Coulometric stripping potentiometry for arsenic(III)

Abstract

Calibration-free determination of As"' in the presence of AsV using coulometric stripping potentiometry is described. AS"', in the concentration range 0.01-2 mgL, is quantitatively reduced to elemental arsenic and simultaneously dissolved in gold codeposited onto a glassy carbon substrate by electrolysis for 4 minutes at -0.50V (vs. Ag/AgCl (0.01 M Cl-)) in 12 pL samples containing 3 M hydrocbloric acid and 10mg/L golduw. Selectivity between arsenicciri) and (v) is achieved by proper control of the deposition potential and by minimizing the goldm) concentration and the time between addition of gold(rI1) and commencement of analysis.Keywords: Coulometric stripping potentiometry, Arsenic(m1, Arsenic(v), Glassy carbon, Gold film electrode Coulometric stripping potentiometry [ 11 is based on exhaustive analyte ion reduction and simultaneous amalgamation or dissolution in gold, i.e., where the mercury or gold phases normally are codeposited with the analyte atoms onto an inert substrate such as glassy carbon, mercurycn) or gold(m) ions being added to the sample prior to analysis. Subsequent to reduction the analyte atoms are reoxidized by means of applying a constant current and simultaneous monitoring of the electrode potential. In this way the time needed for complete reoxidation of the analyte atoms, t ,~p r can be determined. From the known sample volume, v , typically 10-20 pL, the analyte concentration in the sample, C, can be calculated from Faraday's law aswhere istrip is the applied oxidative current and n the number of electrons involved in the oxidation according to Equation 1. Equation 2 is, however, valid only if the applied current is the sole oxidation mechanism, i.e., that the oxidative contribution from oxidants present in the sample, the "chemical current" [l, 21, can be neglected. This requirement is normally fulfilled for noble elements. The reason for this is that oxidants, present in the sample and capable of oxidizing the analyte atoms, are quantitatively reduced simultaneous with analyte ion reduction and that dissolved dioxygen, present in all samples in contact with air, is not capable of oxidizing these elements.In a recent communication Jagner et al.[2] reported on the coulometric stripping potentiometry determination of ten elements using either mercury or gold films. In the determination of arsenic, using in situ plated gold film, it was concluded that it was not possible to differ between As"' and AsV since the Au"' ions, added to the acidic samples prior to analysis, oxidised As"' to AsV. In this communication it will be demonstrated that As"' can be selectively determined in the presence of As". The main differences between the proposed procedure and that described earlier [2] are, that the gold(In) concentration has been decreased from 1.25 to 0.05 mM in order to decrease the rate of oxidation of As"' to AsV and that the sample, and the gold(m) containing matrix, are mixed immediately before commencement of analysis.The optimum potential for selectively determining As"' in t...

Speciation of Arsenic by Potentiometric Stripping Analysis Using Gold(III) Solution as Chemical Reoxidant and a Wall‐Jet Flow Cell

“…While operating at high acid concentration is clearly better, it is also obviously desirable to minimize the generation of acidic waste and to prevent degradation of the working electrode since the gold film was oxidized in the presence of a high chloride concentration to the AuCl 4 À complex [23]. In order to avoid or decrease the rate of oxidation of As(III) to As(V) [24], we used a small gold(III) concentration and mixed it immediately before commencement of analysis. The amount of chemicals needed for one analysis has been reduced be because of expensive gold(III).…”

Determination of As(III) and As(V) in oilseeds by chronopotentiometric stripping analysis: Development of a method

“…As(III) + As(V) was determined in the presence of reducing agent [13]. Chronopotentiometric stripping analysis (CSA) has previously been used for arsenic speciation chiefly by the use of gold-plated electrodes and anodic dissolution currents [10,14,15]. Selectivity between arsenic(III) and arsenic(V) was achieved either by application of selective potential [14] or by sample analysis with and without reduction of pentavalent arsenic [15].…”

“…Chronopotentiometric stripping analysis (CSA) has previously been used for arsenic speciation chiefly by the use of gold-plated electrodes and anodic dissolution currents [10,14,15]. Selectivity between arsenic(III) and arsenic(V) was achieved either by application of selective potential [14] or by sample analysis with and without reduction of pentavalent arsenic [15]. Constant current cathodic stripping potentiometry of arsenic has also been performed with the implementation of mercury film electrodes which enabled overcoming interferences related to the usage of gold electrodes [16].…”