bismuto experiencia

8/13/2019 Bismuto Experiencia

1/4

Substitution of Mercury Electrodes by Bismuth-Coated Screen-Printed Electrodes in the Determination of Quinine in Tonic Water

Arstides Alberich, Nuria Serrano, Jose Manuel Daz-Cruz, Cristina Arino, and Miquel Esteban*

Departament de Qumica Analtica, Facultat de Qumica, Universitat de Barcelona, Mart i Franques 1-11, E08028 Barcelona, Spain

*S Supporting Information

ABSTRACT: The bismuth-coated screen-printed electrode is shown tobe a suitable and safe alternative to the classical mercury electrode in thevoltammetric determination of quinine in tonic water. This experiment isappropriate for undergraduate analytical courses as it helps studentsunderstand the fundamentals of electroanalysis using preplated screen-printed electrodes and the importance of green electrochemistry. Studentinterest 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

Quantitative analysis by voltammetry is a typical experi-ment that can help undergraduate chemistry studentsunderstand the fundamentals and the analytical applications ofelectrochemistry. However, most academic institutions are

reticent to maintain these experiments in their curricula becausemercury electrodes are used. The risk of toxicity that working

with mercury entails has led to proposals for the removal ofmercury-containing devices, ranging from thermometers to

vacuum lines, from educational laboratories.15

In the case of working electrodes, many compounds can bemeasured using nontoxic electrodes such as glassy carbon, gold,platinum, and so forth, and these electrodes are particularlyuseful for designing Hg-free voltammetry experiments.

Although these electrodes are well suited to the oxidation ofanalytes, they are not suited for reduction processes, that is, themain application of the mercury electrodes.

The quantication of quinine in tonic water by means ofvoltammetric analysis employing a static mercury dropelectrode (SMDE) has been successfully carried out at ourinstitution for many years.6 In addition to the primary goal ofteaching the procedures of voltammetric quantication, quinineprovides a good example of the electrochemical reductionmechanisms of organic substances, and the results can alsobecompared with those obtained from uorometric analysis.7

With the objective of making our laboratories safe, a numberof nal-year undergraduate students conducted the quantica-tion of quinine using a screen-printed carbon electrode coated

with a bismuth lm (Bi-SPCE). The overall goal was toincorporate the procedure within an experimental course foradvanced students in analytical chemistry. Bismuth lm

electrodes have the advantage of being environmentally friendlywhile providing electrochemical features that are very similar tothose ofmercury, which accounts for their use over the pastdecade.8 Various attempts have been made to use bismuth in

the determination of metals typically measured with a mercuryelectrode,5,9 but to the best of our knowledge its use forquantifying organic compounds has not been recommended

before by any article in the science education literature. Screen-printed electrodes have been used in low-cost cyclic

voltammetry experiments10 and for examining metal electro-deposition and Faradays law.11 The proposed changestheelectrode type and the electrode materialare not trivial as isevidenced by the fact that in both the educational and thescientic literature phenolic compounds are the only organicsubstances analyzed using a Bi-SPCE.12

The experiment proposed here provides students with threelearning outcomes:

Learning how to coat the electrode substrate with abismuth lm, thereby gaining rsthand experience in thepreparation of preplated electrodes.

Introducing the use of screen-printed electrodes, carryingout the determination with a single strip comprising anintegrated three electrode (working, counter, andreference) device. This outcome is important as screen-printed electrodes allow measurements to be taken in theeld, which is an increasingly important trend inelectroanalysis.

Published: November 15, 2013

Laboratory Experiment

pubs.acs.org/jchemeduc

2013 American Chemical Society andDivision of Chemical Education, Inc. 1681 dx.doi.org/10.1021/ed400288a| J. Chem. Educ. 2013, 90, 16811684
http://localhost/var/www/apps/conversion/tmp/scratch_6/pubs.acs.org/jchemeduchttp://localhost/var/www/apps/conversion/tmp/scratch_6/pubs.acs.org/jchemeduc


2/4

Acquiring knowledge about the reduction mechanisms oforganic substances.

In addition, it should be noted that the modicationproposed for this laboratory experiment does not representan increase in costs, as the instrumentation used with an SMDEcan also be used for the SPCE measurements simply byconnecting the electrode to the potentiostat via a new exiblecable. Furthermore, the initial outlay (not great) in purchasingthe disposable screen-printed carbon electrodes is counter-

balanced by the savings made from not having to acquiremercury and to treat the toxic waste.

