Activity problem. Citric acid is a triprotic acid (H₃A) whose anion (A^3-) forms stable complexes with many metal ions.

When a Ca²⁺ ion-selective electrode with a slope of 29.58 mV was immersed in a solution having A_Ca2+ = 1.00 × 10^-3 , the reading was +2.06 mV. Calcium citrate solution was prepared by mixing equal volumes of solutions 1 and 2.

Solution 1: 

[Ca²⁺] = 1.00 × 10^-3 M, pH = 8.00, μ = 0.10 M

Solution 2: 

[Citrate]_total = 1.00 × 10^-3 M, pH = 8.00, μ = 0.10 M

The calcium citrate solution gave an electrode reading of 25.90 mV. 

(a) Refer to the discussion with Figure B-2 in Appendix B. Calculate the activity of Ca²⁺ in the calcium citrate solution. 

(b) Calculate the formation constant, K_f, for CaA^-. Assume that the size of Ca_A^- is 500 pm. At pH 8.00 and μ = 0.10 M, the fraction of free citrate in the form A^3- is 0.998.

Figure B-2

Ke Ca2 + + --CaA