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Designing and Using 3D-Printed Components That Allow Students To Fabricate Low-Cost, Adaptable, Disposable, and Reliable Ag/AgCl Reference Electrodes Benjamin Schmidt, David King, and James Kariuki* Department of Science, University of Alberta, Augustana Campus, 4901-46th Avenue, Camrose, Alberta T4 V 2R3, Canada
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S Supporting Information *
ABSTRACT: A reference electrode is an essential component of all three-electrode electrochemical measurements. Common commercial reference electrodes, including the saturated calomel electrode and Ag/AgCl reference electrode, are expensive with the former containing toxic mercury. Cheaper alternatives have been proposed including Ag/AgCl references made from pipets and test tubes. However, electrodes prepared in this way are difficult to work with and are limited by the size and shape of glass casings that are available. This paper proposes an in-house manufactured Ag/AgCl reference electrode that uses some 3Dprinted components in the fabrication process. This electrode is cheap to manufacture ($5 vs $60−100 CAD for the commercial reference electrode), and the design can be quickly altered due to the 3D printer’s capabilities in rapidly printing new electrode shapes to suit different analysts’ needs. The lab-made reference electrodes demonstrated stability and consistency in peak potential measurements in the cyclic voltammetry (CV) experiments. In ferricyanide CV tests, the recorded differences in anodic and cathodic peak potential (ΔEp) values for the commercial reference electrode and both lab-made electrodes were 68 ± 9%, 70 ± 12%, and 69 ± 13%, respectively. For all tests, the results were statistically comparable with those of the commercial Ag/AgCl reference electrode. KEYWORDS: Second-Year Undergraduate, Analytical Chemistry, Hands-On Learning/Manipulatives, Testing/Assessment, Electrochemistry, Laboratory Equipment/Apparatus
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INTRODUCTION A major area in electrochemical research involves the production of low-cost electrodes for laboratory and field analysis.1−3 Commercial Ag/AgCl electrodes are expensive and range from $60 to $100 CAD (depending on make and model),4 which poses significant costs for laboratory equipment for institutions with budgetary restrictions. A number of different designs of low-cost lab-made reference electrodes have been explored in the literature5−7 with the Ag/AgCl reference electrode being the most common.8−15 These designs include the use of glass wool plugs,8 agar plugs,9 and platinum wire junctions11 to allow for electrolytic contact between the sample and the silver wire. The in-house manufactured Ag/AgCl references are relatively inexpensive and can be made using common lab materials including glass tubes (pipets/test tubes) for the outer casing.12 Despite the relative availability of these materials, the use of a glass casing is cumbersome and time-consuming as the glass needs to be carefully heated to create a hole for electrolytic contact. Another major drawback of both the pipet and glass-tube-designed electrodes is their lack of customizability, specifically the inability of finding glass shells or pipet tubes that fit the specific requirements of the electrochemical cell setup. With the increasing availability of 3D printers at educational institutions, including universities and high schools, as well as public libraries,16 the low-cost fabrication of electrochemical products may become more accessible to researchers and students. In this paper, we propose a design for Ag/AgCl © XXXX American Chemical Society and Division of Chemical Education, Inc.
