What are the electrochemical similarities and differences between
potassium hexachloroiridate and acetaminophen?
What is the concentration of acetaminophen in Tylenol®?
Each lab partner must prepare their pre-lab assignment and final report independently
Bring a copy of this document to the lab when you will be performing the experiment.
Advisory Note
This experiment requires you to critically analyze your data for trends, derive relationships
between variables, calculate results, and write responses explaining your results DURING the
laboratory period.
Students who have previously done this experiment like the format; however, they feel it is
very important to thoroughly review and understand the ESD background information and
prelab questions before coming to lab.
The report is a photocopy of your laboratory notebook. (SHORT REPORT Format)
It is critical to clearly label what information is on each notebook page (i.e. header, question #
and letter). Ask the instructor leading this experiment to explain the notebook expectations for
the report before beginning the in-lab portion of the experiment.
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Figure 1. Three-electrode cell and cyclic voltammetry instrument
Background and Pre-Lab Assignment (Bring a photocopy of this to the lab to be graded)
My preference is you document your Prelab answers in your lab notebook. Turn in a photocopy
of the notebook pages or a hardcopy of the assignment at the beginning of lab.
Cyclic voltammetry (CV) is an electrochemical technique that can be used to generate qualitative
information regarding a molecule involved in a reaction (i.e. reaction mechanism) or quantitative
information such as the diffusion coefficient or concentration of a molecule in solution. Each case
requires that the molecule-of-interest is capable of being reduced and/or oxidized (i.e. that the
molecule is electrochemically active).
Figure 1 shows a typical diagram of an instrument used for cyclic voltammetry. The major
components include a computer, a potentiostat, and an electrochemical cell. The cell contains
electrodes, an inert salt (i.e. supporting electrolyte) dissolved in high purity deionized water,
and the electrochemically active compound to be studied.
A CV experiment involves measuring the current generated by the oxidation and/or reduction of
an electrochemically active molecule in response to the energy applied at the working electrode
surface (i.e. potential or voltage). The terms “voltage” and “potential” are commonly used
interchangeably. The potential is an external source of energy applied at the working electrode
surface that converts the compound to its oxidized and/or reduced form. The current generated
as a result of an electrochemical reaction at the electrode surface is referred to as Faradaic
current and involves charge transfer. The output of a voltammetry experiment is referred to as a
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voltammogram. A voltammogram is a plot of the current generated (y-axis) versus the applied
potential (x-axis).
As shown in Figure 1, the current generated as a result of a redox reaction is measured by the
electrical circuit formed between the working electrode, auxiliary electrode, and the potentiostat.
Almost all voltammetry experiments utilize a three-electrode electrochemical cell. The three
electrodes used are the (1) working electrode, (2) reference electrode, and (3) an auxiliary or
counter electrode.
Figure 2 shows a three-electrode cell. In a typical
CV experiment, the solution to be analyzed
contains the electrochemically active compound
dissolved in an electrolyte solution. The electrolyte
is typically an inert salt (e.g. KCl, KNO3) soluble in
highly purified water.
High purity deionized water acts as a resistor
and is not able to support the flow of electrons
in an electrochemical cell. The resistance of
high purity DI water (measured in ohms,) is
extremely high due to it having virtually no ions
(i.e. 18 M ).
Addition of an electrolyte (a water-soluble salt), at
concentrations typically in the 0.1 – 1.0 M range,
reduces the resistance of the DI water to a level
that supports the flow of electrons between the working and auxiliary electrodes. The electrolyte
dissociates into positive and negative ions (i.e. K+
, Cl-
) and provide a means for electrons to flow
between the electrodes while maintaining the charge neutrality of the cell.
The potentiostat performs two key functions as denoted by the terms Eappied and Imeasured in
Figure 1. The potentiostat (1) – controls the potential at the surface of the working electrode
versus the reference electrode, and (2) – measures the current flow in the electrochemical cell
as a function of the applied potential. It is important to note that the reference electrode is held at
a constant potential at all times. This feature of the reference electrode allows the potentiostat to
generate an applied potential of a known quantity at the surface of the working electrode during
the experiment.
The potentiostat parameters you will set for each CV experiment include:
- the voltage applied to the working electrode,
- the rate the voltage is changed during the scan cycle (i.e. scan rate),
- the number of scan cycles or segments for an experiment, and
- the sensitivity of the current follower circuit.
