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sulphur—oxygen compounds
ElectrochimicaActa, Vol. 31, No. 15, pp. 2175-2784, 1992 Printed in Great Britain.
@X3-4686/92 S5.00 + 0.00 0 1992 Pergamon Press Ltd.
THE ELECTROCHEMICAL REACTION OF SULPHUR-OXYGEN COMPOUNDS-PART I. A REVIEW OF LITERATURE ON THE ELECTROCHEMICAL PROPERTIES OF SULPHUR/SULPHUR-OXYGEN COMPOUNDS TOR HEMMINGSEN* Department of Metallurgy, The Technical University of Denmark, Building 204, DK 2800 Lyngby, Denmark (Received 23 January 1992; in revisedform 8 April 1992)
Abstract-A review of literature on the electrochemical hehaviour of sulphur-oxygen compounds is given. The sulphur compounds are presented in order of oxidation states. Potential-pH diagrams based on this literature are presented graphically for each reaction. Key words: sulphur and sulphur-oxygen compounds, redox reactions, literature review.
INTRODUCTION The production of sulphur is in the order of 10’ tons per year, and the use of sulphur products is extensive. The paper industry, vulcanization, asphalt production, transformer liquids, pesticides and the medical industry are some of the variety of areas in which it is used[l]. The number of sulphur compounds in heterocyclic organic chemistry has also grown tremendously, but will not be considered here. Through their many oxidation states, sulphur compounds give a large number of derivatives, and the literature on sulphur compounds is indeed vast. On the other hand, most of the literature is concentrated on the chemical behaviour of these compounds, where the analyses are based on the chemical reduction-oxidation reactions. However, many of the compounds are very unstable, and may be analyzed to advantage by the use of voltammetric methods with various electrodes. Most of the existing studies concern the oxidation reactions in anodic stripping voltammetric (ASV) sweeps, most often on a dropping mercury electrode, while the reduction reactions have been studied to a lesser extent. The Pourbaix atlas gives a collection of potentialpH diagrams for the most common forms of various elements, including sulphur[2]. Several books and papers give additional information on sulphur reactions in aqueous solution[3-101. However, a complete collection of the sulphur reactions seems to be lacking. This review includes the reactions of sulphur/sulphur-oxygen compounds in aqueous solution. The diversity of the chemical reactions is illustrated in Fig. 1. A collection of redox equations based on available electrochemical data on sulphuroxygen compounds are presented graphically for each reaction (Figs 2-5). The potentials refer to the l Present address: Rogaland Research, PO Box 2503 Ullandhau8 4004 Stavanger, Norway.
EA37:15-H
standard hydrogen electrode with a one-molar concentration of the compounds. However, some of the electrochemical data deviate considerably, and more than one source should be consulted. REDOX REACTIONS In this review, sulphur compounds are treated in order of oxidation states. Potential-pH diagrams (Figs 2-5) are constructed on the bases of the data from the equations given in the text. The numbers on the figure legends refer to the numbers of these equations. Oxidation state VII
The most oxidised state of the sulphur compound is VII and concerns the peroxodisulphate ion. Peroxodisulphate is used commercially in the production of flour[ll]. It may, however, be allergenicC121. Peroxodisulphate is colourless, like all other sulphur oxides. It can be synthesized from sulphate or bisulphate at a high potential, as given in equations 1.1 and 1.2 (Table 1). The basic equations used for the construction of potential-pH diagrams are marked with an asterisk. The other values derive from calculations based on the Nernst equation and the acid constants for the involved compounds. Oxidation state VI
The sulphur atom in sulphate is at oxidation state VI. It can be formed by the oxidation of various sulphur compounds, as given in Table 2. The sulphate ion is very stable and difficult to reduce. It can, however, be reduced by bacteria to the sulphide ion[13]. The formation of sulphide in oil wells is highly feared as the cause of corrosion problems. Careful treatment of the injection water with uo light and biocides is therefore necessary to avoid bacteria attacks.
2115
T. HEMMINGSEN
2116
Fig. 1. A simplified diagram of some of the reactions of the sulphur-oxygen compounds[ 15,40,48]. Oxidation state V
phite. This salt will disproportionate
ion can be formed by the oxidation of sulphite. Pure acid cannot be isolated, but relatively pure concentrated acid can be prepared from barium dithionate in sulphuric acid[ 151. Ferric oxide in the presence of sulphur dioxide forms ferric sul-
dithionate and ferrous sulphide[ 151. In spite of the structure and the name, dithionic acid is not really related to the polythionic acids[8]. It will not react with sulphur to give polythionates, and is also more resistant to oxidation than the
The dithionate
to give ferrous
-
H2
_----.
02
-
4.1 4.2
-
4.3
-
4.5
-
Fig. 2. Potential-pH equilibrium diagram for the reduction of S,O:-, SO:- and S,Oz- at a one molar concentration. The numbers on the legends refer to the equation numbers given in the text.
-
46
-
51
Fig. 3. Potential-pH equilibrium diagram for the reduction of SOi- and f&O:- at a one molar concentration. The numbers on the legends refer to the equation numbers given in the text.
Sulphur-oxygen
compounds-I
2777
Fig. 4. Potential-pH equilibrium diagram for the reduction of S,O:- and S,O:- at a one molar concentration. The numbers on the legends refer to the equation numbers given in the text.
Fig. 5. Potential-pH equilibrium diagram for the reduction of S and St- at a one molar concentration. The numbers on the legends refer to the equation numbers given in the text.
polythionates[9]. Furthermore the dithonate ions show high stability towards reductions[16]. See Table 3. When the concentration of the dithionate ion is
Oxidation state IV
increased, a disproportionation into sulphate and sulphite ions will occur as given in Equation 3.1.4[7, 151. S,O;- + H,O = H,SO, + SO:-. (3.1.4)
The sulphite ions are used as a commercial reduction agent to remove oxygen from injection water in the oil industryC17, 181, and in district heating systems[19]. The sulphite salts are also added as preservatives to food, wine or pharmaceutical products[20]. The health risk involved in the addition of sulphite is low, as it is very rapidly
Table 1. The standard hydrogen potential of peroxodisulphate reduction
(1.1.1) (1.1.2)
E,(calc.)
Redox pair
Equation no.
S,O:- + 2e- = 2 SOiS,O:-+2H++2em=2HSO;
2.010 2.123
EJiterature) 2.01o*c2, 71 2.123[2], 2.18[7]
Table 2. The standard hydrogen potential of sulphate reduction* Equation no. (2.1.1) (2.1.2) (2.2.1) (2.2.2) (2.2.3) (2.2.4)
E,(cak.)
Redox pair 2 SO:- + 4 H+ + 2e- = S,O;2 HSO; + 2 H+ + 2e- = S,O,SOiSO:HSO; HSO;
+ + + +
2 3 2 3
H+ H+ H+ H+
+ + + +
2e2e.2e2e-
= = = =
SOiHSO; HSO; H,SO,
+ 2 H,O + 2 H,O + H,O + H,O + H,O + H,O
E&literature)
-0.200 -0.313
-0.200*[7,
- 0.092 0.121 0.065 0.121
-0.092*[14],
(2.3.1) (2.3.2) (2.3.3) (2.3.4)
2 SO;- + 8 H+ + 6e- = S,O:- + 4 H,O 2SO:-+9H++6e-=HS,O;+4H,O 2HSO;+8H++k-=HS,O;+4H,O 2 HSO; + 9 H+ + 6e.- = H&O, + 4 H,O
0.073 0.097 0.078 0.082
0.073*[2]
(2.4.1) (2.4.2) (2.4.3) (2.4.4)
2 SO:- + 10 H+ + 8e- = S,Oi- + 5 H,O 2 HSO; + 9 H+ + 8e- = S,O,- + 5 H,O 2 HSO; + 10 H+ + 8e- = HS,O; + 5 H,O 2HSO;+llH++8e-=H,S,O,+5H,O
0.029 0.015 0.027 0.030
0.029*[7]
143 -0.102[7]
(2.5.1) (2.5.2)
SO:-+8H++6e-=S+4H,O HSO;+7H++6e-=S+4H,O
0.357 0.338
0.357*[2], 0.394[7] 0.339[2]
(2.6.1) (2.6.2) (2.6.3) (2.6.4)
SOi- + 8 H+ + 8e- = S2- + 4 H,O SOi-+9H++8e-=HS-+4H,O SO:- + 10 H+ + Se- = H,S + 4 H,O HSO;+9H++8e-=H,S+4H,O
0.149 0.252 0.304 0.289
0.149*[2] 0.252[2] 0.303[2] 0.289[2]
* The sulphurous acid is presented as H,SO,, although SO, or SO,onH,O are more correct[lO]. Some other acids, which may not exist as pure compounds (e.g. H,S,O,, H,S,O,) are also presented in the table or later in the paper to give a more complete picture of possible reactions.
T.
2778
Table 3. The standard Equation
HEMMINGSEN
hydrogen
Redox pair
no.
