via Anodic Stripping Voltammetry
I. Introduction
Throughout the years, a number of methods have been developed for determining trace amounts of heavy metals in almost, if not all, every tangible object that can be possibly conceived by mankind. These methods include the numerous principles associated with the spectrophotometric methods (i.e., UV/VIS Spectroscopy and Atomic Absorption Spectrophotometry, the potentiometric methods (i.e., Potentiometric Stripping Analysis), and the and the Voltammetric methods (i.e., Stripping Voltammetry, Cyclic Voltammetry).
Stripping Voltammetry is one of few electrochemical techniques used for the determination of metals in samples. It is a sensitive electrochemical technique for the determination of metals in aqueous solutions. This particular method works as follows:
1. The metal ions (if present) are deposited onto an electrode which is held at a suitable potential. The sample is either stirred or agitated during this stage to ensure maximum deposition of the metals on the working electrode.
2. Stirring/agitation is halted to allow “stillness” to occur within the solution containing the sample.
3. The deposited metals are “stripped” from the working electrode via potential scanning. The measured peak currents during this step are then related to the concentration of each possible metal in the sample solution.
The stripping process consists of either positive or negative scan of potentials, creating either anodic or cathodic currents, respectively. Hence, Anodic Stripping Voltammetry (ASV) and Cathodic Stripping Voltammetry (CSV) are two specific stripping techniques. This paper will focus on Anodic Stripping Voltammetry.
In addition to varying the direction of the scan, the manner in which the potential is scanned may also differ. The simplest voltammetric method is the Linear Sweep Voltammetry (L.S.V.), in which the linear scanning potential is expressed as a function of time. One other commonly used voltammetric method is the Differential Pulse Voltammetry (D.P.V.), which is believed to possess a lower detection limit than conventional L.S.V. – this is influenced by the pulsed waveform unique to the method, which measures current in short pulses by taking two measurements and recording the difference as the potential is steadily increased. This helps in reducing background current. The waveforms from each pulse superimpose upon one another to form a staircase waveform since the pulse amplitude is constant while the potential increases in small increments.
The purpose of this paper is to evaluate the sensitivity and reliability of Anodic Stripping Voltammetry in determining the presence of heavy metals in foodstuffs.
II. New Developments and State-of-the-Art
Stripping voltammetry has enjoyed notable attention by scientists and researchers since it is the considered to be the most sensitive electro-analytical method and over the last few years, it has improved to become a very powerful analytical technique. In recent years, it has become a widely used method for food analysis. This method of analysis encompasses a variety of analytical procedures having a common characteristic step. Whatever the type of Stripping Analysis, the analyte of interest in a sample solution is first always allowed to aggregate on a working electrode via potential scanning. After a short period of rest, pre-concentration is then followed by the stripping process, which involves dissolution of the deposited metals when a linear sweep is applied to the working electrode. Thus, a measureable current is produced at the surface of the electrode, following the reduction/oxidation of the sample at a specific potential.
Though mercury-type working electrodes such as the hanging mercury drop electrode (abbreviated as H.M.D.E.) and thin mercury film electrode (M.F.E.) were often applied to the voltammetric method in the past, they have ever since been increasingly banned in certain regions and countries due to implied environmental and health repercussions of using mercury.
Eco-friendly and modified electrodes are becoming the main concern of electrochemical research and have received great attention in electro-analysis. Most popular innovations devised to replace mercury as working electrodes include the carbon paste electrodes, polymer film electrode, surface-bound crown ether electrodes, carbon-nanotube electrodes, boron-doped diamond electrodes, bismuth-film electrodes and screen-printed electrodes.
Figure 3 – A cross-sectional depiction of a Carbon Paste Electrode
Figure 4 – A diagram of an array of Carbon nanotube electrode
III. Background Literature
The discipline of the Electrochemical technique is said to have been developed at roughly the same era as the invention of the battery by Alessandro Volta. Roughly 200 years have passed since the monumental invention of the two-cell system. In this two-hundred year span, the technique has experienced a many favorable and utterly unfavorable periods. On one such occasion, it has often been said that the very concept of voltammetry, was almost on the brink of extinction during middle of the 20th century and then underwent a period of rebirth – a renaissance – in the years following 1960 wherein the availability of low-cost, high quality commercially available instrumentation was apparent.
In the historical sense, the birth of the electro-analytical technique that is known today as voltammetry was considered to have occurred via experimentation by the Czech chemist Jaroslav Heyrovsky. Heyrovsky was the one to essentially invent polarography, the regarded precursor of modern voltammetric methods of analysis. His contributions in 1922 led to his acquisition of the Nobel Prize in Chemistry in the year 1959. He was, as most researchers of the early 20th century, an avid user of mercury for analyses. The nature of the liquid-metal interface had been studied since the late 19th century. Measuring of the surface tension of mercury gave good results in this field.
