OH· n H 2 O (differing in n ) has been estimated and was found to be log L HC =−11.4 at 25 °C and −12.1 at 75 °C, respectively (where L HC =[Ca 2+ ] 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). [ABSTRACT FROM AUTHOR]