Increased sensitivity of anodic stripping voltammetry at the hanging mercury drop electrode by ultracathodic deposition

Abstract

An improved approach to the anodic stripping voltammetric (ASV) determination of heavy metals, using the hanging mercury drop electrode (HMDE), is reported. It was discovered that using very cathodic accumulation potentials, at which the solvent reduction occurs (overpotential deposition), the voltammetric signals of zinc(II), cadmium(II), lead(II) and copper(II) increase. When compared with the classical methodology a 5 to 10-fold signal increase is obtained. This effect is likely due to both mercury drop oscillation at such cathodic potentials and added local convection at the mercury drop surface caused by the evolution of hydrogen bubbles.

Introduction

Electroanalytical techniques have been particularly successful in detecting and quantifying trace quantities of heavy metals in samples, mainly when using stripping voltammetry [1], [2], [3], [4]. In voltammetric stripping the analyte is concentrated within or at the working electrode (the deposition step) prior to electrochemical measurement (the stripping step). Depending on the stripping mode, an electrochemist may adopt cathodic stripping voltammetry, which typically includes adsorptive deposition of a metal complex, or anodic stripping voltammetry (ASV). Typically in ASV the deposition potential is more negative (cathodic accumulation) than all the reduction peak potentials of the target analytes. With mercury electrodes significantly lower detection limits can be achieved because ionic metallic analytes may be reduced and dissolve in the mercury forming an amalgam [5] that has a lower kinetic barrier to formulation than if the analyte grew as particles on a solid electrode surface [6]. Other advantages of mercury electrodes include the extended electrochemical measuring window due to the high overvoltage for hydrogen evolution and the fact that new drops or new thin mercury films can be easily formed can provide a ‘cleaning’ process that avoids interferences from previous measurements [5], [7]. Among the several types of mercury electrodes the hanging mercury drop electrode (HMDE) has become one of the most used due to high reproducibility and low consumption of mercury [8]. While the use of mercury electrodes has declined overall in the last decades, for some applications such as environmental monitoring, their use is still attractive in many laboratories. For mercury (Hg) electrodes the typical electrode reactions in anodic stripping metal (M) analysis are as follows:

Hg + Mⁿ⁺ + ne⁻ → M(Hg)

M(Hg) → Hg + Mⁿ⁺ + ne⁻

Eq. (1) (deposition step) ascribes the electrolytic accumulation of the analyte in the working electrode under controlled conditions [9]. The deposition potential to be applied varies with several factors including type of analyte, solvent and pH, among others; however, in ASV, the common rule is the application of a potential around 300 mV more negative than the peak potential of the least easily reduced analyte to be analyzed. Generally, deposition is enhanced by stirring, which increases mass transport by convection [10]. Eq. (2) (stripping step) describes the anodic stripping where the metals are reoxidized to their cationic form, thus enabling quantification since current intensity is proportional to bulk concentration.

Sensitive and reliable heavy metal analysis is required for several purposes, such as consumer safety, understanding contaminant mobility in the environment, studying natural remediation rates of contaminated water streams, etc. Paracelsus's adage “the dose makes the poison” can be perfectly applied to heavy metals. Even those metallic ions that are indispensable for human survival in minute quantities can become toxic at not much larger levels. Copper (Cu) is an essential trace element, which is an integral part of many important enzymes involved in a number of varied vital biological processes. However in high levels it becomes harmful, in particularly it is hepatotoxic [11]. High levels of cadmium (Cd) are also predominantly injurious for the liver [12]. Lead's (Pb) toxicity is widely known, its nefarious effects throughout time have filled history books with lots of interesting historical details. Besides being neurotoxic and carcinogenic it can cause as well hypertension and renal impairment; there is a wide list of possible maladies and unpleasant symptoms [13], [14]. It is now known that even low levels of exposure, resulting in a Pb concentration in blood below 10 μg L⁻¹, can result in cognitive dysfunctions, neurobehavioral disorders and neurological damages [14]. Moreover, one should keep in mind that drinking water is one of the main sources of Pb uptake [14]. Of the four metals ions analyzed in this work Zinc (Zn) appears to be the most innocuous.

Many efforts have been devoted to the development of adequate analytical methodologies for heavy metals [15]. As mentioned before, ASV is good analytical method, not only for its low maintenance costs, high sensitivity, minor sample treatments and swiftness but also, since metals have different reduction potentials, it is rather simple to detect and quantify Pb, Cd, Cu and Zn in the same voltammetric scan [16], [17].

It is the aim of this paper to describe how useful is the significant increase in ASV sensitivity, at the HMDE, can be attained simply by using a more cathodic deposition potential.