68 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
of the peroxide should be approximately measured by titration with 
permanganate on opening a fresh bottle, and again after a few weeks, 
otherwise very serious error may arise through its deterioration. 
Mere traces of hydrofluoric acid, either in the peroxide or the tita- 
nium solution, render this method inexact, 1 hence care should be exer- 
cised as to the character of the peroxide, which, as sold in the market, 
often contains fluorine. 
Dunnington 2 has pointed out the necessity for the presence of at 
least 5 per cent of sulphuric acid in solutions which are to .be thus 
tested for titanium, in order, as he concludes, to prevent partial rever- 
sion to metatitanic acid, which does not give a color with hydrogen 
peroxide. The standard solution of titanium sulphate, holding con- 
veniently about 1 centigram Ti0 2 in 10 cm. 3 , equivalent to 1 per cent 
of Ti0 2 in 1 gram of rock, contains, therefore, 5 per cent or more of 
sulphuric acid. Of this, 10 cm. 3 are mixed with a sufficiency of hydro- 
gen peroxide (2 cm. 3 of most commercial brands is ample) and diluted 
to 100 cm. 3 in a measuring flask. 
Titanium can be estimated, as a rule, most conveniently in the solu- 
tion which has served for the titration of total iron (p. 57). This, hav- 
ing been evaporated, if necessary, to less than 100 cm. 3 , is to be fully 
oxidized with hydrogen peroxide, and if the color is less intense than 
that of the standard, is made up to 100 cm. 3 with dilute sulphuric acid 
in a measuring flask, and mixed; otherwise, in a flask of sufficient size 
to insure that its color shall be less intense. One of the rectangular 
glasses described below being filled with the solution to be tested, 10 
cm. 3 of the diluted standard are run into the other from a burette, and 
water is added from a second burette until there is no distinction as 
to color. A second and a third portion of the standard can be run in 
and diluted and the mean of several determinations struck, when a 
simple calculation gives the percentage of Ti0 2 in the rock. 
If the convenient but expensive Soleil-Duboscq colorimeter is used, 
or the simple Nessler tubes, it is of course unnecessary to dilute the 
rock solution to the extent above required, should it be stronger than 
the standard. Experience has shown, however, that differences can 
not be sharply estimated in strongly colored solutions, and that the 
results are much more satisfactory when the color intensity is not much, 
if any, greater than that given by a standard of the above concentra- 
tion. For the percentages of titanium found in rocks, clays, and soils, 
usually under 1 per cent, but rising to 2 or even 3 per cent or more 
occasionally, the colorimeter method gives results which are fully equal 
to those of the best gravimetric method, besides being a great time 
iHillebrand: Jour. Am. Chem. Soc.,Vol. XVII, p. 718, 1895; Chemical News, Vol. LXXII, p. 158,1895; 
Bull. U. S. Geol. Survey No. 167, p. 56. 
2 Jour. Am. Chem. Soc., Vol. XIII, p. 210, 1891. 