reference electrodes that utilizes modern 3D printing technology to produce components for an adaptable, lowcost, and disposable reference electrode. The electrode dimensions and design are shown in Figures 1 and 2. This electrode design addresses both the cost concerns of commercial reference electrodes and lack of adaptability of most lab or student-made designs. Furthermore, students can easily fabricate the electrodes for use in the lab providing them with experience in techniques such as bulk electrolysis and insight into the different components used to create a stable reference electrode. The reference electrode was tested for stability over time (2−3 weeks of measurement) via potential measurements versus a saturated calomel electrode and characterized using cyclic voltammetry (CV) in ferricyanide. The in-house manufactured reference electrodes continued to function for ∼3 months. However, if the electrode performance begins to deteriorate, simple modification can be done to replace or repair any of the components unlike commercial Ag/AgCl electrodes, which are often sealed and would require additional porous tips and Teflon heat shrink tubing to be repaired. The performance of the lab-made reference electrodes was comparable with that of commercial counterparts, resulting in an easy-to-make design with a low price point for undergraduate student experiments. Received: June 29, 2018 Revised: August 30, 2018
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DOI: 10.1021/acs.jchemed.8b00512 J. Chem. Educ. XXXX, XXX, XXX−XXX
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TinkerCad.19 This online program was chosen for its ease of use and relatively simple interface in comparison to other free CAD programs, for example, OpenSCAD. The electrode design and dimensions are shown in Figures 1 and 2, respectively. This electrode was printed using clear ABS allowing for ease of visibility of components inside the electrode including the level of supporting electrolyte, the position and quality of the silver wire, as well as the length of the agar plug. Printing in other opaque colors of ABS is possible and may alleviate the nonzero potential shift of a Ag/ AgCl redox couple when exposed to strong light;20 however, the advantages of printing in clear ABS would not be present. The removable lid allowed for ease in refilling the electrode if leakage or evaporation of the internal electrolyte occurred. An ionized agar plug was prepared using 15 g of agarose/L with 1 M KCl and placed at the end of the electrode to allow ion migration through the electrode surface while preventing leakage. After the agarose/KCl solution was heated to boiling, a small volume (2−3 mL) was pipetted into the end of a reference electrode casing that was sealed with parafilm. After the agar plug hardened, the electrode casing was filled to volume with 3 M KCl, this differs from the commercial electrode, which is filled with 3 M NaCl. The Ag wire was then coated with AgCl via controlled current bulk electrolysis in 0.1 M HCl. Successive currents of 1 mA for 10 min, 2 mA for 5 min, 3 mA for 5 min, and 5 mA for 5 min were applied to the wire until the surface was coated with a thin layer of AgCl.
Figure 1. Image and schematic diagram of the in-house manufactured Ag/AgCl reference electrode (for scale of 3D-printed components, see Figure 2).
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REFERENCE ELECTRODE PREPARATION The outer casing and lid for the lab-made reference electrode was printed using acrylonitrile butadiene styrene (ABS) for its higher thermal stability in comparison to other materials such as polylactic acid (PLA).17 ABS also shows stability when exposed to a wide range of chemicals at varying concentrations; however, it will begin to break down with exposure to concentrated acids.18 The printing was conducted with a Lulzbot Taz 6 3D printer (Loveland, CO, USA) taking 45 min to complete the printing process (3D stereolithographic files and 3D printing parameters for printing the electrode are included in the Supporting Information; please note that the 3D stl files must be unzipped before use). The printing process involved fused modeling deposition (FDM) printing as it is easily accessible due to lower cost of FDM printers and produces less waste in comparison to other printing technologies such as stereolithography (SLA) printing. The electrode design was made using a free online program called
Reference Electrode Testing and Results
The commercial and lab-made Ag/AgCl reference electrodes were measured against a saturated calomel electrode in 0.2 M KCl to test their potential measurement stability.21 The potential difference (mV) across the electrodes was measured using a voltmeter after 5 min of stabilization. The measurements were carried out once a day at the start of each analysis. Measurements (n = 8−15) were obtained over the course of 2−3 weeks. The measured mean potential difference (±% RSD) of the commercial and both lab-made reference electrodes were 36 ± 5%, 42 ± 4%, and 43 ± 10%, respectively. Despite the statistically significant difference in
Figure 2. TinkerCad-designed Ag/AgCl reference electrode shell schematic (measurements shown are in mm). B
DOI: 10.1021/acs.jchemed.8b00512 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 3. Representative cyclic voltammograms of commercial and lab-made 1 and 2 Ag/AgCl reference electrodes in 1 mM ferricyanide solution/ 1 M KCl. Initial potential = 0.8 V, vertex potential = −0.2 V, final potential = 0.8 V, scan rate = 100 mV/s.
