Fuming sulphuric acid estimation

The amount of free trioxide in fuming sulphuric acid can be estimated by titration with water in a stoppered cylinder. The water is added until no further fuming occurs when air is blown through a glass tube on to the liquid surface.8 Each molecule of water added indicates the presence of one molecule of sulphur trioxide, thus ... 

Principle. A known weight of the substance is heated with fuming nftric acid in a sealed tube, the organic material being oxidised to carbon dioxide and water, and the sulphur to sulphuric acid. The latter is subsequently washed out of the tube, precipitated as barium sulphate, and estimated as such... 

Crude anthracene (about 40%) is mixed in a vessel fitted with a mechanical stirrer, with 1J times its weight of benzol or solvent naphtha (90% at 160°) this specification means that 90% by volume distils up to 160°. Sodium nitrite to one-tenth of the weight of crude anthracene taken is dissolved in 10 times its weight of water, and sufficient 10% sulphuric acid (7-2 gms. for each 1 gm. of 10% nitrite) to decompose this quantity of nitrite is added to the benzol—anthracene mixture and the temperature maintained at 25°. The nitrite solution is then run in at such a speed that no red fumes escape. When all the solution has been added the mixture is filtered at the pump. The filtrate consists of two layers, one of sodium sulphate solution and one of solvent naphtha, or benzol containing the impurities such as nitroso-carbazol. The purified anthracene on the filter is washed with benzol or solvent naphtha this latter on a large scale is used for the final treatment of a fresh lot of crude anthracene. The initial benzol or solvent naphtha, after separation from the aqueous solution, is recovered by distillation. The anthracene from this treatment will be about 80% pure. It may be purified to 95% by crystallising from heavy bases (pyridine, etc.), and is finally raised by sublimation and re-crystallisation from benzol to 98%. (For the estimation of purity, see p. 497.) (C. T., 23, 8, 21 D.R.P., 122852.)... 

Soluble Lead.—The nitric add solution from the preceding determination is evaporated with a little sulphuric acid first on a water-bath and then on a sand-bath until fumes of sulphuric acid appear. The cdd residue is taken up in water and the lead sulphate estimated as with white lead (q.v., paragraph 6). The lead thus found is that corresponding with the white lead contained in the product under examination. 

Phosphorus combined in organic compounds, or as phosphide in metals, is also estimated after oxidation by precipitation as ammonium phosphomolybdate or magnesium ammonium phosphate, with the subsequent treatment which is described on pp. 181—183. The methods by which the phosphorus is brought into solution vary with the nature of the material which is being analysed. Organic compounds are oxidised in a sealed tube with fuming nitric acid or in a flask by a mixture of concentrated sulphuric and nitric acids. 

Sulphates. These include sulphuric esters together with any inorganic sulphate (CaSO ). Dunnicliff gives the following method g, of nitrocellulose is heated on a water-bath with 50 c,c. cone, nitric acid until the nitrocellulose has dissolved and brown fumes have ceased. About 1 g. sodium chlorate is added in small quantities and the whole evaporated to dryness. The residue is treated with 20 c-c. of hydrochloric acid and 0 5 sodium chlorate (added in small quantities) and the liquid again evaporated to dryness. This latter treatment is repeated. This converts all sulphuric esters to soluble. sulphate, which may be estimated in the usual way. A blank test is made to allow for sulphate in the reagents. 

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