Solid-State pH and Ion-Selective Electrodes

Casting metals consisting of Sb or Bi covered by a thin hydroxide layer and membranes of transition metal oxide bronzes show a relatively high selectivity for hydrogen ions, as recently reviewed by Vonau and Guth (2006). Metal/metal oxide electrodes display non-Nernstian responses, but applications in food control and medicine have 

More recently, lithium tungsten bronzes have been studied as materials for reference electrodes (Gabel et al., 2004). 

The used carbons consist of a skeleton of glassy carbon surrounding a periodic array of uniform spherical pores that are interconnected in three dimensions. Pore sizes are of a few hundred nanometers, whereas skeletal walls are tens of nanometers thick. As a result of the mutually interconnected pore-wall structure of such materials, filling pores with an electrolyte solution results in a nanostructured material with both ionic and electronic conductivities (Lai et al., 2009). 

Solid-state reference electrodes for potentiometric sensors are currently under research. The main problem to be faced in developing this type of electrode lies in connecting the ionic conducting (usually aqueous) solution with an electronic conductor. Since the reference electrode has to maintain a defined potential, the electrochemical reaction with components of the electrolyte has to be avoided. Oxides, mixed oxides, and polyoxometalate salts of transition elements can be proposed for preparing solid-state reference electrodes. Tested compounds include tungsten and molybdenum oxides (Guth et al., 2009). 

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