Experiment 5: Cyclic Voltammetry (20 Marks) Name: Nurul Husna Binti Ibrahim Id: 2018226626 Group: As2224m1
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CHM578/674
2020
Experiment 5:
Cyclic Voltammetry
(20 marks)
Name: Nurul Husna binti Ibrahim
(Refer Lab manual Exp. 5 CHM674 Questions)
ID: 2018226626 Group: AS2224M1
prior to answering the following Post-Lab
The important parameters obtained from a cyclic voltammograms are the anodic peak current (Ipa), cathodic peak current (Ipc), anodic peak potential (Epa) and cathodic peak potential (Epc). All of these values can be readily obtained from the voltammogram. Measuring Ip's involve the extrapolation of the base-line current. The peak current for an electrochemically reversible system (rapid transport of electrons on the surface within the framework of the experiment) is described by the Randles-Ševcík equation: Ip = (2.69x105)n3/2AD1/2 v1/2c Where:
Ip = peak current (in A) n = electron stoichiometry D = diffusion coefficient (cm2/s) c = concentration (mol/cm3) A = electrode area (cm2) v = potential scan rate (V/s)
Oxidation:
Reduction :
Reversible reactions may be diagnosed using the following criteria:
Peak separation ∆E = Epa - Epc = 59 mV
Ip,a / Ip,c = 1
(2.303 RT/F = 59
1
at 298K)
CHM578/674
2020
Ip,a and Ip,c α v1/2
Ep, a and Ep,c
independent of v
Results - CV Parameters
Table 1 :
(8 marks)
Effect of FeCN64- Concentration
Concentration Epa (mV) of Fe(II) Solution
Ip,a (μA)
Epc (mV)
Ip,c (μA)
∆E (mV)
2 mmol
+280
18
+220
-18
60
4 mmol
+280
26
+220
-28
60
6 mmol
+280
45
+220
-47
60
8 mmol
+280
58
+190
-52
90
10 mmol
+310
62
+120
-52
190
unknown
+300
32
+240
-30
60
Area of gold electrode (ie: working electrode) = 0.078 cm 2, ∆E = Epa - Epc
Scan potential:
-0.2 V to +0.65 V back to -0.2 V 2
CHM578/674
2020
Scan rate : Table 2 :
25 mV/s Effect of scan rate (Constant Concentration: 4 mmol)
Scan rate, ν
Epa (mV)
Ip,a (μA)
Epc (mV)
Ip,c (μA)
∆E (mV)
scan rate (v ½ )
25 mV/s (0.025 V/s)
+280
27
+220
-28
+60
0.158
50 (0.050)
+280
42
+220
-42
+60
0.224
75 (0.075)
+280
54
+220
-52
+60
0.274
100 (0.100)
+280
58
+220
-60
+60
0.316
125 (0.125)
+280
66
+220
-68
+60
0.354
200 (0.200)
+280
84
+220
-88
+60
0.447
3
CHM578/674
2020
Plot of Ipa versus v ½ = 1.85 x 10-4 Reference Electrode used: Ag/AgCl (KCl saturated) Post-laboratory Questions
Ecell = +0.195 V vs SHE
(12 marks)
1. From Table 1, estimate the concentration of the unknown. - Ip,q = 3.2×10-5 A, concentration of the unknown is 4.9 mmol 2. Based on CV results in Table 2, determine diffusion coefficient,D of Fe2+ ions and compare it with the literature value (ie: DFe2+ = 7.0 x 10-6 cm2/s). (Hint: use Randles-Servcik Equation to find D, c = mol/cm 3, slope Ipa/v½ = 1.85 x 10-4, n=1, A = 0.078 cm2) 1
Answer: ¿ graph I vs V 2 p, q :
I p, q V
3
1
1 2
=slope=1.85 ×1 0−4
1
Ip=(2.69 × 10¿¿ 5)n 2 AD 2 V 2 c ¿ 1
Ip
D2=
3
1
(2.69 ×10¿ ¿5) n 2 A V 2 c ¿ ¿
1.85× 10−4 3
( 2.69× 105 ) ( 1 2 ) ( 0.078 ) ( 4 ×10−6 )
D=¿
|
Percentage error :
( 7.0 ×10−6 ) ( 4.86 ×10−6 ) 7.0 ×10−6
|
×100 %
¿ 30.57 %
4
CHM578/674
2020
3. Define the following terms: Standard Electrode Potential (Eo) : The measure of the individual potential of reversible electrode at standard state which is 1 mol dm-3 at 1 atm. Cell Potential (E cell) : The measure of potential difference between two half cells in an electrochemical series Formal Electrode Potential (E’) : The reduction potential that applies to a half reaction under specified conditions Half-wave Potential (E ½) : The potential exactly in the middle of the two peaks where there is forward and reverse wave. 5. Based on the CV results in Table 2, determine the formal reduction potential (E’) of the ferro/ferricyanide couple. Correct for the potential of the reference electrode and compare it to the standard potential for the couple (Eo Ferro/Ferriccyanide = +0.436 V vs SHE). Comment on any observed difference. Answer:
(E ¿ ¿ p , a+ E p ,c ) ¿ 2 280+220 2
¿ 250 mV =0.250 V Emeasured =E °−E reference E °=0.250+ 0.195 0.445 V Percentage error =¿
0.436−0.445 ∨×100 0.436
¿ 2.06 % 5
CHM578/674
2020
6. From the CV results, identify the reversibility of Fe(CN) 64-/Fe(CN)63- redox reaction (either reversible, irreversible or quasi-reversible). Justify your answer. - The redox reaction is reversible because the redox system remains in equilibrium throughout the potential scan
