14 
PROFESSOR ROSCOE’S RESEARCHES ON VANADIUM. 
the colour changes to a deep chocolate-brown ; so rapid, indeed, is this change of colour, 
when nearly the whole of the free acid is neutralized by zinc, that such a lavender solu- 
tion may serve as a reagent for the detection of free oxygen not inferior in delicacy to 
an alkaline pyrogallate. When a current of air is passed through the lavender solution 
of sulphate in presence of excess of acid, oxygen is absorbed, the colour of the liquid 
gradually changes to a bright and permanent blue, and the vanadium is contained 
in solution as tetroxide, two atoms of oxygen having been taken up (see tetroxide, page 
16). If the free acid contained in the lavender solution of vanadium sulphate be com- 
pletely neutralized by zinc and air passed through, the liquid assumes a permanent 
brown colour, which on addition of acids turns green, and the vanadium is contained in 
solution as trioxide (see trioxide, page 15). The point of the reduction at which the 
bleaching action commences is easily ascertained by testing the liquid from time to time 
with litmus paper. The changes in colour which the sulphuric-acid solution of vana- 
dium pentoxide undergoes when treated with zinc may be divided into seven stages. 
Stage. 
Colour. 
Reaction. 
State of oxidation of the metal. 
1 
Green 
Acid 
Vanadium pentoxide to tetroxide. 
Vanadium pentoxide to tetroxide. 
Vanadium tetroxide. 
2 
Bluish, green 
Acid 
3 
Blue 
Acid 
4 
Greenish blue 
Acid 
Vanadium tetroxide to trioxide. 
5 
Green 
Bleaches slightly . . 
Bleaches strongly . . 
Bleaches strongly . . 
Vanadium trioxide to dioxide. 
6 
Bluish violet 
Vanadium trioxide to dioxide. 
7 
Lavender or violet . . 
Vanadium dioxide. 
This shows that the bleaching action commences in stage 5 with the formation of the 
dioxide. A quantitative experiment made in the same way showed that the bleaching 
action of the reduced solution ceased when to 100 parts of the dioxide 9*5 parts of 
oxygen had been added; in order to pass completely into trioxide, 11 ‘9 parts of oxygen 
would have been needed. 
The reduction of vanadium to dioxide by means of nascent hydrogen in presence of 
zinc serves as an easy and accurate method of estimating vanadium when mixed with 
certain other metals. 
2. Vanadium Trioxide (the suboxide of Berzelius), V 2 O ;5 =150 - 6. — The anhydrous 
trioxide is obtained, as is well known, by reducing vanadium pentoxide in a current of 
hydrogen at a red heat. It is perfectly stable up to temperatures approaching a white 
heat ; and even when heated in a current of hydrogen to this temperature the trioxide 
does not lose weight. 0*411 grm. vanadium pentoxide, when reduced in hydrogen and 
heated for two hours to whiteness in a wind furnace, yielded 0*366 grm. of black trioxide, 
the theoretical quantity being 0*368 grm. 
In addition to the complete description of the properties of the oxide given by 
Berzelius, I have only to add that it undergoes oxidation, not only rapidly glowing 
when exposed whilst warm to the air, but likewise slowly when exposed to the 
air at ordinary temperatures. Black amorphous trioxide, when allowed to stand 
exposed to the air for some months, undergoes a remarkable change ; it takes up an 
