HILLEBKAND.] TITANIUM. 69 
saver. The error introduced by iron, in consequence of the yellowish 
color of its sulphate solution, is practically negligible unless its percent- 
age is very high; then either the iron must be removed prior to making 
the color test, or correction should be applied for known amounts of 
ferric sulphate in solutions of the requisite dilution. 
The exact correction to be applied in such cases is difficult of deter- 
mination because of the impossibility of matching the colors of titanium 
peroxide solutions with those of ferric sulphate; but tests made go to 
show that the coloring effect of 0.1 gram of Fe 2 3 in 100 cm. 3 5 per cent 
sulphuric-acid solution is about equal to 0.2 milligram of Ti0 2 in 100 cm. 3 
when oxidized by hydrogen peroxide. This amounts to a correction of 
only 0.02 per cent on 1 gram of rock containing the unusual amounts of 
10 per cent Fe 2 3 . It will be more satisfactory, when much iron is 
present, to remove this as described on page 71 and to colorimetrically 
estimate the titanium thus freed from iron. 
ALTERNATIVE MODE OF PREPARING THE TEST SOLUTION. 
As said above (pp. 57 and J38), the solution that has been used for 
volumetric estimation of total iron can most conveniently be used for 
the colorimetric determination of titanium, but if desired this can, of 
course, be made on some other portion of rock powder. At one time 
it was the practice in this laboratory to combine it with the determination 
of barium, as described in Bulletin 148 of the United States Geological 
Survey, by decomposing the powder by sulphuric and hydrofluoric 
acids, 1 expelling the latter by repeated evaporations with sulphuric 
acid, taking up with dilute sulphuric acid, 2 filtering from barium 
sulphate, etc., and estimating the titanium colorimetrically in the 
filtrate. The expulsion of fluorine must be thorough, or else the 
titanium result will be low, as already stated (p. 68), and it is not 
always easy to effect this complete removal, though the time required 
to do so seems to be in no slight degree dependent on the nature of 
the fluorides to be decomposed. Long after every trace of fluorine 
seems to be gone, the formation of a crust on the evaporating solution 
sometimes allows an accumulation of enough hydrofluoric-acid gas to 
become plainly manifest to the smell on breaking the crust. 
J It is to be borne in mind that evaporation with hydrofluorie acid alone results in loss of titanium 
by volatilization, but that there is no loss if excess of sulphuric acid is also present. 
2 With acid rocks solution is very complete, and it can be made nearly so with the most basic by 
transference to a small beaker and gentle boiling. The residue thus obtained may contain, besides 
barium sulphate, a little calcium sulphate, zircon, andalusite, topaz, and possibly a trace of titanium 
in some form. It is therefore to be thoroughly fused with sodium carbonate, leached with water, 
fused with potassium bisulphate, dissolved in dilute sulphuric acid, filtered, and the filtrate added 
to the main one. The insoluble matter will now be chiefly barium sulphate, for the further treatment 
of which see page 74, 
