hillebrand.] COLORIMETRIC ESTIMATION OF CHROMIUM. 
Table III. 
41 
No. 
II. 
III. 
Five grains iron ore 
Five grams iron ore 
Two grams silicate 
Chromic ox- 
ide added. 
Milligrams. 
7.03 
Chromic ox- 
ide found. 
2.99 
1.6 
Milligrams. 
7.18 
7.20 
7.25 
7.21 
3.08 
3.13 
1.53 
1.57 
1.59 
When the percentage of chromic oxide in an ore or mineral is more 
than about 0.2 per cent, and vanadium has not also to be estimated, 
much time may be spared by at once taking the color of the original 
extract from the sodium carbonate fusion after insuring complete 
reduction and removal of manganese, and perhaps concentrating. But 
if the chromium is much less than this amount, and especially if sev- 
eral grams of powder have been operated on, it becomes difficult or 
impossible to obtain a filtrate of sufficiently small bulk to show a 
decided color. Therefore, in such cases, and when vanadium is like- 
wise to be estimated, it is necessary to precipitate as above with 
mercurous nitrate in order to eventually have a small bulk of suf- 
ficiently colored solution. Even then, for very minute amounts, it is 
necessary to use Nessler tubes exactly as in ammonia estimations. 
If niter has been used in the fusion and the crucible has been 
attacked by it, a yellow color of the filtrate may be due to dissolved 
platinum, but neither the proportion of niter nor the temperature of the 
blast should ever be high enough to permit the crucible to be attacked. 
The Colorimetrio Apparatus and its Use. 
The glasses employed were of approximately square section, about 12 
centimeters high and 3.3 centimeters inside measurement, with exactly 
parallel sides, and of course as nearly alike as they could be obtained. 
One pair of opposite sides of each should be blackened. With glasses 
of the thickness mentioned it is generally advisable to use from 5 to 10 
cubic centimeters of standard at a time and to so regulate the strength 
of the solution to be tested that it shall contain more than 2 milligrams 
chromic oxide in 100 cubic centimeters, which is about the limit of 
distinct visibility in a thickness of 3.3 centimeters. 
In order to exclude the effect of side light in this and other similar- 
methods (titanium, for instance), it is very convenient to have a simple 
