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
The complex-formation equilibria in the system BeII–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 Ks0= 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.