In the Laboratory
Electrochemical Regeneration of a Spent Oxidizing Solution: Example of a Clean Chemical Process Marina Inglés, Pedro Bonete, Eduardo Expósito, Vicente García-García, José González-García, Jesús Iniesta, and Vicente Montiel* Departamento de Química-Física, Facultad de Ciencias, Universidad de Alicante, Ap. correos 99 Alicante E-03080, Spain; *
[email protected] In industry and university laboratories, – Figure 1. Diagram of the overall process. chromic salt solutions that generate very toxic and hazardous effluents have frequently been H H used (1, 2). It is often complicated or expenCr Cr sive to substitute for this oxidant. Therefore, Cr Cr O HO regeneration of chromium(VI) using a technique that leaves no residue appears to be a + very interesting solution for this problem. The Cr electrochemical regeneration of a redox reagent solution, such as chromium(III), to its OH O active form, chromium(VI), is a good example PhCHO for students of a clean chemical process and PhCH OH H 3 + 2 Cr + 6 H 3 + 2 Cr of how an oxidizing agent can be used in a closed cycle (3). This paper describes a simple laboratory experiment suitable for degree courses Cr in chemistry or chemical engineering. The oxidation of benzyl alcohol with potassium dichromate gives benzaldehyde and a solution containing the spent oxidizing agent, which is electrochemiDescription of the Apparatus cally oxidized to regenerate the chromium(VI) solution. The The experimental set up shown in Figure 2 was used to experiment is very easy to carry out and provides a simple carry out the regeneration of chromium(VI). It comprises demonstration of the utility of the electrochemical methodtwo glass solution reservoirs, an electrochemical UA63.03 ology. reactor, two circulation pumps (Nikkiso Eiko Co. Ltd Model CP08-PPRV-24) with polypropylene bodies, and a gas measureThe Experiment ment system. All interconnecting tubing was made of flexible The experiment consists of two stages: first obtaining PVC having an internal diameter of approximately 12 mm. benzaldehyde by oxidation of benzyl alcohol using a solution The UA63.03 reactor is shown in Figure 3. This is a of potassium dichromate in sulfuric acid (4 ): divided flow cell called a filter press reactor. In this design the electrodes are attached to the corresponding polypropylene plate 3PhCH2OH + K2Cr2O7 + 4H2SO4 → +
3+
6+
2
3+
6+
2
2
3+
2
6+
3+
+
6+
3PhCHO + Cr2(SO4)3 + K2SO4 + 7H2O and later the regeneration of the chromium(VI) solution (5): 2Cr3+
+ 7H2O → Cr2O7 + 2{
14H+
+
11
11
6e{
At the end of the experiment, the initial chromium(VI) solution is recovered. The overall reaction is the indirect electrochemical oxidation of benzyl alcohol to yield benzaldehyde, because chromium(VI) merely acts as a mediator (electron carrier between the electrode and the substrate to be oxidized) (Fig. 1).
1
2 4
3 6
5
5
PhCH2OH → PhCHO + 2H+ + 2e{ Electrolysis can be performed in two different ways: by setting a controlled potential at the working electrode or by setting a controlled current. Working at constant potential minimizes collateral reactions, but the necessary electronic instruments are expensive. In the electrochemical regeneration of chromium(VI), work is carried out at controlled current assuming that the current efficiency1 is not great. In this case, the most important collateral reaction is the oxygen evolution.
7 999.99
9
999.99
8
999.99
10
Figure 2. Diagram of the experimental equipment: (1) cathode, (2) anode, (3), catholyte reservoir, (4) anolyte reservoir, (5) pumps, (6, 7) multimeters, (8) current supply, (9) shunt, (10) charge integrator, (11) gas purge.
