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Identification of electrooxidation products of thiocyanate ion in acidic solutions by thin-layer spectroelectrochemistry
J. Electroonal. Chem., 177 (1984) 311-315 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
311
Preliminary note IDENTIFICATION OF ELECTROOXIDATION PRODUCTS OF THIOCYANATE ION IN ACIDIC SOLUTIONS BY THIN-LAYER SPECTROELECTROCHEMISTRY
EIKI ITABASHI Miyagi
University
of Education,
Aoba,
Aramaki,
Sendai
980
(Japan)
(Received 31st July 1984)
INTRODUCTION
The electrochemical oxidation of thiocyanate ions has been investigated for many years in aqueous as well as non-aqueous solutions. The oxidation products of SNC in non-aqueous solutions have been identified as thiocyanogen (SCN), and/or trithiocyanate ion (SCN); [ 121. The formation of (SCN); was also proposed in molten thiocyanate medium [ 31. Different results have been given concerning the products from the oxidation of SCN in aqueous solutions. Some authors [4,5] supposed them to consist of sulfate and cyanide ions, while others [6,7] proposed that they are essentially parathiocyanogen (SCN)* . Thiocyanogen has been proposed as an intermediate in the electrooxidation of SCN- [4,5] and the chemical oxidation of SCNby a variety of reagents [8-111, and it was suggested that thiocyanogen underwent hydrolysis to yield sulfate and cyanide ions. However, the actual hydrolysis mechanism of (SCN)* is controversial [ 9,121, and no detailed study has been reported on the oxidation products of SCN- in aqueous solution. This paper presents direct evidence for the oxidation products of SCN- by the combined use of voltammetric and spectroelectrochemical methods at an optically transparent thin-layer electrode cell. EXPERIMENTAL
All electrochemical and spectroelectrochemical experiments were carried out with a potentiostat equipped with a digital coulometer. The UV spectra were measured using a Hitachi 556 spectrophotometer. The gold minigrid optically transparent thin-layer electrode (OTTLE) cell was constructed by clamping a gold minigrid (Buckbee-Mears Co., St. Paul, MN; 200 lines/cm) and a Teflon tape (Dilectrix Corp., Farmingdale, NY) between two quartz slides [ 131. The OTTLE cell had a cell thickness of 0.17 mm. The counterelectrode was a platinum wire electrode. The potential of the working electrode was measured against a saturated calomel electrode (SCE).
312
All chemicals were of analytical reagent grade and were used without further purification. All solutions were made up with distilled water and were deaerated with high-purity nitrogen. RESULTS
Typical absorption spectra before and after the oxidation of SCN- under potential step control in acidic solution at the OTTLE cell are shown in Fig. 1 The acidic solution containing the oxidation products of SCN- exhibits a sym, metrical peak at 320 nm and a broad peak 420 + 5 nm. These peaks disappear completely upon electroreduction of the oxidized species of SCN- , and the resultant spectrum agrees with that of thiocyanate solution before the electrooxidation of SCN- . Figure 2 presents the cyclic voltammograms and the corresponding absorbance-potential curves of SCN- at the OTTLE cell. The anodic current increases up to the upper switching potential since high concentrations of SCN- are employed, whereas the cathodic wave gives a peak near +0.27 V. Coulometric experiments under thin-layer cyclic voltammetric conditions showed that the reduction efficiencies of the oxidized species of SCN- in 0.1 mol dmm3 NaSCN and 0.1 mol dmm3 HC104 solution at a sweep rate of 2 mV s-l were 96, 93 and 92% at the upper switching potentials of +0.500, +0.520 and +0.540 V, respectively, at which no anodic dissolution of the gold minigrid electrode took place during the positive scan. This suggests
300
400
500
Wavelength/nm
Fig. 1. Absorption spectra before (a) and after the oxidation of SCN- at +0.500 V (b) in 0.1 mol dm-3 NaSCN and 0.1 mol dm-” HClO, . Cell thickness, 0.017 cm; charge for the oxidation of SCN-, ca. 20 mC.
