LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Coulometric Analysis Experiment for the Undergraduate Chemistry Laboratory Rajeev B. Dabke,* Zewdu Gebeyehu, and Ryan Thor Department of Chemistry, Columbus State University, Columbus, Georgia 31907, United States
bS Supporting Information ABSTRACT: An undergraduate experiment on coulometric analysis of four commercial household products is presented. A special type of coulometry cell made of polydimethylsiloxane (PDMS) polymer is utilized. The PDMS cell consists of multiple analyte compartments and an internal network of salt bridges. Experimental procedure for the analysis of the acid in a toilet bowl cleaner, base in household clear ammonia, Fe2+ in iron supplement tablets, and iodine in povidoneiodine is provided. Samples were titrated against coulometrically generated reagents, and the end point was detected by a visual color change of the indicator. Experimental results obtained by coulometric titration were in agreement with the results obtained by the volumetric titration and with the data on the manufacturer’s label. The experiments highlight the real-world significance of Faraday’s laws of electrolysis and the mole ratios. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Acids/Bases, Consumer Chemistry, Electrochemistry, Laboratory Equipment/Apparatus, Titration/Volumetric Analysis
T
he coulometric methods of chemical analysis are wellknown,1 and undergraduate experiments based on coulometric methods are common in the quantitative analysis course. Numerous reports on coulometric methods with the pedagogical objectives have appeared in this Journal.218 A variety of forms of a coulometry cell made of glass have been reported, and special care is required to clean and assemble the parts of a cell made of glass. The volume of the electrolyte used in these cells is in the range of tens of milliliters or higher.219 The coulometry autotitrator cells are expensive.20 A “salt bridge” is often used2,4,6,7,1417 to physically separate the anode and cathode compartment contents and to allow the current to flow through the cell. A salt bridge used in a coulometer setup is made of fragile sintered glass or a U-shaped tube filled with an agarelectrolyte mixture. Assembling a cell with a sintered glass membrane as a bridge often demands a glass shop facility. Agarelectrolyte salt bridges easily dehydrate, and the cleaning and refilling of agarelectrolyte mixture in a U-shaped tube can be difficult. Devices made of polydimethylsiloxane (PDMS) polymer including electrochemical cells have appeared in recent articles.2123 PDMS devices are becoming increasingly important because they offer small reaction volumes and can be made by a variety of fabrication techniques.21 A miniature coulometry cell made of PDMS (Figure S1 in the Supporting Information) is presented as an alternative to the glass cell. Multiple anode and cathode compartments were formed by drilling holes in the PDMS polymer block, and the compartments were internally connected by agar KNO3 gel channels. The cell utilizes relatively smaller volumes of the electrolyte, provides internal salt bridge connections, and Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
offers multiple compartments for analysis. The PDMS cell is transparent, unbreakable, reusable, and easy to handle. The agar KNO3 mixture can be easily filled in the channels to serve as salt bridges. Four independent trials of analysis can be readily performed by simply moving the electrode from one compartment to the other after each trial. These features make this cell useful in an undergraduate analytical laboratory. Coulometric analysis of four commercially available household products is described: toilet bowl cleaner, household clear ammonia, iron supplement tablets, and povidoneiodine. The experiments are flexible and may be performed in either a single three-hour analytical laboratory session or two laboratory sessions, depending upon the number of analytes chosen. Students work in groups, and each group works on one household product. Analytical experiments with household products make chemistry more relevant and fascinating. Household products analyzed in this study are readily available in a grocery store or a pharmacy. The pedagogical objectives of this experiment are (i) to apply Faraday’s laws of electrolysis and the mole relationships to quantify the active ingredients in the household products and (ii) to explore the anodic and cathodic reaction products and use them in a chemical analysis.