Figure 2. Three electrode electrochemical cell
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Figure 3 shows a potential waveform used in a CV experiment; note that the waveform is an
isosceles triangle. The potential applied to the working electrode is initially varied from +0.8 V to
-0.2 V over 20 seconds. This initial phase is referred to as the cathodic wave. During this
phase, the working electrode acts as the cathode and reduces the oxidized form of the analyte.
The electron transfer for the reduction of the oxidized form (Ox) is represented by
Ox + ne− → Red (Equation 1)
where n equals the stoichiometric number of electrons involved in the reaction, and (Red) is
the reduced form of the analyte species.
The second phase or waveform segment in Figure 3 shows the applied potential being changed
to the opposite polarity of the initial segment (-0.2 V to +0.8 V over 20 seconds). The second
waveform segment is referred to as the anodic wave. In this case, the working electrode acts
as the anode and oxidizes the reduced form of the analyte. The electron transfer for the
oxidation of the reduced form (Red) is represented by Equation 2.
Red → Ox + ne−
(Equation 2)
The scan polarity is a key experimental parameter you will set (see EChem parameters in Figure
2). The scan polarity refers to the direction that the potential change during the initial waveform
segment. The scan polarity in Figure 2 is negative since the potential is scanned from a more
positive to less positive or negative value. The scan polarity can be set to be either (−) or (+).
Figure 3. CV Potential Waveform and EChem Program Parameters
EChem Program Parameters
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Prelab Questions – Part 1 - Draw a potential waveform with the following characteristics: low E = 0 V, high E = +1.0
V, scan rate = 100 mV/s, scan polarity = positive, cycles = 1. Label axes appropriately. - How many segments are in your drawing?
- Which segment is the cathodic wave? Which segment is the anodic wave?
- During which segment can an electroactive analyte be oxidized? Write the equation
representing oxidation of the analyte. - During which segment can an electroactive analyte be reduced? Write the equation
representing reduction of the analyte. - Draw three waveforms on the same figure representing three scan rates (fast, slow, and
intermediate) where the scan polarity is (−), cycles = 1, low E = 0 V, high E = +1.0 V.
Figure 4 shows the cyclic voltammogram for the potential waveform and the potentiostat
parameters in Figure 3. The measured current varies as a function of the applied potential and
two peaks are observed in the voltammogram. The figure shows a positive current generated
during the portion of the cathodic wave when the analyte is reduced. The opposite is observed
as the potential switches to the anodic wave and the analyte is oxidized (i.e. negative current).
Figure 4. Cyclic Voltammogram based on Waveform in Figure 3
Cathodic Wave (c) →
Segment 1 & 3
Anodic Wave (a)
Segment 2 & 4
( Ipc, Epc )
( Ipa, Epa )
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The two peaks in Figure 4 are significant and provides data used in several calculations that
provide information about the molecule (i.e. peak current value Ip, peak potential value Ep). The
subscripts ‘c’ and ‘a’ for the peak current and potential in Figure 3 refer to information generated
during the cathodic and anodic waveforms respectively (i.e. Ipc, Ipa).
Figure 5 presents the data analysis results for the CV generated from waveform segments 1 and
2 in Figure 3. The parameters and results are presented on the right of the figure. Segments 1
and 2 correspond to the cathodic and anodic waveforms.
The shape of a voltammogram provides information on the chemical stability of an electroactive
compound. Equation 3 represents a compound that can be converted between two forms.
Red ⇌ Ox + ne−
(Equation 3)
The double-sided arrow shows that the reduced form can be regenerated from the oxidized
form. The reversibility of a redox reaction varies according to the chemical stability of the
reduced and oxidized forms on the time scale of the experiment which is defined by scan rate.
In the case of a totally reversible redox reaction, the concentration of [Red] and [Ox] remain
constant on the time scale established by the scan rate and the voltammogram is symmetrical
as in Figure 5. In the case of quasi-reversible redox reaction, the concentration of one or both
forms of the electroactive compound changes and the voltammogram is not symmetrical (i.e. the
Figure 5. Analysis of Cycle 1 CV in Figure 4 (CV based on Waveform Segment 1 & 2 only)
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peak current values for IPC and IPA are not equal). If either the reduced or oxidized form is totally
unstable and cannot be detected on the time scale set by the scan rate, the redox reaction is
said to be irreversible.
