Lab Memo: Cyclic Voltammetry: Identification and Quantification of Unknown Concentrations of Substituted Benzoquinones
1.Introduction
Cyclic Voltammetry (CV) measures the redox behavior of electeroactive species and is useful for a whole spectrum of molecules in inorganic chemistry, biochemistry, electrochemisty as well as organic chemistry. A voltammogram is the readout of the energy scan which can be used to determine unknown concentrations. Voltammogram wave shape is very different than in spectrometry and chromatography. The wave is triangular in shape because the y-axis describes the shift in the potential which is the voltage of a working electrode versus a reference electrode. The x axis describes duration in seconds. (Kissinger and Heineman).
Two samples each with an unknown concentration of benzoquinones were analyzed with the instrument, BASi Epsilon-EC.
2. Qualitative Determination of Unknown Samples
Standard solutions were analyzed and the results were graphed to determine the unknown samples. Table 1(on page 2) displays the standard concentrations made with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone with their corresponding Potential in microVolts and the Peak currents in microAmps. The way the unknown samples were identified is by comparing their peak currents (µAmp) with the catalog of known substances with their peak currents.
Table 1: Concentration and Peak Current of 2,3-dichloro-5,6-dicyano-1,4 -benzoquinone
Concentration(mM)
Potential (µV)
Peak Current (µA)
2
469
8.9937
4
503
12.0546
6
503
39.237
8
500
59.15533
Table 2 presents the same measurements from the cyclic voltammogram for the tetrachloro-1,4-benzoquinone standards.
Table 2: Concentration and Peak Current for Tetrachloro-1,4-benzoquinone
Concentration (mM)
Potential (µV)
Peak Current(µA)
2
14
15.0423
4
14
30.227
6
-1
43.5767
8
-7
57.8775
Table 3 displays the unknown samples and the corresponding peak currents that were measured for each of them.
Table 3: Peak Current for Unknowns
Unknown
Peak Current (µA)
CV 503
43.73995
CV 501
29.9351
3. Standard Solution Calibration Curves
Figure 1 and 2 present standard curves for 2,3-dichloro-5,6 dicyano-1,4 –benzoquinone and Tetrachloro-1,4-benzoquinone respectively using the data obtained in the laboratory and presented in Table 1 and 2. The linearity of the first standard curve gave an R2 of 0.846 and of the second 0.9984 and they were considered adequate. The measured peak currents of the unknown are within the range of the respective standard curves.
Figure 1: Peak Current vs. Concentration for 2,3-dichloro-5,6 dicyano-1,4 –benzoquinone
Figure 2: Peak Current vs. Concentration for Tetrachloro-1,4-benzoquinone
4. Quantitative Determination of Unknown Sample
CV experiments need some type of buffer which will resist change of the reaction’s product. The tertbutylammonium tetrafluoroborate works well as a buffering system so the reduction potentianl won’t be affected and the voltammogram will be easier to read (Skoog).
In order to determine the concentrations of the unknowns, the linearized equations estimated in Figure 1 and 2 are used as well as the peak currents for the two unknowns. The concentrations of the two unknowns were determined using the following equations and table 4 presents the calculated concentrations.
Concentration for CV-503 = Peak area/6.4573
Concentration for CV-501 = Peak Area/7.2956
Table 4: Concentration of Unknowns
Unknown Number
Concentration (mM)
CV 503
6.774
CV 501
4.103
5. Standard Preparation and Error Discussion
The standards were prepared using a 100mM stock in a 25 mL flask. Two different methods were used for the preparation of the standards. For the first standard, the stock solution was added volumetrically (4 different ml volumes in 25 mL flask) and for the second, it was quantitatively weighted. The two methods of dilution for the preparation of the standard could involve different errors in the resulting concentration and the linearity of the standard curve. Based on the R2, the standard curve for 2,3-dichloro-5,6-dicyano-1,4-benzoquinone is worse than the standard curve for tetrachloro-1,4-benzoquinone, suggesting that determination of CV-501 is more accurate than the concentration of CV 503.
References
Fry, Albert J; Touster, Jonathan; “Voltammetry in (Toluene)-Tetrabutylammonium Tetrafluoroborate, a Novel Liquid “Hydrocarbon Electrolyte,” Journal of Organic Chemistry, 1986, 51,
3905-3907.
Kissinger, Peter T.; Heineman, William R.; “Cyclic Voltammetry,” 1983, 60(9), 702-706.
Skoog, Douglas A.;Holler, F. James; Crouch, Stanley R.; Principles of Instrumental Analysis, 6th. ed. Brooks and Cole. Cengage Learning, Canada, 2007; pp 716-753