’ CHEMICALS AND EQUIPMENT Lysol brand toilet bowl cleaner, Publix brand clear ammonia, CVS brand slow release iron supplement tablets, and CVS brand Published: October 04, 2011 1707
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povidoneiodine were purchased from a local grocery store or pharmacy. Platinum wire was ordered from Alfa Aesar (0.762 mm diameter). A Sylgard brand 184 elastomer kit was purchased from Dow Corning Corporation. Other chemicals were ordered from Fisher Scientific. All solutions were prepared in Millipore deionized water. Obbligato-Objectives Faraday MP potentiostat was used as a current source in the “cyclic voltammetry” mode. The initial and switching potentials were set to the fixed potential (1.0 V). The working electrode was shorted to the reference electrodes and served as an anode. The counter electrode served as a cathode. The potentiostat monitored the charge passed through the cell. The current for the titration ranged from 10 to 50 mA. A suitable constant current source can also be used for coulometric titrations. A trial of the coulometric titration of a 1500 μL Lysol sample took about 3 min. Preparation of the PDMS cell, preparation of stock solutions, detailed experimental procedure, and calculations are presented in the Supporting Information. The central compartment and the side compartments of the PDMS cell can hold up to 2.2 and 4.2 mL of electrolyte, respectively. The PDMS cells were made by the instructor and staff and can be constructed in about 5 h.
Figure 1. Plot of cathodic charge versus volume of Lysol.
’ ANALYSIS OF THE ACID CONTENT IN THE TOILET BOWL CLEANER Lysol brand toilet bowl cleaner contains hydrochloric acid as the main ingredient. Coulometric analysis is used to analyze the acid in the cleaner. Passing a direct current through water produces H+(aq) and OH(aq) ions in the anode and cathode compartments, respectively. The compartment reactions are anode : H2 OðlÞ f 2Hþ ðaqÞ þ
1 O2 ðgÞ þ 2e 2
ðoxidationÞ
ð1Þ cathode : 2H2 OðlÞ þ 2e f 2OH ðaqÞ þ H2 ðgÞ
Figure 2. Plot of anodic charge versus volume of household ammonia.
ðreductionÞ
ð2Þ These reactions indicate that electrons travel to the cathode and generate OH(aq) ions. The generated OH(aq) ions and electrons are in a 1:1 mol ratio. Faraday’s laws of electrolysis equate 1 Faraday (or 96485.3 coulombs) of electricity to 1 mol of H+(aq) or OH(aq) ions. The amount of charge passed through the cell is proportional to the quantity of OH(aq) ions generated at the cathode. A known volume of the toilet bowl cleaner was placed in the cathode compartment in the presence of aqueous potassium nitrate as a supporting electrolyte. The cathode generated OH(aq) ions and the ions reacted with the acid present in Lysol: ð3Þ
Figure 3. Plot of anodic charge versus volume of Fe2+(aq) in iron tablet.
The acid and OH(aq) ions are related in a 1:1 mol ratio. The charge required to complete the neutralization of acid was measured, and the amount of acid in Lysol was calculated. The end point of this neutralization titration was monitored by using phenolphthalein indicator.
presented. Coulometrically generated H+(aq) ions (eq 1) react with the base:
HClðaqÞ þ OH ðaqÞ f Cl ðaqÞ þ H2 OðlÞ
’ ANALYSIS OF A BASE IN THE HOUSEHOLD AMMONIA Household ammonia (aqueous ammonium hydroxide) is used as a general purpose surface cleaner. A coulometric method of analysis of the base content in the commercial ammonia is
NH4 OHðaqÞ þ Hþ ðaqÞ f NH4 þ ðaqÞ þ H2 OðlÞ
ð4Þ
A known volume of ammonia and an electrolyte were placed in the anode compartment. Electrical charge was passed until the base had been completely neutralized by the generated H+(aq) ions. The end point of the titration was monitored by using methyl orange indicator. The stoichiometric relation stated in eq 4 and Faraday’s laws of electrolysis were applied to calculate the amount of base present in the household ammonia. 1708
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Table 1. Summary of the Household Product Analysis analysis
coulometric titration
information on manufacturer’s label
volumetric titration
acid in Lysol
2.9 M
2.7 M (equivalent to 9.5%)
2.8 M
base in household ammonia
1.2 M
N/A
1.2 M
Fe(II) sulfate in iron supplement tablets
158 mg per tablet
160 mg per tablet
158 mg per tablet
iodine in povidoneiodine
0.039 M
0.039 M (equivalent to 1%)
0.039 M
’ ANALYSIS OF FE2+ IN THE IRON SUPPLEMENT TABLETS Iron supplement tablets help maintain the proper level of iron in blood. Iron tablets used in this study deliver iron in the form of iron(II) sulfate. Fe2+(aq) ions can be oxidized to Fe3+(aq) by the anodic oxidation process. The current efficiency of this oxidation process is not maintained to 100% due to a competing oxidation reaction (eq 1) resulting in oxygen gas evolution at the anode. Excess of cerium(III) sulfate was added to the electrolyte. Ce3+ (aq) ions oxidize to Ce4+(aq) at a lower potential than that required for the oxygen evolution. As a result the oxidation of Ce3+ (aq) takes place as the primary anodic reaction24,25 and helps maintain 100% current efficiency of the oxidation process. The generated Ce4+(aq) ions chemically oxidize Fe2+(aq) to Fe3+(aq): Ce4þ ðaqÞ þ Fe2þ ðaqÞ f Fe3þ ðaqÞ þ Ce3þ ðaqÞ
ð5Þ
An aqueous solution of iron supplement tablet and excess cerium(III) sulfate in sulfuric acid were placed in the anode compartment. Ferroin indicator was used to determine the stoichiometric end point of the reaction as stated in eq 5. The charge required to reach the end point was measured, and the amount of Fe2+(aq) present in the supplement tablet was determined.
