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Hanna Thomson Chem421 Cyclic Voltammetry

CHEMISTRY 421

EXPERIMENT NAME: CYCLIC VOLTAMMETRY

STUDENT: HANNA THOMSON

EXPERIMENT PREFORMED ON: 3/3/2020

TA’S NAME: PATRICK FISHER

DATE OF SUBMISSION: 3/9/2020

DUE DATE OF EXPERIMENT: 3/10/2020

NAME OF LAB PARTNER: ADNAN ALHABISS

Hanna Thomson Chem421 Cyclic Voltammetry

Voltammetric Determination of Acetaminophen in Tylenol Tablets Abstract The purpose of this experiment was to determine the concentration of acetaminophen, the active ingredient in Tylenol, using cyclic voltammetry to measure redox reactions. A calibration curve is constructed based on current of the anode versus concentration of standard in the solution, and the concentration of the unknown solution containing acetaminophen in a Tylenol tablet is determined using LINEST equations in excel. Background Acetaminophen, the active ingredient in Tylenol, is used in many over the counter and prescription pain relief medications. The mechanism of action for acetaminophen is not very well known, but a popular proposal is that acetaminophen works similarly to ibuprofen by blocking cyclooxygenase, an enzyme that forms prostaglandins, molecules responsible for pain and inflammation signals [ CITATION Car14 \l 1033 ]1. Acetaminophen is used in this experiment because of the reduction potential on several sites of the molecule, such as the hydroxy group and the amino group as pictured below in figure 1. Figure 1: Acetaminophen molecular structure.

Image obtained from https://www.researchgate.net/figu re/Molecular-structure-ofacetaminophen_fig14_312848234 During the reduction reaction, the hydroxy group at the para position gains a double bond as well as the nitrogen with the loss of the hyrogens, however there is not much of an oxidation reaction happening which is why we are mainly concerned with the reduction potential of the molecule by using cyclic voltammetry. Cyclic voltammetry is a widely used method of measuring the current passing through an electrochemical cell with a constant voltage applied to it. The “cyclic” characteristics of this instrument refers to the working electrodes potential fixed in the opposite position to return back to the orignal potential, resulting in a cycle type diagram. In a cyclic voltammogram, the time in seconds is meausred against the potential vs. the reference electrode, and the postitive current represents a reductoin reaction whereas the negative current indicates an oxidation reaction. This realtionship can be described by the Randles-Sevcik expression relating the current with the electrode Figure 2: cyclic area, diffusion coefficient, concentration, and the scan rate. The anodic voltammetry current can be depicted by the forward scan, and the backward scan instrumental set up corresponds to the cathodic current. During this lab we are mainly of-measurementsetup-of-cyclicvoltammetry_fig4_32 2824945

on Chem421 Cyclic Voltammetry concerned with the Ipa or the magnitude of the anodic peak current, due to acetaminophens ability to be reduced and not oxidized. The general outline of the cyclic voltammetetry instrument is pictured below in figure 2, where the potentiostat is the source of the electic potential being applied to our sample, the electochemical cell is characterized by the working platinum electrode where the reduction or oxidation reactions are taking place, the counter electrode responsible for the constant flow of electrons fromt the source through our analyte, and the reference electrode responsible for a constant potential almost acting like a standard potential so that the current at other electrodes can be measured, all submerged in our sample.

Procedure The solutions being analyzed were prepared a week prior by preparing an acetaminophen standard by dissolving 2.6454 g of acetaminophen standard in 500 mL of 0.4% perchloric acid and heating until all of the solid dissolved completely. To prepare the buffer solution, 18.60 g of KCl, 9.420 g of citric acid, and 0.283 g of Na2HPO4 were dissolved in deionized water to 500 mL to a pH of 2.2 using pH papers. To create the unknown, two Tylenol tablets were crushed and weighed to 0.5052 g and diluted to 25 mL in a volumetric flask with the buffer solution prepared above. A set of four standards were prepared following table 1 below Table 1: Acetaminophen Standard Preparation Standard volume (mL) 0.00 5.00 7.50 10.0

Sample solution (mL)

Buffer solution (mL)

Deionized water (mL)

1.50 1.50 1.50 1.50

2.50 2.50 2.50 2.50

21.0 16.0 13.5 11.0

Hanna Thomson Chem421 Cyclic Voltammetry

15 mL of the above standards were taken to analyze by inserting in to the glass cell and attaching the SP-50 to the Pt working electrode, Pt auxiliary electrode, and the Ag/AgCl reference electrode. A source of nitrogen gas was bubbled through each of our samples for 5 minutes prior to analysis to ensure any oxygen in the sample was removed. The SP-50 was turned on, and the EC-lab software was ran under voltamperometric parameters and the cyclic voltammetry technique. The parameters were set as follows; Ewe Ei=0.000 V vs. Eoc, scan rate of dE/dt in mV/s with 50 mV/s for the first buffer solution, E1= 1.200 V vs. Ref, E2= -0.600 V vs. Ref, repeat nc=3, measure over the last 50% of the step duration, record averaged over N=10, E range = -2.5 V to 2.5 V, I range = auto, bandwidth = 5-medium, end scan to Ef = 0.000 V vs. Eoc. The scan was run for each sample 3 separate times to obtain 3 trails of each sample as well as the unknown sample, and graph and parameters were printed out after each analysis. Data and Calculations Acetaminophen standard calculation: 5.3 mg/mL acetaminophen (500 mL) / 1000 = 2.65 g acetaminophen dissolved to 500 mL of 0.4% perchloric acid

