Sensitive Potentiometric Flow Injection Analysis of Redox Species Based on the Detection of an Intermediate Produced by a Redox Reaction with the Potential Buffer

Abstract

This review summarizes a highly sensitive potentiometric flow injection method for the determination of various types of oxidative species, where a large transient potential change due to chlorine or bromine, generated during the reaction of the oxidative species with an Fe(III)-Fe(II) potential buffer containing chloride or bromide as an intermediate, is utilized. The analysis of trace amount of hydrazine based on the detection of a transient change in potential caused by the reaction of hydrazine with a Ce(IV) -Ce(III) potential buffer is also described. 1. Introductionprevious minireview (1), we described a potentiometric flow injection analysis (FIA) method for the determination of redox species using a simple redox reaction of a sample with a potential buffer solution consisting of a redox couple such as Fe(III)-Fe(II), Ce(IV)-Ce(III) and Fe(CN)6 3- -Fe(CN)6 4- . This method is based on the detection of the change in potential of a redox electrode caused by a change in the composition of the potential buffer solution as the result of a reaction of the sample with the potential buffer. One of advantages of the method is that samples in a wide concentration range can be determined by selecting an appropriate concentration of the potential buffer. The excellent characteristics of the FIA method permits its use in detecting a final product at an equilibrium state as well as of an intermediate in transient reactions before chemical equilibrium is reached, where an appropriate chemical reaction was used. Among the many redox reactions, there are several reactions where an intermediate compound with a short lifetime is generated. In such a case, the FIA method would be useful for detecting of the intermediate. During our research on the use of potentiometri c FIA for the determination of redox species using the potential buffer, we discovered an interesting phenomena in which a large transient potential change of the redox electrode appeared, when a bromate solution was added to a Fe(III)-Fe(II) potential buffer containing bromide (2). The large change in potential was found to be due to the fact that bromine, generated as an intermediate, was reduced to bromide by Fe(II) in the potential buffer. In the case of potentiometric FIA, if the electrode potential of the intermediate generated during the redox reaction of an analyte with the potential buffer is much higher than that of the potential buffer, a highly sensitive determination of the analyte could be achieved, if the intermediate in the FIA system could be detected. No report on a highly sensitive potentiometric FIA method for the determination of oxidative species in which the detection of an intermediate formed during a redox reaction has appeared, so far. In this minireview, we describe a highly sensitive potentiometric FIA method based on the detection of a large transient potential change in the redox electrode, which appears a short period after mixing an analyte with the potential buffer. In the case where a Fe(III)-Fe(II) potential buffer is used, it was found that the addition of bromide or chloride to the potential buffer enhances the sensitivity to various types of oxidizing species such as bromate, chrominum(VI), hydrogen peroxide and oxychlorine, etc. A high sensitivity to such oxidative species was obtained by the potentiometric detection of a large transient potential change due to the generation of intermediate bromine or chlorine during the reaction of the analyte with chloride or bromide in the potential buffer.