Sulphur Dioxide and Sulphites

Similar to hydrogen sulphide and sulphides, sulphur dioxide and sulphites can be determined by titration with iodine or by oxidation to sulphate followed by gravimetric determination as barium sulphate. Additionally a spectrophotometric method was developed based on the colour of p-nitroaniline which is discharged on acidification but is restored by adding formaldehyde and sulphur dioxide or sulphite. 

The reaction is rather slow but is best carried out in dilute solution. To avoid the aerial oxidation of the sulphite, it is added to a known excess of standardised iodine solution, back titrating the excess with thiosulphate. 

Prepare freshly 0.05 M iodine solution. Pipette 25.0 cm of this solution into a 400 cm conical flask, add 5 cm 2 M HCl and dilute with water to 180 cm. Prepare freshly a solution which is 0.05M in sulphite or SO2. Add from a burette 15-20 cm of this solution to the iodine solution. This should be done slowly with stirring and keeping the tip of the burette close to the iodine solution. Then titrate the excess of iodine against standardised 0.01 M thiosulphate solution prepared by 5-fold dilution of 0.05 M solution. Follow the procedure in Sec.7.3.1. Alternatively weigh out accurately 0.09-0.13 g of the purest sulphite and dissolve it in the acidified diluted iodine solution and continue as above. 

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There are some differences in the regulation of US and EU antioxidants. The restrictions on synthetic antioxidants are more strict in the EU, e.g. TBHQ, THBP, anoxomer, ethoxyquin, guaiac resin and derivates of thiodipropionic acid are not permitted there. On the other hand, sulphur dioxide and sulphites, citric and tartaric acids and their salts and salts of EDTA are not listed as permitted antioxidants in the US. 

Sometimes BHT and BHA along with citric acid are added to produce more effect. Sulphur dioxide and sulphite are useful antioxidants for wine and beer, sugar syrups and cut, peeled or dried fruits and vegetables. 

Sulphur dioxide and sulphite at pH=8 and pH = 10 buffer at carbon fibre... 

Sorbates, Benzoates and p-Hydroxy Benzoates (Parabens) Sulphur Dioxide and Sulphites Other Preservatives. 

Ough, C.S. Sulphur dioxide and sulphites, pp 177-203 in Antimicrobials in Foods, Branen, A.L. and Davidson, P.M. (Eds.) Marcel Dekker, New York, 1983... 

The alkaline sodium sulphite solution may be replaced by saturated amtnonlum sulphite solution prepared as follows. Pass sulphur dioxide into a mixture of 1 part of concentrated ammonia solution (sp. gr. 0-88) and two parts of crushed ice in a freezing mixture imtil the liquid smells strongly of sulphur dioxide, and then neutralise with ammonia solution. This solution slowly deposits ammonium sulphite crystals and contains about 0-25 g. of SOj per ml. Use 60 ml. of this ice-cold ammonium sulphite solution to which 8 ml. of concentrated ammonia solution are added. After the addition of the solution of p-nitrophenyldiazonium chloride, allow the mixture to stand for 1 hour in a freezing mixture, filter oft the yellow precipitate of ammonium p-nitrophenyUiydrazine disulphonate, heat it on a water bath with 20 ml. of concentrated hydrochloric acid at 70-80° for 7 minutes, cool the blood-red solution, and dissolve the resulting precipitate of p-nitrophenylhydr-azine hydrochloride and ammonium salts in water, and isolate the base as above. 

Annex III lays down the conditions of use for permitted preservatives and antioxidants, with lists of foods and maximum levels in each case. Part A lists the sorbates, benzoates and p-hydroxybenzoates, E 200-E 219 part B lists sulphur dioxide and the sulphites, E 220-E 228 part C lists other preservatives with their uses, including nisin, dimethyl dicarbonate and substances allowed for surface treatment of certain fruits, E 249 potassium nitrite, E 250 sodium nitrite, E 251 sodium nitrate and E 252 potassium nitrate, E 280-E 283 propionic acid and the propionates part D lists the antioxidants E 320 butylated hydroxyanisole (BHA), E 321 butylated hydroxytoluene (BHT), E 310 propyl gallate, E 311 octyl gallate, E 312 dodecyl gallate, E 315 eiythorbic acid and E 316 sodium erythorbate. 

The fact that the ultimate products of the atmospheric oxidation of sodium thiosulphate are sulphur dioxide and sodium sulphate may be explained by assuming a primary decomposition into sulphite and sulphur (traces of the latter always being present in the salt), and subsequent oxidation of these.3... 

Sulphur colouring matters are more or less soluble in water, but readily soluble in alkalies or alkaline sulphides when treated with hydrochloric acid or, better, with hydrochloric add and stannous chloride, they emit hydrogen sulphide when burnt they give an odour of sulphur dioxide and leave a more or less abundant residue containing alkaline sulphates and sulphites. 

In a buffer of pH=8, completely different results were obtained for sulphur dioxide and sodium sulphite as starting species. Starting from sulphite, only one oxidation wave is obtained around 0.7 V vs. AglAgCl. From Table 12.1 it can be seen that sulphite itself is the main compound in solution, therefore it is clear that this wave can be attributed to Equation 12.17. Its slope (Fig. 12.10, curve 6), obtained by plotting the peak current versus concentration, is situated in the range that allows exchange of two electrons ... 

Sodium Pallado-sulphite, Na2Pd(S03)2.2Na2S03, is obtained2 as a white precipitate on saturating a solution of palladous chloride with sulphur dioxide and subsequently neutralising with sodium carbonate. 

Potassium Hydrogen Chlorosrnisulphite, 0sCl4(S03)4KeH2, obtained by the prolonged action of potassium hydrogen sulphite on potassiimi osmichloride, crystallises in dark red, monoclinic prisms. When heated with hydrochloric acid it evolves sulphur dioxide, and is converted into potassium osmichloride. 

It is also precipitated by the action of ammonia on a solution containing sodium chloride and ammonium sulphite in equimolecular proportions.10 Hartley and Barrett11 have described a method of preparation from sulphur dioxide and sodium carbonate. It is a white, crystalline salt. 

Potassium sulphite, K2S03.—The sulphite can be prepared by passing sulphur dioxide into a solution of potassium carbonate till evolution of carbon dioxide ceases or by dissolving a known weight of potassium hydroxide in water, saturating with sulphur dioxide, and adding an equal weight of the hydroxide. Evaporation of the solution yields the rhombic dihydrate, which is transformed into the anhydrous salt at 120° C., a white solid with heat of formation 272-6 Cal.7 A monohydrate is also known. 

The liquid (b.p. —10 ) which has a dielectric constant of 14 and a dipole moment of 1.61D, is a good solvent. There does not appear to be any selfionisation of the type displayed by dinitrogen tetroxide (p. 329). Isotope exchange shows that sulphite sulphur exchanges with sulphur dioxide and that thionyl halide sulphur does not. SO3 exchanges oxygen but not sulphur with solvent SO3. 

Aeid sulphite pulping was one of the earliest proeesses used for the produetion of ehemieal pulp. Sulphur dioxide is obtained by burning sulphur and absorbing the gas in the desired base, for example sodium earbonate, to give a solution of sulphur dioxide and sodium bisulphite. This is further fortified with vented sulphur dioxide from previous cooks to give a liquor which contains about 5 to 6% sulphur dioxide (H2SO3) and from 1.0 to 1.5% combined sulphur dioxide (Na2S03). 

Experiment 61. Properties of Sulphur Dioxide, Sulphurous Acid, and Sulphites. 

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