โ Scribed by Bo Karlberg
No coin nor oath required. For personal study only.
The gel layer develops on an electrode glass on its first contact with water. This hydration process has been shown to involve both an ion exchange of the alkali metal ions in the bulk glass with hydrogen ions from the aqueous solution and a rupture of the silicon-oxygen bonds by reaction with water 1-7.
Wikby 4-7 has characterized the gel layer as a separate.phase between two moving boundaries. The phase boundary, gel layer/bulk glass, moves inwards towards the bulk glass at a rate determined by the hydration velocity. The phase boundary solution/gel layer moves in the same direction due to the continuous dissolution of the gel-layer phase. When the rates of movement of the two phase boundaries are equal the gel layer will have a constant thickness independent of time. These conclusions were based on results obtained by analysing lithium and silicon in etching fractions of the gel layer as well as in aqueous solutions where the electrode glass was allowed to hydrate.
With the fractional etching technique, the concentration profiles of silicon and the alkali metal constituent in the glass were established for glasses containing sodium 1-3 and lithium 5-7. The alkali metal content in the gel layer is low compared with the content in the bulk glass. Within a narrow region between the gel layer and the bulk glass the ratio alkali metal:silicon increases from almost zero up to the bulk glass value.
The thickness of the gel layer is a function of the glass composition. For a given glass composition the thickness will also depend on differences in the rates of movement of the two phase boundaries as outlined above. Consequently, various values of the thickness appear in the literature; this would be the case even if an absolutely accurate determination could be made. For hydrogen selective glass electrodes values between 25 A and 2000/~ have been obtained by interferometric 8, tritium uptake 9, and ion-sputtering methods 1ยฐ. On the basis of the results reported in refs. 5-7, Wikby ix has calculated gel-layer thickness values of 900 A and 3000 A for low temperature electrodes (Ingold LoT) hydrated for 44 and 340 h, respectively and values of 150/~ and 400 A for general-purpose electrodes (Ingold 201) hydrated for 30 and 340 h, respectively.
Very little is known about the ion-exchange capacity of the gel layer, probably because the determination of this quantity cannot be made directly in an aqueous solution. In baNc aqueous solutions there is a rapid dissolution of the external part of the gel layer v' 12,13. This results in a reduction of the gel-layer thickness since the dissolution velocity will exceed the hydration velocity. It is
๐ SIMILAR VOLUMES
Some pH-responsive glass electrodes have been characterised with respect to gel layer thickness, durability and ion exchange capacity. Glasses with a low durability exhibited a thick gel layer and a high ion exchange capacity in comparison with high durability glasses. The electrochemical properties
The glass electrode is calibrated on the concentration scale against the hydrogen electrode from measurements of the potentials of the two electrodes immersed in the same buffer. The p[H+] of the buffer is changed between 2 and 11 by addition of base. The difference in potential between the electrod