The tendency of olefins to chemisorb irreversibly on platinum electrodes is exploited to attach a variety of reactive entities to the electrode surface. Chelates connected to the electrode surface through an olefinic side chain allow metals to be selectively chemisorbed at the electrode surface. The stability of the chemisorbed metal complex is shown to vary with electrode potential, due to the influence of the electrical double layer, in such a way that the chelating ability of the chemisorbed chelate can effectively be "switched" on or off. Studies of the electrode reactions of chemisorbed complexes of Fe and Pt and of organic depolarizers such as substituted polyhydric phenols indicate that only in certain instances are the adsorbed reactants able to achieve a reactive orientation of the sort acquired readily by dissolved species. Evidence is presented that the system of each adsorbed olefin studied undergoes ten-electron oxidation to form the corresponding carboxylic acid and/or CO2. Equations which describe the electrochemistry of chemisorbed molecules are derived and tested.

Problems with film formation on platinum electrodes preclude the use of differential pulse voltammetry for analytical determination of catecholamines in physiological media. A pulse electrochemical technique, differential double pulse voltammetry, utilizes two simultaneously varying unequal square wave potential pulses as an alternative to the linear dc ramp and allows these problems to be avoided. Theory and experiment are in good agreement with a reversible, two-electron oxidation to the o-qulnones, with no further complications from homogeneous chemical reactions Indicated. Chemical modification of platinum surfaces with aqueous Iodide provides an electrode which is essentially devoid of electrochemical and chemical Interference over the potential range of interest. The modified electrodes exhibit a sensitivity toward the catecholamines unachievable with unreacted electrodes. The methodology presented Is discussed in regard to its potential usefulness for the determination of dopamine and norepinephrine In brain tissue.

Publication costs assisted by the National Science Foundation and the Petroleum Research Fund Thin layer voltammetric data for Pt electrodes indicate that F", Cl", Br", and I~form chemisorbed layers which withstand rinsing with typical aqueous electrolytes. The chemisorbed halides are much less reactive toward electrochemical oxidation than the corresponding dissolved ions. Aqueous I" reacts with the Pt surface to form an uncharged adsorbed species, whereas F", Cl", and Br™ are adsorbed in anionic form. Accordingly, F~, Cl", and Br", but not I, induce the adsorption of Fe2+ in a 2:1 ratio of halide to iron. The influence of adsorbed halide on electrode reactions of metal ions such as Pt(II), Pt(IV), Sb(III), and Sb(V) can be understood simply in terms of electrostatic interaction between the reactant and the electrode surface. Chemisorbed F", Cl", and Br" cause the surface to be less positively charged, whereas chemisorbed I has the opposite effect. In the irreversibly chemisorbed form, halide does not serve as an electron transfer bridge in the oxidation of Pt(II) or Sb(III).

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The influence of chemisorbed olefins having ionic substituents on the electrode rates of Pt(II) and Pt(IV) complexes at Pt electrodes has been studied. Chemisorption alters the potential at the reaction plane, n2, with the result that ionic complexes tend to react more rapidly at surfaces coated with ionic olefins of opposite charge than at clean surfaces. A comparison of cationic coatings, such as allylammonium ion with anionic ones (such as allylacetic acid anion) adsorbed at potentials positive of the zero-charge condition, demonstrates that the extent to which r<£2 is altered depends upon the position of the charged substituent in relation to the remainder of the double layer and not merely upon the magnitude of its interfacial excess. The detailed electrostatic effects can be predicted in terms of a Boltzmann distribution, taking into account the chain length, structure, functional group charge, electrodic charge, and position of the reactant during electron transfer. The precise extent to which the reactions are sterically hindered by the chemisorbed surfactant has been determined from the variation of electrode rate with surfactant coverage. The results indicate that two adjacent electrode sites are required to form the rate-limiting intermediate in halide-bridged reduction of Pt(IV) complexes.