(3.1.1) (3.1.2) (3.1.3)
converted to sulphate in uiuo. In the paper pulp industry 100000 tons of sulphite salts are added per year to dissolve lignin and carbohydrates while the cellulose is left undissolved. Further, the sulphite ions may be used for bleaching paper[l]. In all these processes sulphite is oxidized into sulphate. The sulphite ions may also be reduced, and be the source of several products as shown in Table 4. Gossman found that sulphite could be reduced on a mercury electrode only in an acidic medium. The proposed reduction product in acidic medium is dithionite[24]. In a less acidic medium two waves are observed. The first wave is due to the formation of dithionite (Equation 4.7.1). The second wave is thought to be the reduction of SO, formed from the decomposition of dithionite (Equation 4.7.2). The dithionite is supposed to be stable only in acid, since otherwise it would give the sulphoxylate ion as seen in Equation 4.7.3. (4.7.1)
2 SO, + 2e- = S,O:= so,
+ so;-
(4.7.2)
SO, + 2e- = SO:-.
Kolthoff disagrees with this mechanism. In a one-electron process, as proposed by Gossman, the constant is diffusion calculated to be 8.2 x 10-5cm-2s-1[25]. On the other hand, a twoelectron mechanism will give a diffusion constant of
Equation
0.026 0.452 0.564
HSO; + 2 H+ + e- = HSO, + H,O
S,Oi-
H&O5
+ H,O = 2 H,SO,.
potential
of sulphite
+ 2 H,O + 2 H,O + 2 H,O + 2 H,O
0.522 0.100 0.172 0.060 0.07 1
0.522*[21], 0.532[7], 0.099[21], -0.013[2] 0.173[21], 0.060[2] 0.068[21], -0.056[2], 0.079[2 l] 0.581[2] 0.509*[2]
0.662 0.449 0.393 0.418 0.422
SOi-
+ 2 H+ + 2e- = SOi-
;::.:; (4.5:3)
SO:-+6H++4e-=S+3H,O HSO;+5H++4e-=S+3H,O H,SO,+4H++W=S+3H,O
(4.6.1) (4.6.2) (4.6.3) (4.6.4) (4.6.5)
SO’- + 6 H+ + 6e- = SO;- + 7 H+ + 6e- = HSO; + 6 H+ + 6e- = HSO;+7H++6e-=H,S+3H,O H,SO, + 6 H+ + 6e- =
7 vs. ferrocene
in 5.5 M H,PO,[22,23].
H,S + 3 H,O
0.662*[7, 0.491[2]
141, 0.705[2]
-0.47t[22]
+ H,O
S*- + 3 H,O HS- + 3 H,O HS- + 3 Ha0
(4.7.8)
(4.7.9) (4.7.10)
reduction EJiterature)
+ 3 H,O + 3 H,O + 3 H,O + 3 H,O + 3 H,O
(4.4.1)
+ H,SO,
E,(calc.)
2 SO:2 HSO; 2 H,SO, 2 H,SO, 2 H,SO,
S,OiS,O,S,O:H&O; H&O,
+ H,O.
Kolthoff proposes that the increase in the diffusion current from the reduction of sulphite with increased acidity is due to the keto-enol equilibrium. The equi-
(4.3.1) (4.3.2) (4.3.3) (4.3.4) (4.3.5)
= = = = =
(4.7.7)
+ H,O = 2 H&O5
0.868 0.583 0.509
4e4e4e4e4e-
H&O.+ = SZO:- + 2 H+.
2 H&O4
+ 6 H,O + 6 H,O + 6 H,O
+ + + + +
(4.7.6)
+ 2 H+ + 2e- = S,O:-
4 SO:- + 12 H+ + 6e- = S,O;4 HSO; + 8 H+ + 6e- = S,O;4 H,SO, + 4 H+ + 6e- = S,O;H+ H+ H+ H+ H’
(4.7.5)
2 HSO, = H,S,O,
The disappearance of the second wave at a low pH value is explained by the decomposition of dithionite which produces the sulphoxylic acid endproduct.
(4.2.1) (4.2.2) (4.2.3)
6 4 2 3 4
(4.7.4)
The second wave at a potential of - 1.23 V (see) is assumed to be caused by the formation of thiosulphate.
g::.:; (4.1:4) (4.1.5)
+ + + + +
0.56[7]
HSO; + 3 H+ + 2e- = H,S02 + H,O.
2SO:-+4H++2e-=S,Oj’-+2H,O 2 HSO; + 2 H+ + 2e- = 2 HSO; + 3 H+ + 2e- = 2 H,SO, + H+ + 2e- = 2 H,SO, + 2 H+ + 2e- =
S,O:H&O; H&O; H,S,O,
E,(literature) 0.026*[2] 0.455[2] 0.564[2], 0.6[14],
At a pH of 6 dithionite may be formed through the reactions given in Equations 4.7.5-7.
Redox pair
no.
(4.1.1)
hydrogen
reduction
2.0 x 10-scm-2s-1, which is more plausible. The reduction product leading to the only wave in 0.1 M nitric acid, at a potential of -0.37 V (see), is therefore assumed to be the formation of the sulphoxide ion (Equation 4.7.4).
(4.7.3)
Table 4. The standard
of dithonate E,(calc.)
S,Oz- + 2e- = 2 SO:+ 2 H+ + 2e- = 2 HSO; + 4 H+ + 2e- = 2 H,SO,
S,OzS,Og-
s,o:-
potential
0.585 0.478 0.450
0.602[7]
0.231 0.368 0.297 0.366 0.347
0.231*[2]
0.450*[7,
0.3 1[7]
143, 0.449[2]
0.416[2]
0.08[7,
143
Sulphur-oxygen compounds-I librium as given in Equation 4.7.11 has the enol isomer, i.e. (HO),SO, as the weakest acid. At pH 3 most of the keto isomer (HOSO,) is dissociated while most of the anol isomer is undissociated. The enol isomer is assumed to be reducible. SO1 + Hz0 = HOHSOz = (HO),SO.
(4.7.11)
Benayada has also reduced sulphite on mercury but in concentrated phosphoric acid. He observes the formation of the sulphoxylate ion[22]. This ion is very unstable and dimerises rapidly to thiosulphate. Two waves are also observed even in 5.5 M H,PO,, and are proposed to be caused by the reductron of the sulphoxide ion and the reduction of the thiosulphate ion. Jacobsen and Sawyer conclude that the reduced product under acidic conditions on mercury, namely the sulphoxylic acid, decomposes to sulphur and sulphur dioxide[26]. Thus, sulphur is the final product under these conditions. The end product in the reduction of sulphite on platinum is reported to be elemental sulphur[27], while the product formed on gold consists of both sulphide and sulphurC28, 291. Also, Contractor observes the formation of elemental sulphur on platinum at a potential of 0.2V, but in addition he detects traces of sulphide by odour when the potential is lowered to O.OV. The sulphur formed adsorbs very strongly to the platinum surface, and prevents adsorption of hydrogen. The decrease in current from reoxidation of hydrogen can be used to calculate the sulphur coverage provided that the hydrogen reoxidation data is known for a “clean” platinum electrode[29]. Under acidic conditions sulphite may be oxidised to sulphate. When the pH is raised, giving sulphite or bisulphite ions, no oxidation wave can be seen. Thus SO, molecules are the active species[30]. This contrasts with the behaviour of sulphite as an oxygen scavenger, where the reaction between sulphite and free oxygen is fastest at the optimum pH of 8.5[31, 321. Klyanini finds the potential of SO, oxidation in 50mM H,SO, on an oxygen-free platinum surface to be 0.65V, while access of oxygen to the electrode surface raises the oxidation potential to l1.26V. The oxygen is believed to prevent the reaction through a radical mechanism. Similar observations are made by Contractor and La1[33]. In cyclic potential sweeps on platinum with 1OmM SO, in 50mM H,SO, the first oxidation wave at 0.75V disappears, but shifts to a lower potential. Also, the intensity decreases. According to Seo and Sawyer, the second wave at 1.2V is caused by an oxidation of a complex between SO, and pIatinum[34]. On gold the oxidation of SO, in 50mM H,SO, takes place at a potential of 0.42V (sce)[34]. The
2179
peak current is proportional to the square root of the sweep rate in the range of 0.3 to 2.75 V/min, and to the sulphite concentration in the range of l-5mM. Contrary to the activity of a platinum surface, that a gold surface is destroyed when the potential is swept in the hydrogen evolution area rather than in the oxygen evolution area. The concentration of bisulphite may also be determined photometrically by the use of uo-spectroscopy. At a concentration below 3mM bisulphite light is adsorbed at 205pm, while at a concentration above 3 mM bisulphite light is adsorbed at 215 pm. The difference is explained by the two conformers of the bisulphite, the HOSO; and the HSO; isomers. The S,O:ion adsorbs light at a wavelength of 255 pm[35]. Oxidation state III Sodium dithionite (Na,S,O,) is used extensively in the pulp industry as a reducing agent. The dithionite can be oxidized to sulphate according to Equation 2.3. The dithionite salt is prepared from the reduction of sulphite with eg sodium amalgam or zinc, or by electrolysis[36-381. Dithionite may be reduced further to thiosulphate, as given in Table 5. Since dithionite is an unstable compound, it will react and form other products. In acidic environments dithionite will disproportionate to thiosulphate and bisulphiteC37, 391. A colour change from yellow to red will be observed[40]. Even in water this disproportionation will take place, but at a slow rate. The reaction is given in Equations 5.2.1-4[fl. In the presence of air, sulphate and sulphite will be formed[40]. The disproportionation in alkaline solution gives sulphite and sulphide as seen from Equation 5.3.1[39]. S,O:-
+ 2 H+ = H&O,
(5.2.1)
H&O,
(5.2.2)
2 H,SO,
= H,SO, + SO, = S,O;-
+ 2 H+
+ H,O SO, + Hz0 = HSO;
(5.2.3) + H+
3S,O:-+6OH-=5SO:+ S2- + 3 H,O. (5.3.1) The bondlength between the sulphur ions in dithionite is extremely large: 2.389Angstrom[15]. This indicates that the strength of the bond is weak. In electron resonance studies the radical ion SO;* is indeed found to exist in equilibrium with the dithionite ion, and the equilibrium constant is found to be 6.3 x IO-lo M[41]. Like sulphite, dithionite can be analysed by iodometric titration[42]. Dithionite can be determined
Table 5. The standard hydrogen potential of dithionite reduction Equation no. (51.1) (51.2) (5.1.3)
Redox pair S,O:- + 2 HS,O; + H&O; + 2 H&O, + H&O, + 2
H+ H+ H+ H+ H+
+ + + + +
2e2e2eZe2e-
= = = = =
(5.2.4)
S,O*- + H,O S,Oa- + H,O HS,O; + H,O HS,O; + H,O H&O, + H,O
E,(calc.)
