Substitution of Mercury Electrodes by Bismuth-Coated Screen-Printed

Laboratory Experiment pubs.acs.org/jchemeduc

Substitution of Mercury Electrodes by Bismuth-Coated ScreenPrinted Electrodes in the Determination of Quinine in Tonic Water Arístides Alberich, Núria Serrano, José Manuel Díaz-Cruz, Cristina Ariño, and Miquel Esteban* Departament de Química Analítica, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, E−08028 Barcelona, Spain S Supporting Information *

ABSTRACT: The bismuth-coated screen-printed electrode is shown to be a suitable and safe alternative to the classical mercury electrode in the voltammetric determination of quinine in tonic water. This experiment is appropriate for undergraduate analytical courses as it helps students understand the fundamentals of electroanalysis using preplated screenprinted electrodes and the importance of green electrochemistry. Student interest is piqued by using a familiar real-world sample, tonic water.

KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Safety/Hazards, Hands-On Learning/Manipulatives, Electrochemistry, Food Science, Green Chemistry, Instrumental Methods, Quantitative Analysis

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electrodes have the advantage of being environmentally friendly while providing electrochemical features that are very similar to those of mercury, which accounts for their use over the past decade.8 Various attempts have been made to use bismuth in the determination of metals typically measured with a mercury electrode,5,9 but to the best of our knowledge its use for quantifying organic compounds has not been recommended before by any article in the science education literature. Screenprinted electrodes have been used in low-cost cyclic voltammetry experiments10 and for examining metal electrodeposition and Faraday’s law.11 The proposed changesthe electrode type and the electrode materialare not trivial as is evidenced by the fact that in both the educational and the scientific literature phenolic compounds are the only organic substances analyzed using a Bi-SPCE.12 The experiment proposed here provides students with three learning outcomes: • Learning how to coat the electrode substrate with a bismuth film, thereby gaining firsthand experience in the preparation of preplated electrodes. • Introducing the use of screen-printed electrodes, carrying out the determination with a single strip comprising an integrated three electrode (working, counter, and reference) device. This outcome is important as screenprinted electrodes allow measurements to be taken in the field, which is an increasingly important trend in electroanalysis.

uantitative analysis by voltammetry is a typical experiment that can help undergraduate chemistry students understand the fundamentals and the analytical applications of electrochemistry. However, most academic institutions are reticent to maintain these experiments in their curricula because mercury electrodes are used. The risk of toxicity that working with mercury entails has led to proposals for the removal of mercury-containing devices, ranging from thermometers to vacuum lines, from educational laboratories.1−5 In the case of working electrodes, many compounds can be measured using nontoxic electrodes such as glassy carbon, gold, platinum, and so forth, and these electrodes are particularly useful for designing Hg-free voltammetry experiments. Although these electrodes are well suited to the oxidation of analytes, they are not suited for reduction processes, that is, the main application of the mercury electrodes. The quantification of quinine in tonic water by means of voltammetric analysis employing a static mercury drop electrode (SMDE) has been successfully carried out at our institution for many years.6 In addition to the primary goal of teaching the procedures of voltammetric quantification, quinine provides a good example of the electrochemical reduction mechanisms of organic substances, and the results can also be compared with those obtained from fluorometric analysis.7 With the objective of making our laboratories safe, a number of final-year undergraduate students conducted the quantification of quinine using a screen-printed carbon electrode coated with a bismuth film (Bi-SPCE). The overall goal was to incorporate the procedure within an experimental course for advanced students in analytical chemistry. Bismuth film © 2013 American Chemical Society and Division of Chemical Education, Inc.

Published: November 15, 2013 1681

dx.doi.org/10.1021/ed400288a | J. Chem. Educ. 2013, 90, 1681−1684

Journal of Chemical Education

Laboratory Experiment

Scheme 1. Reduction Mechanism of Quinine

• Acquiring knowledge about the reduction mechanisms of organic substances. In addition, it should be noted that the modification proposed for this laboratory experiment does not represent an increase in costs, as the instrumentation used with an SMDE can also be used for the SPCE measurements simply by connecting the electrode to the potentiostat via a new flexible cable. Furthermore, the initial outlay (not great) in purchasing the disposable screen-printed carbon electrodes is counterbalanced by the savings made from not having to acquire mercury and to treat the toxic waste.

