Note 4 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

The following proportions are accordingly taken by weight: 5 parts of powdered manganese peroxide, 11 parts of salt (best fused, to prevent its frothing), and 14 parts of sulphuric acid previously mixed with an equal volume of water. The mixture is heated in a salt bath, so as to obtain a temperature above 100°. The corks in the apparatus must be soaked in paraffin (otherwise they are corroded by the chlorine), and black india-rubber tubing smeared with vaseline must be used, and not vulcanised rubber (which contains sulphur, and becomes brittle under the action of the chlorine).

The reaction which proceeds may be expressed thus: MnO₂ + 2NaCl + 2H₂SO₄ = MnSO₄ + Na₂SO₄ + 2H₂O + Cl₂. The method of preparation of Cl₂ from manganese peroxide and hydrochloric acid was discovered by Scheele, and from sodium chloride by Berthollet.