Edliterature)
0.788 0.716 0.766 0.755 0.764
0.788*[7] 0.88[7]
2780
T. HEMMINGSEN Table 6. The standard
Equation
no.
hydrogen
potential
Redox pair
(6.1.1) (6.1.2) (6.1.3) (6.1.4)
0.090 - 0.052 0.048 0.065
S,O;+ 12 H+ + lCh- = 4 S + 6 H,O HS,O;+llH++lOe-=4S+6H.O
separately in the presence of sulphite by iodometric titration with the addition of formaldehydeC42, 431. A mixture of sulphide, thiosulphate and sulphite can be analysed first by determining the total consumption of iodine, and then masking the sulphite with formaldehyde or masking the sulphide by the addition of zinc ions[44-461. A mixture of dithionite, sulphite and thiosulphate can also be analysed. Three analyses are required. First, the total iodine consumption is found. The sulphite is then masked with formaldehyde prior to the titration with iodine. Then, the solution is titrated with bromine[43,47]. A number of other methods can be used to quantify the amount of sulphur compounds. The methods may include gravimetry, polarography, nephelometry, flame-photometry, spectroscopy, radiometry or chromatogrphy. An informative review is given by Blasius et aI.[48].
0.416 0.388
E,(literature) 0.090*[14],
2 HS- + 2 0, = S,O:2 HS- + 4 HSO;
Thiosulphate is a chemical used as a developing agent for photos. The thiosulphate ion has the ability to dissolve unreacted silver bromide. Thiosulphate may also be used medically. In dermatology the salt has an antimyocotic and antiparasitic effect.
(7.2.1) (7.2.2) (7.2.3) (7.3.1) (7.3.2) (7.3.3) (7.3.4) (7.4.1) (7.4.2) (7.4.3) (7.4.4) (7.4.5) (7.5.1)
potential
of thiosulphate
S,O:-+6H++4e-=2S+3H,O HS20;+5H++4e-=2S+3H,O H,S,O,+4H++4e-=2S+3H,O
S,O:S,O:S,O:HS,O; H&O,
+ + + +
30 32 27 22
H+ H+ H+ H+
+ 6 + 8 + 10 + 9 + 8
H+ H+ HC H+ H+
+ + + + + + + + +
24e24e24e24e-
= = = =
2 2 2 2
S:- + 15 H,O HS; + 15 H,O HS; + 15 H,O HS; + 15 H,O
8e8e8e8e8e-
= = = = =
2 2 2 2 2
S*HSH,S H,S H,S
SsO;-+12H++lk-=5S+6H,O
= 3 S,O;-
+ 3 H,O.
+ 6 H+ = 3 S + 3 H,O.
reduction
E,(calc.)
Redox pair
5 S,O;5 S,O,5 HS,O; 5 H,S,O,
+ H,O
(7.1.1) (7.1.2)
(7.1.3)
On the other hand, very acidic solutions of thiosulphate will not yield sulphur, but instead H2S405 and H,S,O, * S02. Thiosulphate may be oxidised to S,Oi- by Cu(II) or PbO, , or to sulphate by chlorine or bromine. When H,S and H,SO, are mixed under acidic conditions a very complex solution, called Wackenroder’s liquid, is formed[50]. A proposed mechanism for the formation of polythionates in this solution is given by Remy[Sl]. The reduction potentials for thiosulphate to sulphur, sulphide or polysulphides are given in Table 7. The oxidation of thiosulphate on platinum is described by Glasstone and Hickling[53]. At a potential of 0.47-0.67 the thiosulphate ions are oxidised to dithonate and some sulphate. The reaction
Oxidation state II
no.
0.289[2]
The acid H&O, or salts with HS,O; are not isolated, and may only exist as intermediates. Thus, the product formed when sulphide and sulphite are mixed under acidic conditions is not thiosulphate, but rather sulphur as given in Equation 7.1.3.
Tetrathionate can be formed by oxidizing thiosulphate with iodine[48]. The reduction potentials for tetrathionate to yield thiosulphate or sulphur are given in Table 6. Zezula has observed a two electron reduction of tetrathionate on a dropping mercury electrode at a potential range of - 0.280 to -0.380 V (she)[49].
hydrogen
0.17[15],
It decomposes at the site of application to sulphur and sulphite. These compounds have a healing effect on wounds[20]. It can also be used as “antichlorine” since the thiosulphate is oxidised to sulphate while chlorine is reduced under the same conditions. Thiosulphate can be formed when sulphide is carefully oxidised with air, or better, with sulphite in a neutral solution[40].
Oxidation state II,,,
Table 7. The standard
0.08[7],
0.416*[2]
2 S2- + SO:-
Equation
reduction
E,(calc.)
S,Oi+ 2e- = 2 S,O:HS,O, + 2e- = 2 S,O:+ H+ HS,0;+H++2em=2HS,0; HS,O, + 3 H+ + 2e- = 2 H,S,Os
(6.2.1) (6.2.2)
of tetrathionate
+ 3 H,O + 3 H,O + 3 H,O + 3 H,O + 3 H,O
E,(literature)
0.465 0.440 0.436
0.465*[2],
0.331 0.359 0.338 0.335
0.331*[2]
-0.006 0.200 0.303 0.291 0.288 0.484
-0.006*[2] 0.200[2]
0.484*[2]
0.602[7]
Sulphur-oxygen compounds-I
2781
Table 8. The standard hydrogen potential of sulphur reduction Equation no.
E,,(calc.)
Redox pair 5 S + Ze- = S:-
EJliterature)
(8.1.1) (8.1.2)
-0.340 -0.171
-0.340*[2]
5S+H++2e-=HS;
(8.2.1) (8.2.2)
4 S + 2e- = S:4S+H++Ze-=HS;
-0.360 -0.174
-0.360*[52]
(8.3.1) (8.3.2)
3 S + Ze- = S:3S+H++Ze-=HS;
-0.390 -0.168
-0.390*[52]
(8.4.1) (8.4.2)
2 S + Ze- = S:2S+H++2e-=HS;
- 0.428 -0.141
-0.428*[52]
s + 2e- = sS+H++2e-=HSS+2H++2e-=H.S
- 0.476 - 0.065 0.142
-0.476*[2], -0.508[14] -0.065[2, 143 0.142[2, 7],0.141[14]
mechanism is assumed to include the oxygen in the solution. The yield of dithionate can be enhanced by a prepolarixation in anodic direction, which regulates the pH to the optimal value of 8. Oxidation state 0 Sulphur may be formed through the reduction of pentathionate ions as given in Equation 7.5.1. It can further be reduced to sulphide and polsulphides as seen in Table 8. At room temperature elemental sulphur is most often found as cyclic S, . Polysulphides or sulphanes can be prepared by heating metal sulphide or hydrogen sulphide with sulphur in water[54]. Sulphur is reported to cause pitting and crevice corrosion on AISI 304 and AISI 316 stainless steels under oxygenfree conditions with traces of hydrochloric acid and moisture[55]. Table 9. The standard
hydrogen
Oxidation
potential reduction
of sulphide
Equation no.
Redox pair
(9.1.1)
4 St- + Ze- = 5 S:4 Si- + 5 H+ + Ze- = 5 HS; 4 HS; + H* + 2e- = 5 HS;
.. ::.:; (9.2.1)
s:-
state < 0
Polysulphides are of great importance as plant protectors. The best known polysulphide is the calcium salt, which is prepared by boiling sulphur with slaked lime[48]. Sulphides are present in several minerals. The volatile acid is very toxic with a very low level of lethality. The sulphide ions are also very corrosive, and will both increase the corrosion rate and cause hydrogen embrittlement in the steel. The sulphide ion is therefore given an extensive treatment in corrosion literature[56-591. See Table 9. The oxidation of sulphide on a pure platinum electrode takes place at a potential of 0.625VC60, 611. The product formed is platinum polysulphide. When the potential is raised, oxygen evolution will occur. The oxidation of sulphide on the platinum oxide surface will then occur at a potential of 1.4 V at
+ 8e- = 5 s-
E&alc.)