Table 1. Mean Peak Potential and ANOVA Results of CV Experiments in Ferricyanide CV Experiment Ferricyanide-Oxidation Peak, mV, % Ferricyanide-Reduction Peak, mV, % Ferricyanide-ΔEp, mV, %
Commerciala,c
Lab-Made 1a,c
297 ± 5 224 ± 11 68 ± 9
299 ± 11 236 ± 10 70 ± 12
Lab-Made 2a,b 303 ± 3 242 ± 10 69 ± 13
ANOVA p-Valueb−d 0.84 0.22 0.91
Data presented as mean ± %RSD. bN = 8 measurements. cN = 15 measurements. dp = 0.05
a
compared with the commercial electrodes. The %RSD fluctuated between 2 and 12% for the commercial reference, 7 and 13% for lab-made reference 1, and 3 and 13% for labmade reference 2 depending on the measured variable. The comparable relative standard deviations suggest that all measured electrodes exhibit reproducible stability over long periods of measurement and behaved similarly to one another in the cyclic voltammetry experiments.
the measured potential, the in-house manufactured electrodes compared more closely to the literature value of 44 mV for Ag/ AgCl reference electrodes.22 This difference in potentials may be a result of different internal supporting electrolytes being used, 3 M KCl and 3 M NaCl for the lab-made and commercial reference electrodes, respectively. The comparable %RSD values for the measured electrodes also suggests similar stability in the measurements over extended periods of time. The commercial Ag/AgCl reference electrode and lab-made Ag/AgCl electrodes were also analyzed for stability in 1 mM ferricyanide/1 M KCl via cyclic voltammetry. Ferricyanide is a common benchmark reagent that is commonly used for electrode testing as well-defined voltammograms are produced when electrodes are working properly.23,24 The oxidation and reduction peak potentials as well as the ΔEp of (n = 7−14) voltammograms were recorded over a period of 3 weeks. In tests of ferricyanide, well-defined voltammograms were produced with the use of all three reference electrodes with significant overlap in peak position (Figure 3). The applicable oxidation and reduction peak potentials and oxidation/ reduction peak voltage difference (ΔEp) were recorded, and the mean and standard deviation were calculated for all measurements (Table 1). There was no significant difference in the peak potentials and ΔEp of all measured variables between the commercial reference and the two lab-made reference electrodes. This indicates that the variation in measurements is not significant enough to distinguish a difference in performance of the developed electrodes when
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ADVANTAGES OF THE LAB-MADE ELECTRODE As was shown in CV tests, there is no statistically significant difference between the lab-made Ag/AgCl reference electrodes and the commercial electrode. Therefore, comparable performance with commercial references represents one major advantage of the lab-made Ag/AgCl reference electrode. Arguably more important is the customizability of the 3Dprinted design because the electrode shell can be modified significantly to fit different electrochemical cells and setups. This allows for a simple method of modifying the reference electrode to meet the needs of the analyst. Examples of these modifications include increasing the stored volume of the electrode shell, changing the material the shell is printed in to alter its resistance to certain chemicals, or modifying the dimensions of the electrode to suite a variety of electrochemical cells. The lab-made electrode is also relatively low in cost in comparison to most commercial reference electrodes. Commercially purchased reference electrodes often range from C
DOI: 10.1021/acs.jchemed.8b00512 J. Chem. Educ. XXXX, XXX, XXX−XXX
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$60 to $100 CAD (dependent on size and design), whereas the lab-made electrode costs ∼$5 to produce. Other major advantages of the lab-made reference electrode in comparison to standard commercial references include disposability and reusability. Because of the low cost of the labmade electrodes, they can be easily used for undergraduate laboratories without significant risk of damaging expensive electrodes. Furthermore, the 3D-printed shell can be modified to be more robust by increasing the thickness and size of the casing, whereas similar glass casings are much more fragile. Any damaged electrode shell can be easily recycled and replaced while the expensive components (silver wire) can continue to be reused in future reference electrodes with simple modification. If after some time the lab-made electrode no longer functions properly, the agar plug can be easily cleaned out and replaced, the electrolyte solution can be refilled, and the silver wire can be recoated with AgCl. This allows for the electrode to be easily reused with the same components, unlike commercial electrodes that are often in sealed glass vessels making modification difficult if the electrode performance deteriorates.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
James Kariuki: 0000-0003-0465-0206 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was carried out with financial support from the Mazankowski Research Internship administered by the Augustana Campus of the University of Alberta.
