Sulphite detection

The second region is the mixed kinetic transport-controlled region, and the most negative part of it can also be used for kinetic and mechanistic studies of the electron-transfer reaction after the experimental currents have been compensated for transport limitations. Finally, a second wave is observed at potentials higher than 0.5 V vs. AglAgCl, which can be attributed to the oxidation of sulphite to sulphate. However, this wave is not further considered because the oxidation mechanism of sulphite showed poor reproducibility (see section 6.3), and sulphite detection in dyeing processes is not of great importance compared with dithionite detection. 

Absolute diethyl ether. The chief impurities in commercial ether (sp. gr. 0- 720) are water, ethyl alcohol, and, in samples which have been exposed to the air and light for some time, ethyl peroxide. The presence of peroxides may be detected either by the liberation of iodine (brown colouration or blue colouration with starch solution) when a small sample is shaken with an equal volume of 2 per cent, potassium iodide solution and a few drops of dilute hydrochloric acid, or by carrying out the perchromio acid test of inorganic analysis with potassium dichromate solution acidified with dilute sulphuric acid. The peroxides may be removed by shaking with a concentrated solution of a ferrous salt, say, 6-10 g. of ferrous salt (s 10-20 ml. of the prepared concentrated solution) to 1 litre of ether. The concentrated solution of ferrous salt is prepared either from 60 g. of crystallised ferrous sulphate, 6 ml. of concentrated sulphuric acid and 110 ml. of water or from 100 g. of crystallised ferrous chloride, 42 ml. of concentrated hydiochloric acid and 85 ml. of water. Peroxides may also be removed by shaking with an aqueous solution of sodium sulphite (for the removal with stannous chloride, see Section VI,12). 

CAUTION. Ethers that have been stored for long periods, particularly in partly-filled bottles, frequently contain small quantities of highly explosive peroxides. The presence of peroxides may be detected either by the per-chromic acid test of qualitative inorganic analysis (addition of an acidified solution of potassium dichromate) or by the liberation of iodine from acidified potassium iodide solution (compare Section 11,47,7). The peroxides are nonvolatile and may accumulate in the flask during the distillation of the ether the residue is explosive and may detonate, when distilled, with sufficient violence to shatter the apparatus and cause serious personal injury. If peroxides are found, they must first be removed by treatment with acidified ferrous sulphate solution (Section 11,47,7) or with sodium sulphite solution or with stannous chloride solution (Section VI, 12). The common extraction solvents diethyl ether and di-tso-propyl ether are particularly prone to the formation of peroxides. 

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). 

Feubolt, R. and Klein, H., Determination of sulphite and ascorbic acid by high-performance liquid chromatography withe electrochemical detection,. Chromatogr., 640, 271, 1993. 

Phosphorescence can also be detected when the phosphor is incorporated into an ionic micelle. Deoxygenation is still required either by degassing with nitrogen or by the addition of sodium sulphite. Micellestabilized room-temperature phosphorescence (MS RTP) promises to be a useful analytical tool for determining a wide variety of compounds such as pesticides and polyaromatic hydrocarbons. 

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

The sodium chloride and sulphate regularly found in Leblanc soda ash are not usually injurious the insoluble matter should not exceed 1 to 1J per cent. It consists principally of calcium carbonate, alumina, silica, and ferric oxide. The sulphides should not be detectable by lead paper thiosulphates are destroyed in calcining the ash sulphites are usually present and can be detected by iodine soln. and sodium hydroxide, except in the so-called caustic ash, does not usually exceed 1 per cent. The moisture in fresh ash ranges below one per cent. Owing to the mode of preparation, ash by Solvay s process is more pure than that prepared by Leblanc s process. It does not contain sodium hydroxide, sulphides, sulphites, or thiosulphates it may contain a slight excess of carbon dioxide a little sodium sulphate is always present iron, alumina, and silica are present in minute traces sodium chloride is perhaps the only... 

When a solution containing sulphite is added to a few drops of Bettendorf s reagent (stannous chloride in concentrated hydrochloric acid) a yellowish-brown deposit of tin sulphides is formed, owing to reduction of the sulphite 0-6 per cent, of the latter can thus be detected.3... 

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... 

For the detection of small amounts of pentathionate in the presence of sulphites, see Czemotzky, Zeitsch. anorg. Ohem., 1928, 175, 402. 

To detect selenious acid in the presence of selenic acid,3 3 c.c. of the solution may he treated with 5 c.c. of concentrated sulphuric acid, a little sodium sulphite added and the mixture boiled. A red coloration indicates the presence of selenious acid. After filtering off any precipitate, 3 c.c. of concentrated hydrochloric acid and more solid sodium sulphite are added and the solution again boiled a red coloration or precipitate then indicates selenic acid. The sensitiveness of this test is the same as for the preceding test. 

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... 

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. 

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. 

Simultaneous detection of sodium dithionite, sulphite and indigo at a wall-jet electrode... 

For the simultaneous detection of sodium dithionite, sulphite and indigo, a multistep amperometric method was worked out and optimised, as outlined in Table 6.4. It is clear that after each measuring step, the electrode surface should be cleaned to remove indigo that is oxidised at all applied... 

In this section, the use of a wall-jet electrode (with optimal values for its parameters as described in section6.7.2) and the method to detect simultaneously sodium dithionite, sulphite and indigo (see section6.7.4) are evaluated as a function of reproducible dyeing processing. In order to evaluate this, a spectrophotometric method was used to measure the amount of dye absorbed and/or adsorbed by the dyed fabric. 

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