In situ investigation of the ionisation of silica in aqueous ammonia by using a high frequency dielectric method

Abstract : The reaction of silica gel and Stöber beads of silica with ammonia was studied in aqueous medium using a high frequency dielectric method. Measurements of the complex impedance of silica pulps in both static and dynamic modes were found to be a new rapid, sensitive and non-destructive way for the in situ characterisation of the surface silica ionisation process in aqueous ammonia. The influence of various parameters (field frequency, ionic strength, ammonia concentration) was discussed. The apparent equilibrium constant for the following reaction (SiOH) + NH(3)<-->(SiO(-)NH(4)(+)) was found to be strongly affected by the degree of dissociation of both silica samples, so that the complete neutralisation was never observed. The porosity of the Stöber silica towards the NH(3) species was demonstrated experimentally through the formation of (SiO(-)NH(4)(+)) entities in the interior of the silica beads. The reversibility of the neutralisation reaction was applied to successive on-column analysis for which the hydrolysis of ammoniated silica was found to be catalysed by the presence of acids.
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C. Despas, Alain Walcarius, Jacques Bessière. In situ investigation of the ionisation of silica in aqueous ammonia by using a high frequency dielectric method. Talanta, Elsevier, 1997, 45 (2), pp.357-369. ⟨10.1016/S0039-9140(97)00138-0⟩. ⟨hal-01942059⟩

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