313
L
0
0.2
0.4
0 E/Vvs.SCE
0.2
0.4
Fig. 2. Thin-layer cyclic voltammograms (A) and absorbance-potential curves (B) monitored at 320 nm in 0.1 mol dmd3 NaSCN and 0.1 mol drnm3 HClO, . Sweep rate, 2 mV s-’ ; cell thickness, 0.017 cm. Upper switching potential: (1) +0.500; (2) +0.520; (3) +0.540 V.
that the oxidation products of SCN- are stable during the time of the experiment. The absorbance monitored at 320 nm increases as the upper switching potential becomes more positive, reaches a maximum during the negative scan at a potential near the zero current axis on the thin-layer cyclic voltammogram, and falls to zero at the potential at which the cathodic current drops off to an almost zero level. The plots of the absorbance vs. anodic charge (A vs. Qa) for the oxidation of SCN- under potential step control are shown in Fig. 3. The absorbance increases with increase in anodic charge for the oxidation of SCN- . The rising portion of the A vs. Qa plots is non-linear. This could arise from the electrical double-layer charging of the OTTLE cell as well as from the edge effect [ 141, i.e. the ohmic polarization effect in a thin-layer cell. DISCUSSION
The electrooxidation mechanism of SCN- in acidic solutions containing a large excess of SCN- can be well explained by the following sequence of reactions: SCN- -
SCN’ + e-
SCN’ + SCN2 (SCN);-
-
=
(1)
(SCN);-
(2)
(SCN); + SCN-
(3)
Anodic
charga/mC
Fig. 3. Plots of absorbance vs. anodic charge for the oxidation of SCN- at +0.500 V in 0.1 mol dmd3 NaSCN and 0.1 mol dm+ HClO, . Cell thickness, 0.017 cm. Wavelength monitored: (1) 320; (2) 420 nm.
The cathodic wave is attributed to a two-electron reduction of trithiocyanate ion (SCN); to give thiocyanate ions. The SCN’ radical corresponding to the initial oxidation product of SCN- reacts rapidly with SCN- in the bulk of solution to yield the anion radical (SCN);- , which was first identified in pulse radiolysis of aqueous thiocyanate solution [ 151 and then was postulated as an intermediate in the chemical oxidation of SCN- by a variety of reagents [S--11] The rate constant for the formation of (SCN);and its formation constant have been determined to be 7 X lo9 mall’ dm3 s-l and 2 X lo5 mol-’ dm’ at 22”C, respectively [ 151. The anion radical (SCN);-, which exhibits a transient absorption with a peak at 470 nm, has been shown to decay in a diffusion-controlled process following a second-order rate law [ 151. The decay of (SCN);has been attributed to the following reaction: 2 (SCN);-
-(SCN)2
+ 2 SCN-
If the disproportionation reaction (4) proceeds during the anodic oxidation of SCN- , the acidic solution containing oxidation products of SCN- will show an absorption peak at 283 to 285 nm, which is assigned to thiocyanagen, (SCN)? [ 1,161. (SCN)* is stable in anhydrous solvents such as acetonitrile [ 11 and acetic acid [ 161, but is hydrolysed rapidly in aqueous solutions to yield sulfate and cyanide ions as the main products [ 9,121. The acidic solu-
(4)
315
tion containing oxidized species of SCN- exhibits an absorption peak at 320 nm. Cauquis and Pierre [l] followed the absorption spectrum of the acetonitrile solution containing the electrooxidation products of SCN- and observed two peaks at 320 and 283 nm, which were assigned to (SCN), and (SCN), , respectively. Provided the assignment of the absorption of (SCN), is correct, it is reasonable to suppose that the oxidation product of SCN in acidic solution corresponds to (SCN), . The oxidized solution of SCN- also shows a broad peak at 420 nm. This peak could also be due to (SCN), , because the optical density monitored at 420 nm begins to rise immediately upon the initiation of the controlled potential oxidation of SCN- , and also the thin-layer cyclic voltammogram gives a single cathodic peak. The slope of the linear portion of the plot of A vs. Qa in Fig. 3 permits the determination of the molar absorptivity of (SCN); . The values of the molar absorptivity of (SCN); at 420 and 320 nm were determined to be 6.4 X lo2 and 2.2 X lo4 mol-’ dm3 cm-‘, respectively. It seems likely that (SCN); is analogous to the stable trihalide ion X 3, although the structure of (SCN)J is uncertain. The