’ ANALYSIS OF IODINE IN THE POVIDONEIODINE Povidoneiodine often contains 1% iodine as an active ingredient. Iodine is used as a first aid antiseptic for minor cuts, scrapes, and burns. A coulometric back-titration method was used for the quantification of iodine in povidoneiodine. Aqueous iodine, more appropriately, I3(aq), reacts with the thiosulfate ions in 1:2 mol ratios: 2S2 O3 2 ðaqÞ þ I2 ðaqÞ f S4 O6 2 ðaqÞ þ 2I ðaqÞ
ð6Þ
A known volume of povidoneiodine was placed in the anode compartment with potassium iodide, a buffer, and a known excess of sodium thiosulfate. The iodine was chemically reduced to iodide (eq 6), and the mixture was completely discolored. The excess sodium thiosulfate was back-titrated with the coulometrically generated iodine. Iodine was generated (eq 7) by passing the anodic charge. 2I ðaqÞ f I2 ðaqÞ þ 2e ðaqÞ
ð7Þ
The stoichiometric end point was reached when the excess sodium thiosulfate reacted with the coulometrically generated iodine, and no more sodium thiosulfate was present in the mixture. Starch was used as an indicator. The mixture turned brownblack at the end point indicating the completion of the back-titration. Using the stoichiometric relations (eqs 6 and 7), the concentration of iodine in the commercial sample was determined.
’ HAZARDS Approved safety goggles must be used while performing the experiment. Laboratory chemicals including potassium nitrate, sodium acetate, potassium iodide, sodium thiosulfate, and cerium(III) sulfate can cause eye and skin irritation. Sulfuric acid and acetic acid can cause severe burns. Reagents involving acids must be prepared in a fume hood. Instructors must examine and approve the electrode connections before students can begin the titrations. Permitted waste containers must be used to collect the waste chemicals. ’ RESULTS AND DISCUSSION A blank trial for each analyte was conducted to ensure the negligible interference from the electrolyte. The amount of charge required to reach the end point of the titration linearly changed with the volume of Lysol, ammonia, and iron tablet solutions. The slope of the plot (Figure 1) of the cathodic charge versus the volume of Lysol was 2.76 103 C μL1. The concentration of acid in Lysol was found to be 2.9 M. The slope of the plot of the anodic charge versus the volume of household ammonia (Figure 2) was 2.99 103 C μL1. The concentration of the base in household ammonia was 1.2 M. The slope of the plot of the anodic charge versus the volume of iron tablet solution (Figure 3) was 1.30 103 C μL1. The mass of iron(II) sulfate per supplement tablet was 158 mg. For iodine analysis, six trials with excess quantities of sodium thiosulfate in the range of 0.0350 to 0.0568 g were determined. The average concentration of iodine determined from these trials was 0.039 M (standard deviation (1.48 103). The experimentally determined quantities of the analytes agree with the quantity of ingredients presented on the manufacturer’s label (Table 1). In an independent set of experiments, the quantities of active ingredients by volumetric titration were determined. A summary of the results is presented in Table 1. The agreement between coulometry results, the manufacturer’s data, and volumetric titration results underlines the efficacy of the proposed experiments. Exposure to high concentrations of ingredients of the household products presented in this experiment can be hazardous to humans and animals.26,27 The hazards include chemical burns caused by hydrochloric acid, nose and throat irritation caused by ammonia, constipation and nausea caused by iron, and severe allergic reactions caused by iodine. In view of this discussion, analysis of household products presented in this paper is both a thought-provoking learning experience and relevant in view of human health. ’ CONCLUSION The PDMS cell and experiments were developed and tested as an undergraduate independent study research project.28 PDMS cell utilized smaller sample and electrolyte volumes compared to the glass cell. Multiple compartments with internal salt bridges 1709
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’ ASSOCIATED CONTENT
LABORATORY EXPERIMENT
householdproducts.nlm.nih.gov/http://www.fsis.usda.gov/ophs/clg/ clg-ars.03.pdf (both accessed in Sep 2011). (27) National Institutes of Health. Office of Dietary Supplements. http://ods.od.nih.gov/factsheets/ (accessed in Sep 2011). (28) One student performed the coulometric titrations presented in this article; however, the PDMS cell has been used by multiple students for other analyte samples in our laboratory.