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REFERENCES
(1) Jomma, E. Y.; Bao, N.; Ding, S.-N. A Pencil Drawn Microelectrode on Paper and Its Application in Two-Electrode Electrochemical Sensors. Anal. Methods 2017, 9 (23), 3513−3518. (2) Metters, J. P.; Tan, F.; Kadara, R. O.; Banks, C. E. Electroanalytical Properties of Screen Printed Shallow Recessed Electrodes. Anal. Methods 2012, 4 (10), 3140. (3) Wonsawat, W.; Dungchai, W.; Motomizu, S.; Chuanuwatanakul, S.; Chailapakul, O. Highly Sensitive Determination of Cadmium and Lead Using a Low-Cost Electrochemical Flow-through Cell Based on a Carbon Paste Electrode. Anal. Sci. 2012, 28 (2), 141−146. (4) Reference Electrodes. https://www.basinc.com/products/ec/ref (accessed Aug 2018). (5) Randle, T. H.; Kelly, P. J. An Inexpensive, Commercial-Type, Reference Electrode. J. Chem. Educ. 1984, 61 (8), 721. (6) Kusuda, K. A Versatile Compact Reference Electrode. J. Chem. Educ. 1989, 66 (6), 531. (7) Massé, R. C.; Gerken, J. B. Assembly of a Robust and Economical MnO 2 -Based Reference Electrode. J. Chem. Educ. 2015, 92 (1), 110−115. (8) Ahn, M. K.; Reuland, D. J.; Chadd, K. D. Electrochemical Measurements in General Chemistry Lab Using a StudentConstructed Ag-AgCl Reference Electrode. J. Chem. Educ. 1992, 69, 74. (9) Inamdar, S. N.; Bhat, M. A.; Haram, S. K. Construction of Ag/ AgCl Reference Electrode from Used Felt-Tipped Pen Barrel for Undergraduate Laboratory. J. Chem. Educ. 2009, 86 (3), 355−356. (10) Brewer, P. J.; Leese, R. J.; Brown, R. J. C. An Improved Approach for Fabricating Ag/AgCl Reference Electrodes. Electrochim. Acta 2012, 71, 252−257. (11) East, G. A.; del Valle, M. A. Easy-to-Make Ag/AgCl Reference Electrode. J. Chem. Educ. 2000, 77 (1), 97. (12) Barlag, R.; Nyasulu, F.; Starr, R.; Silverman, J.; Arthasery, P.; McMills, L. A Student-Made Silver−Silver Chloride Reference Electrode for the General Chemistry Laboratory: ∼ 10 min Preparation. J. Chem. Educ. 2014, 91 (5), 766−768. (13) Stoica, D.; Brewer, P. J.; Brown, R. J. C.; Fisicaro, P. Influence of Fabrication Procedure on the Electrochemical Performance of Ag/ AgCl Reference Electrodes. Electrochim. Acta 2011, 56 (27), 10009− 10015. (14) Thomas, J. M. Student Construction of a Gel-Filled Ag/AgCl Reference Electrode for Use in a Potentiometric Titration. J. Chem. Educ. 1999, 76 (1), 97−98. (15) Qiao, G.; Xiao, H.; Hong, Y.; Qiu, Y. Preparation and Characterization of the Solid-State Ag/AgCl Reference Electrode for RC Structures - ProQuest. Sens. Rev. 2012, 32 (2), 118−122. (16) Bharti, N.; Singh, S. Three-Dimensional (3D) Printers in Libraries: Perspective and Preliminary Safety Analysis. J. Chem. Educ. 2017, 94 (7), 879−885. (17) FDM 3D printing materials compared. https://www.3dhubs. com/knowledge-base/fdm-3d-printing-materials-compared (accessed Aug 2018).