-0.441 0.490 -0.184
and
polysulphide E,(literature) -0.441’[2]
-0.511 0.003*[2]
KI (9:2:4)
S:+ 10 H+ + 8e- = 5 H,S S;-+5H++&-=5HSHS; + 5 H+ + 8e- = 5 H,S
0.003 0.262 0.051
(10.1.1) (10.1.2) (10.1.3)
3 s:- + 2e- = 4 s:3S:-+4H++2e-=4HS; 3HS;+H++2e-=4HS;
-0.478 0.410 -0.150
(10.2.1) (10.2.2) (10.2.3) (10.2.4) (11.1.1) (11.1.2) (11.1.3)
s:- + 6e- = 4 s2S2-+4H++6e-=4HSS!- + 8 H+ + 6e- = 4 H,S HS;+7H++6e-=4H,S 2 s:- + 2e- = 3 sj2S:-+3H++k-=3HS; 2HS;+H++2e-=3HS;
-0.515 0.033 0.309 0.247 -0.506 0.355 - 0.089
(11.2.1) (11.2.2) (11.2.3) (11.2.4)
s:-+4e-=3s2St-+3H++4e-=3HSHS;+2H++4c-=3HSHS;+5H++k-=3H,S
-0.520 0.097 -0.014 0.296
(12.1.1) (12.1.2) (12.1.3) (12.1.4)
S:- + 2e- = 2 S2Si- + 2 H+ + 2e- = 2 HSHS;+H++2e-=ZHSHS;+3H++2e-=2H,S
- 0.524 0.298 0.014 0.425
-0.478*[2]
0.033*[2] -0.506*[2]
0.097*[2]
- 0.524[2] 0.298*[2]
T. HEMMINGSEN
2182
Table 10. Values of dissociation constants (pKJ. Values marked with an asterisk are used in the
construction of the pH-potential diagrams HSO;/SO:H,SO,/HSO; HSO;/SO:H,S,OJHS,O; HS,O;/S,O:HS,O;/S,O;H,S,OJHS,O; HS,O;/S,O:H,S/HS HS-/S2HS;/S;HS;/S:HS;/S:HS;/S:HS,/S;HS;/S; HS,/S;-
1.70(64] 1.77[65] 6.99[7] 0.35*[7, 21, 663 2.45*[7, 211 4.80*[6] 0.29*[2] 1.69*[2] 7.00*[2] 13.9*[2] 9.7*[67] 7.5*[67] 6.3*[67] 5.7*[67] 5.2*[68] 4.8*[68] 4.4*[68]
1.91*[2] 1.80[64,663 7.00[64,66]
1.81[7] 7.21*[21]
2.46[66]
2.50[2]
HAO,
KS
HA
KS?,
H,S,~,
HA+ L?,
H&O,
SO2 H,SzO, ;03 S0,--I;12S0,SO2 LO,
SO,-H,S,O,
Polymeric sulphur compounds
Sulphur, like carbon, can form both covalent bonds and polymerize. Thus the most stable form of sulphur at room temperature is a ring-shaped molecule of eight atoms. The n-electrons stabilise the ring as is the case with the benzene molecule. In analogy to benzene, sulphite may attack the ring as a nucleophile, but opens the ring. The product is a sulphonic acid. The sulphite ion, however, quickly attacks the sulphonic acid, and degrades the acid stepwise through an S,2 mechanism to thiosulphate[lS, 691. Higher orders of sulphonic acid can be generated by adding sulphur trioxide to sulphanes[ 151. The simplest sulphane is H,S. When one equivalent sulphur trioxide is added, the simplest monosulphonic acid, thiosulphate (H&O& is formed. When one more equivalent sulphur trioxide is added, the simplest disulphonic acid, trithionic acid (H&O,), is formed. One should note that dithionic acid is not a disulphonic acid, but rather an oxidation product from sulphur dioxide. The general reaction scheme for monosulphonic acid and disulphonic acid (also called polythionic acid) is given in Equations 13.1-2. = H-S,-SO;
H,S, + 2 SO: - = -O,S-$-SO;.
&OX
p03
compounds are only valid at a limited pH range. It should also be noted that not all species are stable in the total pH range. However, they may exist as intermediates, and are therefore included in the diagrams.
H,S, + SO;-
Table 11. Oxyacids of sulphur[75]
So3-H2SP4 803
1.97[2]
060[7] 1.74[7]
room temperature. At this potential elemental sulphur is formed. Elemental sulphur can be removed from the electrode surface with acetone. The remaining platinum sulphide film is pale and yellow[61]. When the temperature is raised to 80°C two oxidation peaks appear[62]. The peak at 0.97 V refers to weakly bonded sulphur species, while the peak at 1.1 V refers to strongly bonded sulphur. The two kinds of adsorbed species are also observed from the adsorption reaction of SO,[34]. As the adsorbed sulphur prevents the hydrogen from adsorbing to the platinum surface and from being oxidized, the cyclic potential sweep technique can be used as a tool to quantify the amount of adsorbed sulphur[63]. Potential-pH equilibrium diagrams for the redox reactions of sulphur-oxygen compounds at a one molar concentration are constructed from data available in literature. In order to present the data more clearly, the diagram is split up into four. Figure 2 shows the reduction reactions of sulphur at oxidation states + V to + VIII, Fig. 3 shows the reactions at oxidation states III and IV, Fig. 4 shows the reactions at oxidation states between II and 0, and Fig. 5 shows the reactions at oxidation states lower than or equal to 0. No dissociation constants are found for the polythionates. It should therefore be kept in mind that a change in the slope will occur at a pH equal to the pK, value of these acids. Thus, the curves for these
WC
1.89*[21] 7.26[2]
SO3
SO3
W&O,
KSn+zO6
+ H+
(13.1.1) (13.1.2)
Until now the H,S,O, with n = 7 has been the highest member of the monosulphonic acids synthesizedC70, 713, and H,S,O, with n = 8 the disulphonic acids member of the highest synthesized[72]. However, a disulphonic acid with at least nine sulphur atoms has been detected by the use of high-voltage electrophoresis in Wackenroder’s liquid[73, 743. The oxyacids of sulphur may be classified as derivatives of sulphur dioxide and sulphur trioxide as given in Table 11[75]. When one of the hydrogen atoms in a sulphane is substituted with an alkyl or aryl group, one of the
Sulphur-oxygen compound+1 bridges to organic chemistry is made through syntheses of alkyl or aryl thiosulphonic acid[76]. CONCLUSIONS Like carbon, sulphur atoms have the ability to form covalent bonds. Therefore, a large number of sulphur compounds can be formed. Several of these compounds are unstable and will decompose. Thus, most often a mixture of sulphur compounds will be found in a aqueous solution. Due to this fact, it is often necessary to use sophisticated analytical methods to analyse such a mixture. Since most of the sulphur reactions involve electrons, electrochemical methods seem to be most applicable. Acknowledgement-The author would like to thank Prof. Torstein VBland and Prof. Ernst Maahn for helpful discussions. I would also like to give my thanks to the Nordic Council of Ministers for the scholarship granted under the “Nordic Energy Research Programme” and Rogaland Research for opportunity to write this paper.
REFERENCES
2783
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Elsevier, Amsterdam (1984). 14. R. C. Weast, Handbook of Chemistry and Physics, 63rd edn. (1982). 15. A. Senning, Sulfur in Organic Chemistry, Vol. 2, 71. Marcel Dekker, New York (1972). 16. A. Miiller and B. Krebs, Studies in Inorganic Chemistry 1. Sulfur, Its Significance for Chemistry, for the Gee-, Bio-, and Cosmosphere and Technology, Vol. 5, 259. Elsevier, Amsterdam (1984). 17. R. W. Mitchell, J. Pet. Tech. 877 (June 1978). 18. C. K. Chang, J. Pet. Tech. 1689 (Sept. 1985). 19. E. T. Eliasson and A. Einarsson, International Symposium on Solving Corrosion and Scaling Problems in Geothermal Systems. San Francisco, USA (1983). 20. T. Sollmann, A Manual of Pharmacology and its Applications to Therapeutics and Toxicology, 3rd edn. W. B.
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37. F. k. Andry&hchenko, Vkr. Khim. Zh. 29, 125 (1963). 38. C. C. Pate1 and M. R. A. Rao, Prot. Natl. Acad. Sci. lndia 15, 115 (1949). 39. E. M. Marshak, Khim. Neukai. Prom. 2,524 (1957). 40. J. H. Karchmer, The Analytical Chemistry of Sulfur and its Compounds. Wiley-Interscience, New York (1970). 41. S. Lynn, R. E. Rinker and W. H. Concoran, J. phys. Chem. 68,2363
(1964).
42. F. de Bacho, Z. Anal. Chem. 61,209 (1922). 43. R. Wollak, Z. Anal. Chem. 80, 1 (1930). 44. A. Kurtenacker and K. Bittner, Z. Anorg. Allgem. Chem. 141,297 (1924). 45. A. Kurtenacker and R. Wollak, Z. Anorg. Allgem. Chem. 161,201(1927).
46. R. Wollak, Z. Anal. dhem. 77,401 (1929). 47. R. R. Jay. Anal. Chem. 25.288 119531. 48. G. Nickikss, Inorganic Silphu; Chdmistry, p. 201. Elsevier, Amsterdam (1968). 49. I. Zezula, Chem. Listy 47,492 (1953). 50. R. C. Brasted, Comprehensive Inorganic Chemistry, Vol. 8. Van Nostrand, New York (1961). 51. H. Remy, Treatise on Inorganic Chemistry, Vol. I. Elsevier, New York (1956). 52. G. Maronny and G. Valensi, Proc. Int. Comm. Electrothem. Thermodynam. Kinet. 9th meeting, 155 (1957). 53. S. Glasstone and A. Hickling, J. Chem. Sot. 2345 (1932).
