350 HEAVY METALS OF GROUPS VI, VII, AND VIII
- Chromate and Dichromate. To a yellow solution of
potassium chromate add 6iV H2SO4, and note that the color
changes to orange red. To an orange red solution of potas-
sium dichromate add 6N NaOH and note that the color
changes to yellow.
The yellow color of the chromate solution is due to the CrOi~~
ion, the orange red of the dichromate to the CtaO?"" ion. The
changes take place according to the reversible reaction.
~ + 2OH" ^2CrO 4 "" + H 2 O
It is obvious that presence of OH~ ions must favor the formation
of chromate, and presence of H+ ions, since they remove OH~ ions,
must favor the formation of dichromate.
It should be noted that the valence of chromium is 6, both in
chromate and dichromate. The difference between manganate
and permanganate lies in the changing valence of the manganese.
The difference between chromate and dichromate lies merely in
the state of hydration and can be referred back to the acids.
CrO 3 + H 2 O -»• H 2 Cr0 4 chromic acid
2CrO 3 + H 2 O —• H 2 Cr 2 07 dichromic acidGENERAL QUESTIONS XI
HEAVY METALS OF GROUPS VI, VII, AND VIII- In which groups of the periodic system do the elements
chromium, manganese, iron, nickel, and cobalt fall? What is
peculiar about the position of the last three? What other metals
belong to the same family as chromium? In what relation do they
stand to sulphur, selenium, and tellurium? In what relation does
manganese stand to the halogens? What other elements occur
in the eighth group in triads similar to iron, nickel, and cobalt? - How do the monoxides of chromium, manganese, iron, cobalt,
and nickel compare in basic strength with the oxides of copper
and zinc and with the oxides of the alkali and alkaline earth metals?
How do the sesquioxides, R2O3, compare with the monoxides of
this group as regards basic strength?
What is true as regards the base- or acid-forming properties of
the oxides higher than the sesquioxides, e.g., of CrO3, MnO 3 ,
Mn 2 O 7?