Electrochemical methods for speciation of inorganic arsenic

摘要

Arsenic is found in the environment in several oxidation states as well as in a variety of organoarsenic compounds. This situation puts additional demands on the analysis in that it is desirable to measure the amount of each species, not just all of the arsenic. The reason for this is that the different species have greatly different toxicities; of the major inorganic forms, As(III) is much more toxic than As(V). The goal of this research was to develop a convenient method for the analysis of mixtures of As(III) and As(V) at trace levels. Electroanalytical methods are inherently sensitive to oxidation states of elements and therefore are a natural choice for this problem. In fact, a method was developed some years ago for As(III) that used differential pulse polarography: the detection limit is 0.3 parts per billion (ppb). However, As(V) was not detected since in its usual form as an oxyanion it is electrochemically inactive. There are coordinate compounds formed with catechol, AsL(,n)(n = 1-3), that can be reduced at a mercury electrode, but the active species, AsL, is only a small fraction of the major species, AsL(,3), so the detection limit is only 500 ppb. Many details of the electrochemistry of this unusual compound were examined in this work. In order to improve detection limits, a method involving cathodic stripping was developed. It involves codeposition of copper with arsenic on a mercury electrode to effectively concentrate the analyte. Then the elemental arsenic is converted to arsine, AsH(,3), during a cathodic potential scan. The resulting current peak is proportional to As(III) in the absence of catechol and to the sum of As(III) and As(V) in the presence of catechol. It was observed that the current peak was considerably larger than expected and additional experiments revealed that there was evolution of hydrogen during the formation of arsine. This is rather unusual in electrochemical reactions and so some of the details of this catalyzed coreaction were examined. The result is a fortunate enhancement of detection limit so that As(v) at 40 ppb can be measured.