yield of (SCN); produced in acidic thiocyanate solutions under potential step control decreases drastically with decrease in hydrogen ion concentration below 0.04 mol dmm3. This suggests that (SCN); is stabilized by reacting with hydrogen ion to produce an acid molecule such as H(SCN)B. It has been found that the yield of (SCN); also decreases with increase in pH of the solution [ 171. Previous investigations [ 4-71 have shown that the anodic process of SCN- in aqueous solutions proceeds totally irreversibly to yield some species which are not reduced during the negative scan. This irreversibility could result from the decomposition of (SCN); , because the previous experiments were carried out in neutral and non-buffered electrolyte solutions. The identification of the decomposition products of (SCN); is now in progress. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
G. Cauquis and G. Pierre, Bull. Sot. Chim. Fr., (1972) 2244. M.E. Martins, C. Castellano, A.J. Calandra and A.J. Arvia, J. Electroanal. Chem., 92 (1978) 45. F. Pucciarelli, P. Cescon and M. Heyrovsky, J. Electrochem. Sot., 126 (1979) 972. R. Gaugin, Anal. Chim. Acta, 5 (1951) 200. M.M. Nicholson, Anal. Chem., 31 (1959) 128. F. See1 and E. Miiller, Chem. Ber., 88 (1955) 1747. D.A. Holtzen and A.S. Allen, J. Electroanal. Chem., 69 (1974) 153. R.H. Betts and F.S. Dainton, J. Am. Chem. Sot., 76 (1953) 5721. G. Stedman and P.A.E. Whincup, J. Chem. Sot. A, (1969) 1145. F.T.T. Ng and P.M. Henry, Can. J. Chem., 53 (1975) 3319. D.M. Stanbury, W.K. Wilmarth, S. Khalaf, H.N. PO and J.E. Byrd, Inorg. Chem., 19 (1980) 2715, M. Schoneshofer, G. Beck and A. Henglein, Ber. Bunsenges. Phys. Chem., 74 (1970) 1011 T.P. DeAngelis and W.R. Heineman, J. Chem. Educ., 53 (1976) 594. M. Petek, T.E. Neal and R.W. Murray, Anal. Chem., 43 (1971) 1069. J.H. Baxendale, P.L.T. Bevan and D.A. Stott, Trans. Faraday Sot., 64 (1968)2389. C.E. Vanderzee and A.S. Quist, Inorg. Chem., 6 (1966) 1238. D. Duonghang, J. Ramsden and M. Griitzel, J. Am. Chem. Sot., 104 (1982) 2977.
311
Preliminary note IDENTIFICATION OF ELECTROOXIDATION PRODUCTS OF THIOCYANATE ION IN ACIDIC SOLUTIONS BY THIN-LAYER SPECTROELECTROCHEMISTRY
EIKI ITABASHI Miyagi
University
of Education,
Aoba,
Aramaki,
Sendai
980
(Japan)
(Received 31st July 1984)
INTRODUCTION
The electrochemical oxidation of thiocyanate ions has been investigated for many years in aqueous as well as non-aqueous solutions. The oxidation products of SNC in non-aqueous solutions have been identified as thiocyanogen (SCN), and/or trithiocyanate ion (SCN); [ 121. The formation of (SCN); was also proposed in molten thiocyanate medium [ 31. Different results have been given concerning the products from the oxidation of SCN in aqueous solutions. Some authors [4,5] supposed them to consist of sulfate and cyanide ions, while others [6,7] proposed that they are essentially parathiocyanogen (SCN)* . Thiocyanogen has been proposed as an intermediate in the electrooxidation of SCN- [4,5] and the chemical oxidation of SCNby a variety of reagents [8-111, and it was suggested that thiocyanogen underwent hydrolysis to yield sulfate and cyanide ions. However, the actual hydrolysis mechanism of (SCN)* is controversial [ 9,121, and no detailed study has been reported on the oxidation products of SCN- in aqueous solution. This paper presents direct evidence for the oxidation products of SCN- by the combined use of voltammetric and spectroelectrochemical methods at an optically transparent thin-layer electrode cell. EXPERIMENTAL
All electrochemical and spectroelectrochemical experiments were carried out with a potentiostat equipped with a digital coulometer. The UV spectra were measured using a Hitachi 556 spectrophotometer. The gold minigrid optically transparent thin-layer electrode (OTTLE) cell was constructed by clamping a gold minigrid (Buckbee-Mears Co., St. Paul, MN; 200 lines/cm) and a Teflon tape (Dilectrix Corp., Farmingdale, NY) between two quartz slides [ 131. The OTTLE cell had a cell thickness of 0.17 mm. The counterelectrode was a platinum wire electrode. The potential of the working electrode was measured against a saturated calomel electrode (SCE).
