Sulphite interference

Tests with silver nitrate solution -Sulphide, cyanide, and sulphite interfere in tests with silver nitrate solution, hence if any of these anions was detected in the preliminary test with dilute sulphuric acid, it must be removed first as follows. Acidify 1 ml of the soda extract with dilute acetic acid (use litmus paper) and boil gently in a small conical flask or crucible in the fume cupboard to expel H2S, HCN or S02 (1-2 minutes). It is important that the solution be acid throughout. Centrifuge, if necessary, and allow to cool. If the volume has been reduced appreciably, add water to restore the original volume. 

Tests with silver nitrate solution Sulphide and sulphite interfere in the tests with silver nitrate solution, hence must be removed first as follows. Acidify... 

Chloride ion-selective electrodes The most important region of application is the determination of chlorides in waters, including sea water (for a review, see [167]), in serum [110,112,371] (review in [167]) and in soil [151,219,341], The determination of chloride ions in sweat made screening for cystic fibrosis possible in new-born babies (review, [45,55a, 262]). Br , I and S " interfere in the determination of chlorides in phosphate rocks [81]. Sulphite can be determined directly using an electrode with an Hgj CI2 - HgS membrane [398] on the basis of the reaction... 

The various kinds of alkali, cf commerce usually contain a. greater or less proportion of chlorides and sulphates. The presence of these dees not interfere In the slightest degree with the above methods of estimation. In some cases, however, sulphides, sulphites, and hyposulphites aro also present, and these, neutralizing a certain quantity of the tost acjd, Tender the determinations more or less inaccurate. The first of these salts evolves sulphide of hydrogen, the second sulphurous acid, and the last hyposulphurous acid, which is immediately decomposed into sulphurous ecitl... 

A sensitive colour test for sulphite ions consists in adding, drop by drop, a 0-01 per cent, solution of Fast Blue R crystals, shaking after each addition, until the violet coloration disappears and a yellow solution is produced the test is sensitive to one part of sulphurous acid in about 175,000. Thiosulphates and polythionates do not interfere, but sulphides and hydroxides must be absent.1... 

The following interfere with the test strong reducing agents (hydrogen sulphide, dithionites, sulphites and selenites) V, U, Te, Hg, Bi, Au, Pd, Se, Te, Sb, Mo, W, Co and Ni. The reaction is not selective, but is fairly sensitive it can be used in the analysis of the Group IIB precipitate. Since iron(II) ions have no influence on the test, it may be applied to the tin solution which has been reduced with iron wire. 

Sulphides, sulphites, thiosulphates, cyanides, cyanates, fluorides, nitrites, and acetates interfere. The sulphur-containing anions can be quantitatively oxidized to sulphates by hydrogen peroxide ... 

Fuchsitt test Dilute solutions of triphenylmethane dyestuffs, such as fuchsin (for formula, see Section IV. 15, reaction 9) and malachite green, are immediately decolourized by neutral sulphites. Sulphur dioxide also decolourizes fuchsin solution, but the reaction is not quite complete nevertheless it is a very useful test for sulphur and acid sulphites carbon dioxide does not interfere, but nitrogen dioxide does. If the test solution is acid, it should preferably be just neutralized with sodium hydrogen carbonate. Thiosulphates do not interfere but sulphides, polysulphides, and free alkali do. Zinc, lead, and cadmium salts reduce the sensitivity of the test, hence the interference of sulphides cannot be obviated by the addition of these salts. 

Sulphites, sulphates, tetrathionates, and thiocyanates do not interfere, but hydrogen sulphide and ammonium sulphide decompose the reagent with the precipitation of nickel sulphide. 

Bromides and iodides interfere because of the liberated halogen the test is not trustworthy in the presence of chromates, sulphites, thiosulphates, iodates, cyanides, thiocyanates, hexacyanoferrate(II) and (III) ions. All of these anions may be removed by adding excess of nitrate-free Ag2S04 to an aqueous solution (or sodium carbonate extract), shaking vigorously for 3-4 minutes, and filtering the insoluble silver salts, etc. 

The precipitate is soluble in ammonia solution and in solutions of caustic alkalis. Large quantities of hydrochloric acid interfere with the test and should preferably be removed by evaporation to a small volume with excess concentrated nitric acid. Reducing agents, such as sulphides, sulphites, hexacyano-ferrate(II)s, and tartrates, seriously affect the reaction, and should be destroyed before carrying out the test. 