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CONCLUSIONS In this study, lab-made Ag/AgCl reference electrodes were developed utilizing an agar plug, a AgCl-coated silver wire, and a 3D-printed electrode shell. The developed reference electrodes were compared with a commercial Ag/AgCl reference electrode to determine stability and reproducibility of measurements taken over the course of 2−3 weeks. In CV tests of ferricyanide, the developed reference electrodes compared favorably in both the %RSD of measurements and stability over time. The lab-made reference electrode could be a viable alternative to commercial products as it is relatively inexpensive, disposable, reusable, and adaptable due to the versatility of 3D printing. Ag/AgCl reference electrodes are used in many electroanalytical techniques including sensors such as combination pH electrodes and other ion-selective electrodes. With the adaptability of the 3D-printed design, these electrodes can be readily changed to have wide applicability in different techniques and sensors. By modifying the electrode shell through 3D-printing software, the analyst can develop unique designs better suited for the particular techniques or sensors requiring the reference electrode. The lab-made reference electrode can also be used alongside lowcost pencil graphite working electrodes25 and low cost potentiostats such as the CheapStat to further lower the price barrier into electrochemical analysis.26 This reference electrode will be beneficial to undergraduate researchers and laboratories that require low-cost electrodes to reduce the operating cost of electroanalytical experiments.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00512. Basic and advanced parameters for 3D printing (PDF) Basic and advanced parameters for 3D printing (DOCX) STL files for the design of the lab-made reference electrode (ZIP) D
DOI: 10.1021/acs.jchemed.8b00512 J. Chem. Educ. XXXX, XXX, XXX−XXX
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(18) Chemical Resistance Guide. http://www.gilsoneng.com/ reference/ChemRes.pdf (accessed Aug, 2018). (19) Tinkercad. https://www.tinkercad.com/ (accessed Aug 2018). (20) Janz, G. J.; Taniguchi, H. The Silver-Silver Halide Electrodes. Preparation, Stability, and Standard Potentials in Aqueous and NonAqueous Media. Chem. Rev. 1953, 53 (3), 397−437. (21) Yosypchuk, B.; Barek, J.; Yosypchuk, O. Preparation and Properties of Reference Electrodes Based on Silver Paste Amalgam. Electroanalysis 2011, 23 (9), 2226−2231. (22) Harris, D. C.; Lucy, C. A. Quantitative Chemical Analysis, 9th ed.; Murphy, B., Bristow, A., Anderson, M., Eds.; W.H. Freeman and Company: New York, NY, 2016. (23) Kariuki, J. K. An Electrochemical and Spectroscopic Characterization of Pencil Graphite Electrodes. J. Electrochem. Soc. 2012, 159 (9), H747−H751. (24) Mistry, K. K.; Sagarika Deepthy, T.; Chau, C. R.; Saha, H. Electrochemical Characterization of Some Commercial ScreenPrinted Electrodes in Different Redox Substrates. Curr. Sci. 2015, 109 (8), 1427−1436. (25) King, D.; Friend, J.; Kariuki, J. Measuring Vitamin c Content of Commercial Orange Juice Using a Pencil Lead Electrode. J. Chem. Educ. 2010, 87 (5), 507−509. (26) Rowe, A. A.; Bonham, A. J.; White, R. J.; Zimmer, M. P.; Yadgar, R. J.; Hobza, T. M.; Honea, J. W.; Ben-Yaacov, I.; Plaxco, K. W. Cheapstat: An Open-Source, “Do-It-Yourself” Potentiostat for Analytical and Educational Applications. PLoS One 2011, 6 (9), e23783.
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DOI: 10.1021/acs.jchemed.8b00512 J. Chem. Educ. XXXX, XXX, XXX−XXX
