Stability constants of ytterbium cyclohexaphosphate complexes

The compositions of complexes formed in solution are calculated by the iteration (in three approximations) complex method and using the earlier study of the solubility in the YbCl₃-Li₆P₆O₁₈-H₂O system, in which one poorly soluble compound Yb₂P₆O₁₈ · 16H₂O and soluble ytterbium cyclohexaphosphate complexes are formed. The composition are Yb₆(P₆O₁₈)]¹²⁺, Yb₂(P₆O₁₈)]⁰, Yb₃(P₆O₁₈)₂]^3?, Yb(P₆O₁₈)]^3?, and Yb₂(P₆O₁₈)₃]¹². The stability constants of these complexes are determined: β₆₁ = 1.2 × 10¹², β₂₁ = 6 × 10⁴, β₃₂ = 9 × 10¹², β₁₁ = 29.6, and β₂₃ = 3.9 × 10¹² at an ionic strength of ～1. The solubility product of Yb₂P₆O₁₈ · 16H₂O is found to be 2 × 10^?10. The curves of residual concentrations of ytterbium and cyclohexaphosphate plotted with reference to the stability constants of the complexes and the solubility product of Yb₂P₆O₁₈ · 16H₂O agree with experimental data.