(1) 10 parts of bisulphate of potash.
(2) 5 parts of bisulphate of potash and 5 parts of potassium fluoride.
(3) 5 parts of hydric potassic fluoride.
(4) 12 parts of bisulphate of potash; and, afterwards, with 6 parts of
carbonate of soda and 6 parts of nitre.
(5) 8 parts of borax; afterwards, with carbonate of soda till it ceases
to effervesce; then, with 3 parts of carbonate of soda and 3 of
nitre.
(6) 4 parts of borax and 6 parts of fusion mixture.
(7) 12 parts of caustic potash.
(8) 10 parts of caustic soda and 30 of magnesia.
(9) 5 parts of caustic soda and 3 of magnesia.
(10) 2 parts of carbonate of soda and 1 of lime.
(11) 6 parts of soda-lime and 2 of chlorate of potash.
(12) Sodium peroxide.
Of these, numbers 1, 2, and 3 yield the chromium in a form soluble in dilute acids, as chromic salt. The rest in a form soluble in water, as potassium or sodium chromate.
On boiling an insoluble chromium compound with chlorate of potash and nitric acid, the chromium passes into solution as chromate. This method, however, does not answer for chrome iron ore. In the fusion methods the ore must be very finely powdered, well mixed with the fluxes, and subjected to a prolonged fusion in a platinum vessel at a high temperature. Undecomposed particles require re-fusion.
The aqueous extract containing the chromate is ready for volumetric work, except in those cases where nitre has been used. For gravimetric work the solution is acidified with hydrochloric acid, then mixed with ammonia in slight excess, boiled, and filtered. The filtrate is acidified with hydrochloric acid, and treated with sulphuretted hydrogen, warmed, rendered slightly alkaline with ammonia, and the gas again passed. The chromium is precipitated as chromic hydrate mixed with sulphur from the reduction with sulphuretted hydrogen. It is filtered off, washed with hot water, and ignited. It is weighed as chromic oxide.
GRAVIMETRIC DETERMINATION.
The solution containing the chromium, freed from other metals and earths and in the form of (green) chromic salt, is heated to boiling. If any chromate is present reduce it with sodium sulphite or sulphuretted hydrogen. Add ammonia in slight excess, boil till the liquid is free from a red tint, and allow to settle for a few minutes. Filter, wash with hot water, dry, and ignite strongly in a loosely-covered crucible. Cool, and weigh. The substance is chromic oxide, Cr2O3, and contains 68.62 per cent. of chromium. It is a dark-green powder insoluble in acids.
When, as is generally the case, the chromium exists altogether as chromate (phosphates and arsenates being absent) it is best to proceed as follows:—Render the solution acid with acetic acid, then add sodium acetate to the solution and heat nearly to boiling; next treat with a slight excess of acetate of lead, and boil. Allow to settle, and filter. Wash the precipitate with hot water, dry in the water-oven or at a low temperature. Transfer the precipitate to a weighed Berlin crucible, burn the filter separately, ignite below redness, cool in the desiccator, and weigh. The substance is lead chromate, PbCrO4, and contains 16.1 per cent. of chromium, or 23.53 per cent. of chromic oxide (Cr2O3).
VOLUMETRIC METHOD.
This is based on the oxidation of ferrous iron by the solution containing the chromium as chromate. A known weight of iron (0.5, 1, or 1.5 gram, according to the quantity of chromate) is dissolved in 50 c.c. of dilute sulphuric acid. The solution containing the chromate is added, and the remaining ferrous iron titrated with the permanganate or bichromate of potassium solution, as described under Iron. The iron thus found is deducted from that taken, and the difference gives the iron oxidised by the chromate. This multiplied by 0.3101 gives the chromium, Cr, and when multiplied by 0.4529 gives the chromic oxide, Cr2O3.