HYDROGEN PEROXIDE. 105
peroxide, and then shake with about 1 c.c. of ether; an intensely
blue compound of chromium is. formed which is more soluble
in ether than in water and passes, therefore, into the ether layer.- A very dilute solution of potassium iodide on being treated
with a small amount of hydrogen peroxide slowly turns yellow (or
blue if starch is also added) as a result of the separation of iodine.
This reaction is catalytically accelerated, somewhat by acetic acid,
more by mineral acids, and most of all by ferrous sulphate.
Quantitative Determination of Hydrogen Peroxide. Dilute
10 c.c. of the preparation to about 300 c.c, acidify strongly with
sulphuric acid, and titrate with potassium permanganate:
2 KMnO 4 + 3 H 2 SO 4 + 5 H 2 O 2 = K 2 SO 4 + 2 MnSO, + 8 H 2 O + 5 O 2.
1 c.c. of 0.1-normal KMnO 4 solution = 0.0017 g. H 2 Oj.
C. Hydrogen Peroxide as a By-product in Slow Atmospheric
Oxidations.
Schoenbein, in 1864, established the fact that hydrogen peroxide is produced
when metals, or their amalgams, are shaken with water containing dissolved
oxygen:
Zn + 2 H 2 O + O 2 = Zn(OH) 2 + H 2 O 2.
It was shown by M. Traube,^1 in 1893, that the formation of hydrogen per-
oxide according to the equation becomes quantitative if the reaction is allowed
to take place in the presence of calcium hydroxide, for then the peroxide is
precipitated as the difficultly soluble calcium peroxide and is thus withdrawn
from the sphere of action.
Such an oxidation in which twice as much oxygen is used as is necessary for
the primary process itself, the other half of the oxygen being used in some
secondary reaction — mostly in the formation of hydrogen peroxide — is of
frequent occurrence both in organic and inorganic chemistry and is known as
" auto-oxidation." Inasmuch as both the primary and secondary reactions
require the same quantity of oxygen and since they do not take place inde-
pendently of one another, •— hydrogen peroxide is not formed at ordinary tem-
peratures from water and oxygen (see above), — it is necessary to assume the
existence of some peculiar form of reaction-mechanism. Views regarding this,
however, differ widely; for example, the hydrogen peroxide may be regarded
as a decomposition product of some higher oxide, such as zinc peroxide, which
is formed in the primary reaction; or it maybe assumed with Traube that a
dissociation of the water molecules takes place, the hydroxyl groups combining
with the metal and the hydrogen atoms combining with undissociated oxygen
molecules.
Place 200 c.c. of water, 6 g. of slacked lime, 2.5 g. of potas-
sium hydroxide, and 30 g. of mercury in a thick-walled liter
(^1) Ber. 26, 1471 (1893).