Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 2(10), 55-59, October (2012) Res.J.Chem. Sci. International Science Congress Association 55 Studies Relating to Cathodic Reduction of Hypochlorite in Neutral Chloride Solutions – Used in Chlorate ProcessesVasudevan S. CSIR - Central Electrochemical Research Institute, Karaikudi - 630 006, INDIAAvailable online at: www.isca.in Received 20th April 2012, revised 26th April 2012, accepted 25th October 2012Abstract The inhibition effect of the chromium hydroxide film for the reduction of hypochlorite has so far been studied on gold, platinum and iron electrodes in sodium hydroxide and sodium chlorate solution. But there is no reports were available in the case of alkaline earth metal chlorates. In order to more clearly distinguish, the effects of the film, the addition of chromate and its effect on the cathodic reduction of hypochlorite in sodium and alkaline earth metal chlorates solutions, the studies were carried out in platinum electrode. It was found that the addition of chromate to the sodium chlorate solution suppresses the hypochlorite reduction by forming a thin layer of chromium hydroxide on the electrode surface. In the case of magnesium, strontium and barium chlorate solutions the hypochlorite reduction was suppressed even without the addition of chromate in the electrolyte due to the formation of corresponding metal hydroxides on the electrode surface. Key words: cathodic reduction, chromate, chlorate, cyclic voltammetry, hypochlorite. Introduction Cathodic hydrogen and anodic chlorine evolution reactions, from neutral chloride media are primary reactions in the technology of chlorate production. Chromate and its electrochemical reduction are of practical importance in the industrial process for the production of alkali metal chlorates where chromate is added to the electrolyte, since this is empirically known to prevent the hypochlorite being reduced to chloride ions at the cathode surface. It was found that a thin film of Cr(OH) is formed, which hinders almost completely the hypochlorite reduction reaction at the cathode surface. A widely accepted explanation of this effect is that Cr(OH) acts as diaphragm on the cathode1-7. It is suggested that the reduction of hypochlorite ions is prevented by an adverse potential gradient, which hinders the transport of the negatively charged ions into the cathode covered by a ‘diaphragm’. It was found that the reduction of chlorate to be almost completely suppressed on a polymer coated steel cathode and the cathodic losses in the absence of chromate have been found to decrease with an increased current density8,9. All these results can be explained by the effect of an increased potential gradient and seem to support the ‘diaphragm’ theory. It is reported that6,7 the effect of chromate addition on the hypochlorite reduction at a platinum electrode in alkaline solution obtain similar results. They found that reduced chromate formed a Cr(III) on the electrode surface, which suppressed not only the reduction of hypochlorite but also the reduction of several other negatively charged ions and uncharged molecules. This effect has also been observed with much lower concentrations of chromate than generally used in the chlorate process. However, the effect of addition of chromate for hypochlorite reduction has not been studied extensively in alkaline earth metal chlorates. Hence, the cyclic voltammetric studies have been undertaken with sodium, magnesium, strontium and barium chlorides. Material and MethodsA Princeton Applied Research (PAR) model 371 Potentiostat-Galvanostat and a Wenking Scan Generator (VSG 83) in combination with a Kontron digital multimeter for potential indicating and HIL 2103 multimeter for current indication were used. The curves were recorded using a X-Y/t recorder (Rikadenki, Japan). All experiments were carried out in an ordinary three-electrode glass cell, under nitrogen atmosphere, which had an external jacket for circulation of water to maintain electrolyte temperature. Platinum wire embedded in Teflon (area = 0.126 cm) and platinum foil was used as working electrode and counter electrode respectively. Standard Calomel Electrode (SCE) was used as reference electrode. The working electrode was polished with alumina polish on micro cloth for 2 minutes and cleaned with distilled water. This was then followed by the ultrasonic cleaning for 5 minutes to remove any residual alumina particles from the surface and finally rinsed with acetone. The experiments were carried out in 2M sodium, magnesium, strontium and barium chlorate (CDH) solutions, of 0.1L, at the scan rate of 50mv.s-1. NaCrO supplied by Merck was used. The hypochlorite was added to the electrolyte and its concentration was determined by potentiometric titration. Cyclic Voltammograms were recorded at a temperature of 25±1ºC between the potential range of +0.9V to –1.0V (vs SCE). Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 2(10), 55-59, October (2012) Res. J. Chem. Sci. International Science Congress Association 56 Results and DiscussionDuring the electrolytic oxidation of chloride solution to obtain chlorate, the primary products of the anode and cathode reactions are chlorine and alkali respectively, which react in the bulk electrolyte between the electrodes to form hypochlorous acid and hypochlorite. These are subsequently reacting chemically to form chlorate. The main reactions in the chlorate electrolysis process can thus be written as: At the anode, (chlorine is formed) 2Cl Cl + 2e (1) At the cathode, (hydroxyl ions and hydrogen are formed) 2HO + 2e 2OH + H2 (2) At the bulk, chlorine ions formed at the anode react rapidly with water to form hypochlorous acid, Cl + HO HClO + Cl- (3) Since the electrodes are close together and have no diaphragm, the chlorine can react with hydroxyl ions formed at the cathode to produce hypochlorite ions, Cl + 2OH OCl + Cl + HO (4) As a result, the solution around the anode becomes strongly acidic and the solution in contact with the cathode is alkaline. In addition to this pH gradient, the concentration of hypochlorous acid also establishes a gradient relatively high around the anode and relatively low around the cathode. It is usually assumed that chlorate is formed by the reaction of hypochlorite ion by free hypochlorous acid (reaction 5) when pH is 6.0 to 6.5. However when pH is slightly alkaline the electrochemical formation of chlorate through discharge of the hypochlorite 2HOCl + ClO ClO + 2H + 2Cl (5) 6ClO + 3HO 2ClO + 6H + 4Cl + 1.5 O + 6e (6) ion at a potential equal to that for discharge of the chloride ion (reaction 6). According to reactions (5) and (6) chlorate may be formed in two ways, either by a purely chemical reaction of the hypochlorite ion with hypochlourous acid in the bulk solution (chemical chlorate formation) or by electrochemical oxidation of the hypochlorite at the anode under simultaneous oxygen evolution (anodic chlorate formation). If all chlorate is formed by the chemical reaction (5), 6 and 12 Faradays are consumed in the oxidation of one mole chloride to chlorate in the case of alkali and alkaline earth metal respectively. This is said to correspond to a maximum current efficiency of 100%. If all chlorate is formed by reaction (6) the current efficiency cannot be higher than 66.5% since one third of the current is used for the evolution of oxygen. In other words the reaction (6) can be considered as anodic loss reaction. It has been established that the reaction (5) is more plausible than by reaction (6). The most favorable operating conditions for reaction (5) are a temperature of 333 to 348K with a pH of 6.0 – 6.5. Under these conditions, a number of subsidiary or parasitic reactions occur, thereby reducing the current efficiency seriously. The main parasitic reactions are the cathodic reduction of hypochlorite and chlorate, ClO + HO + 2e Cl +2OH (7) ClO + 2e Cl- + ½ O (8) ClO + 3HO + 6e Cl + 6OH (9) In accordance with the well known chemical properties of these ions, hypochlorite is much more easily reduced than chlorate. Reaction (8) is much slower than chlorate formation, but this reaction plays a negligible role under industrial conditions, unless determental corrosion products from anode coating or impregnation are present. In the case of alkaline earth metal chlorides the formation of metal hydroxide will be encountered due to their low solubility. According to M-HO Pourbaix diagram12, hydroxide formation is predicted by the following reaction 2+ + 2OH M(OH) (10) log [M2+] = log K -2pH Where M is magnesium, strontium or barium. The magnesium, strontium and barium hydroxide forms a layer over the cathode surface and hinder cathodic loss reactions. The above reaction (10) was not possible in the case of alkali metals due to their higher solubility of corresponding metal hydroxides. To study and overcome the cathodic loss reactions and to increase the current efficiency for alkaline earth metal chlorate formation, the effect of chromate on cathodic reaction has been carried out in electrolytes like magnesium, strontium and barium chlorides at 303K and compared with sodium and potassium chloride electrolytes. Figure 1 shows a typical cyclic voltammograms with a platinum electrode in 2M magnesium chlorate solutions with and without the addition of sodium chromate. From the figure it is observed that the cathodic peak for magnesium chlorate at - 0.20V appear only when sodium chromate is present. This peak is due to the reduction of chromate to Cr (III) was reported elsewhere10, which in turn forms chromium (III) hydroxide or chromium (III) oxide on the electrode surface. The decrease of current after the peak potential indicates the formation of this chromium (III) hydroxide or chromium (III) oxide film preventing its further growth and the anodic peak in the reverse direction corresponds to the oxidation and thereby the dissolution of the film10,11. The peaks clearly indicate the irreversible nature of the reaction. From the curve (1) of figure 1, it is seen that no reduction peak for chlorate in the absence of chromate has taken place in the cathodic side under these conditions12. Figure 2 shows the voltammogram when hypochlorite is present in the sodium chlorate electrolyte. In the absence of chromate (curve 1 of figure 2) a cathodic current for hypochlorite reduction is obtained. When chromate is present in the electrolyte, at different concentrations (curves 2, 3 and 4), the Research Journal of Chemical Sciences ____ _ Vol. 2(10), 55-59, October (2012) International Science Congress Association current fo r hypochlorite reduction first increases upto after which it decreases and finally reaching zero current. It is clear from the figure 1 that the cathodic current starts decreasing at the potential at which the reduction of chromate takes place. In t he anodic direction at +0.60V hypochlorite reduction again takes place. This indicates that the inhibiting chromium (III) hydroxide or chromium (III) oxide layer has disappeared (figure 1), because the chromium hydroxide layer has dissolved electrochemical ly at that potential, thus allowing hypochlorite reduction. Curve (5) of figure 2 shows the voltammograms of hypochlorite of higher concentration (1.5 g/l) in the electrolyte. The curve obtained is similar to that of curves, 3 and 4 of figure 2 (i.e.) init ially hypochlorite reduction current increases after which it decreases and finally reaching zero current. Figure 3 shows the voltammogram when hypochlorite (0.5, 1.0 and 1.5g/l) is present in the magnesium chlorate electrolyte. From the figure 3, it was found that no hypochlorite reduction peak was observed even in the absence of chromate. This is due to the formation of Mg(OH) on the electrode surface, which is a less efficient alternative to chromate addition in the chlorate production process. The sam e trend was observed in the case of strontium and barium chlorate electrolytes also. Figure 4 show the effect of the sweep rate for the reduction of hypochlorite of concentration of 1.0g/l. The deviation with different sweep rates is due to the continuous consumption of hypochlorite in the homogeneous chlorate formation reaction, which lowers the hypochlorite concentration between successive runs10. Figure–1 Cyclic voltammogram for magnesium chlorate electrolyte at 338Kand at pH 6 with a sweep rate of 50mv s speed of 2000rpm. 1 .Background without chromate 2g l-1 chromate _ _____________________________________________ _ International Science Congress Association r hypochlorite reduction first increases upto – 0.4V after which it decreases and finally reaching zero current. It is clear from the figure 1 that the cathodic current starts decreasing at the potential at which the reduction of chromate takes place. he anodic direction at +0.60V hypochlorite reduction again takes place. This indicates that the inhibiting chromium (III) hydroxide or chromium (III) oxide layer has disappeared (figure 1), because the chromium hydroxide layer has dissolved ly at that potential, thus allowing hypochlorite reduction. Curve (5) of figure 2 shows the voltammograms of hypochlorite of higher concentration (1.5 g/l) in the electrolyte. The curve obtained is similar to that of curves, 3 and 4 of figure ially hypochlorite reduction current increases after which it decreases and finally reaching zero current. shows the voltammogram when hypochlorite (0.5, 1.0 1.5g/l) is present in the magnesium chlorate electrolyte. found that no hypochlorite reduction peak was observed even in the absence of chromate. This is due on the electrode surface, which is a less efficient alternative to chromate addition in the chlorate e trend was observed in the case of strontium and barium chlorate electrolytes also. show the effect of the sweep rate for the reduction of hypochlorite of concentration of 1.0g/l. The deviation with different sweep rates is due to the continuous consumption of hypochlorite in the homogeneous chlorate formation reaction, hypochlorite concentration between successive Cyclic voltammogram for magnesium chlorate electrolyte at 338Kand at pH 6 with a sweep rate of 50mv s -1 and rotation .Background without chromate 2.With Figure – Cyclic voltammogram for sodium chlorate electrolyte with1.0g/l of hypochlorite at 338 K and at pH6 with a sweep rate of 50mv.s-1. 1 .without chromate, 2 g/l of chromate, 4. 3 g/l of chromate, hypochlorite Figure – Cyclic Voltammogram for magnesium chlorate electrolyte at 338 K and at pH 6 with a sweep rate of 50mv s hypochlorate, 2. 