Iodine Coulometry of Various Reducing Agents Including Thiols with

Mar 26, 2018 - Iodine Coulometry of Various Reducing Agents Including Thiols with Online Photocell Detection Coupled to a Multifunctional Chemical Ana...
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Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

Iodine Coulometry of Various Reducing Agents Including Thiols with Online Photocell Detection Coupled to a Multifunctional Chemical Analysis Station To Eliminate Student End Point Detection by Eye Jeralyne B. Padilla Mercado, Eri M. Coombs, Jenny P. De Jesus, Stacey Lowery Bretz, and Neil D. Danielson* Department of Chemistry and Biochemistry, Miami University, 651 E. High Street, Oxford, Ohio 45056, United States S Supporting Information *

ABSTRACT: Multifunctional chemical analysis (MCA) systems provide a viable alternative for large scale instruction while supporting a hands-on approach to more advanced instrumentation. These systems are robust and typically use student stations connected to a remote central computer for data collection, minimizing the need for computers at every student workspace. MCA networks offer multiple measurement capabilities; however, constant-current coulometry is still an uncommon MCA option. The characterization of an assembled teaching coulometry instrument for online data acquisition using an MCA system is described. A constantcurrent source connected to nonisolated Pt electrodes in a 150 mL beaker serving as the reaction cell caused the iodine titrant to be electrochemically generated for subsequent reaction with the analyte reducing agent. A photoresistor mounted on the stir plate below the beaker monitored the darkening of the solution due to formation of the starch−iodine complex. The change in voltage from an operational amplifier circuit permitted this titration curve to be stored by the MCA system for subsequent graphical analysis by Excel. Characterization of the instrument for direct analyte determination is performed with ascorbic acid, thiols, thiosulfate, and bisulfite. The number of electrons per mole of thiol for iodine titration of glutathione and N-acetylcysteine varied differently as a function of pH, indicating different reaction pathways. Ascorbic acid and the thiols are determined in dietary supplements with a recovery of 90−100%, and % RSDs of triplicate measurements ranged from 0.7% to 5%. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Electrochemistry, Instrumental Methods, Hands-On Learning/Manipulatives

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tions.6,7,10,11 Some authors have used two or more different end point determination methods to compare their precision. Comparison of eye detection versus graphical analysis, potentiometry versus eye detection, potentiometry versus amperometry and eye detection, and spectrophotometry versus amperometry have all been published in this Journal and elsewhere.2,5,12−14 The multifunctional chemical analysis (MCA) system from MeasureNet consists of student workstations which can be used to work independently.15 Fifteen workstations, interfaced to a controller that connects to a computer, can send data simultaneously for storage as files that students can e-mail or copy to a flash drive. The data can then be transferred to Excel for plotting or linear regression analysis. The probes available for the workstations include temperature, pH, conductivity electrodes, pressure sensor, and multifunction drop counter,

INTRODUCTION The substantial body of teaching literature that covers coulometric determinations shows the continued importance of this electrochemical technique. This analytical method uses in-situ-generated titrants for analyte determination. Different titrants can be electrogenerated to determine electroactive species, acids and bases, and even complexing reactants. Iodine, bromine, hydroxide, and hydrogen ions have been reported in the literature.1−13 These home-built instruments recommend either isolation of the anodic and cathodic electrodes or instrument simplification using nonisolated electrodes.11,13,14 Coulometric analyses can be coupled to various end point detection methods to find the time in which titrations are finished. For example, colorimetric end point determinations require the use of chemical indicators that signal the completion of the analyte reactions. Starch is used as the visual indicator when iodine is the electrogenerated titrant.1,3,4,8,9 Twin polarized electrodes for biamperometric measurements have been used for cyclohexene, hydrazine, biologically active thiols, and carbimazole determina© XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: June 22, 2017 Revised: March 9, 2018

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DOI: 10.1021/acs.jchemed.7b00445 J. Chem. Educ. XXXX, XXX, XXX−XXX

