Direct Estimation of Total Arsenic Using A Novel Metal Side Disk Rotating Electrode

Abstract

A special type of metal side disk rotating electrode has been demonstrated for the direct estimation of total arsenic [As(III) and As(V)] at low ppb (mg/L) level using anodic stripping differential pulse voltammetry (ASDPV). A conventional three electrodes cell is used equipped with side disk rotating gold electrode as working, graphite/ platinum electrode as auxiliary and Ag/AgCl/3M KCl as reference electrodes. Arsenic is estimated in various acidified samples without any digestion, containing a trace amount of copper at low ppb level. The major problems associated with ASDPV of high acidic condition (acid hazards), irreproducible results due to the interference of hydrogen bubbles, evolved at the cathode during the deposition of arsenic in acidified samples and poor detection level are overcome with the help of the specially designed gold side disk rotating electrode and modified electrolyte. The presence of a trace amount of copper(II) salt in the electrolyte is found to enhance the sensitivity of the technique. The shape and position of the metal disk at the electrode, rotation speed of the electrode and electrolyte are optimized to have less hydrogen gas bubbles interference and high reproducibility in the detection of arsenic up to 2 ppbAE 15% level in various samples. The electrode has been found very stable and reproducible even for more than 200 estimations.

Determination of As(III) by anodic stripping voltammetry using a lateral gold electrode: Experimental conditions, electron transfer and monitoring of electrode surface

“…In fact, it is well know that Cu forms an intermetallic compound with As [11,46] that causes the decrease of the analyte signal. Some researchers, working with mercury electrodes, added copper to promote the deposition of As on the electrode surface [34]. In the presence of Cu(II), we noticed a strong memory effect, that remained for long time, even after some cleaning steps.…”

“…Both DP [21,34,43,51] and SW [7,30,52] are used in literature for the determination of As(III). SW permitted to obtain higher signals but it gave rise to a lower repeatability and a greater background signal.…”

“…This determination is generally specific for As(III), whilst arsenate (As(V) is detected with difficulty. In fact, As(V) has traditionally been regarded as poorly electroactive in alkaline, neutral or acidic media, unless in extreme conditions of acid concentrations [34,35] or at high negative potentials. In practice, As(V) is chemically reduced to As(III), using reductants such as iodide [36], mannitol [37], sulfur dioxide [38], prior to the electrochemical measurement of total arsenic.…”

“…We choose to work with the lateral gold electrode (for As(III) determination,) with a view to a future application of this procedure to the determination of total As (As(V)+As(III)) and then to speciation studies. In fact the evolution of hydrogen gas (first in form of nascent hydrogen) at the cathode and of chlorine gas at the anode is one of the major problems for the reliable and reproducible quantification of total arsenic [34]. The problem of the evolution of hydrogen gas can be minimised using a mild acid electrolyte or using a lateral gold electrode, instead of a frontal gold electrode, that mechanically removes the hydrogen bubbles just after their formation.…”

“…In this way we monitored the effect of the treatments (mechanical and electrochemical) on the gold surface. Finally, we studied the applicability of the equations commonly used for flow systems [34] to the investigated stirred system in order to evaluate the possibility to obtain information on the number of electrons transferred per molecule.…”