Prelab Questions – Part 2 - What are values for the peak cathodic potential and current in Figure 5?
- What are values for the peak anodic potential and current in Figure 5?
- Write a reversible redox reaction for the hexachloroiridate ion.
- What is the oxidation state of iridium in its oxidized and reduced form based on 3?
- Write the Randles-Sevcik equation for a simple diffusion-controlled redox reaction (see
Equation 4, page 14). Define each term in the equation. - Based on the peak current values in Figure 5 for IPC and IPA, and the relationship between
current and concentration in the Randles-Sevcik equation, do you think the redox reaction
for IrCl6
+2 is reversible or irreversible? Why? - What is the y-axis and x-axis of a voltammogram? Which axis is experimentally varied
when generating a voltammogram for an electroactive compound?
Prelab Questions – Part 3 - Calculate the mass of potassium nitrate to prepare 200 mL of 0.10 M KNO3.
- Calculate the mass of acetaminophen to prepare 100 mL of a 16.00 mM stock solution.
Note: See ‘Chemicals & Samples’ section for molecular weight information.
The CV experiments with potassium hexachloroiridate will be performed with the solution in the
cell with no stirring. When an experiment is performed without stirring, the only way the
electroactive compound can reach the working electrode surface via diffusion. As a result, the
reaction is said to be diffusion-controlled when the solution is not stirred.
Voltammograms, such as in Figure 5, which are generated under diffusion-controlled
conditions are duck-shaped as a result of the concentration of the electroactive compound
decreasing in the interfacial region of the electrode – solution interface. This is due to the
reduction and/or oxidation of the electroactive compound as the potential at the working
electrode surface is varied.
The structure of the electrode-solution interfacial region is key to understanding the duckshaped voltammograms obtained using a conventional disc electrode. Figure 6 illustrates the
interfacial region under diffusion-controlled conditions (no stirring).
The solution nearest to the electrode surface is part of the electrical double layer. The double
layer is formed based on charge at the electrode surface and the attraction of electrolyte ions of
the opposite charge. The electroactive compound is also present on the solution side of the
double layer and will only react in this region in response to a specific applied potential value.
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The next region of the solution is the
diffusion layer. The concentration of the
electroactive compound will decrease in
this layer based on the rate of the reaction
in the double layer region. Electroactive
compounds freely move between the
solution side of the double layer and the
diffusion layer under diffusion-controlled
conditions (no stirring). There is
negligible movement of electroactive
compounds between the diffusion and
bulk solution regions under diffusion-controlled conditions.
Prelab Questions – Part 4 - Draw the working electrode – solution interfacial region for an electrochemical cell
under diffusion-controlled conditions. Label the following features:
a. working electrode surface
b. double-layer interfacial zone
c. diffusion-controlled solution zone
d. bulk solution zone
References – Portions of scanned images and text adapted from: - Basic Physics of Galvanic Cells, MIT Lecture Notes. (See BB)
- Kissinger, P. T., Heineman, W. R., J. Chem. Ed., 60(9), 1983, 702. (See BB)
- Faulkner, L. R., J. Chem. Ed., 60(4), 1983, 262. (See BB)
- Petrovic, S.; Cyclic Voltammetry of Hexachloroiridate, Chem. Educator 2000, 5, 231.235.
- Smith, W.H.; Electroanalytical Chemistry: Basic Principles and Applications, J. Chem. Educ.,
1984, 61 (6), p A185. - Van Benschoten, J.J., Cyclic Voltammetry Experiment, J. Chem. Ed., 60(9), 1983, 772.
- Skoog, D. A.; F. J. Holler, F. J.; West, D. M. Principles of Instrumental Analysis, 6th Ed.,
Brooks/Cole, Belmont, CA, 2007, Chapter 25. - Kelly, R.S.; Analytical Electrochemistry: The Basic Concepts, Accessed 15-Jan-2016.
http://www.asdlib.org/onlineArticles/ecourseware/Kelly_Potentiometry/EC_CONCEPTS1.HTM
Figure 6. Electrode-solution interfacial region under
diffusion-controlled conditions (stationary disc electrode).