During the dawn of the 1940s, the then known polarographic technique was steadily improved. All researches at the time have one goal of interest, and that is to essentially increase the overall sensitivity of the polarographic methods and the speed of experimentations based on the discipline. However, the instrumentation itself was still considered to be too complex and cost-inefficient; the strip chart recording process was slow, and the methods were not widely used at the time. By the year 1950, a new type of voltammetric method, named as the Constant-Current Voltammetry by Delahay became increasingly popular. The new method emphasizes the need for an application of a constant current to the electrode and the potential response per unit is recorded.
Figure 7 – Early Polarographic Instrumentation
The early versions of voltammetric techniques encountered numerous difficulties, making them seem less than ideal for usage in routine analysis. However, the advent of the middle 20th century brought about significant advances in all areas of the voltammetric method (e.g., theory, methodology, instrumentation, etc.), which improved the sensitivity and subsequently allowed the expansion of the overall repertoire of analytical methods. The emergence of these advances coupled with the steady rise of low-cost voltammetric instruments facilitated the rapid commercialization and overall development of fairly inexpensive voltammetric instrumentation.
IV. Critical Review
Voltammetry is one of many electrochemical methods of analysis wherein the current-potential behavior of an analyte at an electrode surface is measured. In
some systematic manner, the potential is varied to cause electro active chemical species to be oxidized or reduced at the surface of the working electrode. There exists a directly proportional relationship between the measured peak current and the concentration of an analyte sample.
Elemental Mercurry was, and is still used to this day, as working electrodes in measuring the current-potential behavior. These electrodes are aptly named as Hanging Mercury Drop Electrodes (H.M.D.).
Figure 8 – A diagram depicting different Mercury Electrodes
In conventional stripping voltammetry, the D.M.Electrode is not used unlike with the polarographic method since the working electrode must be kept still and stationary. An ideal working electrode for stripping voltammetry must exhibit the following qualities:
1. The electrode must have a reproducible surface to ensure constant and precise measurements.
2. The electrode should only cause very minimal levels of residual current.
Examples of solid-based electrodes fulfilling the above criteria are glassy carbon, gold, platinum, wax-impregnated graphite and carbon paste electrodes. Perhaps the only perceived limitation in using the above mentioned electrodes is that fact that they may only be used for one species per analysis at a time. Prior to the stripping step, it is nearly impossible to obtain the required homogeneity of the deposited material when a solid electrode is employed in the analysis of several species. Stripping voltammetry employs mercury as the electrode surface as the most practical electrode.
Figure 9 – A selection of Gold (Au) Electrodes
Another type of sub-method found in stripping voltammetric analysis is the use of three electrode system. The electrode where the metal deposition and subsequent stripping process occurs is named as the working electrode. Meanwhile, the electrode through which the current flows is called the counter electrode. And finally, the electrode that ensures constant maintenance of the generated potential of the working electrode is achieved is called the reference electrode. The reference electrode aids in the reduction of the electrical field that tends to accumulate on the working electrode. The three electrode system is also sometimes used in tandem with the H.D.M.E. system because it enables attainment of lower detection limits.
Figure 10 – Sample Diagram of a Three-Electrode System
With small but significant change in procedure, stripping voltammetry is very similar to polarography. With the different techniques using preconcentration step, anodic stripping voltammetry (ASV) will be the first to be developed. This step uses hanging mercury drop electrode which mainly applied to trace analysis of heavy metal ions. Electrolytic dissolution of a metal which previously deposited on a mercury electrode is the basis of ASV for metals. By using cathodic deposition, the pre-concentration process is achieved at a controlled tie and potential. The potential is scanned anodically and linearly in the measurement step. The amalgamated metals are stripped out of the electrode during the anodic scan procedure which reoxidized and dissolved back to the bulk of the sample solution. The resulting anodic current is recorded as a function of the applied voltage. When an anodic reaction can form an insoluble compound (e.g. mercury salts or complexes) the cathodic stripping voltammetric (CSV) technique is possible with the electrode material. The oxidation of the analyte is used for its preconcentration as an insoluble film on the electrode in this technique. Then the concentrated analyte is reduced and measured during at a negative potential scan. This is the reason why it is sometimes called as the “mirror image” of ASV technique. By measuring the height of the resultant reduction peak current, quantification of the metal is achieved.
Figure 11 – Diagram showcasing the different steps in ASV Analysis
Alternative principles have been proposed by scientists for the aggregation of the samples because of the rising interest of applying such sensitive methods to analytes that are impossible to be pre-concentrated via electrolysis at the electrode surface by the utilization of the adsorption phenomena. Accumulation of the analyte in the adsorption stripping voltammetry (AdSV) occurs by physical adsorption rather than by electrolytic deposition. Nevertheless, it is determined by scanning the potential in appropriate negative or positive direction depending on the oxidation-reduction properties of the accumulated analyte. Most of the organic molecules have the capacity to be adsorbed from the aqueous solution onto a mercury surface. Furthermore, AdSV technique can determine many inorganic cations after their complexation with surface-active complex agents.
Figure 12 – Block Diagram of Adsorptive Stripping Voltammetry
Being highly sensitive and selective in general, anodic stripping voltammetry has acquired an overall positive reputation throughout the many years of cases where it has been commercialized and introduced to the analytical field.
