Analysis of Hydroquinone/Quinone Redox Couple Through the Use of Cyclic Voltammetry

Transcription

1 Analysis of Hydroquinone/Quinone Redox Couple Through the Use of Cyclic Voltammetry Adam Woodard and Katrin Henry Ta: Jie Ding Abstract: The hydroquinone/quinone redox couple was studied under various ph levels to determine whether or not the redox couple was dependent on ph according to the Nernst relationship. Upon investigation it was determined that hydroquinone/quinone redox couple does appear to follow the Nernst relationship within a 3% error.

2 Introduction Cyclic voltammetry uses a time varying electric potential to determine redox properties of a compound. From these properties compounds can be identified and quantified. Cyclic voltammetry produces a plot of current versus potential of the working electrode with respect to a reference electrode. This plot has a fairly distinctive shape as can be seen in figures 1 through 5, and it is commonly described as a duck shape. The peak current is linearly dependent on the concentration of the analyte, and the average voltage between the two peaks gives the redox potential for the analyte couple being studied. Sometimes the analyte couple being studied involves a protonation and therefore the ph is also important. According to the Nernst equation for a ph dependent redox couple: E ½,app = E ½.59 ph Eqn-1 Where E ½,app is the apparent voltage of the redox couple (measured voltage) and E ½ is the true half-cell potential. In this lab the Nernst relationship was tested using the hydroquinone/quinone redox couple. Experimental First approximately ml of a mixed buffer solution was made consisting of approximately.1 M each of phosphoric and acetic acids. Then hydroquinone was added to achieve an approximate concentration of 1. mm. From this stock solution five solutions were made at various ph using a 5 M solution of NaOH. Once the solutions were made the electrodes were cleaned in a one molar sulfuric acid solution while cycling the potential from mv to 16mV with a ramp speed of 1 mv/sec for 1 cycles and then rinsed with DI water. This procedure was repeated after each sample to ensure accurate measurements. The lowest ph sample was then run cycling the potential from -5 mv to 6 mv for 1 cycles at three different scan rates: 5, 1, and mv/sec. From this data it was determined that 1 mv/sec gave a good balance between time and resolution. The rest of the samples were then run at 1 mv/sec. The cyclic voltagrams are given in figures 1 to 5. Discussion From the voltagrams the following potentials of peak currents and E ½,app for each of the five ph levels were determined: Table-1: ph Epc Epa E ½,app Where E pc is the potential at the peak current when the working electrode is the cathode and E pa is the potential at the peak current when the working electrode is the anode. From this data a plot of the ph dependence of E ½,app created and a line of best fit was calculated.

3 E ½ [mv] Apparent E ½ Dependcence on ph y = x R² = ph Plot 1 When the slope of the line of best fit is compared to the slope of eqn-1 it is found that there is a -3% error from the expected value of 59. mv/ph unit. This is fairly good agreement and shows that the hydroquinone/quinone redox couple does obey the Nernst relationship. Further analyzing the different scan rates at 1.61 ph showed that peak current increased as scan rate increased as would be expected. Conclusion This lab set out to test the hydroquinone/quinone redox couple to determine whether it obeyed the Nernst relationship as predicted by the chemical theory. The tests successfully showed that it does within a 3% margin of err.

4 Current [μa] Current [μa] Figures: ph S.R. 5 mv/s S.R. 1 mv/s S.R. mv/s Figure 1: The average cyclic voltagram at ph 1.61 for three scan rates ph Figure : The average cyclic voltagram at ph.14

5 Current [μa] Current [μa] ph Figure 3: The average cyclic voltagram at ph ph Figure 4: The average cyclic voltagram at ph 4.43.

6 Current [μa] ph Figure 5: The average cyclic voltagram at ph 5.65.