JChemEd.chem.wisc.edu • Vol. 76 No. 10 October 1999 • Journal of Chemical Education
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In the Laboratory
used for the solution inlet and outlet and for internal flow distributors. Polypropylene frames and EPDM (ethylenepoly(propylene)-diene monomer) gaskets are used. The interelectrode gap is ca. 6 mm. The 63 cm2 projected area electrodes are a lead dioxide anode, generated over lead, and a lead cathode. The anolyte and catholyte compartments are separated by an MA-3470 Sybron anionic membrane.2 Constant current is supplied by an AMEL 2055 power supply. High impedance digital multimeters are used to monitor cell voltage and cell current. A shunt 10 A/60 mV connected between the anode and the power supply allows accurate measurement of the current passed. Method of Analysis In this experiment, the oxidation of the benzyl alcohol was monitored by gas chromatography on a Hewlett Packard G1800A with the following parameters: injector temperature, 275 °C; mass detector temperature, 280 °C; initial temperature, 70 °C; initial time, 3 min; rate, 20 °C/min; final temperature, 270 °C; final time, 3 min; run time, 16 min; capillary column, cross-linked 5% phenyl methyl silicone, HP-5, 30 m × 0.25 mm × 0.25 µm; carrier gas, helium; flow 1.0 mL/min. Retention times were benzaldehyde, 4.50 min; benzyl alcohol, 5.40 min; p-methoxybenzaldehyde, 7.56 min. UV–visible absorption spectroscopy provides a very convenient experimental means for determining chromium(III) (6 ) and chromium(VI) concentrations in the electrochemical regeneration of the chromium(VI) solution. Spectroscopic methods are based upon application of the Beer–Lambert law: A = εbC which states that the measured absorbance, A, is directly proportional to the molar concentration, C, of the lightabsorbing species, where b is the path length (in cm) through the solution and ε is the molar absorptivity (in M{1 cm{1) at the absorption wavelength. When two light-absorbing species are present the individual absorbances are additive, assuming that no interaction occurs between them. During the electrochemical regeneration of chromium(VI), samples were taken to spectrophotometrically study3 the conversion of chromium(III) to chromium(VI) and the efficiency of the current flowing. At 580 nm only the Cr(III) absorbs, so its concentration can be measured directly. At 490 nm both Cr(III) and Cr(VI) absorb, so the contribution of Cr(III) concentration to the absorbance measurements must be taken into account. A Spectronic 20D visible spectrophotometer from the Milton Roy Co. was used. The ε-values were obtained from Beer–Lambert law plots for the separate standard chromium(III) and chromium(VI) solutions of known concentrations in 0.04 M sulfuric acid. The blank solution was 0.04 M sulfuric acid. Experimental Procedure
Oxidation of Benzyl Alcohol A 250-mL three-neck round-bottomed flask is fitted with a Liebig condenser and addition funnel. The flask is charged with benzyl alcohol (3.2 mL, 30 mmol) and THF (15 mL). Then, an aqueous solution (200 mL) 0.1 M in potassium dichromate and 0.4 M in sulfuric acid is added dropwise over a period of 50 min. The reaction mixture is magnetically 1424
Figure 3. Exploded view of the UA 63.03 electrochemical reactor: (1) backplate, (2) gasket, (3) distributor, (4) cathode, (5) frame, (6) membrane, (7) anode.
stirred for 30 min at 40 °C and then extracted with diethyl ether (3 × 30 mL). The aqueous layer, which contains the chromium(III) salts, is saved for subsequent electrochemical oxidation. The ether phase is washed with saturated NaHCO3 solution (3 × 15 mL) and water (3 × 15 mL), dried over anhydrous Na2SO4, and filtered, and the volume is adjusted to 100 mL with diethyl ether. It is analyzed by gas chromatography using p-methoxybenzaldehyde as internal standard. A 77% yield was obtained in the oxidation reaction of benzyl alcohol with potassium dichromate. Before electrochemical regeneration of chromium(VI), the salt solution containing chromium(III) was poured into a gas-washing bottle through which air was bubbled for 50 min, using a vacuum pump, to eliminate as much of the remaining dissolved diethyl ether as possible. This aqueous solution was then used as the anolyte for regeneration of the chromium(VI).