54. A. Senning, Sulfur in Organic Chemistry, Vol. 3, p. 19. Marcel Dekker, New York (1972). 55. B. D. Craig, Handbook of Corrosion Data. ASM International, Metals Park, U.S.A. 56. G. Wranglen, Corros. Sci. 9, 585 (1969). 57. J. S. Smith and J. D. A. Miller, Br. Corros. J. 10, 136 (1975). 58. B. G. Pound, G. A. Wright and R. M. Sharp, Corrosion 45, 386 (1989).
59. M. Kimura, N. Totsuka and T. Kurisi, Corrosion paper 237 (1985). 60. E. Najdeker and E. Bishop, Electroanal. Chem. Int. Electrochem. 41,79 (1973).
61. N. Ramasubramanian, J. electroanal. Chem. 64, 21 (1975). 62. A. Q. Contractor and H. Lal, J. electroanal. Chem. %, 175 (1979). 63. E. Lamy-Pitara, L. Bencharif and J. Barbier, Electro-
2784
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chim. Acta 30,971 (1985). 64. S. P. Parker, McGraw-Hill Encyclopaedia of Chemistry. McGraw-Hill, New York (1983). 65. R. B. Heslop and K. Jones, Inorganic Chemistry. Elsevier, Amsterdam (1976). 66. R. C. Brasted, Comprehensive Inorganic Chemistry, Vol. 8, p, 113. Van Nostrand, New York, (1961). 67. G. Schwarzenbach and A. Fischer, Helv. Chim. Acta 43, 1365 (1960). 68. B. Meyer, L. Peter and K. Spitzer, Inorg. Chem. 16, 27 (1977). 69. A. Senning, Sulfur in Organic Chemistry, Vol. 3, p. 1.
Marcel Dekker, New York (1972). 70. M. Schmidt, 2. Anorg. Allg. Chem. 289, 158 (1957). 71. M. Schmidt, Z. Naturforsch. 13b, 624 (1958). 72. M. Schmidt and H. Dersin, Z. Anorg. Allg. Chem. 289, 175 (1957). 73. E. Blasius and H. Wagner, J. Chromatog. 26, 549 (1967). 74. E. Blasius and H. Thiele, Z. Anal. Chem. 197, 347
(1963). 75. M. Schmidt, Anorganische Chemie. Bibliographisches Institut, Mannheim (1967). 76. M. Schmidt and G. Talsky, Chem. Ber. 94,1352 (1961).
@X3-4686/92 S5.00 + 0.00 0 1992 Pergamon Press Ltd.
THE ELECTROCHEMICAL REACTION OF SULPHUR-OXYGEN COMPOUNDS-PART I. A REVIEW OF LITERATURE ON THE ELECTROCHEMICAL PROPERTIES OF SULPHUR/SULPHUR-OXYGEN COMPOUNDS TOR HEMMINGSEN* Department of Metallurgy, The Technical University of Denmark, Building 204, DK 2800 Lyngby, Denmark (Received 23 January 1992; in revisedform 8 April 1992)
Abstract-A review of literature on the electrochemical hehaviour of sulphur-oxygen compounds is given. The sulphur compounds are presented in order of oxidation states. Potential-pH diagrams based on this literature are presented graphically for each reaction. Key words: sulphur and sulphur-oxygen compounds, redox reactions, literature review.
INTRODUCTION The production of sulphur is in the order of 10’ tons per year, and the use of sulphur products is extensive. The paper industry, vulcanization, asphalt production, transformer liquids, pesticides and the medical industry are some of the variety of areas in which it is used[l]. The number of sulphur compounds in heterocyclic organic chemistry has also grown tremendously, but will not be considered here. Through their many oxidation states, sulphur compounds give a large number of derivatives, and the literature on sulphur compounds is indeed vast. On the other hand, most of the literature is concentrated on the chemical behaviour of these compounds, where the analyses are based on the chemical reduction-oxidation reactions. However, many of the compounds are very unstable, and may be analyzed to advantage by the use of voltammetric methods with various electrodes. Most of the existing studies concern the oxidation reactions in anodic stripping voltammetric (ASV) sweeps, most often on a dropping mercury electrode, while the reduction reactions have been studied to a lesser extent. The Pourbaix atlas gives a collection of potentialpH diagrams for the most common forms of various elements, including sulphur[2]. Several books and papers give additional information on sulphur reactions in aqueous solution[3-101. However, a complete collection of the sulphur reactions seems to be lacking. This review includes the reactions of sulphur/sulphur-oxygen compounds in aqueous solution. The diversity of the chemical reactions is illustrated in Fig. 1. A collection of redox equations based on available electrochemical data on sulphuroxygen compounds are presented graphically for each reaction (Figs 2-5). The potentials refer to the l Present address: Rogaland Research, PO Box 2503 Ullandhau8 4004 Stavanger, Norway.
EA37:15-H
standard hydrogen electrode with a one-molar concentration of the compounds. However, some of the electrochemical data deviate considerably, and more than one source should be consulted. REDOX REACTIONS In this review, sulphur compounds are treated in order of oxidation states. Potential-pH diagrams (Figs 2-5) are constructed on the bases of the data from the equations given in the text. The numbers on the figure legends refer to the numbers of these equations. Oxidation state VII
The most oxidised state of the sulphur compound is VII and concerns the peroxodisulphate ion. Peroxodisulphate is used commercially in the production of flour[ll]. It may, however, be allergenicC121. Peroxodisulphate is colourless, like all other sulphur oxides. It can be synthesized from sulphate or bisulphate at a high potential, as given in equations 1.1 and 1.2 (Table 1). The basic equations used for the construction of potential-pH diagrams are marked with an asterisk. The other values derive from calculations based on the Nernst equation and the acid constants for the involved compounds. Oxidation state VI
The sulphur atom in sulphate is at oxidation state VI. It can be formed by the oxidation of various sulphur compounds, as given in Table 2. The sulphate ion is very stable and difficult to reduce. It can, however, be reduced by bacteria to the sulphide ion[13]. The formation of sulphide in oil wells is highly feared as the cause of corrosion problems. Careful treatment of the injection water with uo light and biocides is therefore necessary to avoid bacteria attacks.
2115
T. HEMMINGSEN
2116
Fig. 1. A simplified diagram of some of the reactions of the sulphur-oxygen compounds[ 15,40,48]. Oxidation state V
phite. This salt will disproportionate
ion can be formed by the oxidation of sulphite. Pure acid cannot be isolated, but relatively pure concentrated acid can be prepared from barium dithionate in sulphuric acid[ 151. Ferric oxide in the presence of sulphur dioxide forms ferric sul-
dithionate and ferrous sulphide[ 151. In spite of the structure and the name, dithionic acid is not really related to the polythionic acids[8]. It will not react with sulphur to give polythionates, and is also more resistant to oxidation than the
The dithionate
to give ferrous
-
H2
_----.
02
-
4.1 4.2
-
4.3
-
4.5
-
Fig. 2. Potential-pH equilibrium diagram for the reduction of S,O:-, SO:- and S,Oz- at a one molar concentration. The numbers on the legends refer to the equation numbers given in the text.
-
46
-
51
Fig. 3. Potential-pH equilibrium diagram for the reduction of SOi- and f&O:- at a one molar concentration. The numbers on the legends refer to the equation numbers given in the text.
Sulphur-oxygen
compounds-I
2777
Fig. 4. Potential-pH equilibrium diagram for the reduction of S,O:- and S,O:- at a one molar concentration. The numbers on the legends refer to the equation numbers given in the text.
Fig. 5. Potential-pH equilibrium diagram for the reduction of S and St- at a one molar concentration. The numbers on the legends refer to the equation numbers given in the text.
polythionates[9]. Furthermore the dithonate ions show high stability towards reductions[16]. See Table 3. When the concentration of the dithionate ion is
Oxidation state IV
increased, a disproportionation into sulphate and sulphite ions will occur as given in Equation 3.1.4[7, 151. S,O;- + H,O = H,SO, + SO:-. (3.1.4)
The sulphite ions are used as a commercial reduction agent to remove oxygen from injection water in the oil industryC17, 181, and in district heating systems[19]. The sulphite salts are also added as preservatives to food, wine or pharmaceutical products[20]. The health risk involved in the addition of sulphite is low, as it is very rapidly
Table 1. The standard hydrogen potential of peroxodisulphate reduction
(1.1.1) (1.1.2)
E,(calc.)
Redox pair
Equation no.
S,O:- + 2e- = 2 SOiS,O:-+2H++2em=2HSO;
2.010 2.123
EJiterature) 2.01o*c2, 71 2.123[2], 2.18[7]
Table 2. The standard hydrogen potential of sulphate reduction* Equation no. (2.1.1) (2.1.2) (2.2.1) (2.2.2) (2.2.3) (2.2.4)
E,(cak.)
