THE EFFECTS OF TRACE ANIONS ON THE VOLTAMMETRY OF SINGLE-CRYSTAL GOLD SURFACES

The development of a new technique for mounting single crystals as a Hanging Meniscus Rotating Disk (HMRD) electrode has facilitated the study of the effect of traces of anionic impurities on the voltammetry of gold single crystals. It is shown that the cause for most of the differences between voltammetry curves reported in the literature for the same crystal face is the presence in the solution of low levels (almost-equal-to 10(-7) M) of Cl- and SO4(2-) ions which were present in the original reagent. Variation of the rotation rate from 0 to 10,000 r.p.m. permitted a change of the effective concentrations of these impurities by a factor of almost-equal-to 100. The resultant curves are compared to those produced by deliberate addition of these anions. The results obtained indicate that the previously reported specific adsorption of ClO4- and F- on Au may well have been due to the presence of the same contaminants (Cl- and SO4(2-). The capacitance minimum values reported in the literature as pzcs appear to coincide with the potential at which SO4(2-) (or HSO4-) desorption finishes and Cl- begins. The reported values for pzc may be erroneous. The validity of the "fingerprint" concept is discussed, as is the use of modifications (changes) of the voltammetric features produced by particular pre-polarisation regimes as electrochemical evidence of "reconstruction".