Sulphur 740 dioxide

Sulphur dioxide is reduced by pentacyanocobaltate is several stages (c/. the reduction of p-benzoquinone), viz. 

Sulphur dioxide is used as a preservative for beer, wine and meats in the produetion of sulphites and hydrosulphites in solvent extraetion of lubrieating oils as a general bleaehing agent for oils and foods in sulphite pulp manufaeture in the eellulose and paper industries and for disinfeetion and fumigation. 

Gaseous sulphur dioxide is highly irritant and praetieally iiTespirable. Effeets on the body are summarized in Table 5.3. It ean be deteeted at about 3.5 ppm and the irritating effeets would 

Liquid sulphur dioxide may cause eye and skin burns resulting from the freezing effects upon evaporation. Dry sulphur dioxide is non-corrosive to common materials of construction except zinc. The presence of moisture renders the environment corrosive. 

In addition to the precautions listed in Table 9.3, the following controls are appropriate  

Wear eye/face protection, approved footwear and rubber gloves. 

Latent heat of vaporization at boiling point 92.8cal/g 

Latent heat of fusion at melting point 27.6cal/g 

Sulphur Dioxide. The. ST-ray emission and absorption spectra of sulphur in the SO2 molecule have been investigated both experimentally and theoretically. The spectra are linked energetically, thus making possible an analysis of iJie arrangement of occupied and free levels in the molecule. The relative positions of the energy levels obtained from the JT-ray spectra compare favourably with those obtained by theoretical calculations. The electronic structures of SO2 and SO have been investigated within the framework 

The reduction of SO2 in DMF has been investigated by electrochemical methods. At a gold electrode, SO2 is reduced by a one-electron process to a product which when complexed by SO2 is a blue species  

The product also dimerizes to dithionite, SaOl , which is also complexed by SO2, to give a red species  

The electrical conductances of some 34 electrolytes in liquid SO2 at 25 °C have been measured. The poisoning of y-alumina by CO2 has been shown to suppress the catalytic reaction  

Using i.r. evidence, a mechanism may be postulated whereby SO2 is adsorbed on hydroxyl sites and COS on aluminium sites on the oxide. The latter sites become blocked by the irreversible chemisorption of COg. The reaction of S02 with carbon at elevated temperatures has been shown to involve the following reactions  

Sulphur Dioxide. The observed products and high rates of insertion of sulphur dioxide into tin-carbon bonds in organotin allenes and alkynes have been interpreted in terms of an electrophilic cleavage mechanism. Such a mechanism is also proposed for sulphur dioxide insertion into mercury-carbon and into lead-carbon bonds. The mechanism of sulphur dioxide insertion into transition metal (Mo, W, or Mn)-carbon bonds is again similar.  

Intramolecular Insertion.—This occurs in metal insertion into phenyl-ort/ru-carbon to hydrogen bonds during the intramolecular oxidative addition of triphenylphosphine complexes of iridium(i). Intramolecular insertion also takes place in palladium(n)-catalysed decomposition of vinyl 

Hirabayashi, K. Itoh, S. Sakai, and Y. Ishii, J. Organometallic Ghent., 1970, 21, 

As a postscript it should be mentioned that the reaction mechanism for tin(ii) halides with FeI(7r-C5H5)(CO)2 and related compounds does not, as was previously thought, involve insertion.  

Sulphur dioxide has been used in the preservation of food products for many centuries (Barnett 1985). It is routinely used for drying apricots because it is inexpensive and effective. Sulphur in the form of free sulphur dioxide is mostly responsible for the preservative action in fruit. There are four main reasons for adding sulphur to preserve fruit  

Sulphur dioxide is an effective inhibitor of the non-enzymatic browning that occurs during storage of dried apricots. 

The micro-organisms are inhibited by the presence of sulphur dioxide. 

It has antioxidant properties that are useful in the preservation of the carotenoids present (Barnett 1985). 

Residual sulphur may not exceed 2000 ppm for the world market (World Health Organization). A level of 2000ppm of sulphur dioxide indicates that the dried product should maintain acceptability for up to a year in storage at ambient temperature and should remain of good quality. Darkening occurs rapidly in storage if the sulphur level in the dried product is lower than 1500 ppm (McBean et al. 1967). 

