Lab 3 Report

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Chemistry

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Apr 3, 2024

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UTEP Department of Chemistry and Biochemistry Laboratory for Instrumental Methods of Analytical Chemistry Identification of Unknown Compounds by IR Spectroscopy Jacqueline Guzman James February 27, 2024

Objective For this experiment, oxidation of acetaminophen will be performed with cyclic voltammetry which occurs by an electrochemical cell, and the process will be optimized by selection of a buffer solution at a particular pH and a scan rate in units of mV/s. Conditions that give a maximum response for oxidation of acetaminophen in solution will then be applied to a prepared solution which contains dissolved Tylenol. Peaks in the collected voltammograms will be compared and utilized to determine the concentration of acetaminophen in Tylenol. Introduction The electroanalytical technique known as cyclic voltammetry studies electron transfer reactions where the flow of electrons is related to chemical changes. For electrochemical redox reactions, a molecule undergoes reduction or oxidation by means of an electrode that transfers an electron heterogeneously. With the electrode as an electrical conductor, this transfer process is initiated by a voltage from an external power source which is applied to the electrode to control the energy of the electrons within it. The transfer occurs when the electrons in the electrode have a greater energy than the LUMO of the molecule. A trace is visualized in a graph where the x-axis represents the applied potential (E), or voltage, and the y-axis is the response of system, or the current (i) that passes through it. An arrow is assigned to each trace to indicate the sweep direction of the potential that was scanned to obtain data, and it also indicates the starting point of the “forward scan.” The forward scan refers to the cathodic trace where there is a negative sweep from the starting potential E1 to the switching potential E2; the anodic trace follows in the reverse scan that sweeps positively back to E1. A significant parameter known as the scan rate (v) describes the speed (V/s) at which the

potential was varied linearly during the reaction. Peaks arise in cyclic voltammograms due to the equilibrium between the oxidized and reduced form of a molecular species as described by the Nernst equation. The species’ standard potential (E0) along with the respective activities of the redox analytes, termed as Ox (for oxidized) and Red (for reduced), are correlated to the standard potential (E) of the electrochemical cell: E = E 0 + RT nF ln ( Ox ) ( ¿ ) = E 0 + 2.3026 RT nF log 10 ( Ox ) ( ¿ ) Eq. 1 where R is the universal gas constant, T is the temperature, n is the number of electrons, and F is Faraday’s constant. For practicality, the Nerst equation may be modified by replacement of the activities with the concentrations of the oxidized and reduced forms of the species that undergoes the electron transfer so that n is equal to 1 and a formal potential E0’ replaces the standard potential E0: E = E 0 ' + RT nF ln ¿¿ Eq. 2 In this way, the influence of variation in electrode potential and species’ concentration allows prediction of how the system will respond in respect to time. A schematic of the electrochemical cell used in cyclic voltammetry is given below.

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The solution remains electrically neutral by migration of ions as the electron transfer occurs. This means that ion movement balances out the charge differences in solution to close the electrical circuit as the electrode transfers an electron to the analyte. To minimize solution resistance, an adequate supporting electrolyte which is classified as a salt is dissolved in an adequate solvent, and this mixture is the electrolyte solution. Solution conductivity is concentration-dependent in reference to the salt, and charge balance will be inhibited without the electrolyte. An electrochemical cell consists of a set-up with three electrodes: working, counter, and reference. The electrochemical event takes place by the working electrode which must consist of material that is redox-inert within a potential range. Its applied potential is a function of the potential of the reference electrode and is controlled by a potentiostat. The equilibrium potential of the reference electrode is well-defined, stable, and independent of the electrolyte used. A potential applied to the working electrode causes the analyte to undergo a redox process and allows the flow of current. This flow of electrons occurs between the working electrode and counter electrode at which the electrical circuit is completed. Materials and Methods

Results and Discussion -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1 -6.00E-04 -5.00E-04 -4.00E-04 -3.00E-04 -2.00E-04 -1.00E-04 0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 ATPM in pH 1.8 Buffer Series2 Series4 Series6 E/V vs. Ag/AgCl i/A Figure 1. Cyclic voltammetry at scan rates of 40, 100, and 250 mV/s, respectively. Step 1: Prepare 3.5 mM acetaminophen solutions 1, 2, and 3 with pH buffer of 1.8, 2.0, and 6.0, respectively. Deoxygenate each solution. Also, prepare 3.5 mM acetaminophen solution with 1.8 M sulfuric acid. Step 2: Clean the electrodes, and set the conditions for computer software. Assemble the cell with the electrodes and electrolyte solution. Step 3: With the pH 6.0 buffer solution, perform the scan at the given rates (40, 100, 250 mV/s). Check for bubbles between scans and remove by lifting the electrodes out of solution. Step 4: Repeat step 2 & 3 with the other solutions. Save all data as .csv file for analysis. Step 5: After determination of optimal conditions, weigh tylenol tablet and crush into a fine powder with a mortar and pestle. Prepare solution with appropriate pH buffer and deoxygenate. Step 6: Perform scan with appropriate scan rate. Compare peak height of acetaminophen and the tylenol tablet and determine concentration.

-0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1 -1.00E-03 -5.00E-04 0.00E+00 5.00E-04 1.00E-03 1.50E-03 2.00E-03 ATPM in pH 2.0 Buffer Series2 Series4 Series6 E/V vs. Ag/AgCl i/A Figure 2. Cyclic voltammetry at scan rates of 40, 100, and 250 mV/s, respectively. -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1 -1.50E-04 -1.00E-04 -5.00E-05 0.00E+00 5.00E-05 1.00E-04 1.50E-04 2.00E-04 2.50E-04 ATPM in pH 6.0 Buffer Series2 Series4 Series6 E/V vs. Ag/AgCl i/A Figure 3. Cyclic voltammetry at scan rates of 40, 100, and 250 mV/s, respectively.

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-0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1 -2.00E-04 -1.00E-04 0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 ATPM in 1.8 H2SO4 Series2 Series4 E/V vs. Ag/AgCl i/A Figure 4. Cyclic voltammetry at scan rates of 40 and 100 mV/s, respectively. Visualization and analysis of the CV data shows that the optimal conditions were of the pH 2.0 buffer with a scan rate of 250 mV/s. These were the conditions used for the Tylenol tablet. Peaks are evident in all graphs to demonstrate the oxidation of acetaminophen in aqueous solution. This affirms that the metabolism of acetaminophen is a pH-dependent process to allow the deprotonation of acetaminophen. Although the procedure called for a calibration curve, it was not able to be constructed because there were not multiple concentrations of acetaminophen in solution. Conclusion Buffer solutions at constant concentration of acetaminophen (3.5 mM) were used in an electrochemical cell to select the optimal conditions for its oxidation to occur. Various scan rates in cyclic voltammetry along with differences in pH were compared to determine maximum

response between the applied potential and resultant current. The applied potential that stimulated the oxidation reaction gave rise to peaks which showed the electrical conductivity of the system. Concentration of acetaminophen in Tylenol was unobtainable as there was not enough data to construct a calibration curve. References  Elgrishi, N.; Rountree, K.J.; Brian D. McCarthy, B.D.; et al. A Practical Beginner’s Guide to Cyclic Voltammetry. J. Chem, Ed. 222000111888 95 (2), 197-206.