Hanna Thomson Chem421 Cyclic Voltammetry CYCLIC VOLTAMMETRY Ipa concentration (mA) trial 1 trial 2 trial 3 0 0.156 0.163 0.179 1.06 0.176 0.188 0.194 1.59 0.431 0.451 0.482 2.12 0.603 0.634 0.665 unknown 0.499 0.534 0.538

average 0.166 0.186 0.454667 0.634 0.523667

Table 2: Concentration vs. Ipa of 4 known acetaminophen standards

Acetaminophen Standards 0.7 0.6 f(x) = 0.22 x + 0.1 R² = 0.8

Ipa (mA)

0.5 0.4 0.3 0.2 0.1 0

0

0.5

1

1.5

2

2.5

concentration (mg/mL)

Calculation of unknown concentrations: Trial 1: 0.499 = 0.2222x + 0.0952, x = 1.8173 mg/mL Trial 2: 0.534 = 0.2222x + 0.0952, x = 1.9766 mg/mL Trial 3: 0.538 = 0.2222x + 0.0952, x = 1.9946 mg/ mL Average concentration of 3 trials: (1.8173 + 1.9766 + 1.9946)/3 = 1.9296 mg/mL Standard deviation of unknown concentrations: 0.097584 (calculated using Microsoft excel STDEV function) Standard error: Average + (t*standard deviation) / sqrt(number of samples) = 1.9296 + (2.92 * 0.097584)/sqrt(3) = 1.9296 +/- 0.163 mg/mL. % RSD:

Hanna Thomson Chem421 Cyclic Voltammetry

Standard deviation / average x100 = 0.097584 / 1.9262 x 100 = 0.05 % *cyclic voltammograms and data attached at the end of this report Results and Discussion The purpose of this lab was to determine the concentration of acetaminophen in Tylenol tablets using cyclic voltammetry by analyzing the redox potential of acetaminophen and a standard addition method amongst a set of known standard acetaminophen concentrations. After running cyclic voltammetry for the known samples and the unknown Tylenol tablet, we determined the average concentration of acetaminophen in 0.5052 g of Tylenol tablets to be 1.92926 +/- 0.163 mg/mL, with a percent relative standard deviation of about 0.05%. During this experiment we also noticed that we are only observing a peak in the redox portion of the cyclic voltammogram corresponding to our Ipa values graphed in the data and calculations portion of this lab report. This is because due to the molecular structure of acetaminophen, it is primarily capable of undergoing a redox reaction versus an oxidation reaction due to the functional groups present in this molecule. The area most focused on during this experiment was the anode potential portion of the graph, or the positive right hand side. Questions Why is a platinum electrode used instead of a carbon or gold electrode? Platinum electrodes are used over gold or carbon electrodes because of it’s substantial electrochemical inertness, but is sometimes disfavored because of it’s possibility to reduce hydrogen in to hydrogen gas in molecules containing water or acids. Platinum is used over gold or carbon electrodes because gold is less useful in the sense that it sometimes oxidizes on its surface, resulting in limited use in the positive potential range. Carbon electrodes are disfavored because of the difficulty to make and its high price, as well as it’s prone to being mechanically damaged during experiments [ CITATION Wor19 \l 1033 ]. Discuss any interference of O2 that is apparent from the series of voltammograms During this experiment we eliminated the interference of oxygen in our samples by bubbling them with nitrogen gas for 5 minutes before analyzing our samples, since nitrogen is an inert gas it will not interfere with the reactions taking place. Whereas oxygen is capable of oxygenating our molecule and throwing off our results significantly. However, I do not observe any possible oxygen interference in our samples. Discuss why your acquired data does not look like the example in figure 2 Our results do not look like the data in figure 2 because we do not have any oxidation reactions taking place during this experiment with acetaminophen, only reduction reactions at the

Hanna Thomson Chem421 Cyclic Voltammetry

multiple oxygen functional groups present in our molecule, so we do not observe the anode curve under negative current as shown in figure 2 in the laboratory manual.

References Drahl, C. (2014, july). How Does Acetaminophen Work? Researchers Still Aren’t Sure. Retrieved from chemical and engineering news: https://cen.acs.org/articles/92/i29/Does-Acetaminophen-WorkResearchers-Still.html Working Electrodes. (2019, June). Retrieved from Chem Libre Texts: https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analyti cal_Chemistry)/Analytical_Sciences_Digital_Library/JASDL/Courseware/Analytical_Electrochemis try%3A_The_Basic_Concepts/05_Experimental_Hardware/C._Working_Electrodes L.J Mathias, M. H. (1992). Chem. Ed. 69 and references therein. In Instrumental Analysis Laboratory AY 2019-2020 (pp. 32-34)...