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The plastic lid is used to mount the photocell sensor, a cadmium-sulfide photoresistor (Radioshack), which monitors the darkening of the solution due to the formation of the blue starch−iodine complex. The beaker is covered with aluminum foil to minimize sources of error in photometric readings due to the surroundings. Figure S5 in the Supporting Information shows the proper placement of the stir bar, the photocell, and the electrodes. This alignment ensures that the photocell is not blocked by the other two components of the cell, minimizing sources of signal noise. The leads from the photocell sensor are connected to a home-built operational amplifier circuit shown in Figure 1 to the right, on the blue tray. A diagram of the operational amplifier circuit is shown in Figure S6, included in the Supporting Information. The circuit, which was constructed using a Global Specialties breadboard, is essentially a standard voltage operational amplifier circuit.18 A close-up picture of the breadboard is shown in the Supporting Information in Figure S7. One lead of the photoresistor is connected to the variable output of the trimmer resistor (Radioshack), and the other lead is connected to the inverting input of the 741 operational amplifier (Radioshack). The noninverting input of the 741 chip is grounded. The photoresistor detects the change in ambient light during the titration; its resistance increases as a function of solution darkening due to the starch indicator. The ratio of the circuit feedback resistor Rf to the resistance of the photocell times the input voltage from the trimmer resistor is the output voltage.19 The iodine titrant is electrogenerated at the anode from I−, and the reduction of hydrogen ion to hydrogen gas takes place as the cathode. During the titration, the electrogenerated iodine oxidizes the analyte in solution, and the photocell senses ambient light giving a stable baseline reading. The darkening of the solution due to the formation of the blue starch−iodine complex signals the end point of the titration.20 The change in color of the indicator blocks ambient light from the photocell, causing an increase in its resistance and a subsequent increase in the voltage output (smaller negative number). The voltage output of the amplifier circuit is recorded as a function of time using the MeasureNet MCA system. It is important not to exceed the ±2500 mV limits when using the MeasureNet data station (see Hazards section). The data when plotted in Excel look similar to a titration curve as shown in Figure 2. The initial portion at around −1000 mV is the baseline reading when the analyte titration is taking place. At

among others. Constant-current coulometry is not an option for this system. Just as previous research in which flow injection analysis and liquid chromatography have been adapted to be used with a MCA system, the present research sets out to incorporate constant-current coulometry as an option.16 The primary goal of this research is to assemble and characterize a constant-current coulometry instrument that provides online detection that is compatible with a MCA system. A simple detection system based on a photocell mounted at the bottom of the reaction beaker monitors the darkening of the solution due to the starch−iodine complex. The change in resistance of the photocell is monitored by an operational amplifier circuit, and the voltage output is sent to the MCA workstation to provide a graphical output to determine the titration end point. Electrogenerated iodine is the titrant used to determine ascorbic acid, glutathione, Nacetylcysteine, thiosulfate, and bisulfite in standards and real samples.

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APPARATUS

Instrument Design

The instrument that we have designed is shown in Figure 1.

Figure 1. Picture of constant-current coulometry instrument. From left to right: current source, electrodes in coulometric cell on top of the photoresistor held in a Styrofoam cup lid on a magnetic stirrer, and current-to-voltage converter circuit. MeasureNet station directly in back of the electrodes.

The constant-current source, Lake Shore Cryotronics Model 121, with selectable current values (100, 300 nA; 1, 3, 10, 30, 100, 300 μA; 1, 3, 10, 30, 100 mA), was set to 10 mA and connected using wires with alligator clips to the generating and counter electrodes.17 The electrodes, 1 and 2 cm concentric platinum wire and gauze cylinders, are contained in a 150 mL beaker that serves as the titration cell. This nonisolated electrode combination was found to work best as described in a comparison study found in the Supporting Information. A variety of electrode pairs with different surface areas of Pt were compared with respect to precision for the titration of standard ascorbic acid. Representative titration curves are shown in Figures S1−S4. It should be noted that nonisolated electrodes can reduce the efficiency of the system; iodine can be potentially reduced at the cathode. However, the agreement between the experimental and expected values for all our standard analytes was very good, indicating that this potential problem was not apparent. As shown in Figure 1, there is a Styrofoam cup lid between the beaker and the magnetic stirrer.

