A potentiometric study of the stability of aqueous yttrium–acetate complexes from 25 to 175 °C and 1–1000 bar

Abstract The stability of yttrium–acetate (Y–Ac) complexes in aqueous solution was determined potentiometrically at temperatures 25–175 °C (at P s ) and pressures 1–1000 bar (at 25 and 75 °C). Measurements were performed using glass H + -selective electrodes in potentiometric cells with a liquid junction. The species YAc 2+ and YAc 2 + were found to dominate yttrium aqueous speciation in experimental solutions at 25–100 °C (log [Ac − ]  YAc 3 0 into the Y–Ac speciation model was necessary. The overall stability constants β n were determined for the reaction Y 3 + + n Ac - = YAc n 3 - n β n = YAc n 3 - n [ Y 3 + ] · [ Ac - ] n · γ YAc n 3 - n γ Y 3 + · γ Ac - n where terms in square parentheses stand for the molal concentrations [mol/kg H 2 O], and γ i is the activity coefficient of the i th species. With temperature increase from 25 to 200 °C, the values of log  β 1 , log  β 2 and log  β 3 increase from 2.27 to 4.51, 3.86 to 7.86 and 4.63 to 10.24, respectively. Increase of pressure results in decrease of the stability of all three complexes: log  β 1 decreases by 0.21 and 0.37, log  β 2 by 0.40 and 0.73, and log β 3 by 0.57 and 1.01 when pressure increases from 1 to 1000 bar at 25 and 200 °C, respectively. However, the effect of pressure on Y speciation is much less pronounced than that of temperature and results only in a decrease of the predominance field of YAc 3 0 in high-pressure solutions. The calculated values of the partial molal thermodynamic functions for the overall formation reactions are: Δ r S ∘  = 23.4, 42.5, and 54.5 cal/mol K, Δ r C p ∘ = 73.2 , 122.2, and 182.5 cal/mol K, and Δ r V ∘  = 16.1, 30.9, 44.3 cm 3 /mol for YAc 2+ , YAc 2 + and YAc 3 0 , respectively. The revised Helgeson–Kirkham–Flowers (HKF) equation of state parameters were generated for Y–Ac complexes.