6
2020
Experiment 5:
Cyclic Voltammetry
(20 marks)
Name: Nurul Husna binti Ibrahim
(Refer Lab manual Exp. 5 CHM674 Questions)
ID: 2018226626 Group: AS2224M1
prior to answering the following Post-Lab
The important parameters obtained from a cyclic voltammograms are the anodic peak current (Ipa), cathodic peak current (Ipc), anodic peak potential (Epa) and cathodic peak potential (Epc). All of these values can be readily obtained from the voltammogram. Measuring Ip's involve the extrapolation of the base-line current. The peak current for an electrochemically reversible system (rapid transport of electrons on the surface within the framework of the experiment) is described by the Randles-Ševcík equation: Ip = (2.69x105)n3/2AD1/2 v1/2c Where:
Ip = peak current (in A) n = electron stoichiometry D = diffusion coefficient (cm2/s) c = concentration (mol/cm3) A = electrode area (cm2) v = potential scan rate (V/s)
Oxidation:
Reduction :
Reversible reactions may be diagnosed using the following criteria:
Peak separation ∆E = Epa - Epc = 59 mV
Ip,a / Ip,c = 1
(2.303 RT/F = 59
1
at 298K)
CHM578/674
2020
Ip,a and Ip,c α v1/2
Ep, a and Ep,c
independent of v
Results - CV Parameters
Table 1 :
(8 marks)
Effect of FeCN64- Concentration
Concentration Epa (mV) of Fe(II) Solution
Ip,a (μA)
Epc (mV)
Ip,c (μA)
∆E (mV)
2 mmol
+280
18
+220
-18
60
4 mmol
+280
26
+220
-28
60
6 mmol
+280
45
+220
-47
60
8 mmol
+280
58
+190
-52
90
10 mmol
+310
62
+120
-52
190
unknown
+300
32
+240
-30
60
Area of gold electrode (ie: working electrode) = 0.078 cm 2, ∆E = Epa - Epc
Scan potential:
-0.2 V to +0.65 V back to -0.2 V 2
CHM578/674
2020
Scan rate : Table 2 :
25 mV/s Effect of scan rate (Constant Concentration: 4 mmol)
Scan rate, ν
Epa (mV)
Ip,a (μA)
Epc (mV)
Ip,c (μA)
∆E (mV)
scan rate (v ½ )
25 mV/s (0.025 V/s)
+280
27
+220
-28
+60
0.158
50 (0.050)
+280
42
+220
-42
+60
0.224
75 (0.075)
+280
54
+220
-52
+60
0.274
100 (0.100)
+280
58
+220
-60
+60
0.316
125 (0.125)
+280
66
+220
-68
+60
0.354
200 (0.200)
+280
84
+220
-88
+60
0.447
3
CHM578/674
2020
Plot of Ipa versus v ½ = 1.85 x 10-4 Reference Electrode used: Ag/AgCl (KCl saturated) Post-laboratory Questions
Ecell = +0.195 V vs SHE
(12 marks)
1. From Table 1, estimate the concentration of the unknown. - Ip,q = 3.2×10-5 A, concentration of the unknown is 4.9 mmol 2. Based on CV results in Table 2, determine diffusion coefficient,D of Fe2+ ions and compare it with the literature value (ie: DFe2+ = 7.0 x 10-6 cm2/s). (Hint: use Randles-Servcik Equation to find D, c = mol/cm 3, slope Ipa/v½ = 1.85 x 10-4, n=1, A = 0.078 cm2) 1
Answer: ¿ graph I vs V 2 p, q :
I p, q V
3
1
1 2
=slope=1.85 ×1 0−4
1
Ip=(2.69 × 10¿¿ 5)n 2 AD 2 V 2 c ¿ 1
Ip
D2=
3
1
(2.69 ×10¿ ¿5) n 2 A V 2 c ¿ ¿
1.85× 10−4 3
( 2.69× 105 ) ( 1 2 ) ( 0.078 ) ( 4 ×10−6 )
D=¿
|
Percentage error :
( 7.0 ×10−6 ) ( 4.86 ×10−6 ) 7.0 ×10−6
|
×100 %
¿ 30.57 %
4
CHM578/674
2020
3. Define the following terms: Standard Electrode Potential (Eo) : The measure of the individual potential of reversible electrode at standard state which is 1 mol dm-3 at 1 atm. Cell Potential (E cell) : The measure of potential difference between two half cells in an electrochemical series Formal Electrode Potential (E’) : The reduction potential that applies to a half reaction under specified conditions Half-wave Potential (E ½) : The potential exactly in the middle of the two peaks where there is forward and reverse wave. 5. Based on the CV results in Table 2, determine the formal reduction potential (E’) of the ferro/ferricyanide couple. Correct for the potential of the reference electrode and compare it to the standard potential for the couple (Eo Ferro/Ferriccyanide = +0.436 V vs SHE). Comment on any observed difference. Answer:
(E ¿ ¿ p , a+ E p ,c ) ¿ 2 280+220 2
¿ 250 mV =0.250 V Emeasured =E °−E reference E °=0.250+ 0.195 0.445 V Percentage error =¿
0.436−0.445 ∨×100 0.436
¿ 2.06 % 5
CHM578/674
2020
6. From the CV results, identify the reversibility of Fe(CN) 64-/Fe(CN)63- redox reaction (either reversible, irreversible or quasi-reversible). Justify your answer. - The redox reaction is reversible because the redox system remains in equilibrium throughout the potential scan
6
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