Generation of the Lead Dioxide Anode Before the regeneration of the chromium(VI) solution, a lead dioxide electrode was generated electrochemically from a lead electrode. The catholyte and anolyte containers were each filled with 200 mL of a 0.4 M sulfuric acid solution. A current density of 10 mA/cm2 was allowed to flow until 16 C/cm2 of electrode area was passed. Sulfuric acid solution from the anolyte reservoir was then drained off. The anodic reaction is the formation of a lead dioxide layer at the electrode; hydrogen evolution occurs on the cathode: Anode: Pb + 2H2O → PbO2 + 4H+ + 4e{ Cathode: 2H+ + 2e{ → H2
Regeneration of the Chromium(VI) Solution The chromium solution to be regenerated was ca. 0.20 M in chromium species and 0.4 M in sulfuric acid. This solution was introduced into the anolyte reservoir after the lead dioxide anode had been generated. The 0.4 M sulfuric acid solution was maintained in the catholyte reservoir. Electrolysis was carried out at a current density of 90 mA/cm 2 until the current circulating was 150% of the theoretical charge4 (11,580 C), and then at a current density of 50 mA/cm2 until 300% of the theoretical charge. One-
Journal of Chemical Education • Vol. 76 No. 10 October 1999 • JChemEd.chem.wisc.edu
In the Laboratory
Table 1. Experimental Results
Q a (%) ∆V/ V b
A490
A580
[Cr(III)]/ [Cr(VI)]/ Yieldc C.E.d (%) mol L{1 mol L{1 (%)
0
3.6
0.480
0.229
0.136
0.064e
0
—
25
3.6
0.551
0.194
0.115
0.075
15
22
50
3.6
0.654
0.165
0.098
0.091
28
27
75
3.8
0.784
0.142
0.084
0.111
38
31
100
3.9
0.841
0.106
0.063
0.120
54
28
150
4.1
0.984
0.064
0.038
0.143
72
26
200
4.2
1.138
0.036
0.021
0.166
85
26
250
4.2
1.225
0.021
0.012
0.179
91
23
300
4.2
1.305
0.010
0.006
0.190
96
21
aReferred
to the theoretical charge. bVoltage difference between electrodes. cMaterial yield: consumed Cr(III)/initial Cr(III). dCurrent efficiency referred to chromium (VI). eRemaining Cr (VI) from the previous chemical process.
milliliter samples were taken at 0, 25, 50, 75, 100, 150, 200, 250, and 300% of the theoretical charge and the potential difference between the electrodes was measured. The chromium(VI) solution was then ready to be used in a new experiment. Anode:
Cr(III) → Cr(VI) + 3e{ 2H2O → O2 + 4H+ + e{
Cathode: 2H+ + 2e{ → H2 The samples were diluted to 10 mL with water, the absorbances were measured in the spectrophotometer, and the concentrations of Cr(III) and Cr(VI) were calculated. Energy consumption5 was calculated from the value of the potential difference between the electrodes and current efficiency was calculated from the chromium(VI) concentration. Experimental Results The experimental results are shown in Table 1. The current efficiency of the electrolysis is low because the concentration of chromium(III) that was used is low for the current flowing and collateral reactions take place. In this case the oxidation of water is the most important collateral reaction. During electrolysis the chromium(VI) concentration increased and when 300% of the theoretical charge was passed
!
a 96% conversion of chromium(III) to chromium(VI) was achieved. The energy consumption was 7.97 kWh/kg when the density of the current passed was 90 mA/cm 2 and 14.48 kWh/kg when current density was 50 mA/cm2. Conclusions Because the laboratory practice consisted of two parts, it allowed the study of both a traditional organic electrochemical oxidation and other analytic methods such as gas chromatography and visible spectrophotometry to be used. This closed process in which there are no residues demonstrates a new aspect of redox reactions to students. Electrochemistry is a clear and simple way to solve some problems of environmental contamination. Notes 1. The current efficiency is defined as the ratio between the charge employed in the Cr(III) oxidation and the total charge passed. 2. Membrane supplied by the Ionac Chemical Company, a division of the Sybron Corporation, Birmingham, NJ. 3. The absorbance measurements were at 490 and 580 nm; these are represented as A490 and A580, respectively. 4. The theoretical charge is defined as the charge necessary for all the Cr(III) to pass to Cr(VI) without considering the existence of collateral reactions. 5. The energy consumption is defined as the energy necessary to obtain a certain quantity of the product; it is normally expressed in kWh/kg. The expression that permits the calculation is the following:
kWh = ∆V × z × 2680 M × C.E.(%) kg where ∆V is the average voltage difference between electrodes measured in volts, M is the molecular weight in grams of the potassium dichromate (M = 294 g), z is the number of electrons required to produce potassium dichromate from chromium(III) (in this case z = 6), and C.E.(%) is the current efficiency. For instance, at 150% of the theoretical charge, ∆V = 3.8 V, C.E.(%) = 26, and the energy consumption is 7.97 kWh/kg.
Literature Cited 1. 2. 3. 4. 5. 6.
Katz, S. A.; Salem, H. J. Appl. Toxicol. 1993, 13, 217. Jones, R. B.; Dreisbach, J. H. J. Chem. Educ. 1994, 71, 158. Alcedo, J. A.; Wetterhahn, K. Int. Rev. Exp. Pathol. 1990, 3, 85. Schwab, D.; Martinez, P. J. Chem. Educ. 1989, 66, 528. Pamplin, K. L.; Johnson, D. C. J. Electrochem. Soc. 1996, 7, 143. Pandey, S.; Powell, J. R.; MacHale, M. E. R.; Acree, W. E. Jr. J. Chem. Educ. 1997, 74, 848.
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