MS-307

Metal Recovery from a Dilute Solution by Electrochemical Method

Date of Experiment: February 14, 2012

Date of Presentation: February 17, 2012

Group A9-Rig II

Chaitanya Talnikar (09002013)

Sangeeta Mahala (09002017) - Report

Saket Kumar Choudhary (09D02007)

Objectives:

In this experiment, we perform an electrochemical deposition to reduce toxic metal ions (Cu⁺²) concentration in waste water and study electrochemical deposition of cupric ions from copper sulphate solution onto the graphite cathode, to calculate Reaction rate constant,Current efficiency, Energy consumption.

Motivation & Theory:

Most industrial waste streams contain a number of different heavy metals. Electrolysis is a method to remove dissolved metals off the waste water streams.

The experiment works on the principle of electrolysis where in a direct electric current drives a chemical reaction i.e., interchange of atoms and ions by the removal or addition of electrons from the external circuit.

At cathode, reduction:

Cu2++ 2e-→Cu

2H2O+2e-→ H2+2OH-

At anode, oxidation:

H2O→12O2+ 2H++ 2e-

The copper electro-deposition is a first order reaction with respect to cupric ion concentration in the waste water.

Concentration of cupric ion at any time, assuming uniform concentration throughout the solution is given by,

C= Coexp⁡(-kat)

Where,

C = Concentration of cupric ion at any time (t)

Co= initial concentration of cupric ion

k = rate constant for deposition of Cu⁺² ions at cathode

a = area (of cathode)/Volume (of solution) = 4dm²/9 litres

t = time

Instantaneous current efficiencyηi%= VSoldCdtInFx 100

Where, F = 96500 C/equivalent

dCdt=slope of Conc. vs timegraph

n = number of electrons (2 equiv. per mole)

Average current efficiencyηavg%=WMnF0θI dtx 100

Where, θ = total electrolysis time (sec.)

M = molecular weight of Cu (63.5 gms/mole)

W = mass of copper deposited at cathode = VsolC0-Cf

Energy consumption per kg of copper (kWh/kg) = 0θIEcelldt3600×1000W

Experimental and Calculation Procedure:

Figure 1: Experimental and calculation procedure of the experiment

Experimental Setup:

Figure 1: Schematic of the experimental setup

Figure 2: Schematic of the electrolytic cell

Data Analysis:

Sample Calculation:

For an absorbance of 0.197, the value of Concentration was obtained to be= (0.194*4920.2+6.3411) = 975.62ppm

At t=0, the value of (dC/dt) was observed to be= - (0.0028*exp(1.3899-0.0028*0)) =

-0.01124mol/min*m³.

W= 0.009*(3.9075-3.001)*63.5= 0.149

Results and Discussion:

Table 1: Absorbance at 573nm

Graph 1: Semi-log plot of lnC v/s Time

From the slope of above graph we get the reaction rate constant which is 0.0028 min^-1.

Table 2: Computational data over 96 minutes

Time (mins)

Voltage (V)

Current (A)

dC/dt (mol/m3 min)

Instantaneous current efficiency (%)

IE_cell

0

2.5

2.74

-0.011240

11.876

6.85

4

2.5

1

-0.011115

32.179

2.50

8

2.5

0.97

-0.010991

32.804

2.43

12

2.5

0.97

-0.010869

32.439

2.43

16

2.6

1.33

-0.010748

23.395

3.46

20

2.6

1.34

-0.010628

22.962

3.48

24

2.5

1.42

-0.010510

21.427

3.55

28

2.4

1.46

-0.010393

20.608

3.50

32

2.4

1.49

-0.010277

19.968

3.58

36

2.4

1.54

-0.010163

19.104

3.70

40

2.6

1.67

-0.010049

17.421

4.34

44

2.5

1.73

-0.009938

16.630

4.33

48

2.4

1.84

-0.009827

15.461

4.42

52

2.4

1.86

-0.009717

15.125

4.46

56

2.5

1.9

-0.009609

14.641

4.75

60

2.5

2.11

-0.009502

13.037

5.28

64

2.5

2.13

-0.009396

12.771

5.33

68

2.5

2.15

-0.009292

12.511

5.38

72

2.5

2.17

-0.009188

12.258

5.43

76

2.4

2.19

-0.009086

12.011

5.26

80

2.4

2.20

-0.008985

11.823

5.28

84

2.4

2.21

-0.008885

11.638

5.30

88

2.4

2.22

-0.008786

11.457

5.33

92

2.4

2.21

-0.008688

11.381

5.30

96

2.4

2.2

-0.008591

11.305

5.28

Graph 2: Plot of Current v/s Time (mins)

Graph 3: Plot of IE_cell v/s Time (mins)

We can see in both the above graphs, inconsistency in the data values. This can be attributed to voltage fluctuations. As a result, the ammeter and voltmeter reading may be slightly inconsistent.

Graph 4: Instantaneous Current Efficiency % v/s Time (mins)

The above plot shows inconsistency owing to the fluctuation in reading. The average current efficiency can be calculated by finding the area under the current versus time graph and using the given formula. Similarly, the energy required can be calculated using the area under Graph 3 and the given formula.

Thus we have:

Average current efficiency = 12.3%

Energy Requirement= 16.9 kWh/kg of metal deposited

Error Analysis:

Table 3: Error Analysis

Ln( C )

Ideal ln(C)

% Error in ln C

1.363

1.387575181

1.77875768

1.343

1.342775181

0.01939589

1.322

1.297975181

-1.82875313

1.279

1.253175181

-2.04170049

1.228

1.208375181

-1.63470793

1.163

1.163575181

0.08840873

1.099

1.118775181

1.77224637

The ideal value listed above is the value as predicted by our best-fit linear regression model. The value in the left-most column of the table shows the actual value of Ln C, and the error is in the rightmost column. As can be seen the error ranges from as low as 0.02% to about 2.0%. On an average we can say there is a 1.8% error in the plot.

Other errors can also be present, such as the error involved in measuring the total volume of 9 litres (since aqueous copper sulphate will itself give out some amount of water), error in the concentration v/s absorbance data used to make correlations, errors in taking measurements by the spectrometer, errors in the readings shown by the voltmeter and ammeter, and these all translate into errors in the energy consumed in the process. If we assume a 5% error in all the parameters concerned, and use the least count as a measure of the errors in the voltmeter (0.1V) and ammeter (0.01A), then we end up with an effective net error of 10% in the energy consumed in the process.

Conclusions and Remarks:

Depending on the energy we have available, we can use this data as an estimate to see whether this is a viable technique for purification of water of cupric ions.

The results obtained verify our hypothesis that the reaction is first order

Since we are not changing any of the mass transfer influencing parameters in this experiment, i.e. we are not changing the cell voltage or the flow-rate of the electrolyte, we are unable to see or draw any kind of conclusion about how these influence the mass transfer and hence the reaction rate.

Precautions:

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This work is licensed by Saket Choudhary under a Creative Commons Attribution License (CC-BY 3.0), and is an Open Educational Resource.

Last edited by Saket Choudhary on Apr 1, 2012 6:03 am -0500.