An equilibrium involving chromate(VI) and
dichromate(VI) ions
Class practical
Dilute sulfuric acid is added to an aqueous solution of potassium chromate(VI). Colour changes are
observed. The reaction is then reversed using aqueous sodium hydroxide.
Lesson organisation
This experiment is most appropriate with A-level students, given the potential hazards with solutions
containing chromate(VI) and dichromate(VI) ions. Otherwise it could be carried out as a teacher
demonstration.
This experiment can be carried out individually by students, but the potassium chromate(VI) solution used
should be prepared beforehand by the teacher or technician, given the hazards presented by the solid.
It should take no more than 5 min.
Chemicals
Potassium chromate(VI) solution, 0.2 M (TOXIC, OXIDISING, DANGEROUS FOR THE ENVIRONMENT),
about 1 cm3
Sodium hydroxide solution, 1.0 M (CORROSIVE), about 10 cm3
Dilute sulfuric acid, 1.0 M (IRRITANT), about 5 cm3
Refer to Health & Safety and Technical notes section below for additional information.
Apparatus
For one demonstration:
Eye protection: goggles
Test-tube
Test-tube holder
Dropping pipette
Health & Safety and Technical notes
Read our standard health & safety guidance [1]
Wear goggles.
Potassium chromate(VI) solution, K2CrO4(aq) (TOXIC, OXIDISING, DANGEROUS FOR THE
ENVIRONMENT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
Sodium hydroxide solution, NaOH(aq) (CORROSIVE) - see CLEAPSS Hazcard and CLEAPSS Recipe
Book.
Dilute sulfuric acid, H2SO4(aq), (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
Procedure
a Put 10 drops of potassium chromate(VI) solution in a test-tube.
b Add about 5 cm3 of sulfuric acid and shake to mix thoroughly. What do you observe?
c Now add about 10 cm3 of sodium hydroxide solution and shake the mixture. Do you get back to where
you started?
d Can you repeat the whole process by adding sulfuric acid and sodium hydroxide alternately all over
again?
Technical notes
The process occurring involves the equilibrium between chromate(VI), dichromate(VI) and hydrogen ions:
The addition of acid encourages the equilibrium towards the right, producing more orange-coloured
dichromate(VI) ions. The addition of hydroxide ions causes the concentration of hydrogen ions to decrease,
and this brings the equilibrium back to the left-hand side, regenerating yellow chromate(VI) ions.
It is important that students realize that both ions are always present and exist in equilibrium with one
another, but that the yellow chromate(VI) ions predominate under alkaline conditions and the orange
dichromate(VI) ions predominate in acidic solutions.
Students may be asked if this a redox reaction. Their answer is often ‘yes’, but examination of oxidation
numbers will show that chromium remains in the +6 oxidation state throughout.
Health & Safety checked August 2008
Weblinks
Journal of Chemical Education [2] - a set of still pictures showing the colour changes.
Source URL: http://www.nuffieldfoundation.org/practical-chemistry/equilibrium-involving-chromatevi-anddichromatevi-ions
Links:
[1] http://www.nuffieldfoundation.org/node/1634/
[2] http://www.jce.divched.org/JCESOFT/CCA/CCA8/MAIN/8/08/32/thumbs.html