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Experimental
Instrumentation
• CH Instruments 621E Electrochemical Analyzer
a) Potentiostat
▪ potential control range: ±10V
▪ current sensitivity: picoamperes
b) Control and data analysis software
• Electrochemical Cell Components
Note: only one working electrode is used with the auxiliary and reference electrode
a) Three-electrode glass electrochemical cell and cell stand
b) Platinum (Pt) disc working electrode (2 mm diameter)
c) Glassy Carbon (C) disc working electrode (2 mm diameter)
d) Platinum wire auxiliary electrode
e) Silver/silver chloride (1M KCl) reference electrode
Chemicals & Samples
• 18 M deionized water
• Potassium nitrate (MW 101.1 g/mole)
• Potassium hexachloroiridate IV, K2IrCl6, (MW 483.13 g/mole)
• 0.20 M Disodium phosphate, DSP (MW 141.96 g/mole)
• 0.10 M Citric acid, CA (MW 192.13 g/mole)
• pH 2.2 McIlvaine buffer: 10 mL DSP + 490 mL CA
• Acetaminophen, APAP (MW 151.16 g/mole)
• Tylenol ® Regular Strength Tablets (325 mg APAP / tablet)
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Experiment 1: Determining the Working Electrode Operating Range
Solution Preparation: Supporting Electrolyte Solution (0.10 M KNO3).
Prepare 200 mL of the supporting electrolyte using a volumetric flask
and 18 M DI water.
Part A: Determining the Role of the Supporting Electrolyte
Ohm's law states that the current ( I, amperes)
between two points of a conducting material is
directly proportional to the voltage ( V, volts) across
the two points. The resistance (R, ohms) between the
two point is the proportional constant. Ohm’s law is
represented by equation 1 and figure 5 shows a
simple electrical circuit with I, V, and R.
Equation 1 (Ohm’s Law) 𝐼 =
𝑉
𝑅
Locate the electronic circuit in the lab bench to be used to detect current. The circuit
components include a power source (battery box), a red light emitting diode (LED), red and
blue wire leads, and connectors. Verify that there is a battery in the box before proceeding.
Locate the connectors at the end of the red and blue wires. They represent the two points
described above and in figure 5. Discuss and answer the following questions with your partner.
These do not need to be recorded in your notebook. - Touch the metal connectors at the end of the wires together
a. Does the LED light up?
b. Is metal a conducting material? - Arrange the wires so the metal connectors do not touch each other.
a. Does the LED light up?
b. Is air a conducting material? - Fill a small beaker with 18 MOhm (M) deionized water and place the leads in solution.
a. Does the LED light up?
b. Is 18 M water a conducting material? - Fill a small beaker with tap water and place the leads in solution.
a. Does the LED light up?
b. Is tap water a conducting material? - Fill a small beaker with the 0.10 M KNO3 solution and place the leads in the solution.
a. Does the LED light up?
b. Is 0.10 M KNO3 a more or less conducting material than tap water? Why?
Figure 5
Figure 5
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Questions (document answers in your notebook)
Current only flows between the leads when there is a conducting material completing the
circuit between the negative and positive poles of the power source (i.e. battery). - Based on varying the material or medium, what is required for the LED to light up?
- What type of particle (i.e. protons, neutrons, other) flows through the circuit when the
LED lights up? - Which solution generated the brightest light? Why?
- Write a chemical equation showing what happens to KNO3 when it is dissolved in water?
- In terms of Ohm’s law, what term varies based on the salt concentration in water?
- Why is the 0.10 M KNO3 solution called the supporting electrolyte?
- Why is it necessary to have ions in solution when using an electrochemical technique?
Part B: Determining the Working Range and Settings for the Electrochemical System
The electrochemical cell you will use in this experiment consists of a Pt Disc working electrode,
a Pt wire counter electrode, an Ag/AgCl reference electrode, and the 0.10 M KNO3 supporting
electrolyte solution.
Prior to conducting the CV analysis of potassium hexachloroiridate, you need to determine the
voltage range for the cell and supporting electrolyte where there is minimal to no change in
current (I) as the potential (V) at the working electrode is varied (i.e. I
V
⁄ ≈ 0).
This is a critical step because once the range is established, any current detected when
analyzing the K2IrCl6 solution can be assumed to be due to a redox reaction at the surface of
the working electrode.