Figure 2. Typical titration plot for ascorbic acid using an applied current of 10 mA. B

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Table 1. Analytes Determined in Commercial Products

an average end point time of 299 s (n = 3) with an RSD of 0.77 and a percent error of 1.86%. The analogous data for the two line method was 295 s (n = 3) with an RSD of 0.34 and a percent error of 3.28%. We also felt that the voltage uptick method was easier to use, not requiring choosing points in the linear upturn range.

around 300 s the solution turns a uniform blue color signaling the titration end point. The final stable portion near 0 V, after 350 s, is the result of the intense color of the starch−iodine complex. Representative titration curves for N-acetylcysteine and glutathione are shown in the Supporting Information as Figures S8 and S9, respectively. The minimum distinguishable analytical signal Sm is defined as the addition of three times the standard deviation of the blank signal to the mean blank signal. Sm can be calculated by taking 20−30 measurements of the blank signal and doing the appropriate statistical treatment.21 In our case, the end point is defined as the first definite uptick in voltage at the end of the first titration plateau. This is determined by selecting a range of 20 data points before the uptick from baseline. The standard deviation of the voltage signal (σ) of this range is multiplied by 3 and added to the absolute voltage value of the last data point from this range, as shown in eq 1. The time corresponding to this calculated end point voltage is considered the end point of the titration.

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General Procedure

Standards of known concentration are prepared from reagents (sodium salts of bisulfite and thiosulfate as well as the acid forms of ascorbic acid, N-acetylcysteine, and glutathione, all obtained from Sigma-Aldrich) and titrated in triplicate prior to analyzing the commercial product samples. The amount of standard in aliquot needed to reach a predetermined end point is calculated using Faraday’s electrolysis equation. Equation 2 shows a sample calculation when the target end point is 300 s, where I is the current, t is the time, n is the number of electrons transferred per mole of analyte, and F is the Faraday constant. The mean gram found of the standard triplicates is compared to the theoretical value. A small percent error between the theoretical and experimental amounts can be used as a check to ensure that the titration setup works at essentially 100% current efficiency.

end point voltage (mV) = 3σ (mV) + |voltage of last point of the first plateau (mV)|

PROCEDURE AND RESULTS

(1)

Alternatively, the y = mx + b trendline could be found for the first plateau region and the trendline determined for the upturned straight portion of the titration curve (two line method). Setting those equations in terms of y (voltage) equal to each other will allow calculation of x (time) that corresponds to the intersection of the two lines which can be considered the end point. A comparison of these two methods (uptick and two lines) showed that both methods were precise, but the voltage uptick method was a little more accurate. For an expected titration time for ascorbic acid of 305 s, the uptick method gave

I×t × molar mass n×F 0.0100 A × 300 s = × molar mass C n × 96485.31 mol e−

grams analyte =

(2)

The titration of various analytes by this instrument with electrogenerated iodine follows a basic setup. A clean 150 mL C

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Table 2. Standards and Commercial Products Determined with the Home-Built Coulometer Name Ascorbic acid NAcetylcysteine Glutathione Thiosulfate

Titration Media

Standard Recovery, % (% RSD) n = 3

H2SO4, pH 2.0 H2SO4, pH 2.0

99.7% (0.885% RSD) 107% (4.34% RSD)

K2HPO4/KH2PO4, pH 7.0 K2HPO4/KH2PO4, pH 7.0

99.3% (1.54% RSD) 100% (0.693% RSD)

Commercial Product Apple juice N-Acetyl-L-cysteine dietary supplement (500 mg) by Jarrow Formulas Glutathione reduced dietary supplement (500 mg) by Jarrow Formulas BettaSafe Water Conditioner for Betta Fish Ultra-Swim Chlorine Removal Shampoo

