A solution of
tungstate acidulated with hydrochloric acid becomes intensely blue on
the addition of stannous chloride and warming. Fused in a bead of
microcosmic salt it gives a clear blue colour (reddish-brown if iron is
also present) in the reducing flame, but is colourless in the oxidising
flame.
~Solution and Separation.~--The decomposition and solution of natural
tungstates is difficult to effect owing to the separation of tungstic
acid; the method of treatment is as follows:--Boil the finely-powdered
substance with hydrochloric acid or aqua regia till it apparently ceases
to be attacked; dilute, filter, and wash with dilute hydrochloric acid.
Cover with dilute ammonia, and filter the solution, which contains
ammonic tungstate, into an evaporating dish. Treat the residue again
with acid, and again dissolve out the separated tungstic acid with
ammonia, and repeat this operation until decomposition is complete. By
this means there will be obtained--(1) a solution containing tungstate
of ammonia; (2) an insoluble residue with silicates, and oxides of tin,
niobium, tantalum, &c.; and (3) an acid solution containing the soluble
bases. The tungstate of ammonia requires simple evaporation on the
water-bath and gentle ignition in order to cause the tungstic acid to be
left in an almost pure state; possibly, it may carry a little silica.
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