THEORY

Quinine is a natural alkaloid with a bitter taste that can beadded to certain beverages, including tonic water and bitterlemon. Given the numerous reports detailing health problemsrelated 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 describedin this Journal using such methods as molecular uorescencespectroscopy,1416 capillary electrophoresis,17,18 and even

visually.19 As the quinine molecule contains an aromaticquinoline fused ring, it can be reduced electrochemically at pH

values that are not too high (Scheme1) and then determinedby voltammetry. This reaction is favored by the subsequentequilibrium in which a water molecule is split off by

elimination.

EXPERIMENTAL SECTION

Reagents and Instrumentation

Concentrated sulfuric acid, concentrated acetic acid, sodiumhydrogen phosphate, sodium dihydrogen phosphate, andsodium acetate were obtained from Fluka. Quinine sulfateand bismuth(III) nitrate pentahydrate were obtained fromSigma-Aldrich. Reagent grade chemicals were used for the bulkof the work as this quality provides good results.

Deposition of the bismuth lm and measurements bydifferential pulse voltammetry (DPV) were performed on a757 VA Computrace (Metrohm) attached to a personal

computer with data acquisition software also obtained fromMetrohm. Bismuth screen-printed electrodes were prepared bycoating a bismuth lm on the carbon disk (4 mm diameter) of athree-electrode screen-printed sensor (ref DPR-110, Drop-sens). Each Bi-SPCE consists of a bismuth lm workingelectrode, a silver reference electrode, and a carbon counterelectrode. The electrode was connected to the potentiostatusing a exible cable (ref CAC, Dropsens). Both the screen-printed carbon electrodes and the exible cable can bepurchased at the Dropsens web site,20,21 although other brandsare available.

All measurements were conducted in a glass cell at roomtemperature, and deaeration was achieved with nitrogen.

Preparation of the Bismuth Film

A 0.2 mol L1 acetic acid/acetate buffer (pH = 4.5) with 100ppm Bi(III) solution was used to deposit the lm on the carbondisk of the screen-printed sensor. The SPCE was connected tothe potentiostat and immersed in 20 mL of the solution in anelectrochemical cell. Care was taken to ensure that the liquid

just covered the three electrodes but did not come into contactwith the exible cable (which was further protected bywrapping it in a paraffin lm). After deaerating the solutionfor 5 min, the bismuth lm was prepared in accordance with theprocedure shown in Table 1. Once the bismuth lm wasdeposited, the screen-printed was rinsed carefully with water.This methodology provided very high repeatability andreproducibility.22

Voltammetry

Prior to performing the measurements, the following steps wereconducted: (i) the tonic water was degassed by magneticstirring, thus eliminating the CO2in the sample, (ii) a 0.2 molL1 phosphate buffer solution was prepared with a pH value of7.0, and (iii) a 100 ppm standard solution of quinine in 0.05mol L1 sulfuric acid was prepared.

Once the electrode had been coated, the Bi(III) solution wasremoved 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 todegas the cell solution. After adjusting the experimentalparameters listed in Table1 and ensuring the blank measure

was satisfactory, 2 mL of the tonic water sample was added tothe buffer solution in the cell (also an exact volume) and a

voltammogram was recorded. Standard additions of 0.4 mL ofquinine solution and data collection were repeated until thedesired number of additions (typically 34) was completed. Amicropipet was used to make the additions, but a microsyringe

would work equally well. Ideally, at least three determinationsfor the same sample of quinine should be performed anddifferent tonic waters brands can be analyzed.

Scheme 1. Reduction Mechanism of Quinine

Table 1. Experimental Parameters

preparation of the bismuth lm

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

nal potential 1.65 V

step increment 0.005 V

step amplitude 0.050 V pulse time 0.05 s

voltage step time 1 s

Journal of Chemical Education Laboratory Experiment

dx.doi.org/10.1021/ed400288a| J. Chem. Educ. 2013, 90, 168116841682


3/4

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 isammable and bismuth nitrate may intensify re.

RESULTS

The Bi-SPCE exhibits a well-behaved response to quinine intonic water samples. A single peak was clearly identied at1.44 V (Figure 1A), the intensity of which increased withsubsequent standard additions. No other peaks were observedin the working potential range used.