Preparation of the Bismuth Film

A 0.2 mol L−1 acetic acid/acetate buffer (pH = 4.5) with 100 ppm Bi(III) solution was used to deposit the film on the carbon disk of the screen-printed sensor. The SPCE was connected to the potentiostat and immersed in 20 mL of the solution in an electrochemical cell. Care was taken to ensure that the liquid just covered the three electrodes but did not come into contact with the flexible cable (which was further protected by wrapping it in a paraffin film). After deaerating the solution for 5 min, the bismuth film was prepared in accordance with the procedure shown in Table 1. Once the bismuth film was deposited, the screen-printed was rinsed carefully with water. This methodology provided very high repeatability and reproducibility.22

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THEORY Quinine is a natural alkaloid with a bitter taste that can be added to certain beverages, including tonic water and bitter lemon. Given the numerous reports detailing health problems related to quinine intake, its use is regulated in some countries. For example, the U.S. Food and Drug Administration (FDA) limits the quinine content in tonic water to 83 ppm.13 The determination of quinine has been previously described in this Journal using such methods as molecular fluorescence spectroscopy,14−16 capillary electrophoresis,17,18 and even visually.19 As the quinine molecule contains an aromatic quinoline fused ring, it can be reduced electrochemically at pH values that are not too high (Scheme 1) and then determined by voltammetry. This reaction is favored by the subsequent equilibrium in which a water molecule is split off by elimination.

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Table 1. Experimental Parameters preparation of the bismuth film parameter value deposition potential −0.80 V deposition time 300 s (with stirring) rest time 20 s (without stirring) differential pulse voltammetry parameter value initial potential −1.10 V final potential −1.65 V step increment 0.005 V step amplitude 0.050 V pulse time 0.05 s voltage step time 1s

EXPERIMENTAL SECTION

Reagents and Instrumentation Voltammetry

Concentrated sulfuric acid, concentrated acetic acid, sodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium acetate were obtained from Fluka. Quinine sulfate and bismuth(III) nitrate pentahydrate were obtained from Sigma-Aldrich. Reagent grade chemicals were used for the bulk of the work as this quality provides good results. Deposition of the bismuth film and measurements by differential pulse voltammetry (DPV) were performed on a 757 VA Computrace (Metrohm) attached to a personal computer with data acquisition software also obtained from Metrohm. Bismuth screen-printed electrodes were prepared by coating a bismuth film on the carbon disk (4 mm diameter) of a three-electrode screen-printed sensor (ref DPR-110, Dropsens). Each Bi-SPCE consists of a bismuth film working electrode, a silver reference electrode, and a carbon counter electrode. The electrode was connected to the potentiostat using a flexible cable (ref CAC, Dropsens). Both the screenprinted carbon electrodes and the flexible cable can be purchased at the Dropsens web site,20,21 although other brands are available. All measurements were conducted in a glass cell at room temperature, and deaeration was achieved with nitrogen.

Prior to performing the measurements, the following steps were conducted: (i) the tonic water was degassed by magnetic stirring, thus eliminating the CO2 in the sample, (ii) a 0.2 mol L−1 phosphate buffer solution was prepared with a pH value of 7.0, and (iii) a 100 ppm standard solution of quinine in 0.05 mol L−1 sulfuric acid was prepared. Once the electrode had been coated, the Bi(III) solution was removed from the cell and 20 mL of the phosphate buffer solution was transferred via a class A transfer pipet. As usual, when bismuth electrodes are employed, there is no need to degas the cell solution. After adjusting the experimental parameters listed in Table 1 and ensuring the blank measure was satisfactory, 2 mL of the tonic water sample was added to the buffer solution in the cell (also an exact volume) and a voltammogram was recorded. Standard additions of 0.4 mL of quinine solution and data collection were repeated until the desired number of additions (typically 3−4) was completed. A micropipet was used to make the additions, but a microsyringe would work equally well. Ideally, at least three determinations for the same sample of quinine should be performed and different tonic waters brands can be analyzed. 1682




Figure 1. (A) Voltammograms from a quinine tonic water sample and four successive standard additions of quinine. (B) A representative data set of quinine peak height vs the total concentration of quinine added to the sample.

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HAZARDS Sulfuric acid and acetic acid can cause severe skin burns and eye damage. Quinine sulfate and bismuth nitrate can cause skin, eye, and respiratory irritation. Moreover, acetic acid is flammable and bismuth nitrate may intensify fire.