Sulphur dioxide gas is produced in the process from the combustion of sulphur under controlled conditions. Exposure to sulphur dioxide gas may occur due to leaks in the process or if the sulphur store accidentally catches fire. Leaks can be detected by placing the stopper of a bottle containing ammonium hydroxide close to the area of the suspected leak. Dense white fumes are released by the reaction of sulphur dioxide with ammonia. Continuous cycle sequential monitoring systems e.g. Gelman type are recommended for the early detection of sulphur dioxide leaks, particularly if the plant is located inside a building. 

In the event of a sulphur fire, large quantities of sulphur dioxide may be released. The procedures for extinguishing the fire outlined in section 3.1.2 should be followed. It is important to stay up-wind of any sulphur dioxide source, away from low lying areas where the gas could collect. Any closed space should be well ventilated before entering and self- contained, positive pressure breathing apparatus worn. In the event of exposure to sulphur dioxide breathing may become difficult. Oxygen must however only be administered by a qualified physician. 

Sulphur dioxide is extremely irritating. A concentration of 0.3 to 1 ppm can be detected by the average individual, 3 ppm has an easily noticeable odour and 20 ppm is the least amount which is irritating to the eyes. In the absence of local legislation the UK (HSE) or USA (ACIGH) covering maximum allowable exposure limits should be applied  

1 Bleaching of wool with sulphur dioxide (stoving process) 

In this method moist woollen yam in the hank form is exposed to the action of sulphur dioxide produced by burning sulphur (3 to 6% o.w.f) placed in an iron or earthenware pot inside the chamber. Sulphur is converted to sulphur dioxide and 

The solvent properties of sulphur dioxide were discovered some time ago several reviews are available . 

Specific Conductivity at —10° (Ohm i cm i). Enthalpy of Vaporization (kcal mole i)  

Apart from many covalent substances sulphur dioxide dissolves metal sulphites (with formation of disulphites) as well as tetraalkylammonium halides, alkali halides and pseudohalides to some extent, and numerous covalent compounds. 

Lichtin, N. N. Progress in Physical Organic Chemistry, Vol. I, Wiley, New York 

Waddington, T. C. Non-Aqueous Solvent Systems, Chapter 6, Academic Press, London-New York 1965. 

Cleavage of the iron-carbon bond in reactions of sulphur dioxide with complexes of the type [FeR( j -C5H6)(CO)2] is an electrophilic process which proceeds with a large and negative entropy of activation and with inversion of configuration at the a-carbon. The inversion of stereochemistry observed on insertion of sulphur dioxide 

Studies on the cleavage of iron-carbon r-bonds by sulphur dioxide have usually been carried out in neat sulphur dioxide. In chloroform solution the reaction of [Fe(CHMe2)( j -C6H5XCO)2] with SOa is first order in the iron-alkyl and SOa, with Ah — 8.7 0.6kcalmol and AS = — 45 2 cal mol The second- 

Although reactions of sulphur dioxide with [Fe(CHDCHDCMe3)( / -C5H5)(CO)2] proceed with inversion in organic solvents the concentrations of sulphur dioxide are apparently higher than those used in the kinetic studies described above where a different mechanism may operate. Similarly the report that [Fe(Me)( -l-Me-3-Ph-CsHaXCOXPPhg)] inserts sulphur dioxide with high stereospecificity at iron (probably retention) both in liquid sulphur dioxide and in methylene chloride solution does not resolve this problem.  

Reactions of sulphur dioxide with [Fe( -allylX -QH6XCO)2] are known to afford S-bonded sulphinate complexes. It is generally accepted that these reactions proceed via dipolar metal-alkene intermediates, and low-temperature n.m.r. studies have now provided evidence for this reaction intermediate (8). The rates of re- 

Although there is only one oxidation reaction possible with sulphur dioxide and hence a selectivity problem does not exist, recent results from kinetic research are included in this chapter, since there is a close analogy with other oxidations, especially on V2Os-based catalysts. 