The determination of quinine in tonic water was performedusing the standard addition method (Figure 1B), plotting theabsolute value of the individual peak current (yaxis) against thetotal concentration of quinine added and adjusting the dilution

factor (x axis). The peak height values were automaticallymeasured by the software, but alternatively, the baseline can bedrawn by hand and the peak heights then measured. Thequinine concentration in the sample was determined from theabsolute value of the x intercept of the linear t. Note that thestandard addition was performed with just one screen-printedelectrode. However, the screen-printed electrode is replaced forevery replicate. In fact, the same Bi-SPCE might be reused fordifferent replicates, but given that the surface of the electrode isa source of variability, we think that it can be minimized whenpreparing a new bismuth lm, in a new screen-printedelectrode, for each determination. The quinine concentrationdata obtained by an undergraduate student from thedetermination of three replicates performed using a Bi-SPCE

can be seen in Table2. Here, the average content of quinine inthe tonic water was 73 (3) ppm, a value comparable to theresults obtained with a static mercury drop electrode.

DISCUSSION

To evaluate the substitution of mercury electrodes withbismuth-coated screen-printed electrodes in the determinationof quinine in tonic water, the experiment was performed by apilot group of three pairs of students who compared the mainanalytical features of the Bi-SPCE and the SMDE. The averagequinine contents determined were very similar for bothelectrodes (74 ppm for Bi-SPCE and 73 ppm for SMDE);however, the average relative standard deviation was somewhat

higher for the Bi-SPCE (4.1% vs 2.5%, respectively). Thisdifference could be attributed to the greater variabilityintroduced when different bismuth lms are coated incomparison to the mercury drop electrode, which is alwaysmore regular in its characteristics. Despite the slightly worsestandard deviation, the use of the nontoxic electrodes providesconsiderable advantages.

Determination of quinine in tonic water using a Bi-SPCEcould be implemented in a course in which 2024 students(working in pairs) rotate through a set of establishedlaboratories in different branches of analytical chemistry. Inour institution, the course is recommended for third-yearundergraduates. For classes in which instructors do not prepareany solutions beforehand, a four-hour period is sufficient tocomplete the set of three determinations, including the bismuthlm formation step. Even though the use of a Bi-SPCE insubstitution of an SMDE requires completing this preliminary

stage, the experimental procedures are otherwise similar in thetime requirements. Moreover, students are given the chance tolearn how to prepare a preplated electrode. Thus, students areexposed to some of the essential problems of experimentalanalytical chemistry, including the application of differentelectrochemical techniques and the undertaking of a determi-nation by standard addition with a real-world sample, whichmakes the experiment more attractive and relevant to thestudents. Finally, the experiment presents new educational

values regarding the replacement of classical methods with agreener electrochemistry and shows students how screen-printed electrodes can facilitate on-site measurements for a real-

world problem.

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

Table 2. Student Data of the Determination of QuininePerformed with a Bi-SPCE

current (A) for each replicate

Vquinine added (mL) Cquinine added(ppm) 1 2 3

0 0 0.60 0.58 0.62

0.4 1.83 0.81 0.79 0.82

0.8 3.60 0.96 0.90 0.94

1.2 5.31 1.11 1.07 1.10

1.4 6.96 1.26 1.24 1.27

quinine content 73 71 76

R2 0.997 0.993 0.995




4/4

ASSOCIATED CONTENT

*S Supporting Information

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

AUTHOR INFORMATION

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

Notes

The authors declare no competing nancial interest.

ACKNOWLEDGMENTS

The authors acknowledge nancial support from the SpanishMinisterio de Ciencia e Innovacion (MICINN, projectCTQ2009-09471) and from the Spanish Ministerio deEconomia y Competitividad (MCOC, project CTQ2012-32863).

REFERENCES(1) Foster, B. L. Mercury Thermometer Replacements in Chemistry

Laboratories. J. Chem. Educ. 2005, 82, 269270.(2) Ongley, L. K.; Kern, C. S.; Woods, B. W. A Non-Mercury

Thermometer Alternative for Use in Older Melting Point Apparatuses.J. Chem. Educ.2008, 85, 12631264.

(3) McGregor, D.; Sweeney, W. V.; Mills, P. A Simple Mercury-FreeLaboratory Apparatus To Study the Relationship between Pressure,Volume, and Temperature in a Gas. J. Chem. Educ.2012,89, 509512.

(4) Linn, D. E., Jr. An Accessible Mercury-Free Vacuum Schlenk Linefor Air-Free Techniques. J. Chem. Educ. 2012, 89, 14791480.