312
All chemicals were of analytical reagent grade and were used without further purification. All solutions were made up with distilled water and were deaerated with high-purity nitrogen. RESULTS
Typical absorption spectra before and after the oxidation of SCN- under potential step control in acidic solution at the OTTLE cell are shown in Fig. 1 The acidic solution containing the oxidation products of SCN- exhibits a sym, metrical peak at 320 nm and a broad peak 420 + 5 nm. These peaks disappear completely upon electroreduction of the oxidized species of SCN- , and the resultant spectrum agrees with that of thiocyanate solution before the electrooxidation of SCN- . Figure 2 presents the cyclic voltammograms and the corresponding absorbance-potential curves of SCN- at the OTTLE cell. The anodic current increases up to the upper switching potential since high concentrations of SCN- are employed, whereas the cathodic wave gives a peak near +0.27 V. Coulometric experiments under thin-layer cyclic voltammetric conditions showed that the reduction efficiencies of the oxidized species of SCN- in 0.1 mol dmm3 NaSCN and 0.1 mol dmm3 HC104 solution at a sweep rate of 2 mV s-l were 96, 93 and 92% at the upper switching potentials of +0.500, +0.520 and +0.540 V, respectively, at which no anodic dissolution of the gold minigrid electrode took place during the positive scan. This suggests
300
400
500
Wavelength/nm
Fig. 1. Absorption spectra before (a) and after the oxidation of SCN- at +0.500 V (b) in 0.1 mol dm-3 NaSCN and 0.1 mol dm-” HClO, . Cell thickness, 0.017 cm; charge for the oxidation of SCN-, ca. 20 mC.
313
L
0
0.2
0.4
0 E/Vvs.SCE
0.2
0.4
Fig. 2. Thin-layer cyclic voltammograms (A) and absorbance-potential curves (B) monitored at 320 nm in 0.1 mol dmd3 NaSCN and 0.1 mol drnm3 HClO, . Sweep rate, 2 mV s-’ ; cell thickness, 0.017 cm. Upper switching potential: (1) +0.500; (2) +0.520; (3) +0.540 V.
that the oxidation products of SCN- are stable during the time of the experiment. The absorbance monitored at 320 nm increases as the upper switching potential becomes more positive, reaches a maximum during the negative scan at a potential near the zero current axis on the thin-layer cyclic voltammogram, and falls to zero at the potential at which the cathodic current drops off to an almost zero level. The plots of the absorbance vs. anodic charge (A vs. Qa) for the oxidation of SCN- under potential step control are shown in Fig. 3. The absorbance increases with increase in anodic charge for the oxidation of SCN- . The rising portion of the A vs. Qa plots is non-linear. This could arise from the electrical double-layer charging of the OTTLE cell as well as from the edge effect [ 141, i.e. the ohmic polarization effect in a thin-layer cell. DISCUSSION
The electrooxidation mechanism of SCN- in acidic solutions containing a large excess of SCN- can be well explained by the following sequence of reactions: SCN- -
SCN’ + e-
SCN’ + SCN2 (SCN);-
-
=
(1)
(SCN);-
(2)
(SCN); + SCN-
(3)
Anodic
charga/mC
Fig. 3. Plots of absorbance vs. anodic charge for the oxidation of SCN- at +0.500 V in 0.1 mol dmd3 NaSCN and 0.1 mol dm+ HClO, . Cell thickness, 0.017 cm. Wavelength monitored: (1) 320; (2) 420 nm.
