The diffusion of dissolved silica in dilute aqueous solution

The diffusion coefficient of dissolved silica at 25.5 ± .5°C was determined as a function of concentration using a non-steady-state method whereby agar-gelled solutions containing dissolved silica from 0.09 to 1.50 mM (pH = 5.5) were placed in contact with distilled water in glass cells. Diffusion coefficients were obtained by measuring the dissolved silica content of the distilled water after a given length of time. The measured diffusion coefficients decreased as a function of increasing dissolved silica concentration, which is thought to reflect an increase in dimeric silica according to the equilibrium: 2Si(OH)₄ = Si₂O(OH)₆ + H₂O. The tracer diffusion coefficients for Si(OH)₄ and Si₂O(OH)₆ and an association constant for the above reaction were determined by fitting the following equation to the experimental data: D_obs = αD_monomer + (1 − α)D_dimer where α is the fraction of total dissolved silica which is Si(OH)₄. The best fit yielded tracer D's for Si(OH)₄ and Si₂O(OH)₆ of 2.2 and 1.0 (in units of 10⁻⁵ cm² sec⁻¹), respectively, and an association constant of 330.