Percent Found Based on Label Value 121% 89% 100% 4.27% by weight 0.718% by weight

taken and titrated in 1 M acetic acid/1 M sodium acetate, pH 4.7. A value of 1.42 ± 0.02 mg was found (95.9% recovery, 1.21% RSD, n = 3). The commercial product samples selected were dried apricots, Mediterranean Apricots by Mariani, and dried golden raisins, California Golden Raisins by Sun Maid. To determine any sample matrix effect, 3.5 g portions of dried fruit were blended with 140 mL of 0.2 M NaOH, acidified to pH 1−2 and sonicated to eliminate any sulfite as SO2. Portions of that solution were then brought to pH 7 before coulometric titration with end point times averaging 200 s. The same procedure was done, but the acidification step was omitted to determine sulfite plus the matrix; titration times were about 350 s. After subtracting for the matrix, the raisins and apricots were found to range from 715 to 750 mg bisulfite/kg. The Codex Alimentarius Commission from the Food and Agriculture Organization of the United Nations and the World Health Organization have determined that the maximum level of sulfites in apricots are 2000 mg/kg in the final product and 1500 mg/kg for bleached raisins.23,24 Possibly, this procedure could also be adopted for the determination of sulfite in wine.9

beaker is covered on the outside with aluminum foil. A 1 mL volume of 3% starch is added to the beaker along with 0.5 g of KI salt, giving [I−] of 0.03 M as suggested by the literature.1 The analyte aliquot to be titrated is added, followed by addition of 50 mL of buffer (specified in table form in the Commercial Product Analyses section), and water is added to reach a total volume of 130 mL. Ascorbic acid, N-acetylcysteine, bisulfite, glutathione, and thiosulfate were analyzed following this general procedure. The chemistry and sample preparation of each analyte is explained below. Coulometric Analyses by Graduate Student

Table 1 shows the structures of the analytes that have been determined in this experiment along with their reaction with molecular iodine, the titrant. The reaction chemistry as indicated in Table 1 is as expected except for N-acetylcysteine (NAC). The reason for not identifying the reaction product (indicated as a question mark) will be explained in the pH study of thiol compounds (see Supporting Information). Briefly, the unexpected conclusion made was that cysteine and N-acetylcysteine generally involve a two electron transfer per mole while glutathione reacts with iodine as expected with a one electron transfer per mole at neutral or acidic pH (Table 1S). Ascorbic acid, also known as vitamin C, is an organic acid found in foods and is also used as a food additive because it is a biologically important compound. Its structure is shown in Table 1. Since it is a reducing agent it can undergo a redox reaction with iodine as the oxidizing agent. Iodine is electrogenerated, and its reaction with ascorbic acid (AA) to form dehydroascorbic acid (DHA) and iodide is shown in Table 1. A 2.78 mg amount of the AA standard was taken and titrated in 3.18 × 10−4 M HNO3 at pH 3.14; a quantity of 2.73 ± 0.02 mg was found (98.2% recovery, 0.772% RSD, n = 3). We have also used 0.03 M H2SO4 which can be a buffer at low pH. N-Acetylcysteine (NAC) is a thiol used to support liver and lung function. Its structure is shown in Table 1 along with its reaction with iodine, where SH is highlighted as the moiety that participates in the redox reaction and R represents the rest of the molecule. A 5.07 mg portion of the NAC standard was taken and titrated in 0.03 M H2SO4. A value of 4.80 ± 0.10 mg was found (94.7% recovery, 2.14% RSD, n = 3). The N-acetylL-cysteine dietary supplement with a label value of 500 mg of NAC per capsule by Jarrow Formulas was the commercial product analyzed, and an amount of 507 ± 19 mg was found (101% recovery, 3.76% RSD, n = 3). Bisulfite is a food additive to dried fruits and vegetables and is intended to increase the shelf life of those products. The structure of the metabisulfite ion and its reaction with iodine in the presence of water as described previously22 are shown in Table 1. A 1.48 mg amount of a metabisulfite standard was

Commercial Product Analyses by Undergraduate Student

An undergraduate student and coauthor determined a variety of analyte standards and commercial products. The results of titrations of ascorbic acid, N-acetylcysteine, and glutathione are shown in Table 2. Glutathione is an important thiol that has antioxidant properties. The structure of this organic compound and its reaction with iodine are shown in Table 1. On the basis of the one electron transfer found in acidic and neutral pH (Table S1), glutathione follows a more standard reaction pathway to the oxidized dimer as compared to the cysteine compounds. The percent found based on the label value for glutathione is very good, but that for N-acetylcysteine is significantly low; the high result as compared to the likely label value (90 mg ascorbic acid in 200 mL apple juice) is not unexpected for a natural fruit-containing product. Thiosulfate is a sulfur-containing compound that is commonly used to dechlorinate water. Its structure and its redox reaction with iodine are shown in Table 1. The results of thiosulfate titrations are shown in Table 2. Both commercial products lacked label values for the analyte (although the patent of the Ultra Swim Shampoo indicated 1% thiosulfate by weight), so a standard addition method was used to analyze these commercial products.

