Studies of metal carbonate equilibria. Part 16. The beryllium(II)–water–carbon dioxide(g) system in neutral-to-alkaline 3.0 mol dm–3 perchlorate media at 25 °C

Abstract

The complex-formation equilibria in the system Be^II–H₂O–CO₂(g) have been studied by potentio-metric and solubility measurements in 3.0 mol dm^–3NaICO₄ at 25 °C. The e.m.f. measurements were performed in the range 6.4 ⩽–log h⩽ 8.5 (h= free hydrogen-ion concentration), the total metal concentration B was varied from 0.97 to 30 mmol dm^–3 and the partial pressure of CO₂ from 0.01 to 0.97 atm. The solubility measurements were carried out in the range 4.95 ⩽–log h⩽ 7.0, at partial pressure of CO₂ of 0.97 and 0.29 atm. The e.m.f. and solubility data can be explained by assuming formation of the mononuclear complexes Be(OH)₂(CO₂), [Be(OH)₃(CO₂)]^–, and [Be(OH)₄(CO₂)]^2–. The potentiometric data can be best explained by also taking into consideration the formation of one polynuclear complex of stoicheiometry [Be₃(OH)₉(CO₂)₃]^3– or [Be₃(OH)₁₀(CO₂)₃]^4–. The set of stability constants obtained from the solubility measurements are log K_s0= 6.18 ± 0.03, log β₁₁₀=–6.02 ± 0.09, log β₁₃₁=–16.82 ± 0.02, and log β₁₄₁=–24.2 ± 0.1. From the potentiometric measurements log β₁₂₁=–10.12 ± 0.06, log β₁₃₁=–16.68 ± 0.08, log β₁₄₁=–24.22 ± 0.04, and log β₃₉₃=–45.5 ± 0.5 or log β₃₁₀₃=–52.0 ± 0.5. The Raman spectra of concentrated beryllium(II) carbonate solutions in the range pH 7.15–11 indicate that the CO₂ is present as carbonate in the two predominant complexes. Consequently these complexes should be written as [Be(OH)₂(CO₃)]^2– and [Be₃(OH)₃(CO₃)₃]^3– or [Be₃(OH)₄(CO₃)₃]^4–. Tentative structures for the two possible polynuclear complexes are proposed and discussed.