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Estimation of Sulphur

Estimation of Sulphur is often performed by extraction with carbon disulphide, especially where a distinction is to be made from combined sulphur. Total sulphur is usually determined by oxidation to sulphuric acid, the latter being weighed as barium sulphate. The oxidising agents commonly applied are bromine water, nitric acid, perchloric acid, potassium nitrate, potassium chlorate and sodium peroxide; the last three are usually mixed with anhydrous sodium carbonate and applied by fusion; they are suited to the estimation of sulphur in organic compounds.

Determination of Sulphur in Organic Compounds
Electrolytic Apparatus for Determination of Sulphur in Organic Compounds.
In the estimation of sulphur in organic compounds by oxidation with nitric acid, the acid is sometimes mixed with bromine or an alkali chloride, or, more frequently, fuming nitric acid is used, in which case the oxidation is commonly effected in a sealed glass tube at 150° to 300° C. A very convenient electrolytic apparatus (fig.) has, however, been described, in which only ordinary concentrated nitric acid is necessary; the lower electrode is the anode, and the bubbles of gas, escaping periodically from beneath the concave surface, cause sufficient agitation and stirring. The great advantage of such a method as this is its safety, the oxidation proceeding at the ordinary pressure, although requiring no longer time than the sealed-tube method.

Whenever nitric acid a nitrate has been used for the oxidation, it is advisable to remove excess by evaporation with hydrochloric acid before proceeding to the precipitation of the sulphuric acid with barium chloride. Another method applicable to organic compounds consists in completely converting the sulphur into hydrogen sulphide. The substance is vaporised or decomposed by heating in a current of hydrogen and the mixture of vapours and hydrogen then passed through a heated quartz tube containing platinised asbestos. The resulting hydrogen sulphide is absorbed in alkali and estimated iodometrically.

A rapid volumetric method for the determination of sulphur is as follows: The sulphur is dissolved in a known volume of hot standard sodium hydroxide solution and after cooling is oxidised to sulphate by the addition of hydrogen peroxide; the excess of alkali is then titrated with standard acid. The method is applicable to gases containing any common sulphur compound except thiophen (e.g. coke oven gas).

The Estimation of Sulphur content of coals, oils, tars, asphalts, etc., may be determined by combustion in an oxygen bomb, a little ammonium nitrate being added to the charge to ensure complete oxidation to sulphuric acid. After washing out the bomb the sulphur is determined as barium sulphate.

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