Sulphur dioxide detection

The Karl Fischer procedure has now been simplified and the accuracy improved by modification to a coulometric method (Chapter 14). In this procedure the sample under test is added to a pyridine-methanol solution containing sulphur dioxide and a soluble iodide. Upon electrolysis, iodine is liberated at the anode and reactions (a) and (b) then follow the end point is detected by a pair of electrodes which function as a biamperometric detection system and indicate the presence of free iodine. Since one mole of iodine reacts with one mole of water it follows that 1 mg of water is equivalent to 10.71 coulombs. 

Gaseous sulphur dioxide is highly irritant and practically irrespirable. Effects on the body are summarized in Table 4.3. It can be detected at about 3.5 ppm and the irritating effects would preclude anyone from suffering prolonged exposure at high concentrations unless unconscious, or trapped. 

Reduction to S(IV) is the basis of two spot tests of Feigl, although these apply to other S(VI) classes, e.g. sulphonamides. Thus Feigl and Lenzer fused the sample with alkali to yield sulphite, then treating with hydrochloric acid and warming to expel sulphur dioxide they detected the latter with nickel(II) hydroxide on test paper, which yielded ultimately the black Ni(IV) oxyhydrate (see also Section C). In the other test FeigF fused the sample with sodium formate/alkali, cooled and acidified with sulphuric acid to liberate sulphur dioxide in this case also. This was detected by a ferric chloride/potassium ferricyanide reagent which yielded a blue colour (Prussian, Turnbull s). 

Verhoef and co-workers suggested omitting the foul smelling pyridine completely and proposed a modified reagent, consisting of a methanolic solution of sulphur dioxide (0.5 M) and sodium acetate (1M) as the solvent for the analyte, and a solution of iodine (0.1 M) in methanol as the titrant the titration proceeds much faster and the end-point can be detected preferably bipoten-tiometrically (constant current of 2 pA), but also biamperometrically (AE about 100 mV) and even visually as only a little of the yellow sulphur dioxide-iodide complex S02r is formed (for the coulometric method see Section 3.5). 

The detection and determination ot the perchlorates.—The perchlorates give no precipitates with silver nitrate or barium chloride soln. cone. soln. give a white crystalline precipitate with potassium chloride. Unlike all the other oxy-acids of chlorine, a soln. of indigo is not decolorized by perchloric acid, even after the addition of hydrochloric acid and they do not give the explosive chlorine dioxide when warmed with sulphuric acid unlike the chlorates, the perchlorates are not reduced by the copper-zinc couple, or sulphur dioxide. Perchloric acid can be titrated with —iV-alkali, using phenolphthalein as indicator. The perchlorates can be converted into chlorides by heat and the chlorides determined volumetrically or gravimetrically they can be reduced to chloride by titanous sulphate 28 and titration of the excess of titanous sulphate with standard permanganate they can be fused with zinc chloride and the amount of chlorine liberated can be measured in terms of the iodine set free from a soln. of potassium iodide and they can be... 

Sulphur, detection of, 1041, 1045 Sulphur dioxide, 185, 607 test reagent, 553 ... 

Because atmospheric humidity must be avoided, the reaction flask is isolated from the atmosphere with drying tubes. Moreover, since the solvent is rarely perfectly anhydrous and will contain traces of water due to its hygroscopic nature, its water content must be measured prior to the determination. The equivalence point of the titration reaction is detected by an electrical method instead of a visual method. The current intensity that passes between two platinum electrodes inserted in the reaction medium is measured (see Fig. 19.10). The reagent, which is a mixture of sulphur dioxide, iodine and a base, is characterised by the number of mg of water that can be neutralised by 1 cm3 of this reagent. This is referred to as the equivalent mass concentration of water, or the titre T of the reagent. 

Detection.—Sulphur in the free state is readily recognisable by its general appearance and characteristics, and especially by its combustion to sulphur dioxide. Both in mixtures and compounds the presence of the element can be demonstrated by heating with charcoal and an alkali carbonate,2 or even better, on a small scale, by heating with an equal bulk of sodium or potassium,3 or with powdered iron 4 in each case some of the sulphur is converted into sulphide, which may be detected by the action of an aqueous extract on mercury or silver, or on sodium nitroprusside the metals are blackened, whilst the nitro-prusside is very sensitive in giving a purple coloration (see p. 62). Alternatively, the solution of the alkali sulphide may be acidified and tests applied for hydrogen sulphide to the vapours evolved on warming. 

Various microchemical tests are available for the detection of minute quantities of sulphur, both free and combined. The substance under examination may be treated with a little sodium hydroxide solution, the extract evaporated just to dryness, a few drops of aqueous sodium cyanide (0-1 per cent.) added and the evaporation repeated. The residue, moistened with dilute sulphuric acid and a drop of ferric chloride, gives the characteristic ferric thiocyanate colour if sulphur is present.6 In the ease of minerals, traces of sulphur dioxide produced on heating may be detected 6 by the colour change of an alkaline solution of Bromocrcsol Green or by the deeolorisation of starch-iodine solution. 

