ANION DETECTION BY ELECTRO-INSERTION INTO N,N,N',N'-TETRAHEXYLPHENYLENEDIAMINE (THPD) MICRODROPLETS STUDIED BY VOLTAMMETRY, EQCM, AND SEM TECHNIQUES

The electrochemical oxidation and re-reduction of N',N',N,N-tetrahexyl phenylene diamine (THPD) deposited in form of microdroplets on a basal plane pyrolytic graphite or gold electrode is shown to be a chemicall y reversible process in the presence of aqueous electrolyte media cont aining NO3-, SCN-, ClO4-, or PF6-. Sharp voltammetric responses with a mid point potential, E-mid, characteristic of the type of anion and i ts concentration are observed. The oxidation product, an ionic liquid, undergoes rapid ion exchange when the anion of the aqueous electrolyt e is exchanged with anions of lower E-mid replacing anions of higher E -mid. Although the effect of the supporting electrolyte cation in the case of the alkali metals K+, Na+, and Li+ is not significant, a consi derable change in the voltammetric peak shape and E-mid occurs in the presence of protons. This effect, attributed to the protonation of THP D, is also sensitive to the type of anion present with anions of lower E-mid causing more facile protonation. After protonation of THPD, oxi dation and re-reduction can be shown to be associated with H+ expulsio n and uptake. Deposited onto the rough surface of a gold coated planoc onvex quartz crystal oscillator the [THPD+ClO4-](oil) deposit can be o bserved in form of micron-sized droplets in SEM images. A strong frequ ency response of the crystal oscillator in an electrochemical quartz c rystal microbalance experiment associated with the oxidation and re-re duction of THPD can be detected but is not related to changes in mass. Rather, this frequency response may be attributed to changes in the v iscosity and/or coverage of the oily deposit.