Stability constants of aluminium fluoride complexes

Abstract

Pointwise titrations of aluminium nitrate solutions with sodium fluoride were made at ionic strengths of 0.5, 0.2, 0.1 and 0.05 M and at temperatures of 25° and 37°. The total Al 3+ concentration was kept constant at 5 × 10 −4 M and the total F − concentration covered the range 5–20 × 10 −4 M with a minimum of 20 points per titration. It is shown that only mononuclear complexes were present with these solution compositions. Determinations of H + and F − in solution were made by the use of quinhydrone and lanthanum fluoride electrodes, respectively. Formation constants were calculated by three different procedures. Best results were obtained by a non-linear generalized least-squares method that ( a) permits calculation of the formation constants with the simultaneous presence of all the complexes being taken into account, ( b) does not require segregation of data according to the values of the formation function ( n ), and ( c) yields standard errors that reflect the errors in all the experimental measurements. Values for the formation constants at infinite dilution were obtained by a linear least-squares adjustment of the values obtained for the four finite ionic strengths. The formation constants thus obtained for the reaction AlF n−1 + F ⇌ AlF n (charges omitted) at 25° have logarithmic values of 6.69 ± 0.02, 5.35 ± 0.03, 3.68 ± 0.01 and 2.75 ± 0.04 for n = 1, 2, 3 and 4, respectively. The corresponding values at 37° are 6.68 ± 0.10,5.34 ± 0.02,3.94 ± 0.04, and 3.29 ± 0.05.