1.0g/l of hypochlorite and hypochlorite _ ________ ISSN 2231-606X Res. J. Chem. Sci. 57 – 2 Cyclic voltammogram for sodium chlorate electrolyte with1.0g/l of hypochlorite at 338 K and at pH6 with a sweep .without chromate, 2. 1.0 g/l of chromate, 3. 3 g/l of chromate, 5. 1.5g/l of hypochlorite – 3 Cyclic Voltammogram for magnesium chlorate electrolyte at 338 K and at pH 6 with a sweep rate of 50mv s -11. 0.5 g/l of 1.0g/l of hypochlorite and 3. 1.5g/l of hypochlorite Research Journal of Chemical Sciences ____ _ Vol. 2(10), 55-59, October (2012) International Science Congress Association Figure–4 Cyclic Voltammogram for magnesium chlorat 338 K and at pH 6 with 0.5 g/l of hypochlorite, 1. 100 mv s 150 mv s-1 and 200 mv s-1 It is reported13 that the suppression of hypochlorite reduction is more effective when chromate is used as addition agent, even though the thickness of chromium hydroxide film is six orders of magnitude thinner than Ca(OH) film (when calcium salt is used as addition agen t). But a porous film of alkaline earth metal hydroxides could be an interesting alternative to chromate addition. Hence, in the case of preparation of alkaline earth metal chlorates, the cathodes become covered with alkaline earth metal hydroxide layer du e to the precipitation of M (where M = Mg, Ca, Sr or Ba) ions with OH low solubility. For example, according to Mg diagram14 , magnesium hydroxide formation is predicted by, Mg2+ + 2OH Mg(OH) log [Mg2+] = log K -2pH This visible layer gives further practical support to the popular concept of a “diaphragm” in the technology of chlorate production in the case of alkaline earth metals. Hence, from these studies it is concluded that the addition of chromate to the electrolyte in the sodium chlorate process suppresses the hypochlorite reduction by forming a thin layer of chromium hydroxide on the electrode surface, and in the case of alkal ine earth metal chlorates, the formation of metal hydroxide on the electrode surface suppresses the hypochlorite reduction. It is confirmed that, the higher current efficiencies were achieved in our lab scale and pilot scale studies for the electrochemical preparation of chlorates, bromates and perchlorate15-25 without the addition of any chromate. Conclusion It is concluded that, in the case of sodium chlorate electrolyte, the addition of chromate suppresses the hypochlorite reduction _ _____________________________________________ _ International Science Congress Association Cyclic Voltammogram for magnesium chlorat e electrolyte at 338 K and at pH 6 with 0.5 g/l of hypochlorite, 1. 100 mv s -1, that the suppression of hypochlorite reduction is more effective when chromate is used as addition agent, even though the thickness of chromium hydroxide film is six orders film (when calcium salt is t). But a porous film of alkaline earth metal hydroxides could be an interesting alternative to chromate addition. Hence, in the case of preparation of alkaline earth metal chlorates, the cathodes become covered with alkaline e to the precipitation of M 2+ (where M = Mg, Ca, Sr or Ba) ions with OH ions due to their low solubility. For example, according to Mg -HO Pourbaix , magnesium hydroxide formation is predicted by, (11) (12) This visible layer gives further practical support to the popular concept of a “diaphragm” in the technology of chlorate production in the case of alkaline earth metals. Hence, from these studies it is concluded that the addition of chromate to the electrolyte in the sodium chlorate process suppresses the hypochlorite reduction by forming a thin layer of chromium hydroxide on the electrode surface, and in the case of ine earth metal chlorates, the formation of metal hydroxide on the electrode surface suppresses the hypochlorite reduction. It is confirmed that, the higher current efficiencies were achieved in our lab scale and pilot scale studies for the preparation of chlorates, bromates and without the addition of any chromate. It is concluded that, in the case of sodium chlorate electrolyte, the addition of chromate suppresses the hypochlorite reduction by forming a thin layer of chromium hydroxide on the electrode surface, which prevents the hypochlorite reduction. In the case of magnesium, strontium and barium chlorates, the cathodes become covered with metal hydroxide, which will act as a porous film and prevent the hypochlorite reduction on the cathode surface, which can avoid the addition of chromate to the electrolyte makin g the chlorate process echo friendly. Acknowledgement The author wishes to express their sincere gratitude to The Director, Central Electrochemical Research Institute, Karaikudi for his permission to publish this paper. References1.Wagner C., The Cathodi c Reduction of Anions and the Anodic Oxidation of Cations, 181- 185 (1954)2. Tilak B.V., Viswanathan K., and Rader C.G. mechanism of sodium chlorate formation, Electrochem. 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