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COMPREHENSIVE STUDY ON THE DISSOLVED Ca(OH)2 IN STRONGLY ALKALINE SOLUTIONS

Kutus, B. Gácsi, A. Pallagi, A. Pálinkó, I. Sipos, P. Peintler, G.

In order to reveal the possible formation of Ca(OH)2(aq) solution species, NaOH solutions saturated with solid calcium oxide were prepared in the concentration ranges of 0–1 M for ICP-OES measurements (5–75 °C) and 0–4 M NaOH for precipitation titrations and potentiometric investigations (both at 25 °C). The results of these ICP measurements proved that the observed and calculated data are in good agreement only when Ca2+, CaOH+ and Ca(OH)2 solution species are considered together in the chemical model. The calculated thermodynamic constants at 25 °C are as follows: L(Ca(OH)2(s)) = 8.8×10–5 is the solubility product, while K(CaOH+) = 1.5 and β(Ca(OH)2) = 4.7 are the stability products. The distribution diagram calculated on the basis of these constants shows convincingly that the Ca(OH)2 cannot be omitted above the concentration of ~ 0.2 M NaOH. Moreover, dissolved Ca(OH)2 becomes the most significant species above 2 M base concentration. An important consequence is that the total Ca(II) concentration cannot be decreased below ~ 3×10–4 M even at the highest concentrations of the base.