THE TRANSITION ELEMENTS 379+ + 6e~ ^2Cr^3 + (aq) 4- 21H 2 O:
E^= + 133V
A standard solution of potassium dichromate can be made up
by accurately weighing the pure salt. (A standard solution of potas-
sium manganate(VII) cannot be made up by direct weighing, since
the salt always gives a little manganese(IV) oxide in water.) Sulphuric
acid is added to the solution to be titrated, but hydrochloric acid
can be present, since the chloride ion is not easily oxidised by
dichromate [cf. manganate(VII)]. The end-point is not easy to detect
with dichromate, since the orange colour of the latter has merely
been replaced by the green colour of the hydrated Cr3+ ion. It is
therefore usual to use an oxidation-reduction indicator, such as
diphenyl amine, which turns from colourless to blue at the end
point.
The dichromate ion oxidises iron(II) to iron(III), sulphite to
sulphate ion*, iodide ion to iodine and arsenic(III) to arsenic(V)
(arsenate). Reduction of dichromate by sulphite can be used to
prepare chrome alum, since, if sulphur dioxide is passed into potas-
sium dichromate acidified with sulphuric acid, potassium and
chromium(III) ions formed are in the correct ratio to form the alum,
which appears on crystallisation :K 2 Cr 2 O 7 + H 2 SO 4 + 3SO 2 -> K 2 SO 4 4- Cr 2 (SO 4 ) 3 4- H 2 O2[KCr(SO 4 ) 2 .12H 2 O]Chrome alum is also obtained if the acidified dichromate is
boiled with ethanol, the ethanal formed distilling off.
Reduction of dichromate by strong reducing agents yields the
chromium(II) ion, Cr2+ (see p. 383).
The addition of concentrated sulphuric acid to a solid dichromate
mixed with a chloride produces a red vapour, chromium(VI)dioxide
dichloride, CrO 2 Cl 2 (cf. sulphur dioxide dichloride, SO 2 C1 2 ).
Chromium(VI) dioxide dichloride reacts with water immediately:
2CrO 2 Cl 2 4- 3H 2 O -> Cr 2 O|~ + 6H+ + 4C1~If it is passed into a concentrated solution of a chloride, however,
a chlorochromate(VI) is formed:Cr0 2 Cl 2 + Cr + H 20 -> [CrO 3 Cl]T + 2HC1- Thus, filter paper which has been dipped into a solution of potassium dichromate
turns green in the presence of sulphur dioxide. This reaction provides the usual test
for sulphur dioxide.