84 SOME PRINCIPLES AND METHODS OF ROOK ANALYSIS, [bull. 176. 
in a small pressure bottle. 1 If the color of the precipitate indicates 
absence of arsenic the filter with its contents is carefully ignited in 
porcelain, and the delicate sulphuric-acid test for molybdenum is 
applied as follows: The molybdenum compound is heated in porcelain 
with a single drop of strong sulphuric acid till the acid is nearly vola- 
tilized. On cooling a beautiful blue color is proof of the presence of 
molybdenum. 
The filtrate, in bulk from 25 cm. 3 to 100 cm. 3 is boiled to expel hydro- 
gen sulphide, and titrated at a temperature of 70° to 80° C. with a 
very dilute solution of permanganate representing about 1 mg. of 
V 2 C) 5 per cubic centimeter as calculated from the iron strength of the 
permanganate, one molecule of V 2 5 being indicated for each one of 
Fe 2 3 . One or two checks are always to be made by reducing again 
by means of a current of sulphur dioxide gas, boiling this out again, 2 
and repeating the titration. The latter results are apt to be a very 
little lower than the first and are to be taken as the correct ones. 
In case the volume of permanganate used is so small as to make 
doubtful the presence of vanadium, it is necessary to apply a qualita- 
tive test, which is best made as follows: The solution is evaporated and 
heated to expel excess of sulphuric acid, the residue is taken up with 
two or three cubic centimeters of water and a few drops of dilute nitric 
acid, and a couple of drops of hydrogen peroxide are added. A char- 
acteristic brownish tint indicates vanadium. Unless the greater part 
of the free sulphuric acid has been removed the appearance of this 
color is sometimes not immediate and pronounced, hence the above 
precaution. It is also necessary that the nitric acid shall be in con- 
siderable excess, since in a neutral or only faintly acid solution the 
color does not appear strongly. 
The above is a surer test to apply than the following: Reduce the 
bulk to about 10 cubic centimeters, add ammonia in excess and intro- 
duce hydrogen sulphide to saturation. The beautiful cherry-red color 
of vanadium in ammonium-sulphide solution is much more intense 
than that caused by hydrogen peroxide in acid solution, but the action 
of ammonia is to precipitate part or all of the vanadium with the 
chromium or aluminum that may be present or with the manganese 
used in titrating, and ammonium sulphide is unable to extract the 
vanadium wholly from these combinations. Usually, however, the 
1 From a sulphuric solution the separation of platinum and molybdenum by hydrogen sulphide is 
much more rapid and satisfactory than from a hydrochloric solution. 
2 The direct use of a solution of sulphur dioxide or of an alkali sulphite is inadmissible unless the. c e 
have been freshly prepared, since after a lapse of time they contain other oxidizable bodies than sul- 
phurous acid or a sulphite. The sulphur dioxide is best obtained as wanted by heating a flask contain- 
ing a solution of sulphur dioxide, or of a sulphite to which sulphuric acid has been added. 
The expulsion of the last traces of sulphur dioxide is said to be more effectively accomplished by 
boiling with simultaneous passage of a rapid current of carbon dioxide for a few minutes at the last 
than by boiling alone. Because of the small amount of air carried with it, long passage of the gas is 
said to result in slight oxidation of the vanadium (Manasse, Ann. Chem. u. Pharm., Vol. CCXL, p. 
23, 1887; Zeitschr. fur anal. Chemie, Vol. XXXII, p. 225, 1893). 