Redox pair 2 SO:- + 4 H+ + 2e- = S,O;2 HSO; + 2 H+ + 2e- = S,O,SOiSO:HSO; HSO;
+ + + +
2 3 2 3
H+ H+ H+ H+
+ + + +
2e2e.2e2e-
= = = =
SOiHSO; HSO; H,SO,
+ 2 H,O + 2 H,O + H,O + H,O + H,O + H,O
E&literature)
-0.200 -0.313
-0.200*[7,
- 0.092 0.121 0.065 0.121
-0.092*[14],
(2.3.1) (2.3.2) (2.3.3) (2.3.4)
2 SO;- + 8 H+ + 6e- = S,O:- + 4 H,O 2SO:-+9H++6e-=HS,O;+4H,O 2HSO;+8H++k-=HS,O;+4H,O 2 HSO; + 9 H+ + 6e.- = H&O, + 4 H,O
0.073 0.097 0.078 0.082
0.073*[2]
(2.4.1) (2.4.2) (2.4.3) (2.4.4)
2 SO:- + 10 H+ + 8e- = S,Oi- + 5 H,O 2 HSO; + 9 H+ + 8e- = S,O,- + 5 H,O 2 HSO; + 10 H+ + 8e- = HS,O; + 5 H,O 2HSO;+llH++8e-=H,S,O,+5H,O
0.029 0.015 0.027 0.030
0.029*[7]
143 -0.102[7]
(2.5.1) (2.5.2)
SO:-+8H++6e-=S+4H,O HSO;+7H++6e-=S+4H,O
0.357 0.338
0.357*[2], 0.394[7] 0.339[2]
(2.6.1) (2.6.2) (2.6.3) (2.6.4)
SOi- + 8 H+ + 8e- = S2- + 4 H,O SOi-+9H++8e-=HS-+4H,O SO:- + 10 H+ + Se- = H,S + 4 H,O HSO;+9H++8e-=H,S+4H,O
0.149 0.252 0.304 0.289
0.149*[2] 0.252[2] 0.303[2] 0.289[2]
* The sulphurous acid is presented as H,SO,, although SO, or SO,onH,O are more correct[lO]. Some other acids, which may not exist as pure compounds (e.g. H,S,O,, H,S,O,) are also presented in the table or later in the paper to give a more complete picture of possible reactions.
T.
2778
Table 3. The standard Equation
HEMMINGSEN
hydrogen
Redox pair
no.
(3.1.1) (3.1.2) (3.1.3)
converted to sulphate in uiuo. In the paper pulp industry 100000 tons of sulphite salts are added per year to dissolve lignin and carbohydrates while the cellulose is left undissolved. Further, the sulphite ions may be used for bleaching paper[l]. In all these processes sulphite is oxidized into sulphate. The sulphite ions may also be reduced, and be the source of several products as shown in Table 4. Gossman found that sulphite could be reduced on a mercury electrode only in an acidic medium. The proposed reduction product in acidic medium is dithionite[24]. In a less acidic medium two waves are observed. The first wave is due to the formation of dithionite (Equation 4.7.1). The second wave is thought to be the reduction of SO, formed from the decomposition of dithionite (Equation 4.7.2). The dithionite is supposed to be stable only in acid, since otherwise it would give the sulphoxylate ion as seen in Equation 4.7.3. (4.7.1)
2 SO, + 2e- = S,O:= so,
+ so;-
(4.7.2)
SO, + 2e- = SO:-.
Kolthoff disagrees with this mechanism. In a one-electron process, as proposed by Gossman, the constant is diffusion calculated to be 8.2 x 10-5cm-2s-1[25]. On the other hand, a twoelectron mechanism will give a diffusion constant of
Equation
0.026 0.452 0.564
HSO; + 2 H+ + e- = HSO, + H,O
S,Oi-
H&O5
+ H,O = 2 H,SO,.
potential
of sulphite
+ 2 H,O + 2 H,O + 2 H,O + 2 H,O
0.522 0.100 0.172 0.060 0.07 1
0.522*[21], 0.532[7], 0.099[21], -0.013[2] 0.173[21], 0.060[2] 0.068[21], -0.056[2], 0.079[2 l] 0.581[2] 0.509*[2]
0.662 0.449 0.393 0.418 0.422
SOi-
+ 2 H+ + 2e- = SOi-
;::.:; (4.5:3)
SO:-+6H++4e-=S+3H,O HSO;+5H++4e-=S+3H,O H,SO,+4H++W=S+3H,O
(4.6.1) (4.6.2) (4.6.3) (4.6.4) (4.6.5)
SO’- + 6 H+ + 6e- = SO;- + 7 H+ + 6e- = HSO; + 6 H+ + 6e- = HSO;+7H++6e-=H,S+3H,O H,SO, + 6 H+ + 6e- =
7 vs. ferrocene
in 5.5 M H,PO,[22,23].
H,S + 3 H,O
0.662*[7, 0.491[2]
141, 0.705[2]
-0.47t[22]
+ H,O
S*- + 3 H,O HS- + 3 H,O HS- + 3 Ha0
(4.7.8)
(4.7.9) (4.7.10)
reduction EJiterature)
+ 3 H,O + 3 H,O + 3 H,O + 3 H,O + 3 H,O
(4.4.1)
+ H,SO,
E,(calc.)
2 SO:2 HSO; 2 H,SO, 2 H,SO, 2 H,SO,
S,OiS,O,S,O:H&O; H&O,
+ H,O.
Kolthoff proposes that the increase in the diffusion current from the reduction of sulphite with increased acidity is due to the keto-enol equilibrium. The equi-
(4.3.1) (4.3.2) (4.3.3) (4.3.4) (4.3.5)
= = = = =
(4.7.7)
+ H,O = 2 H&O5
0.868 0.583 0.509
4e4e4e4e4e-
H&O.+ = SZO:- + 2 H+.
2 H&O4
+ 6 H,O + 6 H,O + 6 H,O
+ + + + +
(4.7.6)
+ 2 H+ + 2e- = S,O:-
4 SO:- + 12 H+ + 6e- = S,O;4 HSO; + 8 H+ + 6e- = S,O;4 H,SO, + 4 H+ + 6e- = S,O;H+ H+ H+ H+ H’
(4.7.5)
2 HSO, = H,S,O,
The disappearance of the second wave at a low pH value is explained by the decomposition of dithionite which produces the sulphoxylic acid endproduct.
(4.2.1) (4.2.2) (4.2.3)
6 4 2 3 4
(4.7.4)
The second wave at a potential of - 1.23 V (see) is assumed to be caused by the formation of thiosulphate.
g::.:; (4.1:4) (4.1.5)
+ + + + +
0.56[7]
HSO; + 3 H+ + 2e- = H,S02 + H,O.
2SO:-+4H++2e-=S,Oj’-+2H,O 2 HSO; + 2 H+ + 2e- = 2 HSO; + 3 H+ + 2e- = 2 H,SO, + H+ + 2e- = 2 H,SO, + 2 H+ + 2e- =
S,O:H&O; H&O; H,S,O,
E,(literature) 0.026*[2] 0.455[2] 0.564[2], 0.6[14],
At a pH of 6 dithionite may be formed through the reactions given in Equations 4.7.5-7.
Redox pair
no.
(4.1.1)
hydrogen
reduction
2.0 x 10-scm-2s-1, which is more plausible. The reduction product leading to the only wave in 0.1 M nitric acid, at a potential of -0.37 V (see), is therefore assumed to be the formation of the sulphoxide ion (Equation 4.7.4).
(4.7.3)
Table 4. The standard
of dithonate E,(calc.)
S,Oz- + 2e- = 2 SO:+ 2 H+ + 2e- = 2 HSO; + 4 H+ + 2e- = 2 H,SO,
S,OzS,Og-
s,o:-
potential
0.585 0.478 0.450
0.602[7]
0.231 0.368 0.297 0.366 0.347
0.231*[2]
0.450*[7,
0.3 1[7]
143, 0.449[2]
0.416[2]
0.08[7,
143
Sulphur-oxygen compounds-I librium as given in Equation 4.7.11 has the enol isomer, i.e. (HO),SO, as the weakest acid. At pH 3 most of the keto isomer (HOSO,) is dissociated while most of the anol isomer is undissociated. The enol isomer is assumed to be reducible. SO1 + Hz0 = HOHSOz = (HO),SO.
