Measuring the true proton conductivity

A knowledge of the true, bulk proton conductivity has been both desirable and difficult to achieve in the characterization of most known solid proton conductors. It was almost 100 years after the first electrical measurements on ice that von Hippel demonstrated that single crystals of pure ice must have a proton conductivity far below the values reported previously.  

Quantitative data on bulk proton transport are required especially for the understanding of proton transport mechanisms (see Chapters 29 31) including the implications for the use of solid proton conductors in operational electrochemical cells (see Chapters 32 39). 

In this Chapter, proton conductivity refers to the displacement of protonic species (e.g. H , HjO, OH ) in small electric fields across a sample close to thermodynamic equilibrium. This conductance relates directly to the self diffusion coefficient of the corresponding species, which may be significantly smaller than the chemical diffusion coefficient, in particular in the presence of another highly mobile species (e.g. conduction electrons in hydrogen bronzes see Chapter 7). 

There is no standard procedure for the measurement of proton conductivity and experimental techniques as well as structural and chemical considerations have to be adapted to the material under investigation. There are some peculiar features common to most solid proton conductors which make it difficult to identify the proton as the charge carrier, to determine its accurate conductance (self diffusion) and to separate bulk proton transport from artefacts (such as transport along surfaces, grain boundaries, domain walls, dislocations, second phases). How to cope with these will be discussed in the following section. 

A prior condition is to have well-defined samples. Of great advantage is the use of rather perfect single crystals, but these are available 

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Any charge transport by protons must directly correspond to an adequate transport of protonic mass. A straightforward way to determine the true proton conductivity, therefore, is to record directly the proton selfdiffusion coefficient. The traditional way to do this is through tracer techniques. They may be very accurate but require difficult experimental work and measurement periods of the order of days or weeks for a single component and temperature. A further disadvantage is the inherent system perturbation by isotope substitution, which is particularly critical for the substitutions H/ H and... 

ATP synthase equilibrate with the aqueous phases of the matrix and cytosol (or incubation medium). While this may seem intuitive, in view of the extremely high conductance of water to protons, there have in recent years been a number of reports of significant anomalies in the thermodynamic and kinetic relationships between respiration, ATP synthesis and the measured AjUH+ sufficient to lead some investigators to suggest that this last parameter does not represent the true intermediate in the coupling of respiration to phosphorylation [24],... 

In a study of the kinetics of nitration of benzene, toluene, m-xylene, mesity-lene, and anisole by nitric acid in AC2O, the rate of nitration and the order with respect to acid are both sensitive to prior solvent treatment. Conductivity measurements show that the effect is due to some trace impurities in the solvent which on protonation give rise to NOs" ions. Rates of nitration in purified solvent are sensitive to added NOs ions and water but almost unchanged by added AcOH. It has been deduced from arguments based on the true rate constant for electrophilic attack that reaction occurs through the N02 ion. ... 

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