Table 2. Student Data of the Determination of Quinine Performed with a Bi-SPCE current (μA) for each replicate

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RESULTS The Bi-SPCE exhibits a well-behaved response to quinine in tonic water samples. A single peak was clearly identified at −1.44 V (Figure 1A), the intensity of which increased with subsequent standard additions. No other peaks were observed in the working potential range used. The determination of quinine in tonic water was performed using the standard addition method (Figure 1B), plotting the absolute value of the individual peak current (y axis) against the total concentration of quinine added and adjusting the dilution factor (x axis). The peak height values were automatically measured by the software, but alternatively, the baseline can be drawn by hand and the peak heights then measured. The quinine concentration in the sample was determined from the absolute value of the x intercept of the linear fit. Note that the standard addition was performed with just one screen-printed electrode. However, the screen-printed electrode is replaced for every replicate. In fact, the same Bi-SPCE might be reused for different replicates, but given that the surface of the electrode is a source of variability, we think that it can be minimized when preparing a new bismuth film, in a new screen-printed electrode, for each determination. The quinine concentration data obtained by an undergraduate student from the determination of three replicates performed using a Bi-SPCE can be seen in Table 2. Here, the average content of quinine in the tonic water was 73 (±3) ppm, a value comparable to the results obtained with a static mercury drop electrode.

Vquinine added (mL)

Cquinine added (ppm)

1

2

3

0 0.4 0.8 1.2 1.4 quinine content R2

0 1.83 3.60 5.31 6.96

−0.60 −0.81 −0.96 −1.11 −1.26 73 0.997

−0.58 −0.79 −0.90 −1.07 −1.24 71 0.993

−0.62 −0.82 −0.94 −1.10 −1.27 76 0.995

higher for the Bi-SPCE (4.1% vs 2.5%, respectively). This difference could be attributed to the greater variability introduced when different bismuth films are coated in comparison to the mercury drop electrode, which is always more regular in its characteristics. Despite the slightly worse standard deviation, the use of the nontoxic electrodes provides considerable advantages. Determination of quinine in tonic water using a Bi-SPCE could be implemented in a course in which 20−24 students (working in pairs) rotate through a set of established laboratories in different branches of analytical chemistry. In our institution, the course is recommended for third-year undergraduates. For classes in which instructors do not prepare any solutions beforehand, a four-hour period is sufficient to complete the set of three determinations, including the bismuth film formation step. Even though the use of a Bi-SPCE in substitution of an SMDE requires completing this preliminary stage, the experimental procedures are otherwise similar in the time requirements. Moreover, students are given the chance to learn how to prepare a preplated electrode. Thus, students are exposed to some of the essential problems of experimental analytical chemistry, including the application of different electrochemical techniques and the undertaking of a determination by standard addition with a real-world sample, which makes the experiment more attractive and relevant to the students. Finally, the experiment presents new educational values regarding the replacement of classical methods with a “greener” electrochemistry and shows students how screenprinted electrodes can facilitate on-site measurements for a realworld problem.

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DISCUSSION To evaluate the substitution of mercury electrodes with bismuth-coated screen-printed electrodes in the determination of quinine in tonic water, the experiment was performed by a pilot group of three pairs of students who compared the main analytical features of the Bi-SPCE and the SMDE. The average quinine contents determined were very similar for both electrodes (74 ppm for Bi-SPCE and 73 ppm for SMDE); however, the average relative standard deviation was somewhat 1683



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Fluorimeter for Middle School, High School, and Undergraduate Chemistry Labs. J. Chem. Educ. 2011, 88, 1182−1187. (17) Herman, H. B.; Jezorek, J. R.; Tang, Z. Analysis of Diet Tonic Water Using Capillary Electrophoresis. J. Chem. Educ. 2000, 77, 743− 744. (18) Holland, L. A. Capillary Electrophoresis: Focus on Undergraduate Laboratory Experiments. J. Chem. Educ. 2011, 88, 254−256. (19) Sacksteder, L.; Ballew, R. M; Brown, E. A.; Demas, J. N.; Nesselrodt, D.; DeGraff, B. A. Photophysics in a Disco. J. Chem. Educ. 1990, 67, 1065−1067. (20) DropSens. http://www.dropsens.com/en/screen_printed_ electrodes_pag.html (accessed Oct 2013) (21) DropSens. http://www.dropsens.com/en/accesories_pag. html#conectores (accessed Oct 2013) (22) Serrano, N.; Diaz-Cruz, J. M.; Ariño, C.; Esteban, M. Ex situ Deposited Bismuth Film on Screen-Printed Carbon Electrode: A Disposable Device for Stripping Voltammetry of Heavy Metal Ions. Electroanalysis 2010, 22, 1460−1467.

ASSOCIATED CONTENT

S Supporting Information *

Notes for the instructors; student handout. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. Esteban. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the Spanish Ministerio de Ciencia e Innovación (MICINN, project CTQ2009-09471) and from the Spanish Ministerio de Economia y Competitividad (MCOC, project CTQ201232863).

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REFERENCES

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Substitution of Mercury Electrodes by Bismuth-Coated Screen-Printed

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