Nils Ingri

Equilibrium and structural studies of silicon(iv) and aluminium(iii) in aqueous solution. V. Acidity constants of silicic acid and the ionic product of water in the medium range 0.05–2.0 M Na(CI) at 25°C

Abstract The apparent ionization constants for silicic acid, k 1 and k 2 , and the ionic product of water, k w , have been determined in 0.05, 0.1, 0.2, 0.4 and 2.0 M Na(CI) media at 25°C. The medium dependence of these constants was found to fit equations of the form log k i = log K i +a i I 1 2 (1+I 1 2 )+b i I where K 1 is the ionization constant in pure water, α i and b i are parameters of which b i has been adjusted to present data. The following results were obtained (α i , b i ): p K 1 = 9.84, (1.022, −0.11); p K 2 = 13.43, (2.044, −0.20); and p K w = 14.01 (1.022, −0.22). k i values are collected in Tables I and II. Attempts have been made to explain the medium dependence of k 1 and k 2 with weak sodium silicate complexing according to the equilibria Na + + SiO(OH) − 3 ⇌ NaSiO(OH) 3 ;k 11 Na + + SiO 2 (OH) 2 2 ⇌ NaSiO 2 (HO) − 2 ; k 21 giving k 11 = 0.37M −1 and k 21 = 3.0M −1 . However, these weak interactions cannot be interpreted unambiguously from potentiometric data at different 1 -levels. Probably the medium dependence could equally well be expressed by variations in the activity coefficients. The measurements were performed as potentiometric titrations using a hydrogen electrode. The average number of OH - reacted per Si(OH) 4 , Z, has been varied within the limits 0 ⩽ Z ⩽ 1.1 and B 1 , the total concentration of Si(OH) 4 , between 0.001 M and 0.008 M. k 1 was evaluated from experimental data with B ⩽ 0.003 M, and k 2 with B ⩽ 0.008 M and Z ≳ 0.95.

Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution. II. Formation constants for the monosilicate ions SiO(OH)3− and SiO2(OH)22−. A precision study at 25°C in a simplified seawater medium

The hydrolysis of silicic acid, Si(OH)4, was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25°C. The measurements were performed as potentiometric titrations (hydrogen electrode) in which OH− was generated coulometrically. The total concentration of Si(OH)4, B, and log[H+] were varied within the limits 0.00075 ⩽ B ⩽ 0.008 M and 2.5 ⩽ -log[H+] ⩽ 11.7, respectively. Within these ranges the formation of SiO(OH)3− and SiO2(OH)22− with formation constants log β−11(Si(OH)4 ⇌ SiO(OH)3− + H+) = −9.472 ±0.002 and log β−21(Si(OH)4 ⇌ SiO2(OH)22− + 2H+) = −22.07 ± 0.01 was established. With B > 0.003 M polysilicate complexes are formed, however, with -log[H+] ⪆ 10.7 their formation does not significantly affect the evaluated formation constants. Data were analyzed with the least squares computer program LETAGROPVRID.

Equilibrium aluminium hydroxo-oxalate phases during initial clay formation; H+-Al3+-oxalic acid-Na+ system

Abstract The conditions necessary for initial clay formation have been studied in different model systems comprising different organic acids besides Si and Al. In the present paper the solid phases and the precipitation boundary characterizing the subsystem H+-Al3+-oxalic acid (H2L) are discussed. pH and tyndallometric measurements were performed in an ionic medium of 0.6 M Na(Cl) at 25 °C. The two phases Al3(OH)7(C2O4) · 3H2O (phase I) and NaAl(OH)2(C2O4) · 3H2O (phase II) determine the precipitation boundary. The following formation constants for the two phases were deduced: lgβ1 = lg([Al3+]−3[H2C2O4]−1[H+]9 = −21.87 ± 0.08 and lgβ11 = lg([Al3+]−1[H2C2O4]−1[H+]4 = −5.61 ± 0.06. Phase I exists in the range [Al]tot≥ 10−4.4 mol dm−3,[H2C2O4]tot ≥ 10−4.9 mol dm−3 and at pH The solid phases have been characterized by X-ray analysis of powders, TGA and IR spectra, and tentative structures are proposed. Phase I seems to be an octahedral layer structure, in which 3 5 of the octahedral sites between two close packed oxygen sheets are occupied by Al3+ and the oxalate ion acts as a bridge ligand between two aluminium atoms. Phase II forms a more open sheet structure and has ion exchange properties. Powder data for a phase crystallized from the studied solution after a year are also presented. This phase, Na4Al2(OH)2(C2O4)4 · 10H2O, supports the results from the equilibrium analysis of recent solution data by Sjoberg and O hman (1985), who have found the dinuclear complex Al2(OH)2(C2O4)44− to exist in a solution in which the ligand is in excess.