80 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS. [bull. 176. 
GRAVIMETRIC METHOD. 
Having obtained chromium in solution as chromate and free from 
all else but a little alumina, as at the conclusion of the preceding sec- 
tion on phosphorus, proceed as follows: 
Concentrate if necessary and add fresh ammonium sulphide, or 
introduce hydrogen sulphide. The chromium is reduced and appears 
as a precipitate of sesquioxide mixed with the rest of the alumina. 
This precipitate is now treated according to Baubigny * by dissolving 
in nitric acid, evaporating to near dryness and heating with strong 
nitric acid and potassium chlorate, finally evaporating to dryness to get 
rid of the acid. Oxidation is complete and very speedy. On dilution 
with cold water, acid sodium carbonate is added in slight excess, and 
after two or three hours the precipitated alumina is filtered off. From 
the filtrate the chromium is then thrown out by fresh ammonium sul- 
phide, redissolved and reprecipitated to free from alkali, and weighed. 
The separation of aluminum from chromium by hydrogen peroxide 
in ammoniacal solution, as recommended by Jannasch and Cloedt, 2 has 
been shown by Friedheim and Briihl, 3 together with most of the other 
separations of Jannasch based on the use of lrydrogen peroxide and 
from which so much was hoped, to be valueless. 
COLORIMETRIC METHOD. 
For this very accurate and by far the quickest method 4 for deter- 
mining chromium in rocks and ores where the amount does not exceed 
a few per cent, there is needed the aqueous extract of a sodium-car- 
bonate fusion of the rock (as obtained, for instance, under Phosphorus, 
p. 79) in order to compare its color with that of a standard solution. 
Preparation and strength of standard solution. — This standard solu- 
tion is made by dissolving 0.25525 gram or double that amount of 
pure potassium monochromate in one liter of water made alkaline b}^ 
a little sodium carbonate. Each cubic centimeter then corresponds to 
one-tenth milligram or two-tenths milligram of chromic oxide (Cr 2 3 ), 
in which condition chromium is usually reported in rocks and ores. It 
is probably inadmissible to increase the strength of the standard much 
above the figure given. 
Preparation of the test solution. — Before filtering the aqueous 
extract of the sodium-carbonate fusion a few drops of alcohol (ethyl or 
methyl) are added to destroy the color of sodium manganate. If the 
yellow color of the filtrate is very faint, concentration by evaporation 
iBull. Soc. Chimique (n. s.) Vol. XLII, p. 291, 1884; Chemical News, Vol. L, p. 18, 1885. 
2 Zeitschr. furanorg. Chemie, Vol. X, p. 402, 1895. 
3 Zeitschr. fur anal. Chemie, Vol. XXXVIII, p. C81, 1899. 
4 W. F. Hillebrand, Jour. Am. Chem. Soc., Vol. XX, p. 454, 1898; Chemical News, Vol. LXXVIII.pp. 
227, 239, 1898; Bull. U. S. Geol. Survey No. 167, p. 37; First applied by L. de Koningh (Nederl. Tyds. VOOI 
Pharm. Chem. and Tox.. 1889) for the estimation of chromium in foodstuffs. 