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HAZARDS Gloves and eye protection are required when handling concentrated mineral acids such as nitric and sulfuric. Eye protection is required when handling all other chemicals and solutions described in this experiment. There are no significant instrumental hazards. On the basis of the comment by one of the manuscript reviewers, it is recommended that a Zener diode D

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circuit (Figure S10) be added to the amplifier circuit (Figure S6) to limit the output voltage to the MeasureNet station. However, on the basis of a personal communication with the MeasureNet company, the station input is rated to ±5000 mV.

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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.7b00445. Supplementary figures (PDF, DOCX) Instructor notes (PDF, DOCX) Student experiment coulometry handout with long report guidelines (PDF, DOC)

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DISCUSSION We have shown how the classic constant-current coulometry experiment, often done using eye detection, can be easily updated inexpensively to provide unattended online detection that is compatible with an MCA system. Students should be able to achieve the following objectives: (1) to understand and put together the important components of a constant-current coulometry instrument, (2) to titrate a drink or food containing ascorbic acid or a supplement with either glutathione or Nacetyl cysteine, (3) to calculate the grams of the analyte of interest and compare back to the label value, and (4) to predict and understand the versatility of I2 coulometry titrations. The basis of this paper is the first research chapter in the MS thesis by one of us (J.B.P.M.).25 The coulometry student experiment has been part of our instrumental analysis course every semester since Fall 2015. Initially, more traditional samples containing ascorbic acid such as fruit drinks and fruit flavored snacks were analyzed. To provide more experiment variation on a semester basis, the determination of sulfur compounds has and will be continued to be implemented. About 60−80 students, mainly junior level chemistry and biochemistry majors, have taken this required three credit course (one credit lecture, two credits lab) per year. Student rotation through the experiments was required due to limited instrumentation. The Thursday “lecture” was taught as a guided self-study in which the students would work in groups of four to answer key questions about the technique using an online textbook as the primary reference. This would give them the necessary fundamentals to do the corresponding experiment the following Tuesday. Students also work in groups of 3−4 students to complete the work in a 3.5 h lab period. Coulometry is taught in conjunction with cyclic voltammetry because both experiments are amperometric in nature. This electrochemistry experiment is one of four long laboratory reports that are written up in journal format; the other three are fluorescence, atomic emission spectroscopy, and liquid chromatography/flow injection. The other experiments conducted in this course are Beer’s Law and its limitations, Raman spectroscopy, atomic absorption, mass spectrometry, and gas chromatography. Additional information about any of these experiments can be obtained from the corresponding author (N.D.D.). In addition, students work on a two period project which is intended to extend the scope of samples that can be analyzed for a particular instrument. Certainly, extensions of the coulometry student experiment can be envisioned. The indirect determination of zinc after thiol complexation at alkaline pH and then titration by iodine using this same coulometry instrument, a second research chapter in ref 25, has been recently published by us.26 Students could be involved in a redesign of the circuit to gain experience working with operational amplifiers. One of the manuscript reviewers alerted us to the possibility of an alternative circuit using the OPT 101 chip which combines the photocell and operational amplifier into a single device. A comparison between the present circuit design and a circuit with the OPT 101 would be an interesting future project. Miniaturization of the electrodes (perhaps using carbon) and titrating smaller samples using lower applied currents should be feasible.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stacey Lowery Bretz: 0000-0001-5503-8987 Neil D. Danielson: 0000-0003-4857-5789 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant 1432466, Principal Investigator S. L. Bretz. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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REFERENCES

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