Stability Constant of the Calcium-Citrate(3-) Ion Pair Complex

By measuring the change in potential caused by the addition of sodium citrate to the solutions containing calcium and sodium chlorides, the stability constants (K) of the calcium-citrate(3-) ion pair were determined at ionic strengths 0.02-0.17 mol L^-1 and temperatures 18-45 °C. No significant effect of temperature was observed on the values of the stability constants from 18 to 45 °C as the values of K occurred between 6.8 × 10⁴ and 7.1 × 10⁴ L mol^-1 in this temperature range. The K value at 7.0 × 10⁴ L mol^-1 was also found in good agreement with the 7.16 × 10⁴ L mol^-1 obtained by calcium oxalate monohydrate solubility measurements. Thermodynamic functions, ΔG°, ΔH°, ΔS°, and ΔC_p°, for the Ca²⁺-CH^3- ion pair association reaction were calculated by the use of a van't Hoff isochore and the values of the thermodynamic stability constant of the ion pair complex at different temperatures. These results indicated that, similar to other calcium ion pair complexes, the calcium-citrate(3-) ion pair complex is also entropically stabilized. The ion pair association reaction was almost thermoneutral in the temperature range 18-45 °C, as ΔH° values were <±1 kcal mol^-1.