Multicomponent diffusion of aqueous sodium carbonate and aqueous sodium bicarbonate

Abstract

Diffusion coefficients of Na2CO3 + H2O and NaHCO3 + H2O have been measured by a conductimetric technique at 25 °C. Above 0.1 mol L−1, diffusion of aqueous Na2CO3 is typical of a simple binary electrolyte. At lower concentrations a significant proportion of the Na2CO3 component diffuses as NaHCO3 and NaOH produced by hydrolysis: [Formula: see text]. Because OH− is more mobile than HCO3−, binary Na2CO3 + H2O solutions undergoing diffusion separate spontaneously into two ternary solutions: Na2CO3 + NaOH + H2O ahead of the diffusion boundary and Na2CO3 + NaHCO3 + H2O left behind the boundary. A procedure is developed for measuring multicomponent diffusion coefficients of hydrolyzing salts. Results for Na2CO3 are in satisfactory agreement with predicted ternary values. As a consequence of the disproportionation reaction: [Formula: see text], diffusion of aqueous NaHCO3 is formally a ternary process. However, because reactants and products have similar mobilities and because only about 1% of the salt disporportionates, diffusion of aqueous NaHCO3 is effectively binary.

Thermal diffusion with hydrolysis in aqueous solutions of sodium carbonate

“…Before the thermal diffusion of aqueous sodium carbonate is discussed, it is helpful to summarize briefly its unusual isothermal diffusion behavior (7,8).…”

“…Dilute sodium carbonate solutions contain four major solute species: Na+ and C032-ions together with HC03-and OHions produced by the hydrolysis reaction (7,8) [3] co3'-(aq) + H 2 0 = HC03-(aq) + OH-(aq) K(25"C) = 2.15 x Electroneutrality and local reaction equilibrium place two constraints on the ionic concentrations. At thermodynamic equilibrium, there is a third constraint: the concentrations of HC03-and OH-are identical.…”

“…Diffusion of dilute aqueous sodium carbonate is therefore a ternary process described by the extended Fick equations (7,8) C1 represents the concentration of the total sodium carbonate component (hydrolyzed plus unhydrolyzed forms):…”

“…Soret coefficients and heats of transport are then reported for aqueous Na2C03 at 25°C and salt molalities down to 0.001 mol kg-'. This salt was chosen because isothermal diffusion data (7,8) are available, which simplifies the analysis enormously. In the most dilute solutions that 'To whom correspondence should be addressed.…”

A mathematical model for non-catalytic liquid-solid reversible reactions

“…It was assumed, for the base case, that the order of magnitude of the effective diffusion coefficients was l0 -I~, which has been verified for a number of alcohols in water (Blasifiski and Amanowitz, 1979). The parameter DeB was always assumed to be twice greater than DeA, which is approximately verified for the diffusion coefficients of NaOH and Na2CO 3 in aqueous solutions (Leaist and Noulty, 1985;Noulty and Leaist, 1984). Supposing that a simple relationship such as …”