The cathodic wave is attributed to a two-electron reduction of trithiocyanate ion (SCN); to give thiocyanate ions. The SCN’ radical corresponding to the initial oxidation product of SCN- reacts rapidly with SCN- in the bulk of solution to yield the anion radical (SCN);- , which was first identified in pulse radiolysis of aqueous thiocyanate solution [ 151 and then was postulated as an intermediate in the chemical oxidation of SCN- by a variety of reagents [S--11] The rate constant for the formation of (SCN);and its formation constant have been determined to be 7 X lo9 mall’ dm3 s-l and 2 X lo5 mol-’ dm’ at 22”C, respectively [ 151. The anion radical (SCN);-, which exhibits a transient absorption with a peak at 470 nm, has been shown to decay in a diffusion-controlled process following a second-order rate law [ 151. The decay of (SCN);has been attributed to the following reaction: 2 (SCN);-
-(SCN)2
+ 2 SCN-
If the disproportionation reaction (4) proceeds during the anodic oxidation of SCN- , the acidic solution containing oxidation products of SCN- will show an absorption peak at 283 to 285 nm, which is assigned to thiocyanagen, (SCN)? [ 1,161. (SCN)* is stable in anhydrous solvents such as acetonitrile [ 11 and acetic acid [ 161, but is hydrolysed rapidly in aqueous solutions to yield sulfate and cyanide ions as the main products [ 9,121. The acidic solu-
(4)
315
tion containing oxidized species of SCN- exhibits an absorption peak at 320 nm. Cauquis and Pierre [l] followed the absorption spectrum of the acetonitrile solution containing the electrooxidation products of SCN- and observed two peaks at 320 and 283 nm, which were assigned to (SCN), and (SCN), , respectively. Provided the assignment of the absorption of (SCN), is correct, it is reasonable to suppose that the oxidation product of SCN in acidic solution corresponds to (SCN), . The oxidized solution of SCN- also shows a broad peak at 420 nm. This peak could also be due to (SCN), , because the optical density monitored at 420 nm begins to rise immediately upon the initiation of the controlled potential oxidation of SCN- , and also the thin-layer cyclic voltammogram gives a single cathodic peak. The slope of the linear portion of the plot of A vs. Qa in Fig. 3 permits the determination of the molar absorptivity of (SCN); . The values of the molar absorptivity of (SCN); at 420 and 320 nm were determined to be 6.4 X lo2 and 2.2 X lo4 mol-’ dm3 cm-‘, respectively. It seems likely that (SCN); is analogous to the stable trihalide ion X 3, although the structure of (SCN)J is uncertain. The yield of (SCN); produced in acidic thiocyanate solutions under potential step control decreases drastically with decrease in hydrogen ion concentration below 0.04 mol dmm3. This suggests that (SCN); is stabilized by reacting with hydrogen ion to produce an acid molecule such as H(SCN)B. It has been found that the yield of (SCN); also decreases with increase in pH of the solution [ 171. Previous investigations [ 4-71 have shown that the anodic process of SCN- in aqueous solutions proceeds totally irreversibly to yield some species which are not reduced during the negative scan. This irreversibility could result from the decomposition of (SCN); , because the previous experiments were carried out in neutral and non-buffered electrolyte solutions. The identification of the decomposition products of (SCN); is now in progress. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
G. Cauquis and G. Pierre, Bull. Sot. Chim. Fr., (1972) 2244. M.E. Martins, C. Castellano, A.J. Calandra and A.J. Arvia, J. Electroanal. Chem., 92 (1978) 45. F. Pucciarelli, P. Cescon and M. Heyrovsky, J. Electrochem. Sot., 126 (1979) 972. R. Gaugin, Anal. Chim. Acta, 5 (1951) 200. M.M. Nicholson, Anal. Chem., 31 (1959) 128. F. See1 and E. Miiller, Chem. Ber., 88 (1955) 1747. D.A. Holtzen and A.S. Allen, J. Electroanal. Chem., 69 (1974) 153. R.H. Betts and F.S. Dainton, J. Am. Chem. Sot., 76 (1953) 5721. G. Stedman and P.A.E. Whincup, J. Chem. Sot. A, (1969) 1145. F.T.T. Ng and P.M. Henry, Can. J. Chem., 53 (1975) 3319. D.M. Stanbury, W.K. Wilmarth, S. Khalaf, H.N. PO and J.E. Byrd, Inorg. Chem., 19 (1980) 2715, M. Schoneshofer, G. Beck and A. Henglein, Ber. Bunsenges. Phys. Chem., 74 (1970) 1011 T.P. DeAngelis and W.R. Heineman, J. Chem. Educ., 53 (1976) 594. M. Petek, T.E. Neal and R.W. Murray, Anal. Chem., 43 (1971) 1069. J.H. Baxendale, P.L.T. Bevan and D.A. Stott, Trans. Faraday Sot., 64 (1968)2389. C.E. Vanderzee and A.S. Quist, Inorg. Chem., 6 (1966) 1238. D. Duonghang, J. Ramsden and M. Griitzel, J. Am. Chem. Sot., 104 (1982) 2977.