Physical Properties.—Sulphur dioxide is a colourless gas possessing a pungent, choking odour, and exerting an extremely irritating action on the mucous membrane. In consequence of this it has been used in poison gas warfare, but it is not efficient against well-equipped troops on account of the ease with which it is extracted from the air by respirators. Approximately 0-0005 per cent, can be detected by the sense of smell, whilst 0-05 j>er cent, is unendurable.2 Vegetation is injured if the concentration of the gas exceeds 0-003 per cent. 

Detection and Estimation.—Sulphurous acid is usually detected by its reducing action, for instance on potassium diehromate solution, the acid being warmed in order that the test may be effected with the evolved gas the odour of sulphur dioxide is also a fairly trustworthy indication. For special purposes9 many of the reactions already mentioned may be applied. 

If gaseous sulphur dioxide is passed into a solution of thiosulphate, a yellow solution is formed which on keeping becomes colourless it then yields a precipitate of sulphur when treated with formaldehyde and sodium hydroxide, but no polythionate can be detected. When the colourless solution is neutralised with sodium hydroxide, it is found to contain sulphite and thiosulphate, but sulphur is not precipitated. These results may be due to the formation of an additive compound such as that mentioned in (ii) above.5... 

Detection and Estimation.—The decomposition of thiosulphates by means of hydrochloric acid to yield sulphur dioxide with separation of sulphur serves as a primary identification test, the limit of sensitiveness being about 0-1 mg. S203 per c.c.3 The alkali thiosulphates produce with silver nitrate solution a white precipitate of thiosulphate which gradually turns yellow, then brown, and finally black, due to the formation of sulphide the change is accelerated by warming 4... 

Properties.—The blue solution is unstable and decomposes slowly, with formation of sulphuric acid, sulphur dioxide and nitrogen dioxide. When shaken with air or submitted to oxidation by chlorine, nitric acid or hydrogen peroxide, conversion into nitrosulphonic acid is effected, brown fumes being liberated. Dilution -with water also destroys the coloured substance. If strongly cooled, the solution changes to an intense red, so that if a solution is too weak to possess a marked colour at the ordinary temperature, the presence of the purple acid can easily be detected by cooling in a mixture of acetone and solid carbon dioxide. 

The detection and estimation of selenium in the mother-liquors from the preparation of sulphite-cellulose is rendered difficult by the colour of these liquors, due to the organic substances present. The presence of the element is indicated by the fact that a red deposit is formed on the sides of the combustion tube when the organic matter is being burnt off. Estimation may be carried out by evaporating the liquor until all the free sulphur dioxide has been evolved, acidifying with hydrochloric acid and boiling to remove the last traces of sulphur dioxide. After allowing to stand in a warm place for a few days the selenium settles to the bottom of the container and may be estimated as described.4... 

Another method 4 for the detection of tellurium in the presence of selenium depends on the fact that the former is not precipitated by sulphur dioxide in the presence of concentrated hydrochloric acid. A solution in the concentrated acid (dens. 1T6) is heated to 90° C. and saturated with sulphur dioxide the precipitated selenium is removed by filtration, the filtrate diluted with an equal volume of water and saturated in the cold with sulphur dioxide. If tellurium is present a black precipitate is formed immediately. 

Disadvantages associated with sulphur dioxide are that some tasters can detect it as an unpleasant banknote or taint and it has a tendency to provoke allergic reactions in some individuals. Asthma sufferers tend to be affected by gaseous sulphur dioxide, small traces of which can promote an asthmatic attack. There is a risk with foods containing sulphites of gas liberation upon swallowing. 

Sulphur dioxide is a widely used preservative for foods and soft drinks, particularly dilutables, and in principle is quite easy to detect and quantify. Owing to the importance of sulphur dioxide as a preservative an extensive review of its chemistry in foods was undertaken in the mid-1980s (Wedzicha, 1984). 

There is a very simple and quick method that can be used to detect the reducing power of sulphur dioxide, developed in the last century and often called the Ripper titration (Ough, 1988). In this method, sulphur dioxide is titrated against iodine or potassium iodate/potassium iodide solution in the presence of starch. When all the sulphur dioxide has been oxidised, a blue colour is produced by the reaction of free iodine with the starch. This is a very quick method but will give only an estimate of the level of sulphur dioxide as other reducing substances, such as ascorbic acid, will interfere consequently, this method is not particularly appropriate for juices with high ascorbate levels. 

Detection of Sulphur Dioxide.—This test is made on sugars from juices decolorised by means of sulphurous acid. To a solution of 10-15 grams of the sugar in about 25 c.c. of water in a flask are added a scrap of pure zinc (or 0-3-0-4 gram of magnesium wire) and 5 c.c. of pure hydrochloric acid. In presence of sulphur dioxide, hydrogen sulphide is evolved and may be detected by the odour or by means of a strip of lead acetate paper. 

Detection of Sulphur Dioxide.—Sulphurous anhydride, which is often added to tanning extracts, either as such or more often as sulphites, to enhance their keeping qualities, may be detected by treating 10 grams of the extract in a beaker with 20 c.c. of hydrochloric acid and 20 c.c. of water, a piece of pure zinc being added and the beaker covered with a clock-glass under which is suspended a strip of lead acetate paper if the extract contains sulphurous anhydride, the paper becomes brown or assumes a blackish tint with metallic reflection. If the paper remains white for 15 minutes, absence of sulphurous acid may be assumed. 

---