Estimation of Sulphur -- Carius Method

Sulphur is estimated by the Carius method: a precisely known mass of the organic substance is heated strongly, in a sealed tube, together with fuming nitric acid. The carbon and hydrogen present are oxidised to carbon dioxide and water as a side reaction, while the sulphur present is specifically oxidised all the way to sulphuric acid. Treating the resulting solution with excess barium chloride then quantitatively precipitates that sulphuric acid as barium sulphate, and the MASS of that barium sulphate precipitate is what the whole estimation actually measures.

Practically: the weighed organic compound, together with a few millilitres of fuming nitric acid, is sealed inside a Carius tube, which is then placed inside a protective iron tube and heated for around five hours. After the tube is allowed to cool, a small hole is made in it to safely vent any gas pressure that built up, before the (now depressurised) Carius tube is broken open and its full contents collected in a beaker. Adding excess barium chloride solution to this beaker precipitates every trace of the sulphuric acid formed as barium sulphate, which is then filtered off, washed, dried and weighed. …