(5) Goldcamp, M. J.; Underwood, M. N.; Cloud, J. L.; Harshman, S.An Environmentally Friendly, Cost-Effective Determination of Lead inEnvironmental Samples Using Anodic Stripping Voltammetry. J.Chem. Educ.2008, 85, 976979.

(6) Metrohm Herisau Polarographic determination of quinine.

Application Bulletin 126/2 e(7) Sawyer, D. T.; Heineman, W. R.; Beebe, J. M.; Reilley, C. N.

Chemical Experiments for Instrumental Methods, Wiley: Hoboken, NJ,1984; pp 271273.

(8) Svancara, I.; Prior, C.; Hocevar, S. B.; Wang, J. A Decade withBismuth-Based Electrodes in Electroanalysis. Electroanalysis 2010, 22,14051420.

(9) Wilburn, J. P.; Brown, K. L.; Cliffel, D. E. Mercury-Free Analysisof Lead in Drinking Water by Anodic Stripping Square WaveVoltammetry. J. Chem. Educ. 2007, 84, 312314.

(10) Stewart, G.; Kuntzleman, T. S.; Amend, J. R.; Collins, M. J.Affordable Cyclic Voltammetry.J. Chem .Educ. 2009,86, 10801081.

(11) Chyan, Y.; Chyan, O. Metal Electrodeposition on an Integrated,Screen-Printed Electrode Assembly.J. Chem. Educ.2008,85, 565567.

(12) Serrano, N.; Alberich, A.; Diaz-Cruz, J. M.; Arino, C.; Esteban,M. Coating methods, modifiers and applications of bismuth screen-printed electrodes. TrAC, Trends Anal. Chem. 2013, 46, 15

29.

(13) U.S. Food and Drug Administration (FDA) Code of FederalRegulations - Title 21 - Food and Drugs; Sec. 172.575. http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=172.575(accessed Oct 2013)

(14) OReilly, J. E. Fluorescence Experiments with Quinine. J. Chem.Educ.1975, 52, 610612.

(15) Pandey, S.; Borders, T. L.; Hernandez, C. E.; Roy, L. E.; Reddy,G. D.; Martinez, G. L.; Jackson, A.; Brown, G.; Acree, E., Jr.Comparison of Analytical Methods: Direct Emission versus First-Derivative Fluorometric Methods for Quinine Determination in TonicWaters. J. Chem. Educ. 1999, 76, 8587.

(16) Wigton, B. T.; Chohan, B. S.; McDonald, C.; Johnson, M.;Schunk, D.; Kreuter, R.; Sykes, D. A Portable, Low-Cost, LED

Fluorimeter for Middle School, High School, and UndergraduateChemistry Labs. J. Chem. Educ. 2011, 88, 11821187.

(17) Herman, H. B.; Jezorek, J. R.; Tang, Z. Analysis of Diet TonicWater Using Capillary Electrophoresis. J. Chem. Educ. 2000,77, 743744.

(18) Holland, L. A. Capillary Electrophoresis: Focus on Under-graduate Laboratory Experiments. J. Chem. Educ. 2011, 88, 254256.

(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, 10651067.(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.; Arino, C.; Esteban, M. Ex situ

Deposited Bismuth Film on Screen-Printed Carbon Electrode: ADisposable Device for Stripping Voltammetry of Heavy Metal Ions.

Electroanalysis2010, 22, 14601467.


http://pubs.acs.org/mailto:[email protected]://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=172.575http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=172.575http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=172.575http://www.dropsens.com/en/screen_printed_electrodes_pag.htmlhttp://www.dropsens.com/en/screen_printed_electrodes_pag.htmlhttp://www.dropsens.com/en/accesories_pag.html#conectoreshttp://www.dropsens.com/en/accesories_pag.html#conectoreshttp://www.dropsens.com/en/accesories_pag.html#conectoreshttp://www.dropsens.com/en/accesories_pag.html#conectoreshttp://www.dropsens.com/en/accesories_pag.html#conectoreshttp://www.dropsens.com/en/accesories_pag.html#conectoreshttp://www.dropsens.com/en/screen_printed_electrodes_pag.htmlhttp://www.dropsens.com/en/screen_printed_electrodes_pag.htmlhttp://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=172.575http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=172.575http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=172.575mailto:[email protected]://pubs.acs.org/

bismuto experiencia

Documents