Experiments on the electrode reaction process for the determination of potassium ferricyanide by cyclic voltammetry
Experiments on the electrode reaction process for the determination of potassium ferricyanide by cyclic voltammetry
This experiment is from the official website of Wuhan University School of Pharmacy
Operation method
Experiments on the electrode reaction process for the determination of potassium ferricyanide by cyclic voltammetry
Principle
Cyclic voltammetry (CV) is one of the most important electroanalytical chemical research methods. It is widely used in the research fields of electrochemistry, inorganic chemistry, organic chemistry, and biochemistry because of its simple instrumentation, convenient operation, and intuitive graphical analysis. Voltammetric analysis measures the current of a system at a certain potential and obtains a voltammetric curve. Qualitative and quantitative analysis is carried out according to the voltammetric curve. If an isosceles triangle-shaped pulse voltage (triangular wave) is applied to the working electrode, the current-voltage curve obtained includes two branches. If the first half of the potential is scanned in the cathode direction, the electroactive substances are reduced on the electrode, producing a reduction wave, then when the second half of the potential is scanned in the anode direction, the reduction products will be oxidized again on the electrode, producing an oxidation wave. Thus a triangular wave scan, complete a cycle of reduction and oxidation process, so the method is called cyclic voltammetry, and its current ~ voltage curve is called cyclic voltammogram. If the reversibility of the electroactive material is poor, the height of the oxidation wave and the reduction wave are different and the symmetry is poor. The voltage scan rate in cyclic voltammetry can range from several millivolts per second to 1 volt per second. The working electrode can be a suspended mercury electrode, or a solid electrode such as platinum, glassy carbon or graphite. [Fe(CN)6]3-~[Fe(CN)6]4- is a typical reversible redox system, and the standard electrode potential of its redox pair is: [Fe(CN)6]3- + e- = [Fe(CN)6]4- φ θ = 0.36V ( vs. NHE ) The Nernst equation for the electrode potential versus electrode surface activity is: φ = φ θ + RT/Fln ( COx/CRed) At a certain scanning rate, during the positive scanning from the starting potential (-0.2V) to the turning potential (+0.8V), [Fe(CN)6]4- in the solution is oxidized to produce [Fe(CN)6]3-, which generates the oxidation current; when the negative scanning is changed from the turning potential (+0.8V) to the original starting potential (-0.2V), the oxidizing current is generated on the surface of the indicator electrode during the negative scanning. During the negative scan from the turning potential (+0.8 V) to the original starting potential (-0.2 V), the [Fe(CN)6]3- generated on the surface of the indicator electrode is reduced to [Fe(CN)6]4-, generating a reduction current. In order for the liquid-phase mass transfer process to be controlled only by diffusion, electrolysis should be carried out with the electrolyte added and the solution at rest. The diffusion coefficient of K4[Fe(CN)6] in 0.1 MNaCl solution is 0.63 × 10-5 cm-s-1; the rate of electron transfer is large and it is a reversible system (the standard reaction rate constant is 5.2 × 10-2 cm-s-1 at 25°C in 1 MNaCl solution). The dissolved oxygen in the solution is electrically active, and is removed by the passage of inert gas.Epc and Epa are the peak cathodic potential and the peak anodic potential, respectively.Ipc and Ipa are the peak cathodic current and the peak anodic current, respectively. Here P stands for peak, a for anode and c for cathode. Forward scan (scanning to the left) is cathodic scanning: Fe(CN)63- + e- = Fe(CN)62- Reverse scanning (scanning to the right) is anodic scanning: Fe(CN)62- - e- = Fe(CN)63- For the reversible system: 1) ipa / ipc=1 ② Difference between reduction and oxidation potentials: △Φ= Φpa - Φpc= 0.056/n V potential: = (Φpa + Φpc)/ 2 For the forward peak current of the reversible system, it can be expressed by the Randles-Savcik equation as follows: where ip is the peak current (A); n is the number of electron transfers; A is the area of the electrode (cm2); D is the diffusion coefficient (cm2-s-1); v is the scanning speed (V s-1); and c is the concentration ( mol-L-1). Move 1. Configuration solution, potassium ferricyanide standard solution (5.0 x 10-2 mol-L-1), potassium chloride solution (1.0 mol-L-1). Caveat 1. The electrode surface should be treated cleanly before the experiment, which is the main factor affecting the experiment;2. keep the solution stationary during the scanning process. Common Problems 1. Plot the concentration of potassium ferricyanide (c) versus ipa versus ipc at the same scanning speed. For more product details, please visit Aladdin Scientific website.
2. Pretreatment of the working electrode: the glass-carbon electrode was polished with a polishing cloth containing Al2O3 powder (particle size 0.05 ?m), and then ultrasonically cleaned with distilled water.
3. The electrode system was placed in the liquid to be tested, the instrument was opened, the parameters were set and the experiment was carried out.
4．Support the cyclic voltammogram of electrolyte: put 1.0 mol-L-1 KCl solution into the electrolytic cell, insert the electrodes, take the newly treated platinum electrode as the indicator electrode (green clip), the platinum wire electrode as the auxiliary electrode (red clip), and saturated glycerine mercuric electrode as the reference electrode (white clip), and carry out the cyclic voltammetry setup, the scanning rate was 20 mV/s; the starting potential was -0.2 V; The termination potential was +0.8 V, and the sensitivity was 1.e-0.04. After one minute of standing, the cyclic voltammetry scan was started and the cyclic voltammogram was recorded.
5. Cyclic voltammogram of K4 [Fe(CN)6] solution: make cyclic voltammograms of 0.02, 0.04, 0.08, 0.12, 0.16 mol-L-1 of K4 [Fe(CN)6] solution (all containing the supporting electrolyte NaCl concentration of 0.10 mol-L-1), respectively.
6. Cyclic voltammograms of K4 [Fe(CN)6] solution at different scanning rates: the cyclic voltammograms were recorded in 0.16 mol-L-1 K4 [Fe(CN)6] solution scanned over the potential range of -0.2 to +0.8 V at 20, 40, 60, 80, and 100 mV-s-1, respectively.
2. Plot the relationship between ipa and ipc and the corresponding v1/2 at the same potassium ferricyanide concentration.