Diego Ferri

Studies on metal carbonate equilibria. Part 10. A solubility study of the complex formation in the uranium(VI)–water–carbon dioxide (g) system at 25 °C

The complex formation in the system UVI–H2O–CO2(g) was studied by measuring the solubility of UO2(CO3)(s) as a function of the CO2 pressure and the hydrogen-ion concentration. The measurements were made at 25 °C, by using two different ionic media, 0.5 and 3 mol dm–3 NaClO4, respectively. The equilibrium constants for the following reactions were determined. Interaction UO2(CO3)(s)⇌ UO22++ CO32– Ks,o UO22++tCO32–⇌ UO2(CO3)t, t= 1–3 βt,1 3UO22++ 6CO32–⇄[(UO2)3(CO3)6]6–β6,3 coefficients Iµ(i,j) for the various uranium species with ClO4–(or Na+) were determined by using the Bronsted, Guggenheim, Scatchard specific ion-interaction theory. By using these and equilibrium data from the literature, we propose the following set of ‘best’ estimates of equilibrium constants, referred to the pure water reference state: log Ks,o=–14.4 ± 0.1, log β1,1= 9.5 ± 0.2, log β2,1= 16.6 ± 0.2, log β3,1= 21.3 ± 0.2, and log β6,3= 53.4 ± 0.8.

On the hydrolysis of the titanium(IV) ion in chloride media

Abstract Determinations of the [Ti(IV)]/[Ti(III) ratio in solutions of titanium(IV) chloride equilibrated with H2(g), at 25°C in 3 M (Na)Cl ionic medium, have indicated the predominance of the Ti(OH)22+ species in the concentration ranges 0.5 ⩽ [H+] ⩽ 2 M and 1.5 x 10−3 ⩽ [Ti(IV)] ⩽ 0.05 M. From the equilibrium data the reduction potential has been evaluated Ti(OH)22+ + 2 H+ + e ⇋ Ti3+ + 2H2O, EoH = (7.7 ± 0.6) x 10−3 V. The acidification reactions of Ti(OH)22+ were also studied in 12 M(Li)Cl medium at 25°C by measuring the redox potential of the Ti(IV)/Ti(III) couple as a function of [H+]. The potentiometric data in the acidity range 0.3 ⩽ [H+] ⩽ 12 M have been explained by assuming Ti4+ + e ⇋ Ti3+, Eo = 0.202 ± 0.002 V Ti4+ + H2O ⇋ TiOH3+ + H+, log Ka1 = 0.3 ± 0.01 Ti4+ + 2H2O ⇋ Ti(OH)22+ + 2H+, log Ka1Ka2 = 1.38 ± 0.05.

Salt effect on the dissociation constant of acid-base indicators

The effect of the ionic environment on the dissociation constant of acid-base indicators is accounted for by using the specific interaction theory (SIT) of Broensted, Scatchard, and Guggenheim. It is shown that the SIT may be applied to evaluate the activity coefficients of organic ions, regardless of their size and structure, if suitable modifications are introduced.

Studies of metal carbonate equilibria. Part 15. The beryllium(II)–water–carbon dioxide(g) system in acidic 3.0 mol dm–3 perchlorate media

The complex-formation equilibria in the system BeII–H2O–CO2(g) have been studied by e.m.f. techniques in 3.0 mol dm–3(Na)ClO4 at 25 °C in the range 2.0 ⩽ log h⩽ 6.2, where h is the free hydrogen-ion concentration. The total metal concentration, B, has been varied from 1 to 80 mmol dm–3 and the partial pressure of CO2 from 0.1 to 0.97 atm. The e.m.f. data obtained can be explained by assuming, in addition to previously proposed hydrolytic species, the temary equilibria (i)–(iv). The stability constants of the mixed complexes are as follows: log β331=–8.90 ± 0.02, 3Be2++ 3H2O + CO2(g)⇌[Be3(OH)3(CO2)]3++ 3H+(i), 5Be2++ 6H2O + CO2(g)⇌[Be5(OH)6(CO)2]4++ 6H+(ii), 6Be2++ 9H2O + 2CO2(g)⇌[Be6(OH)9(CO2)2]3++ 9H+(iii), Be2++ 2H2O + CO2(g)⇌ Be(OH)2(CO2)+ 2H+(iv) log β561=–17.24 ± 0.04, log β692=–29.46 ± 0.06, and log β121=–10.4 ± 0.1. The Raman spectra of concentrated solutions containing the different mixed complexes have been recorded. From these results tentative structures for the mixed hydroxo–carbonato complexes are proposed and discussed. The (1,2,1), (3,3,1), and (5,6,1) complexes are suggested to possess stoicheiometries BeCO3,[Be3(OH)2(HCO3)]3+, and [Be5(OH)4(CO3)]4+ respectively.

Complex Formation Between Lead(II) and Bromide Ions

Abstract Activity coefficient effects represent the main source of uncertainty in the interpretation of equilibrium data in the Pb(II)-Br- system. This is presumably the reason for a number of contrasting reports on the stoichiometry and formation constants of PbBrn 2-n complexes. Specific Interaction Theory is employed in the present paper to select experimental conditions under which minimal activity coefficient variations are produced as well as to interpret the data. Emf measurements (lead amalgam electrode) based on junctionless cell (sodium ion-sensitive glass electrode, internal reference) show that in the range of bromide concentration 0.03–5.0 m all species PbBrn 2-n (1 ≤ n ≤ 6) are formed. The corresponding formation constants, expressed on the 5.0 m (= 4.1 M) NaC104 activity scale, are reported.