Figure 6 shows the maximum
(+1.7 V) and minimum (-0.7 V)
operating range for the Pt disc
electrode in 0.10 M KNO3.
Figure 6. Maximum and Minimum Operating Range
for the Platinum Working Electrode
+1V 0 -1V
E (Volts vs. SCE Reference Electrode)
Pt Disc Electrode
+1.7 V to -0.7
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Assemble the electrochemical cell as specified in the Appendix.
Transport the cell to the three electrode potentiostat which is located in the Instrument Facility.
A. Locate the Standard Operating Procedure (SOP) for the potentiostat on the instrument
bench. Refer to the SOP for how to set up the experimental parameters, acquire a
voltammogram, and analyze the CV data.
B. Using the SOP, enter the
following information into the
Setup Parameters table
NOTE: The potential values, E(V), may
need to be adjusted based on the working
and reference electrode performance at
the time of the experiment.
C. To start the CV analysis, click on the icon.
o IMPORTANT: You will need to adjust the SENSITIVITY until the current signal (i.e.
Y-axis) is ON-SCALE over the entire voltage range.
NOTE: A voltammogram with OFF-SCALE current will show an increase in current
and then suddenly become constant (i.e. flat) at the maximum current value.
When the scan is reversed, it will behave similarly showing a decrease in current
and then become constant at the minimum current value.
D. Now adjust the VOLTAGE RANGE to generate a voltammogram where (
I
V
⁄ ≈ 0)
over the entire range.
• Record the voltage range in your notebook for use in the next experiment.
E. Review the voltammogram and voltage range you plan to use in the next experiment
with the Lead Instructor for this experiment before proceeding.
F. Disconnect the cell from the potentiostat and transport the cell to the prep lab.
a. Rinse the electrodes with 18 M DI water
b. Place the reference electrode in its glass storage vial (0.10 M KCl solution)
Initial E (V) = +1.7
High E (V) = +1.7
Low E (V) = - 0.7
Init P/N = N
Scan Rate (V/s) = 0.100
Segment = 2
Smpl Interval (V) = 0.001
Quiet Time(s) = 2
Sensitivity (A/V) = 1e-6
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Experiment 2: Potassium Hexachloroiridate (Characterization)
Solution Preparation: 2.00 mM K2IrCl6 in 0.10 M KNO3
Obtain a vial from the lead instructor containing the mass of K2IrCl6 to prepare 50 mL of a
2.00 mM solution. Transfer the powder onto a weighing paper and then into a 50 mL
volumetric flask. Dilute to volume using the supporting electrolyte solution.
Potassium hexachloroiridate is an inorganic molecule that is electrochemically active. It
undergoes a simple redox reaction and is an ideal system for learning the fundamentals of
cyclic voltammetry.
Characterization of Potassium Hexachloroiridate
Assemble the electrochemical cell as specified in the Appendix.
The electrochemical cell you will use in this experiment consists of a Pt Disc working
electrode, a Pt wire counter electrode, an Ag/AgCl reference electrode, and 2.00 mM
K2IrCl6 in the 0.10 M KNO3 supporting electrolyte solution.
Refer to the SOP for instructions to set up and operate the potentiostat.
The purpose of this part of the experiment is for you to learn how to set the potentials
(i.e. High E, Low E) and the sensitivity settings for the analysis of the K2IrCl6 solution.
The goal is to determine the potentials and sensitivity settings to generate a
voltammogram with the following properties as depicted by Figure 4:
• the potentials (i.e. High E, Low E) are set so the peak apexes for the reduction and
oxidation reactions is centered relative to the x-axis,
• the potentials are set so there is sufficient area prior to and after the peaks to
establish a baseline to measure the peak reduction (IPC) and oxidation currents (IPA),
• the sensitivity is set so the peak current apexes are on-scale
Optimization of Experimental Parameters
A. Refer to the SOP to set potential range (Initial E, High E, Low E); use the voltage values
from the last experiment.
• use the default settings for all the other parameters
B. Optimize the potential and sensitivity settings until you achieve a voltammogram
consistent with the goal stated above.
C. Review your voltammogram and results with the lead instructor for this experiment
before proceeding.
