A comprehensive study on the dominant formation of the dissolved Ca(OH) sub(2(aq)) in strongly alkaline solutions saturated by Ca(ii)

• Published in: RSC advances 2016-05, Vol.6 (51), p.45231-45240
• Main Authors: Kutus, B, Gacsi, A, Pallagi, A, Palinko, I, Peintler, G, Sipos, P
• Format: Article
• Language: English
• Subjects: Constants; Electrolytes; Formations; Mathematical models; Slaked lime; Solubility; Speciation; Titration
• ISSN: 2046-2069
• DOI: 10.1039/c6ra05337h

The solubility of calcium hydroxide and the aqueous speciation of Ca(ii) in alkaline medium at various temperatures and background electrolyte concentrations were characterized by solubility measurements applying ICP-OES and potentiometric detection methods. Contrary to suggestions from previous literature, the (dissolved) Ca(OH) sub(2(aq)) was found to be the dominant solution species above pH similar to 13, although the well-known CaOH super(+) sub((aq)) is also formed to a much smaller extent. The solubility product, as well as the formation constants for the species CaOH super(+) and Ca(OH) sub(2) were found to be (8.8 plus or minus 0.2) 10 super(-5) M super(3), (1.5 plus or minus 0.1) M super(-1) and (4.7 plus or minus 0.1) M super(-2), respectively, at 25 degree C, at 1 M ionic strength and expressed in terms of concentrations. The most important implication of this model is that the total concentration of the dissolved calcium(ii) cannot be decreased below ca.2 10 super(-4) M at any base concentration, even if this is increased to the solubility limit of the caustic. This statement was further validated viaprecipitation titrations. The standard enthalpies and entropies of the reactions were also calculated from temperature-dependent solubility measurements.