(4.7.11)
Benayada has also reduced sulphite on mercury but in concentrated phosphoric acid. He observes the formation of the sulphoxylate ion[22]. This ion is very unstable and dimerises rapidly to thiosulphate. Two waves are also observed even in 5.5 M H,PO,, and are proposed to be caused by the reductron of the sulphoxide ion and the reduction of the thiosulphate ion. Jacobsen and Sawyer conclude that the reduced product under acidic conditions on mercury, namely the sulphoxylic acid, decomposes to sulphur and sulphur dioxide[26]. Thus, sulphur is the final product under these conditions. The end product in the reduction of sulphite on platinum is reported to be elemental sulphur[27], while the product formed on gold consists of both sulphide and sulphurC28, 291. Also, Contractor observes the formation of elemental sulphur on platinum at a potential of 0.2V, but in addition he detects traces of sulphide by odour when the potential is lowered to O.OV. The sulphur formed adsorbs very strongly to the platinum surface, and prevents adsorption of hydrogen. The decrease in current from reoxidation of hydrogen can be used to calculate the sulphur coverage provided that the hydrogen reoxidation data is known for a “clean” platinum electrode[29]. Under acidic conditions sulphite may be oxidised to sulphate. When the pH is raised, giving sulphite or bisulphite ions, no oxidation wave can be seen. Thus SO, molecules are the active species[30]. This contrasts with the behaviour of sulphite as an oxygen scavenger, where the reaction between sulphite and free oxygen is fastest at the optimum pH of 8.5[31, 321. Klyanini finds the potential of SO, oxidation in 50mM H,SO, on an oxygen-free platinum surface to be 0.65V, while access of oxygen to the electrode surface raises the oxidation potential to l1.26V. The oxygen is believed to prevent the reaction through a radical mechanism. Similar observations are made by Contractor and La1[33]. In cyclic potential sweeps on platinum with 1OmM SO, in 50mM H,SO, the first oxidation wave at 0.75V disappears, but shifts to a lower potential. Also, the intensity decreases. According to Seo and Sawyer, the second wave at 1.2V is caused by an oxidation of a complex between SO, and pIatinum[34]. On gold the oxidation of SO, in 50mM H,SO, takes place at a potential of 0.42V (sce)[34]. The
2179
peak current is proportional to the square root of the sweep rate in the range of 0.3 to 2.75 V/min, and to the sulphite concentration in the range of l-5mM. Contrary to the activity of a platinum surface, that a gold surface is destroyed when the potential is swept in the hydrogen evolution area rather than in the oxygen evolution area. The concentration of bisulphite may also be determined photometrically by the use of uo-spectroscopy. At a concentration below 3mM bisulphite light is adsorbed at 205pm, while at a concentration above 3 mM bisulphite light is adsorbed at 215 pm. The difference is explained by the two conformers of the bisulphite, the HOSO; and the HSO; isomers. The S,O:ion adsorbs light at a wavelength of 255 pm[35]. Oxidation state III Sodium dithionite (Na,S,O,) is used extensively in the pulp industry as a reducing agent. The dithionite can be oxidized to sulphate according to Equation 2.3. The dithionite salt is prepared from the reduction of sulphite with eg sodium amalgam or zinc, or by electrolysis[36-381. Dithionite may be reduced further to thiosulphate, as given in Table 5. Since dithionite is an unstable compound, it will react and form other products. In acidic environments dithionite will disproportionate to thiosulphate and bisulphiteC37, 391. A colour change from yellow to red will be observed[40]. Even in water this disproportionation will take place, but at a slow rate. The reaction is given in Equations 5.2.1-4[fl. In the presence of air, sulphate and sulphite will be formed[40]. The disproportionation in alkaline solution gives sulphite and sulphide as seen from Equation 5.3.1[39]. S,O:-
+ 2 H+ = H&O,
(5.2.1)
H&O,
(5.2.2)
2 H,SO,
= H,SO, + SO, = S,O;-
+ 2 H+
+ H,O SO, + Hz0 = HSO;
(5.2.3) + H+
3S,O:-+6OH-=5SO:+ S2- + 3 H,O. (5.3.1) The bondlength between the sulphur ions in dithionite is extremely large: 2.389Angstrom[15]. This indicates that the strength of the bond is weak. In electron resonance studies the radical ion SO;* is indeed found to exist in equilibrium with the dithionite ion, and the equilibrium constant is found to be 6.3 x IO-lo M[41]. Like sulphite, dithionite can be analysed by iodometric titration[42]. Dithionite can be determined
Table 5. The standard hydrogen potential of dithionite reduction Equation no. (51.1) (51.2) (5.1.3)
Redox pair S,O:- + 2 HS,O; + H&O; + 2 H&O, + H&O, + 2
H+ H+ H+ H+ H+
+ + + + +
2e2e2eZe2e-
= = = = =
(5.2.4)
S,O*- + H,O S,Oa- + H,O HS,O; + H,O HS,O; + H,O H&O, + H,O
E,(calc.)
Edliterature)
0.788 0.716 0.766 0.755 0.764
0.788*[7] 0.88[7]
2780
T. HEMMINGSEN Table 6. The standard
Equation
no.
hydrogen
potential
Redox pair
(6.1.1) (6.1.2) (6.1.3) (6.1.4)
0.090 - 0.052 0.048 0.065
S,O;+ 12 H+ + lCh- = 4 S + 6 H,O HS,O;+llH++lOe-=4S+6H.O
separately in the presence of sulphite by iodometric titration with the addition of formaldehydeC42, 431. A mixture of sulphide, thiosulphate and sulphite can be analysed first by determining the total consumption of iodine, and then masking the sulphite with formaldehyde or masking the sulphide by the addition of zinc ions[44-461. A mixture of dithionite, sulphite and thiosulphate can also be analysed. Three analyses are required. First, the total iodine consumption is found. The sulphite is then masked with formaldehyde prior to the titration with iodine. Then, the solution is titrated with bromine[43,47]. A number of other methods can be used to quantify the amount of sulphur compounds. The methods may include gravimetry, polarography, nephelometry, flame-photometry, spectroscopy, radiometry or chromatogrphy. An informative review is given by Blasius et aI.[48].
0.416 0.388
E,(literature) 0.090*[14],
2 HS- + 2 0, = S,O:2 HS- + 4 HSO;
Thiosulphate is a chemical used as a developing agent for photos. The thiosulphate ion has the ability to dissolve unreacted silver bromide. Thiosulphate may also be used medically. In dermatology the salt has an antimyocotic and antiparasitic effect.
(7.2.1) (7.2.2) (7.2.3) (7.3.1) (7.3.2) (7.3.3) (7.3.4) (7.4.1) (7.4.2) (7.4.3) (7.4.4) (7.4.5) (7.5.1)
potential
of thiosulphate
S,O:-+6H++4e-=2S+3H,O HS20;+5H++4e-=2S+3H,O H,S,O,+4H++4e-=2S+3H,O
S,O:S,O:S,O:HS,O; H&O,
+ + + +
30 32 27 22
H+ H+ H+ H+
+ 6 + 8 + 10 + 9 + 8
H+ H+ HC H+ H+
+ + + + + + + + +
24e24e24e24e-
= = = =
2 2 2 2
S:- + 15 H,O HS; + 15 H,O HS; + 15 H,O HS; + 15 H,O
8e8e8e8e8e-
= = = = =
2 2 2 2 2
S*HSH,S H,S H,S
SsO;-+12H++lk-=5S+6H,O
= 3 S,O;-
+ 3 H,O.
+ 6 H+ = 3 S + 3 H,O.
reduction
E,(calc.)
Redox pair
5 S,O;5 S,O,5 HS,O; 5 H,S,O,
+ H,O
(7.1.1) (7.1.2)
(7.1.3)
On the other hand, very acidic solutions of thiosulphate will not yield sulphur, but instead H2S405 and H,S,O, * S02. Thiosulphate may be oxidised to S,Oi- by Cu(II) or PbO, , or to sulphate by chlorine or bromine. When H,S and H,SO, are mixed under acidic conditions a very complex solution, called Wackenroder’s liquid, is formed[50]. A proposed mechanism for the formation of polythionates in this solution is given by Remy[Sl]. The reduction potentials for thiosulphate to sulphur, sulphide or polysulphides are given in Table 7. The oxidation of thiosulphate on platinum is described by Glasstone and Hickling[53]. At a potential of 0.47-0.67 the thiosulphate ions are oxidised to dithonate and some sulphate. The reaction
Oxidation state II
no.
0.289[2]
The acid H&O, or salts with HS,O; are not isolated, and may only exist as intermediates. Thus, the product formed when sulphide and sulphite are mixed under acidic conditions is not thiosulphate, but rather sulphur as given in Equation 7.1.3.
Tetrathionate can be formed by oxidizing thiosulphate with iodine[48]. The reduction potentials for tetrathionate to yield thiosulphate or sulphur are given in Table 6. Zezula has observed a two electron reduction of tetrathionate on a dropping mercury electrode at a potential range of - 0.280 to -0.380 V (she)[49].
hydrogen
0.17[15],
It decomposes at the site of application to sulphur and sulphite. These compounds have a healing effect on wounds[20]. It can also be used as “antichlorine” since the thiosulphate is oxidised to sulphate while chlorine is reduced under the same conditions. Thiosulphate can be formed when sulphide is carefully oxidised with air, or better, with sulphite in a neutral solution[40].
Oxidation state II,,,
Table 7. The standard
0.08[7],
0.416*[2]
2 S2- + SO:-
Equation
reduction
E,(calc.)
S,Oi+ 2e- = 2 S,O:HS,O, + 2e- = 2 S,O:+ H+ HS,0;+H++2em=2HS,0; HS,O, + 3 H+ + 2e- = 2 H,S,Os
(6.2.1) (6.2.2)
of tetrathionate
+ 3 H,O + 3 H,O + 3 H,O + 3 H,O + 3 H,O
E,(literature)
0.465 0.440 0.436
0.465*[2],
0.331 0.359 0.338 0.335
0.331*[2]
-0.006 0.200 0.303 0.291 0.288 0.484
-0.006*[2] 0.200[2]
0.484*[2]
0.602[7]
Sulphur-oxygen compounds-I
2781
Table 8. The standard hydrogen potential of sulphur reduction Equation no.
E,,(calc.)
