c. of boiling water.
During effervescence the beaker is covered, the precipitate is stirred,
and 2 c.c. of methylated spirit are mixed in. The precipitate is
collected on a large filter, washed with cold water, and then with hot,
till free from chlorine, which is tested for with starch and potassium
iodide. The acid ferrous sulphate solution (presently described) is then
measured into the beaker, and the precipitate, still in the paper,
added; more acid is added (if necessary), and the solution is diluted
and titrated. In place of bleaching powder solution, 90 c.c. of bromine
water (containing 22 grams per litre) may be used.
FERROUS SULPHATE ASSAY.
This method, which is the one commonly used, is based on the
determination of the amount of ferrous iron oxidised by a known weight
of the ore. It is known that 87 parts of the dioxide will oxidise 112
parts of ferrous iron;[83] therefore 1 gram will oxidise 1.287 gram of
ferrous iron, or 1 gram of ferrous iron oxidised will be equivalent to
0.7768 gram of the dioxide. The finely-divided substance containing the
dioxide is digested in a solution of a known quantity of iron in
sulphuric acid. The iron, of course, must be in excess, which excess is
determined when the ore is dissolved by titrating with standard
permanganate or bichromate of potash solution.
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