YcaOH« 7ca, and yh are the activity coefficients of CaOH+, Ca, and H+, respectively. [CaSO4] is calculated as follows:

CaSO4c KCaSO4c

The activity coefficient /CaSO4c = 1, because CaSO4 is not dissociated. /SO is the activity coefficient of the sulfate ion.

The previous expressions for [CaCO3], [CaHCO+], [CaOH+], and [CaSO4] may now be substituted into Equation (11.31) and the result solved for [Ca2+] to produce

rCaKw YCaYSO4[SO4 ]



Example 11.8 Analysis of a water sample yields the following results: [TDS] = 140 mg/L, [Car] = 0.7 mgmol/L, [Mgr] = 0.6 mgmol/L, [A]mgeq = 0.4 mg/L, sulfate ion = 0.3 mgmol/L, temperature = 25°C, and pH = 6.7. Calculate the concentration of the calcium ion corrected for the formation of the complex ions.


rCaOHcKCaOHcrH[H+] K


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