A chromate–dichromate equilibrium

In this experiment, students add dilute sulfuric acid to an aqueous solution of potassium chromate(VI). They observe the resulting colour changes, before reversing the reaction using aqueous sodium hydroxide.

The 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. The 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 five minutes.

Equipment

Apparatus

• Eye protection (goggles)
• Test tube
• Test tube holder
• Dropping pipette

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

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection (goggles) throughout.
• Potassium chromate(VI) solution, K₂CrO₄(aq) (TOXIC, OXIDISING, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC078a and CLEAPSS Recipe Book RB069.
• Sodium hydroxide solution, NaOH(aq) (CORROSIVE) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085.
• Dilute sulfuric acid, H₂SO₄(aq), (IRRITANT) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.

Procedure

1. Put 10 drops of potassium chromate(VI) solution in a test tube.
2. Add about 5 cm³ of sulfuric acid and shake to mix thoroughly. What do you observe?
3. Now add about 10 cm³ of sodium hydroxide solution and shake the mixture. Do you get back to where you started?
4. Can you repeat the whole process by adding sulfuric acid and sodium hydroxide alternately all over again?

Teaching notes

The process occurring involves the equilibrium between chromate(VI), dichromate(VI) and hydrogen ions:

2CrO₄^2–(aq) (yellow) + 2H⁺(aq) ⇌ Cr₂O₇^2–(aq) (orange) + H₂O(l)

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 is a redox reaction. Their answer is often ‘yes’, but examination of oxidation numbers will show that chromium remains in the +6 oxidation state throughout.