The oxidation of sulphur dioxide to trioxide is one of the oldest heterogeneous catalytic processes. The classic catalyst based on V2Os has therefore been the subject of numerous investigations which are amply reviewed by Weychert and Urbaneck [346]. These authors conclude that none of the 34 rate equations reported is applicable over a wide range of process conditions. Generally, these equations have the form of a power expression, in which the reverse reaction is taken into account within the limits imposed by chemical equilibrium, viz. 

Langmuir—Hinshelwood models have been proposed as well as models based on a redox mechanism. 

Recently, Happel et al. [154] using data from Kadlec et al. [167,217] conclude that a model based on the dissociative adsorption of oxygen, which is rate-determining, fits the experimental results best, viz. 

The industrial catalyst consists of a mixture of V2Os and K2S207 supported on silica. Under technical reaction conditions ( 440°C), this mixture forms a viscous molten phase on the surface of the porous silica structure. Apparently a redox model can also be applied to such a system [154], 

spectroscopy, electrical conductivities, and chemical evidence suggest that the insertion of sulphur dioxide into transition-metal-alkyl and -aryl bonds in complexes of the type [FeR(CO)2(A -C6H5)], [MoR(CO)3-(A -CgHg)], [MnR(CO)s], and [ReR(CO)s] involves the formation of intermediates which almost certainly have O-bonded sulphinate structures of the type (17) rather than S-bonded alkoxysulphenate structures of the type (18). 

The O-bonded sulphinates isomerize slowly to the corresponding S-bonded sulphinates, e.g. (19), which are the final products of insertion, but attempts to isolate the intermediates have not been successful since on removal of the last traces of SO2 rapid linkage isomerization to the S-sulphinates occurs. This isomerization may be analogous to the conversion of [(NHs)5Co(ONO)] + into [(NH3)6Co(NOa)] +.  

Studies on the reaction of sulphur dioxide with threo- Q h -C (CHDCHDCMeaXCO) show that insertion proceeds with inversion of configuration at the a-carbon, However, no conclusion concerning retention or inversion of configuration at carbon can be obtained from the reaction of sulphur dioxide with [Fe(A -C5H6)(CHMePh)(CO)J.2  

Elimination of sulphur dioxide from an S-bonded sulphinate complex is a process related to sulphur dioxide insertion. Kinetic studies on the rearrangement of the five-co-ordinate sulphinato-complexes (20) to the sulphur dioxide complexes (21) show that if the organic group R contains electron-withdrawing 

Use in well ventilated conditions and eliminate sources of ignition. 

Do not rely on the sense of smell to detect hydrogen sulphide leaks. Strips of v/et lead acetate paper turn black on exposure to hydrogen sulphide and offer a simple indicator, as do colour indicator tubes. For plant-scale operations, instrumental multi-point detectors and alarms are likely to be more appropriate. 

Segregate cylinders of hydrogen sulphide from oxygen or other highly-oxidizing or combustible materials. 

Ground all lines and equipment used with hydrogen sulphide. 

Insert traps in the line to prevent suck-back of liquid into the cylinder. 

Preservative E-no. Alternative form used at equivalent level E-no. 

Benzoic acid (m.p. 122°C) E210 Sodium benzoate E211 

Benzene carboxylic acid Calcium benzoate E213 

Sulphur dioxide (gas) E220 Sodium sulphite E221 

Carbenes,—Attempts to insert Cp2 into several metal-metal bonds failed, because the difluorocarbene precursor MeaSnCCFg) underwent exchange reactions at one of the metals, e.g. 

