Electrochemical quartz crystal microbalance studies : (1) tetracyanoquinodimethane conducting salt electrodes, and (2) the deposition of mercury on graphite electrodes in chloride solution

The Electrochemical Quartz Crystal Microbalance (EQCM) was used to study the redox behavior of tetracyanoquinodimethane (TCNQ) thin film electrodes in solutions of 1M Ca (OAc)₂ and 1M KOAc. Subsequent frequency changes were consistent with permselective ion motion for cathodic insertion and anodic expulsion of counter ions originating from the electrolyte solution. The ion motion during redox cycling of the TCNQ(0/1-) couple, as observed by frequency vs. time data pairs during cyclic voltammetry, gave a "square-wave-like" behavior. Cathodic chronoamperometric experiments showed direct incorporation of the predominant cation from the electrolyte into the solid state lattice. Anodic chronoamperometric experiments showed a more complex mass transient behavior consisting of a fast and slow component. This behavior can be explained by initial insertion of anions from the electrolyte in order to maintain electrical neutrality of the film, followed by the slow diffussional loss of the neutral electrolyte from the film. The anodic process accounts for frequency changes at potentials where faradaic processes did not occur. The treatment of reduced TCNQ thin film electrodes with ethidium bromide solution resulted in passivation of the electrode and the cessation of ion motion. EQCM studies of the passivation process showed it to involve an ion exchange step and subsequent anodic oxidation coupled with ejection of counter ions. The electroactivity of the ethidium modified TCNQ thin film electrodes could be restored by exposure to dilute solutions of fish herring DNA. The reactivation resulted in an average 48 ± 17 % recovery of the total charge before passivation along with the restoration of the "square-wave-like" ion motion. Mercury deposition onto microcrystalline graphite and glassy carbon electrodes from aqueous chloride media was studied by EQCM and voltammetric techniques. After deposition of Hg(0) under controlled potential conditions, mercury continued to deposit at open circuit as a calomel film by the reaction, Hg⁰ + Hg²⁺ + 2Cl^- ⇄ Hg₂Cl_{2 (S)}. The implications for the preparation of mercury films and anodic stripping voltammetry are discussed.