Sulphur dioxide test reagent

Acidify the soda extract with dilute hydrochloric acid, pass in sulphur dioxide to reduce the arsenate to arsenite, boil off the excess sulphur dioxide (test with potassium dichromate paper), and pass hydrogen sulphide into the solution to precipitate the arsenic as arsenic(III) sulphide continue the passage of hydrogen sulphide until no more precipitate forms. Filter, boil off the hydrogen sulphide, and test the filtrate for phosphate by the ammonium molybdate test or with the magnesium nitrate reagent. 

An alternative procedure for the above test is as follows. Mix 2-3 ml. of 2 per cent, aqueous paraperiodic acid solution with 1 drop of dilute sulphuric acid (ca. 2 5N) and add 20-30 mg. of the compound. Shake the mixture for 5 minutes, and then pass sulphur dioxide through the solution until it acquires a pale yellow colour (to remove the excess of periodic acid and also iodic acid formed in the reaction). Add 1-2 ml. of Schiff s reagent (Section 111,70) the production of a violet colour constitutes a positive test. 

Aromatic aldehydes react with the dimedone reagent (Section 111,70,2). All aromatic aldehydes (i) reduce ammoniacal silver nitrate solution and (ii) restore the colour of SchifiF s reagent many react with sodium bisulphite solution. They do not, in general, reduce Fehling s solution or Benedict s solution. Unlike aliphatic aldehydes, they usually undergo the Cannizzaro reaction (see Section IV,123) under the influence of sodium hydroxide solution. For full experimental details of the above tests, see under Ali-phalic Aldehydes, Section 111,70. They are easily oxidised by dilute alkaline permanganate solution at the ordinary temperature after removal of the manganese dioxide by sulphur dioxide or by sodium bisulphite, the acid can be obtained by acidification of the solution. 

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). 

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

Nickel(II) hydroxide test The auto-oxidation of sulphur dioxide (or sulphurous acid) induces the oxidation of green nickel(II) hydroxide to the black nickel(III) hydroxide. The colour change is quite distinct, but for very small amounts of sulphur dioxide use may be made of the conversion of benzidine acetate to benzidine blue by the nickel(III) hydroxide. (DANGER THE REAGENT IS CARCINOGENIC.) Thiosulphates give a similar reaction and must therefore be absent sulphides also interfere. 

Sch iff reagent A reagent used for testing for aldehydes and ketones it consists of a solution of fuchsin dye that has been decolorized by sulphur dioxide. Aliphatic aldehydes restore the pink immediately, whereas aromatic ketones have no effect on the reagent. Aromatic aldehydes and aUphatic ketones restore the colour slowly. 

Place 1-2 drops test solution (or a small quantity of the test solid) in the apparatus shown in Fig. 11.53 and place 1 drop sodium carbonate-phenolphthalein reagent on the knob of the stopper. Add 3-4 drops m sulphuric acid and insert the stopper into position. The drop is decolourized either immediately or after a short time according to the quantity of carbon dioxide formed. Perform a blank test in a similar apparatus. 

Place a drop of the alkaline test solution in the distillation apparatus of Fig. 11.59 and evaporate to dryness. Add 5 drops concentrated sulphuric acid and 5 drops pure methanol, stopper the apparatus, and heat to 80°C in a water bath. Collect the methyl borate which distils over in a micro porcelain crucible, waxed on the inside, and containing about 1 ml reagent. A black precipitate forms. For very small amounts of borate it is best to add a few drops of benzidine acetate solution and thus to detect the traces of manganese dioxide by the... 

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