The amount of adsorbed sodium ions in the compact part of the electrical double layer of the vitreous silica electrolyte interface has been estimated at pH 10 with the aid of a sodium-24 and bromide-82 tracer. Etching experiments show a penetration depth of sodium ions in the solid phase less than 0.3 nm; moreover, a gel layer formation at pH 10 is not observed. This experimental evidence is against the application of the gel layer explanation in case of nonporous silica and in favor of the site-binding model.
The tritium-hydrogen exchange and the tritium distribution in tritium-labelled thin microporous silica layers was studied with a washing and etching technique. The tritium distribution was found to be homogeneous along the whole thickness of the layer. The exchange process can be resolved in three diffusion processes which can be related, in order of decreasing rate, to tritium: (i) in the physisorbed water molecules in the micropores; (ii) in the silanol groups in the micropores; and (iii) in bulk intra-skeletal silanol groups, respectively, in agreement with the description of the microporous silica layer given by Grigorovich et al.The diffusion rate of the water molecules resembles the diffusion rate of water molecules in ice I. The diffusion coefficient of the hydrogen in the micropore silanol groups (5 X 10 -17 m 2 s -1) is much higher than the self-diffusion coefficient D H ~ 10 -21 m 2 s-l, calculated from the previously studied sodium-hydrogen exchange process in the silica film using the interdiffusion coefficient according to Doremus. However, the former diffusion coefficient is compatible with the diffusion coefficient of the "free" hydrogen ions, calculated from the sodium-hydrogen exchange process using an interdiffusion coefficient according to Helfferich by which the dissociation of the silanol groups in the micropores is taken into account, if the dissociation constant is about 10 -9.
A poly(viny1 chloride) membrane electrode, based on the ion-pair complex of tetramethylammonium bromide (TMAB) with sodium tetraphenylborate, is described. The electrode exhibits a linear response with a slope of 49 mV per decade of concentration at 20 "C within the range 1 x lo-4-0.1 M TMAB, at an ionic strength of 0.1 M NaBr. The effect of the pH of the test solution and the interference of other cations and sio44-(or silicate ions of lower charge such as HSiO& or H2Si04*-) were studied. Potentiometric titration was used to determine TMAB, in the presence of sio44-, with satisfactory results.
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