D. Record the operating parameter settings in your notebook.
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Collecting Voltammograms at Different Scan Rates
E. Using the optimized operating parameters, generate voltammograms for the 2 mM
K2IrCl6 solution at 5 different scan rates. The rates should range from 20 – 100 mV/s.
i. Save each voltammogram as a .bin file per the SOP.
ii. Use the SOP to determine how to measure the peak potential and current
for the cathodic and anodic waves at each scan rate
iii. Record the values for Epc, Ipc, Epa, Ipa at each scan rate.
o Suggestion: Prepare a table with 5 columns to record the current and
potential in your notebook. Label the first column as “Scan Rate”.
F. Disconnect the cell from the potentiostat and transport the cell to the prep lab.
i. Rinse the electrodes with 18 M DI water
ii. Place the reference electrode in its glass storage vial (0.10 M KCl).
In-Lab Data Analysis
The Randles-Sevcik equation shows the relationship between the peak current and number of
terms for a simple diffusion-controlled redox reaction (equation 4). The current (IP ) can be the
peak cathodic or anodic current (Ipc, Ipa) from a voltammogram.
𝐼𝑝 = (2.69 × 105
) 𝑛
3
2
⁄ 𝐴 𝐷
1
2
⁄
1
2
⁄ 𝐶 (Equation 4)
The terms in equation 4 are as follows: IP = peak current (Amps), n = number of electrons
involved in the reaction, A = electrode area(cm2
), D = diffusion coefficient of the redox species
(cm2 /s), v = scan rate (V/s), and C = concentration of the redox species (moles/cm3
). - Per the SOP, use the software feature to create an Overlay Plot containing all the CV
files that were collected at the different scan rates.
a. Based on the Overlay Plot, briefly describe any trends you observe.
i. Is the peak reduction potential (EPC) constant (< 5%) or variable ( 5%)?
ii. Is the peak oxidation potential (EPA) constant or variable? - Per the SOP, use the Peak Parameter Plot software feature to determine the
mathematical relationship between peak current and scan rate using the following
plot types: (1) – IP vs. scan rate, (2) – IP vs. √𝑠𝑐𝑎𝑛 𝑟𝑎𝑡𝑒
iii. Record the correlation coefficient for each plot type.
iv. How do the results for iii. compare with the behavior predicted based on
equation 4? - Based on equation 4, what is the relationship between current and the concentration of
an electroactive compound in solution?
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- The peak anodic current is proportional to the concentration of which form of Ir(III) or (IV)?
- Explain why the absolute value of IPC / IPA can be used to assess the reversibility of the
IrCl6 redox reaction?
a. What conclusions can be drawn if the ratio is 1 or approximately equal to 1?
b. What conclusions can be drawn if the ratio is much less or much greater than 1? - Calculate the IPC / IPA from the average cathodic and anodic peak current.
- What does the calculated value indicate regarding the reversibility of the IrCl6redox reaction?
- Review your responses with the lead instructor before proceeding.
10.Clean up.
a. Rinse the glass vessel 3X with 18 M DI water and dry with a Chemwipe.
b. Rinse the electrodes with 18 M DI water
c. Polish the Pt disc working electrode, place it into its storage container, and return
it to the drawer.
d. Place the reference electrode in its storage vial (0.10 M KCl solution).
Experiment 3: Acetaminophen (Characterization and Quantitation of APAP in Tylenol)
Acetaminophen (APAP) is an organic molecule that is
electrochemically active (Figure 7). In contrast to an inorganic
compound such as potassium hexachloroiridate, the electrochemistry
of organic compounds can be much more complex.
You will use the techniques and concepts from Experiment 2 to
characterize the electrochemical behavior of APAP and to
measure the concentration of APAP in a pharmaceutical product.
Solution Preparation: - Supporting Electrolyte Solution (pH 2.2 McIlvaine buffer)
Prepare 500 mL of the buffered supporting electrolyte. Add 10 mL of 0.20 M disodium
phosphate to a 500 mL graduated cylinder. Dilute to volume with 0.10 M citric acid.
Transfer the buffer solution to a plastic bottle that is properly labelled. - APAP Calibration Stock Solution (16.00 mM APAP)
Weigh and record the mass of APAP.
Transfer the APAP to the volumetric flask and fill ¾ full with the pH 2.2 electrolyte
solution. Sonicate to dissolve the APAP and dilute to volume with the pH 2.2 solution.