bS
Supporting Information Directions to make the miniature PDMS coulometry cell; detailed experimental procedure and calculations. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT R.B.D. dedicates this article to his mentor T. S. Rao, Pune, India, on his eightieth birthday. We thank James Schreck for helpful discussions. ’ REFERENCES (1) Szebelledy, L; Somoggi, Z. Z. Anal. Chem. 1938, 112, 313. (2) Reilley, C. N. J. Chem. Educ. 1954, 31 (10), 543–545. (3) Evans, D. H. J. Chem. Educ. 1968, 45 (2), 88–90. (4) Vincent, C. A.; Ward, J. W. J. Chem. Educ. 1969, 46 (9), 613–614. (5) Beilby, A. L.; Landowski, C. A. J. Chem. Educ. 1970, 47 (3), 238–239. (6) Tackett, S. L. J. Chem. Educ. 1972, 49 (1), 52–54. (7) Stock, J. T. J. Chem. Educ. 1973, 50 (4), 268–269. (8) Marsh, D. G.; Jacobs, D. L.; Veening, H. J. Chem. Educ. 1973, 50 (9), 626–628. (9) Lieu, V. T.; Kalbus, G. E. J. Chem. Educ. 1975, 52 (5), 335. (10) Grimsrud, E.; Amend, J. J. Chem. Educ. 1979, 56 (2), 131–133. (11) Greenspan, P. D.; Burchfield, D. E.; Veening, H. J. Chem. Educ. 1985, 62 (8), 688–689. (12) Stock, J. T. J. Chem. Educ. 1992, 69 (12), 949–952. (13) Singh, M. M.; Pike, R. M.; Szafran, Z.; Davis, J. D.; Leone, S. A. J. Chem. Educ. 1995, 72 (1), A4–A5. (14) Bertotti, M.; Vaz, J. M; Telles, R. J. Chem. Educ. 1995, 72 (5), 445–447. (15) Swim, J.; Earps, E.; Reed, L. M.; Paul, D. J. Chem. Educ. 1996, 73 (7), 679–683. (16) Lotz, A. J. Chem. Educ. 1998, 75 (6), 775–777. (17) Lowinsohn, D.; Bertotti, M. J. Chem. Educ. 2002, 79 (1), 103–105. (18) Thomas, N. C. J. Chem. Educ. 2007, 84 (10), 1667. (19) Harris, D. C. Quantitative Chemical Analysis, 8th ed.; Freeman: New York, 2010; p 371. (20) Fisher Scientific Catalog. http://www.fishersci.com/ (accessed Jan 2011). (21) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171–1196. (22) Meenakshi, V.; Babayan, Y.; Odom, T. W. J. Chem. Educ. 2007, 84 (11), 1795–1798. (23) Chia, M. C.; Sweeney, C. M.; Odom, T. W. J. Chem. Educ. 2011, 88 (4), 461–464. (24) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; p 432. (25) Sawyer, D. T.; Sobkowiak, A.; Roberts, J. L. Electrochemistry for Chemists, 2nd ed.; Wiley: New York, 1995; pp 152155. (26) United States Department of Health and Human Services. Health and Safety Information on Household Products. http:// 1710
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