Continuous-flow performance of carbon electrodes modified with immobilized Fe(II)/Fe(III) centers Amperometric...

The amperometric performance of two types of chemically modified carbon electrodes developed for the determination of oxides of nitrogen in continuous-flow systems is presented. The modification consists in immobilization of reversible Fe(II)/Fe(III) centers. The first type of electrode is a simple modification made by direct mixing of carbon paste with tris-4,7-diphenyl-1,10-phenanthroline-iron(II) perchlorate; the other is a glassy-carbon surface modified by oxidative electropolymerization of tris-[5-amino-1,10-phenanthroline]iron(II) perchlorate. Detection is accomplished by transporting an injected sample plug to the sensing surface with the aid of gravitational flow of an aqueous solution of supporting electrolyte. The polymer-coated electrochemical detector compares favourably with the chemically modified carbon paste. It offers excellent resistance to poisoning and a competitive limit of detection [about 2 ppb (2 parts in 10(9)) v/v], at + 1.0 V vs. Ag/AgCl, and good selectivity for NO(2) when used in a thin-layer cell. Incorporation of the cell in a continuous-flow system allows injection of about 120 samples per hour. The typical concentration range amenable to determination is 2-25 ppb v/v but depends on the thickness of the polymeric film. Nitrogen monoxide can also be detected but only in undiluted, pure form. Dinitrogen oxide gives no amperometric signal at any of the modified surfaces.