Figure 7. Acetaminophen
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Calculate the exact concentration of the stock solution (mM) based on the APAP mass.
Prepare four calibration solutions from the APAP stock solution.
• Use 25 mL volumetric flasks, the appropriate micropipette, and the pH 2.2
solution as the diluent to prepare the solutions.
• Use the following volumes of stock APAP solution: 200, 400, 800, and 1200 L.
• Calculate the exact concentrations for the standards to four significant figures. - Tylenol ® Tablet Stock Solution
Add one tablet to a 200 mL volumetric flask and fill approximately ¾ with the pH 2.2
solution. Sonicate the solution for 3 – 4 minutes to break up the tablet; not all of the
tablet ingredients will dissolve. Dilute to volume with the pH 2.2 solution.
Record the label claim for the concentration of [APAP / tablet] based on the product’s
primary and secondary packaging. - Tylenol ® Tablet Working Solution
Transfer 1.40 mL of the Tylenol ® stock solution to a 25 mL volumetric flask using a
micropipette. Dilute to volume with the pH 2.2 solution.
Assemble the Electrochemical Cell and Set Up the Analysis Parameters
The electrochemical cell you will use in this experiment consists of a glassy carbon working
electrode, a Pt wire counter electrode, an Ag/AgCl reference electrode, and the APAP
standard working solutions.
Analyze the standard solutions starting from the lowest concentration to the highest. - Rinse the glass vessel 3X with 18 M DI water
and dry with a clean, dry Chemwipe. - Clean the working electrode and assemble the
three electrode electrochemical cell. - Per the SOP, enter the following experiment settings
into the parameter table. Note: the potential will be
scanned initially in the anodic direction.
Analyze the Standard Calibration Solutions - Analyze the standards starting from the lowest to highest concentration. Use the steps
outlined below to insure the concentration of the solutions to be analyzed is not altered.
a. Rinse the vessel with a small volume of the solution to be analyzed.
b. Rinse the electrodes with 18 M DI water and blot the electrodes with a
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Chemwipe to dry them before reassembling the cell. Any residual water will dilute
the [APAP] concentration.
c. Reassemble the cell and acquire the CV.
d. Save each voltammogram as a .bin file per the SOP.
e. Repeat steps a. – d. until all four standard solutions have been analyzed.
Analysis of the Tylenol® Working Sample Solution
Analyze the sample solution in triplicate; use a FRESH sample solution for each measurement.
10.Analyze the Tylenol ® working solution as specified in step 9; remember to use a FRESH
sample solution for each measurement.
11.Dispose of the Tylenol ® solution in the proper waste container and rinse the electrodes
with 18 M DI water.
12.Clean up
a. Rinse the glass vessel 3X with 18 M DI water and dry with a Chemwipe.
b. Rinse the electrodes with 18 M DI water.
c. Polish the glassy carbon working electrode, place into its storage container,
and return to the drawer.
d. Place the reference electrode in its glass storage vial (0.10 M KCl solution).
In-Lab Data Analysis - Measure the peak potential and current for the cathodic and anodic waves for the
standards and samples. Record the values for Epc, Ipc, Epa, Ipa - Pick one standard concentration and calculate IPC / IPA .
- What term describes the reversibility of the APAP redox reaction at pH 2.2? Why?
- Per the SOP, use the Overlay Plot feature to create a figure with all the CV scans for
the APAP standard solutions.
a. Explain whether or not the plot is consistent with the behavior predicted by
the Randles-Sevcik equation. - Per the SOP, use the Calibration Curve software feature to determine the
concentration of diluted APAP solution.
o NOTE: use the average IPC value from the three Tylenol ® working solutions
analyzed.
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Post Lab Questions (not to be done with your lab partner)
Record the response to each question in your laboratory notebook. - Briefly comment on the chemical reversibility of APAP including a rationale to
support your answer. - Briefly describe the differences between an IrCl6 voltammogram and an APAP
voltammogram in terms of their overall shape (symmetrical, non-symmetrical), and
the electrochemical reactions observed for the cathodic and anodic potential waves. - Determine the concentration of APAP in the tablet (i.e. mg APAP / tablet).
a. Determine the % agreement of your value versus the APAP value based on
the label claim on the product’s packaging.
