NMR and IR study of the functionality of electrode glasses

During conditioning of glass electrodes in aqueous solutions a hydrated layer is formed, which is responsible for the pNa+ sensitivity. For a detailed understanding of this effect the structure of the bulk glass as well as of the hydrated layer is of particular importance. 27Al and 11B MAS NMR were used to investigate Na2O-Al2O3-SiO2 and Na2O-B2O3-SiO2 glasses. The 27Al MAS spectra can be interpreted in terms of two different structural units. We can distinguish between AlO4 tetrahedra connected to Q4 groups only and AlO4 tetrahedra linked to Q3m groups. In a similar way two BO4 units can be detected which are linked to SiO4 units or BO3. In both cases only one of the two structural units is removed during the reaction in hydrous solution which can easily be concluded from the changes of the 11B and 27Al MAS lineshape. In summary criteria for a good pNa+ sensitivity could formulate in terms of certain structural units which must be present in the bulk glass. Also, the swelling layer must contain a large content of Q4 and the H2O/SiOH ratio must be sufficiently high.