Sulphur Dioxide.—Insertion reactions of sulphur dioxide into transition-metal-alkyl bonds and into metal-carbon bonds have been the subject of review articles. 

spectroscopy, and chemical evidence, all suggest that the mechanism of insertion of sulphur dioxide into metal-carbon bonds in such compounds as Fe(A -C6H6)(R)(CO)2, Mo(A -C6H6)(R)(CO)3, and MnR(CO)6 involves initial formation of an O-bonded sulphinate, e.g. (23), followed by rearrangement to the stable S-bonded product, e.g. (24). The reaction of sulphur dioxide with 2-alkynyl compounds of transition metals was first considered to produce allenyl(oxy)sulphinyl compounds MS(0) 0C(R)=C=CH2. Later, the products were said to be allenyl-O-sulphinates, MOS(0) C(R)=C=CHa. Now the available evidence 

Jacobson, P. Reich-Rohiwig, and A. Wojcicki, Chem. Comm., 1971, 1526. 

The first kinetic study of sulphur dioxide insertion into a transition-metal-carbon bond, mentioned above, indicated an electrophilic mechanism. Such a mechanism is also indicated by the first kinetic study of sulphur dioxide insertion into tin-carbon bonds. For insertion into the substituted phenyltrimethyltin compounds (26), the variation of second-order rate constant with the nature of the substituent Y gave a reasonable Hammett 

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Edeleanu process An extraction process utilizing liquid sulphur dioxide for the removal of aromatic hydrocarbons and polar molecules from petroleum fractions. 

Sulphur dioxide, SO2, m.p. — 72-7°C, b.p. — I0"C. Colourless gas with characteristic smell. Formed by burning S, metal sulphides, H2S in air or acid on a sulphite or hydrogen sulphite. Powerful reducing agent, particularly in water. Dissolves in water to give a gas hydrate the solution behaves as an acid - see sulphurous acid. Used in the production of SO3 for sulphuric acid. 

Hence, acids can be defined as substances producing cations characteristic of the solvent (solvo-cations, for example NH4, NO ), and bases as substances producing anions characteristic of the solvent (solvo-anions, for example OH , NH, NO3). This concept has been applied to solvents such as liquid sulphur dioxide, liquid hydrogen chloride and pure sulphuric acid. 

Lead reacts only briefly with dilute hydrochloric and sulphuric acids for both lead(Il) chloride and lead(Il) sulphate are insoluble and form a film on the lead which effectively prevents further attack. Lead, however, does slowly dissolve in both concentrated sulphuric and hydrochloric acids. The sulphuric acid is reduced to sulphur dioxide ... 

The gas is passed through caustic soda solution to remove any sulphur dioxide or carbon dioxide produced in side reactions. Carbon monoxide is also obtained when an ethanedioate (oxalate) is heated with concentrated sulphuric acid ... 

At high temperatures oxygen reacts with the nitrogen in the air forming small amounts of nitrogen oxide (p. 210). Sulphur burns with a blue flame when heated in air to form sulphur dioxide SO2, and a little sulphur trioxide SO3. Selenium and tellurium also burn with a blue flame when heated in air, but form only their dioxides, Se02 and Te02. 

These elements are generally unaffected by non-oxidising acids (behaviour expected for non-metallic elements) but they do react when heated with concentrated sulphuric and nitric acids, both powerful oxidising agents. Sulphur is oxidised to sulphur dioxide by hot concentrated sulphuric acid,... 

A method of estimating small amounts of water in organic liquids (and also in some inorganic salts) is that of Karl Fischer. The substance is titrated with a mixture of iodine, sulphur dioxide and pyridine dissolved in methyl alcohol. The essential reaction is ... 

Hydrogen sulphide bums in air with a blue flame yielding sulphur dioxide, but if the air supply is limited, preferential combustion to form sulphur occurs ... 

Sulphur dioxide is formed together with a little of the trioxide when sulphur burns in air ... 

Sulphur dioxide is also produced by the action of an acid (usually concentrated sulphuric since it is involatile) on a sulphite or hydrogensulphiie. for example... 

On the industrial scale it is produced in large quantities for the manufacture of sulphuric acid and the production methods are dealt with later. It was once estimated that more than 4 000 000 tons of sulphur dioxide a year entered the atmosphere of Britain from the burning of coal and oil. 

The molecule of sulphur dioxide has a bent structure. Both S—O distances are equal and short and since sulphur can expand its outer quantum level beyond eight, double bonds between the atoms are likely i.e. 