Answer the following questions based on the article by Heineman and Kissinger posted on BB. - Schematically represent the oxidation mechanism of APAP. Label structures
appropriately. Define any acronyms used. - Use the format shown in equation 3 (p.5) and write the redox reaction for APAP at
pH 6.0. - Based on the voltammogram for APAP at pH 6.0, comment on the reversibility of the
redox reaction including your rationale.
a. Explain whether the extent of reversibility for APAP at pH 6.0 is equivalent or
different at the two scan rates used. Provide a rationale that supports your
explanation. - Based the mechanism and figures 3 – 5 in the article, explain why a scientist would
use different scan rates if they were trying to characterize the reaction chemistry of
an electroactive compound?
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Appendix
Cleaning the Working Electrode
IMPORTANT: Before proceeding, go to the part of the procedure to identify the type of working
electrode to be used and the solution(s) to be analyzed. DO NOT clean the reference or auxiliary
electrodes, clean only the working electrode. - Locate the plastic board in the drawer with two circular cleaning cloths mounted on its
surface. Place it on the bench top. The two cloths are used to polish the working
electrodes; one cloth is used with alumina slurry solution and the other with 18 M DI
water. The alumina solution is in the drawer. - Place 2 – 3 drops of the alumina slurry on the cleaning cloth labeled “polish”. Using
moderate pressure polish the working electrode for 60 – 90 seconds using a figure-8
type motion. Thoroughly rinse the working electrode with 18 M distilled water to remove
any alumina residue. - Place 2 – 3 drops of 18 M DI water on the cleaning cloth labeled “rinse”. ”. Using
moderate pressure clean the working electrode of any residual alumina particles for
60 seconds using a figure-8 type motion. Rinse the working electrode with 18 M DI
water and dry the electrode by blotting the electrode surface with a clean, dry Chemwipe.
DO NOT wipe the electrode with the Chemwipe. - Locate the Cell Top and Stand on the potentiostat and echem computer bench. Move the
stand to the bench where you are working.
Insert the working electrode into the Teflon cap of the electrochemical cell holder shown in
Figure 6.
Figure 6 Cell Cap, Glass Vessel, and Stand
Cap
Top
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Electrochemical Cell Assembly - Locate the Pt wire auxiliary electrode in the drawer. Rinse the wire with 18 M DI water
and dry the electrode by blotting the electrode surface with a clean, dry Chemwipe. DO
NOT wipe the electrode with the Chemwipe. Carefully insert the auxiliary electrode into the
teflon cap of the electrochemical cell holder. - Locate the glass cell vessel in the drawer. Rinse the cell with 18 M DI water and then the
solution to be analyzed. Dispose of the solution used to rinse the cell in the proper waste
container. Fill the cell vessel approximately 2/3 full with the solution to be analyzed. - Carefully assemble the cell vessel to the cell cap making sure not to bump the vessel into
the electrodes. The cell vessel should sit securely on the teflon stand. - Locate the Ag/AgCl reference electrode in the drawer. It is soaking in 0.1 M KCl solution.
Carefully remove it from the soaking solution, rinse with 18 M DI water and dry the
electrode by blotting the electrode surface with a clean, dry Chemwipe. DO NOT wipe the
electrode with the Chemwipe. Carefully insert the reference electrode into the teflon cap. - The three electrodes should be immersed in the analyte solution to be analyzed. All of the
electrodes should be immersed to approximately the same depth in the solution. Use the
O-ring on the electrode if the depth needs to be adjusted. - Ask the lead instructor for this experiment to verify the cell is properly assembled before
proceeding.
Attaching the Cell to the Three-Electrode Potentiostat
The potentiostat is located in the Instrument Facility. - The potentiostat and the computer will be turned ON. If not, ask the lead instructor for help.
- Carefully take the electrochemical cell to the bench where the potentiostat is located.
- Locate the three wires (leads) that have alligator clips on their ends. Identify the lead that is
clipped to each electrode based on the following letters: W = working, R = reference, A =
auxiliary. - Connect each electrode to the potentiostat by securing the clip on the metal rod located
on the top of the electrode. Make sure that the metal clips do not touch each other. - Ask the lead instructor for this experiment to verify the cell is properly assembled.