Equilibrium Constants for Various Complexes of Calcium at 25°C
Complex Constant Symbol Value
CaCO3 CaHCO+ CaOH+ CaSO4
CaHCO3c K CaOHc
These complexes are weak enough that, in the complexometric titration determination using EDTA, they break and are included in the total calcium hardness reported. Thus, the total calcium hardness is composed of the "legitimate" cation, Ca2+, plus the complex ions as shown in the previous equations. This total calcium hardness must be corrected by the concentrations of the complexes in order to determine the correct activities of the calcium ions. Let the total concentration of the calcium species as determined by the EDTA titration be [Car]. Thus, the concentration of the calcium ion [ Ca^q)] is
[Ca2;q)] = [Car] - ([CaCOj] + [CaHCO+] + [CaOH+] + [CaSO4]) (11.31)
Table 11.4 shows the equilibrium constants of the previous complexes at 25°C. For other temperatures, these values must be corrected using the Van't Hoff equation. The use of this equation, however, requires the value of the standard heat of formation AH298. At present, none are available for CaCO^, CaHCO+, CaOH+, and CaSO4. Research is therefore needed to find these values.
Determination of the Calcium Complexes. The whole thrust of the discussion regarding water stability is the determination of whether CaCO3 precipitates at a given solution condition. The concentration of CaCO3 will therefore be determined in relation to the solubility product of CaCO3. Applying Hess's law,
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