Redox pair 5 S + Ze- = S:-
EJliterature)
(8.1.1) (8.1.2)
-0.340 -0.171
-0.340*[2]
5S+H++2e-=HS;
(8.2.1) (8.2.2)
4 S + 2e- = S:4S+H++Ze-=HS;
-0.360 -0.174
-0.360*[52]
(8.3.1) (8.3.2)
3 S + Ze- = S:3S+H++Ze-=HS;
-0.390 -0.168
-0.390*[52]
(8.4.1) (8.4.2)
2 S + Ze- = S:2S+H++2e-=HS;
- 0.428 -0.141
-0.428*[52]
s + 2e- = sS+H++2e-=HSS+2H++2e-=H.S
- 0.476 - 0.065 0.142
-0.476*[2], -0.508[14] -0.065[2, 143 0.142[2, 7],0.141[14]
mechanism is assumed to include the oxygen in the solution. The yield of dithionate can be enhanced by a prepolarixation in anodic direction, which regulates the pH to the optimal value of 8. Oxidation state 0 Sulphur may be formed through the reduction of pentathionate ions as given in Equation 7.5.1. It can further be reduced to sulphide and polsulphides as seen in Table 8. At room temperature elemental sulphur is most often found as cyclic S, . Polysulphides or sulphanes can be prepared by heating metal sulphide or hydrogen sulphide with sulphur in water[54]. Sulphur is reported to cause pitting and crevice corrosion on AISI 304 and AISI 316 stainless steels under oxygenfree conditions with traces of hydrochloric acid and moisture[55]. Table 9. The standard
hydrogen
Oxidation
potential reduction
of sulphide
Equation no.
Redox pair
(9.1.1)
4 St- + Ze- = 5 S:4 Si- + 5 H+ + Ze- = 5 HS; 4 HS; + H* + 2e- = 5 HS;
.. ::.:; (9.2.1)
s:-
state < 0
Polysulphides are of great importance as plant protectors. The best known polysulphide is the calcium salt, which is prepared by boiling sulphur with slaked lime[48]. Sulphides are present in several minerals. The volatile acid is very toxic with a very low level of lethality. The sulphide ions are also very corrosive, and will both increase the corrosion rate and cause hydrogen embrittlement in the steel. The sulphide ion is therefore given an extensive treatment in corrosion literature[56-591. See Table 9. The oxidation of sulphide on a pure platinum electrode takes place at a potential of 0.625VC60, 611. The product formed is platinum polysulphide. When the potential is raised, oxygen evolution will occur. The oxidation of sulphide on the platinum oxide surface will then occur at a potential of 1.4 V at
+ 8e- = 5 s-
E&alc.)
-0.441 0.490 -0.184
and
polysulphide E,(literature) -0.441’[2]
-0.511 0.003*[2]
KI (9:2:4)
S:+ 10 H+ + 8e- = 5 H,S S;-+5H++&-=5HSHS; + 5 H+ + 8e- = 5 H,S
0.003 0.262 0.051
(10.1.1) (10.1.2) (10.1.3)
3 s:- + 2e- = 4 s:3S:-+4H++2e-=4HS; 3HS;+H++2e-=4HS;
-0.478 0.410 -0.150
(10.2.1) (10.2.2) (10.2.3) (10.2.4) (11.1.1) (11.1.2) (11.1.3)
s:- + 6e- = 4 s2S2-+4H++6e-=4HSS!- + 8 H+ + 6e- = 4 H,S HS;+7H++6e-=4H,S 2 s:- + 2e- = 3 sj2S:-+3H++k-=3HS; 2HS;+H++2e-=3HS;
-0.515 0.033 0.309 0.247 -0.506 0.355 - 0.089
(11.2.1) (11.2.2) (11.2.3) (11.2.4)
s:-+4e-=3s2St-+3H++4e-=3HSHS;+2H++4c-=3HSHS;+5H++k-=3H,S
-0.520 0.097 -0.014 0.296
(12.1.1) (12.1.2) (12.1.3) (12.1.4)
S:- + 2e- = 2 S2Si- + 2 H+ + 2e- = 2 HSHS;+H++2e-=ZHSHS;+3H++2e-=2H,S
- 0.524 0.298 0.014 0.425
-0.478*[2]
0.033*[2] -0.506*[2]
0.097*[2]
- 0.524[2] 0.298*[2]
T. HEMMINGSEN
2182
Table 10. Values of dissociation constants (pKJ. Values marked with an asterisk are used in the
construction of the pH-potential diagrams HSO;/SO:H,SO,/HSO; HSO;/SO:H,S,OJHS,O; HS,O;/S,O:HS,O;/S,O;H,S,OJHS,O; HS,O;/S,O:H,S/HS HS-/S2HS;/S;HS;/S:HS;/S:HS;/S:HS,/S;HS;/S; HS,/S;-
1.70(64] 1.77[65] 6.99[7] 0.35*[7, 21, 663 2.45*[7, 211 4.80*[6] 0.29*[2] 1.69*[2] 7.00*[2] 13.9*[2] 9.7*[67] 7.5*[67] 6.3*[67] 5.7*[67] 5.2*[68] 4.8*[68] 4.4*[68]
1.91*[2] 1.80[64,663 7.00[64,66]
1.81[7] 7.21*[21]
2.46[66]
2.50[2]
HAO,
KS
HA
KS?,
H,S,~,
HA+ L?,
H&O,
SO2 H,SzO, ;03 S0,--I;12S0,SO2 LO,
SO,-H,S,O,
Polymeric sulphur compounds
Sulphur, like carbon, can form both covalent bonds and polymerize. Thus the most stable form of sulphur at room temperature is a ring-shaped molecule of eight atoms. The n-electrons stabilise the ring as is the case with the benzene molecule. In analogy to benzene, sulphite may attack the ring as a nucleophile, but opens the ring. The product is a sulphonic acid. The sulphite ion, however, quickly attacks the sulphonic acid, and degrades the acid stepwise through an S,2 mechanism to thiosulphate[lS, 691. Higher orders of sulphonic acid can be generated by adding sulphur trioxide to sulphanes[ 151. The simplest sulphane is H,S. When one equivalent sulphur trioxide is added, the simplest monosulphonic acid, thiosulphate (H&O& is formed. When one more equivalent sulphur trioxide is added, the simplest disulphonic acid, trithionic acid (H&O,), is formed. One should note that dithionic acid is not a disulphonic acid, but rather an oxidation product from sulphur dioxide. The general reaction scheme for monosulphonic acid and disulphonic acid (also called polythionic acid) is given in Equations 13.1-2. = H-S,-SO;
H,S, + 2 SO: - = -O,S-$-SO;.
&OX
p03
compounds are only valid at a limited pH range. It should also be noted that not all species are stable in the total pH range. However, they may exist as intermediates, and are therefore included in the diagrams.
H,S, + SO;-
Table 11. Oxyacids of sulphur[75]
So3-H2SP4 803
1.97[2]
060[7] 1.74[7]
room temperature. At this potential elemental sulphur is formed. Elemental sulphur can be removed from the electrode surface with acetone. The remaining platinum sulphide film is pale and yellow[61]. When the temperature is raised to 80°C two oxidation peaks appear[62]. The peak at 0.97 V refers to weakly bonded sulphur species, while the peak at 1.1 V refers to strongly bonded sulphur. The two kinds of adsorbed species are also observed from the adsorption reaction of SO,[34]. As the adsorbed sulphur prevents the hydrogen from adsorbing to the platinum surface and from being oxidized, the cyclic potential sweep technique can be used as a tool to quantify the amount of adsorbed sulphur[63]. Potential-pH equilibrium diagrams for the redox reactions of sulphur-oxygen compounds at a one molar concentration are constructed from data available in literature. In order to present the data more clearly, the diagram is split up into four. Figure 2 shows the reduction reactions of sulphur at oxidation states + V to + VIII, Fig. 3 shows the reactions at oxidation states III and IV, Fig. 4 shows the reactions at oxidation states between II and 0, and Fig. 5 shows the reactions at oxidation states lower than or equal to 0. No dissociation constants are found for the polythionates. It should therefore be kept in mind that a change in the slope will occur at a pH equal to the pK, value of these acids. Thus, the curves for these
WC
1.89*[21] 7.26[2]
SO3
SO3
W&O,
KSn+zO6
+ H+
(13.1.1) (13.1.2)
Until now the H,S,O, with n = 7 has been the highest member of the monosulphonic acids synthesizedC70, 713, and H,S,O, with n = 8 the disulphonic acids member of the highest synthesized[72]. However, a disulphonic acid with at least nine sulphur atoms has been detected by the use of high-voltage electrophoresis in Wackenroder’s liquid[73, 743. The oxyacids of sulphur may be classified as derivatives of sulphur dioxide and sulphur trioxide as given in Table 11[75]. When one of the hydrogen atoms in a sulphane is substituted with an alkyl or aryl group, one of the
Sulphur-oxygen compound+1 bridges to organic chemistry is made through syntheses of alkyl or aryl thiosulphonic acid[76]. CONCLUSIONS Like carbon, sulphur atoms have the ability to form covalent bonds. Therefore, a large number of sulphur compounds can be formed. Several of these compounds are unstable and will decompose. Thus, most often a mixture of sulphur compounds will be found in a aqueous solution. Due to this fact, it is often necessary to use sophisticated analytical methods to analyse such a mixture. Since most of the sulphur reactions involve electrons, electrochemical methods seem to be most applicable. Acknowledgement-The author would like to thank Prof. Torstein VBland and Prof. Ernst Maahn for helpful discussions. I would also like to give my thanks to the Nordic Council of Ministers for the scholarship granted under the “Nordic Energy Research Programme” and Rogaland Research for opportunity to write this paper.
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2783
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