Liquid sulphur dioxide is a solvent for a number of substances, for example iodine, sulphur, some sulphites, potassium iodide and sulphur dichloride oxide, SOClj (see below). The liquid can be assumed to ionise slightly, thus ... 

Sulphur dioxide is oxidised by chlorine in the presence of charcoal or camphor to give sulphur dichloride dioxide sulphuryl chloride), SOjCl ... 

Sulphur dioxide is an acidic oxide and dissolves readily in water, and in alkalis with which it forms salts ... 

Although sulphur dioxide, as a gas, is a reducing agent in the sense that it unites with oxygen, free or combined (for example in dioxides or peroxides) most of its reducing reactions in aqueous solution are better regarded as reactions of sulphurous acid (in acid solution), or the sulphite ion (in alkaline solution). 

The solution obtained when sulphur dioxide dissolves in water has long been thought to contain unionised sulphurous acid, H2SO3, but more probably contains hydrated sulphur dioxide (cf. NHj solution, p. 217). The solution behaves as a dibasic acid, i.e. 

Two important redox potentials for reduction by sulphur dioxide in aqueous solution are ... 

This reaction is a useful test for a sulphite or for moist sulphur dioxide, which turns dichromate paper (filter paper soaked in potassium dichromate) from yellow to green. 

In strongly acid solution, substances which are normally reducing agents reduce sulphur dioxide solution or sulphites, for example iron(II) and zinc ... 

When a saturated solution of sulphur dioxide is titrated against approximately 2 M sodium hydroxide solution the following pH curve is obtained Figure 10.4) ... 

Figure 10.4. Titration of 25 cm of saturated aqueous sulphur dioxide with 2 M sodium...

Alternatively these salts can be prepared by first saturating a known volume of alkali with sulphur dioxide, giving a solution of the hydrogensulphite, from which sulphite can be prepared by the addition of a second equal volume of alkali. 

Barium sulphite is soluble in dilute hydrochloric acid unlike barium sulphate which is insoluble. Hence this reaction, and the evolution of sulphur dioxide on addition of an acid, distinguishes a sulphite from a sulphate. 

In the laboratory it is commonly prepared by the reaction between sulphur dioxide and oxygen at high temperature in the presence of a platinum catalyst ... 

Sulphur dioxide is obtained in the following three ways ... 

Since arsenic is often found in nature associated with sulphide ores, sulphur dioxide obtained by this method may contain some arsenic(III) oxide as impurity, and in certain processes this is a distinct disadvantage. 

The gases from the kiln contain about 9% sulphur dioxide. (The calcium oxide combines with the silica to form a silicate slag which, when cool, is crushed and mixed with some anhydrite to give cement, a valuable by-product.)... 

In all the above methods, the sulphur dioxide obtained is impure. Dust is removed by first allowing the gases to expand, when some dust settles, then by passage through electrostatic precipitators and finally by washing with water. Water is removed by concentrated sulphuric acid which is kept in use until its concentration falls to 94%. 

The combination of sulphur dioxide and oxygen to form the trioxide is slow and does not proceed to completion ... 

The 94% acid from the sulphur dioxide drying towers (above) is used here and its strength brought up to 98 %. This is concentrated sulphuric acid. Stronger acid up to 106% may also be made. This concentration is suitable for sulphonating in, for example, the detergent industry. 

Concentrated sulphuric acid has a strong affinity for water and great heat is evolved on mixing hence the acid must be added to water to dilute it. Because of this affinity, the acid can be used to dry gases with which it does not react, for example oxygen, chlorine, sulphur dioxide, and is used in desiccators. It will remove water of crystallisation from some compounds, for example... 

Concentrated sulphuric acid is an oxidising agent, particularly when hot, but the oxidising power of sulphuric acid decreases rapidly with dilution. The hot concentrated acid will oxidise non-metals, for example carbon, sulphur and phosphorous to give, respectively, carbon dioxide, sulphur dioxide and phosphoric(V) acid. It also oxidises many metals to give their sulphates cast iron, however, is not affected. The mechanisms of these reactions are complex and the acid gives a number of reduction products. 

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