V. Applications and Societal Impact
Over the recent years, the health of the environment is steadily degrading due to human-associated activities, such as excessive mining, logging, and generation of waste products. More often than not, these activities cause the release of a mixture of pollutants that may exist in different forms, be it airborne, water, or earth-damaging. This steady decline of the quality of the ecosystems can be prevented if only the societies of the world, together with their respective leading bodies and governments, by working collectively on the matter, and set this as the main goal. To achieve this goal, a measure of the current status of the environment must first be developed. This activity is called environmental monitoring which aims to:
1. Control, or better yet, prevent the further spreading of pollution.
2. Get knowledge about the effectiveness of devised ecological programs for the future improvement of such programs.
3. Set ecological standards for the industry sector of the world.
Chemical contaminants in foodstuffs are considered to be worrisome issues that are currently being experienced by consumers, food producers, manufacturers and food-regulatory departments. In the last years or so, the number of cases of heavy metal contamination of food has risen significantly. This further leads to a rise in the cases of reported food poisonings via heavy metals, most notably lead and mercury. The presence of heavy metals in foodstuffs may arise from environmental pollutants and industrial chemicals during food processing and packaging or from the use of agrochemicals and veterinary drugs in food production
Figure 14 – Diagram showcasing the Heavy Metal Cycle
Anodic stripping voltammetric methods have shown promising results that may suggest the feasible usage of such method for determining the presence of trace-ultratrace concentrations of heavy/toxic metals, such as arsenic, antimony, cadmium, lead, mercury, and uranium.
The determination of ultratrace concentrations of lead and cadmium in agriculture crops, totaling to over 1700 crop samples, by DP Anodic Stripping Voltammetry (Differential Pulse) was done by the research team led by Stazger. A collaborative study carried out via 13 laboratories for the direct determination of lead in evaporated milk and apple juice by anodic stripping voltammetry was also reported in the literature. 20 laboratories have also reportedly performed a study for the A.S.Voltammetric determination of lead and cadmium in various foodstuffs, such as beans, beef, fish, milk and infant formulations, fruit extracts, a
nd fortified cereals. Golimowski has published a study the compares the concentrations of detected cadmium and lead in over 36 alcoholic beverage samples via Anodic Stripping Voltammetry.
Furthermore, stripping voltammetric techniques, with ASV being the most prominent method, were utilized for the determination of heavy metal content in a different array of foodstuffs such as green, leafy vegetables, wheat crops and rice plants, kitchen table salt, livestock liver and fish, infant formulas and even preserved food items such as softdrinks.
Food additives are classes of chemicals infused with the foodstuff during processing and manufacturing with the purpose of improving its flavor, color, texture, overall appearance, stability and most often shelf life. These food additives are often allowed to be present in food in very minimal quantities. Natural and synthesized food dyes provide food with an overhauled attractive appearance. On the other hand, use of dyes in colorant agents is causes of undesirable side effects. Although food additive dyes traditionally have been analyzed spectrophotometrically and/or chromatographically, they are also very much suited for analysis via electrochemical methods, most promisingly, using the anodic stripping voltammetric method. The research team of Fogg has reviewed the utilization of anodic stripping voltammetry for the determination of synthetic dyes usually used in the textile and printing industry.
VI. Conclusions
This paper was able to evaluate the sensitivity and reliability of Anodic Stripping Voltammetry in quantitative and qualitative determination of the presence of heavy metals in foodstuffs. The electro-analytical method is indeed highly sensitive, with its capability to detect trace components of heavy metals having concentrations of as little as parts per trillion, depending on the type of voltammetric electrode used. ASV is also seen as a highly selective method of analysis, as it is relatively free from interference of other components in samples because of its characteristic wavelength-based detection and selection of specific analytes in said samples. The method is also seen to require minimal sample preparation, with the only perceived drawback being the requirement that analytes must be in solutions before analysis. Indeed, Anodic Stripping Voltammetry is a highly selective, highly sensitive and relatively easy-to-use method and is therefore highly recommended for trace metal analysis in differing types of sample materials.
VII. References
• Alghamdi A.H. (2009). Applications of stripping voltammetric techniques in food analysis. King Saud University. Saudi Arabia
• Anodic Stripping Voltammetry (1997). Retrieved from Pomona College, November 15, 2015 from pages.pomona.edu/~wes04747/chem160/ASV.doc
• Anodic Stripping Voltammetry (n.d). Retrieved from ASA Analytics, November 16, 2015. http://www.asaanalytics.com/asv.php
• Kounaves, S. (n.d.). Voltammetric Techniques. Retrieved from Brown, November 16, 2015 from http://www.brown.edu/Departments/Engineering/Courses/En123/Lectures/
potentiostat.pdf
• Thomas F.G. & Henze G. (2001). Introduction to Voltammetric Analysis: Theory and Practice. National Library of Australia, CSIRO publishing.
• Vladimirovna, Z.A. (2006). The Improvement of Anodic Stripping Voltammetric (ASV) Method of Cadmium and Mercury Determination. Department of Chemical Engineering and Geosciences. Lulea University of Technology.
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