The test may also be conducted with 2 ml of the soda extract. Carbonate, sulphite, and thiosulphate have no influence upon the reaction. Nitrite interferes, presumably owing to the oxidation of the hydrogen cyanide. In the presence of sulphide, the test is complicated by the precipitation of black iron(II) sulphide when sulphate is added to the alkaline solution. It is best to boil the solution containing the suspended iron(II) sulphide, acidify with hydrochloric acid, and boil again to expel most of the dissolved hydrogen sulphide upon adding a drop of iron(III) chloride solution, a blue precipitate is produced if cyanide is present. 

Tritium is measured by liquid scintillation counting of a portion of a distilled sample. Several reagents (such as sodium sulphite and silver iodide) can be added in the distillation to prevent interference by radioiodine. The allowed concentration of tritium in water for human consumption is relatively high thus the method presented here is normally adequate for routine determinations. However, if required, lower concentrations of tritium in water can be determined by electrolytic enrichment. The principles of the tritium determination procedure are as follows. 

Metal ions forming chloride complexes which give the same reactions as Sb(V) with RB interfere in the determination. These are Au(III), Tl(III). Ga(III), and Fe(III). Gold can be separated after reduction to the element with sulphite. Gallium and iron can be separated by extraction as chloride complexes before the oxidation of antimony to Sb(V). Small amounts of Fe(III) are masked with phosphoric acid. 

Numerous metals, e.g.. Be, Al, Ga, In, Se, Cr(III), Zr, U(V1), Th, V, interfere in the determination. Fe(II) does not react with ECR and CTA. Nevertheless, the colour reaction proceeds slowly as a result of oxidation of Fe(II) to Fe(III). This happens even in the presence of NH2OH or sulphite, but not in the presence of ascorbic acid, which keeps iron(ll) in the lower oxidation state. The method with ECR and CTA becomes highly selective if the determination of Fe is preceded by extractive separation of iron as its thiocyanate complex (see Section 26.2.1). 

Interfering species in the determination of phosphorus by the phosphomolybdenum blue method are As(V), Si, and Ge, which also react with molybdate to form the corresponding acids which are reduced to the respective heteropoly blues. Arsenic(V) does not interfere when reduced to As(III) using sulphite or thiourea. In the presence of vanadium(V), molybdovanadophosphoric acid is produced. Large amounts of vanadium(V) are reduced with Mohr s salt to V(IV) before the molybdate is added. The difference in the rates of formation of the phosphomolybdenum- and silicomolybdenum- blues has been utilized for the determination of phosphorus in the presence of silicon [29]. The interference of silicon can be prevented by the use of a sufficiently acidic medium [30]. ... 

The chief interference is from Fe(lll), which forms a green complex with chromotropic acid. Before the determination of Ti, larger quantities of iron should be separated or smaller ones reduced with ascorbic acid or sulphite. Vanadium in quantities not exceeding those of titanium has no appreciable effect on the determination of Ti. Molybdenum at concentrations below 50 pg/ml does not interfere. Fluoride interferes by masking titanium, but can be removed by fuming with H2SO4. Oxidants (e.g. HNO3) must be absent because chromotropic acid is fairly easily oxidized. 

Sulphides, sulphites, thiosulphates, cyanides, cyanates, fluorides, nitrites, and acetates interfere. The sulphur-containing anions can be quantitatively oxidized to sulphates by hydrogen peroxide. The modified procedure in the presence of these anions is therefore to stir a drop of the test solution with 4 drops 3% hydrogen peroxide, then to add 2 drops m sulphuric acid, and to continue as above. Cyanides are rendered innocuous by treating the test solution with 4 drops of a saturated solution of mercury(II) chloride, followed by 2 drops sulphuric acid, etc. the slightly dissociated mercury(II) cyanide is formed. Nitrites can be removed by treatment with aniline hydrochloride. 

Another important detector for ion chromatography is the electrochemical detector (ECD). This makes it possible to detect anions such as cyanide, nitrite, sulphide, bromide, iodide and sulphite with maximum sensitivity. Thanks to its specific detection capabilities it is also possible to identify these ions without interference alongside high concentrations of other ions which may also display similar retention values (Fig. 75). 

In the presence of oxalic add, and with the reductant added separately, phosphate ions do not interfere. If, however, the oxalic add and the reductant are added together (as in the metol-sulphite method), a slight increase in colour intensity can be observed 1 foaoVL of phosphate corresponding to about 0.05 /imol/L of silicate. 

The following technique was evolved by Allport and Jones for the colorimetric determination of small quantities of procaine in injections and tablets. Adrenaline, ephedrine, boric acid, chlorbutol, sodium sulphite, ammonium chloride, sodium chloride and sodium phosphate, as the substances normally dispensed with procaine, were shown not to interfere, but it should be noted that the method is not applicable to solutions preserved with cresols or other phenols. 

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