AbstractThe solubility‐product constant of LaF,(s) was obtained by potentiometric titration of sodium fluoride with lanthanum perchlorate in sodium perchlorate solutions of various ionic strengths. The limiting value ofpKs.pwere found to be 17.65 and 17.95 at 20° and 25°C, respectively. Thermodynamic calculation from these values givesδG°=‐102.46 kJmol−1,δH°= 100.40 kJmol−1andδS°= 680.40JK−1for the reaction, La+3(aq)+3F‐(aq)=LaF,(s), andδG°=‐1613.8kJmol‐1, δH1° =‐1594.0kJmol−1andS° =433.1JK−1mol−1for the formation of LaF2,(s) at 25°C. The mean activity coefficients and solubilities of LaF2,(s) in NaCIO4solution at various i