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28.02 ‑ Sulphur, sublimed or precipitated; colloidal sulphur.
(A) SUBLIMED OR PRECIPITATED SULPHUR
The sulphur in these two categories is usually about 99.5 % pure.
Sublimed sulphur, or flowers of sulphur, is obtained by slow distillation of crude or impure sulphur, followed by condensation in the solid form (or sublimation) as fine, very light particles. It is chiefly used in viticulture, in the chemical industry or for vulcanising high‑grade rubber.
This heading also includes "washed sublimed sulphur", treated with ammonia solution to eliminate the sulphur dioxide; this product is used in medicine.
The precipitated sulphur classified here is always obtained by precipitating a solution of a sulphide, or of an alkaline or alkaline‑earth polysulphide, with hydrochloric acid. It is more finely divided and paler yellow than sublimed sulphur; its odour is somewhat similar to that of hydrogen sulphide and it deteriorates with age. Its uses are almost entirely medicinal.
The precipitated sulphur of this heading must not be confused with certain "recovered" (triturated or micronised) sulphurs sometimes described as "precipitated" but classified in heading 25.03. (B) COLLOIDAL SULPHUR
Colloidal sulphur is obtained by the action of hydrogen sulphide on a solution of sulphur dioxide containing gelatin. It may also be obtained by the action of mineral acid on sodium thiosulphate, or by cathodic pulverisation. Colloidal sulphur is a white powder which forms an emulsion with water; however it can be preserved in this state only if a protective colloid (albumin or gelatin) is added, and even then it can be kept for only a limited time. The heading includes this prepared colloidal solution. Like all colloidal dispersions, sulphur dispersions have a large surface for adsorption and can take up colouring matter; they are also very active antiseptics used internally in medicine.
The heading excludes unrefined sulphur as obtained by the Frasch process and refined sulphur, even though they are often very pure (heading 25.03).
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