Identifying Chromate and Dichromate

Introduction

Chromate and dichromate are unusually visible inorganic anions. Yellow chromate-rich solution becomes orange as acid favours dichromate-rich solution; adding base can restore the yellow form. The change is a reversible equilibrium shift, not an oxidation-state change. Both ions contain chromium(VI), so colour and pH history must be interpreted together.

Core explanation

The commonly written overall equilibrium is 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l). Adding acid raises H⁺ activity and favours the right-hand, orange side. Adding OH⁻ removes H⁺ through neutralization and favours the left-hand, yellow side. Depending on concentration and pH, hydrogen chromate HCrO₄⁻ is also present; the simple equation summarizes the visible direction without listing every aqueous species.

Check oxidation states before calling the process redox. In CrO₄²⁻, four oxygens contribute −8, so chromium must be +6 for net −2. In Cr₂O₇²⁻, seven oxygens contribute −14, so two chromiums total +12, or +6 each. No electron is gained or lost by chromium in the acid-base/dimerization shift. By contrast, reducing acidic dichromate can produce green Cr³⁺: that is a genuine redox change from +6 to +3 and is not reversed simply by adding base.

The pH response can support an unknown's identity if it is tested reversibly and under appropriate conditions. An originally yellow solution turning orange with acid and back toward yellow with base is consistent with chromate–dichromate chemistry. But an ordinary acid-base indicator may also change yellow to orange, and other coloured ions may be present. A confirmatory precipitation or redox observation, as specified by the syllabus, can strengthen the assignment. For example, Ba²⁺ may form yellow BaCrO₄ under suitable pH and concentration; acid can alter its solubility by protonating chromate. Reagent conditions must be stated.

In a mixture, the ratio of yellow and orange species can create intermediate hues. “Orange means dichromate alone” is therefore too absolute. Concentration affects dimerization because two chromate-derived units participate in the equilibrium. Dilution may shift relative speciation as well as lighten both colours. The dominant colour is an observable clue to distribution, not an exact concentration measurement.

Chromium(VI) compounds require careful handling because of their hazards; classroom reasoning can use supplied colour observations without recommending unsupervised tests. The educational equilibrium experiment at https://chem.libretexts.org/Ancillary Materials/Laboratory Experiments/Wet Lab Experiments/General Chemistry Labs/Online Chemistry Lab Manual/Chem 10 Experiments/12%3A Equilibrium and Le Chatelier%27s Principle %28Experiment%29 gives the yellow chromate–orange dichromate equation. The pH dependence and hydrogen chromate intermediate are also discussed in OpenStax's chemistry answer key at https://openstax.org/books/chemistry/pages/chapter-19.

Step-by-step reasoning

1. Write the chromate–dichromate equilibrium with H⁺ on the reactant side. 2. Predict acid addition favours orange dichromate-related species. 3. Predict base addition consumes H⁺ and favours yellow chromate. 4. Calculate Cr oxidation state in both species to check whether redox occurs. 5. Separate this reversible pH response from a persistent green Cr³⁺ reduction product.

Visual explanation

Draw a yellow box “2CrO₄²⁻” on the left and an orange box “Cr₂O₇²⁻” on the right. An acid arrow points right and a base arrow points left. Below both write Cr(+6). A separate downward arrow to green Cr³⁺ is labelled “reduction, not the same equilibrium.”

Real-world analogy

Two identical small groups can join into one larger group when the room conditions change, then split again when conditions are restored. Chromate-related units shift toward dichromate under acid and back toward chromate under base, while chromium remains chemically in the same oxidation state.

Real-world example

Acidified dichromate has long been used as an oxidizing reagent in laboratory chemistry. When it oxidizes another substance, orange chromium(VI) can become green chromium(III). This practical colour change is distinct from the yellow-to-orange pH shift and illustrates why oxidation states must accompany visual descriptions.

Why?

Why does acid make the solution more orange? H⁺ is a reactant in the overall equilibrium that creates dichromate from chromate. Increasing its activity shifts the composition toward the right. Base removes H⁺, shifting the composition back toward chromate.

Common misconception

“Yellow to orange means chromium was oxidized” is false. Chromium is +6 in both anions. The colour change reflects different molecular species and coordination of oxygen, not electron transfer.

Worked example

A yellow unknown becomes orange after controlled acid addition and returns yellow after base is added. The equation 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O predicts both directions and supports a chromium(VI) oxyanion system. If a separate reducing-agent test instead makes the orange solution green and base does not restore orange, the green product is more consistent with Cr³⁺ formation.

Quick check

1. What is chromium's oxidation state in both chromate and dichromate? Answer: +6 in each, so their acid-base interconversion is not a redox reaction.

Exam focus

Balance the overall equilibrium, put H⁺ on the correct side, and state yellow versus orange. When discussing green chromium(III), name it as a distinct reduction product. Avoid saying that every orange solution contains only dichromate.

Advanced insight

The full aqueous system includes HCrO₄⁻ as an intermediate and equilibria whose positions depend on pH and total chromium concentration. Because dichromate formation combines two chromium centres, dilution can alter the ratio even at similar acidity. A rigorous speciation calculation uses multiple equilibrium constants rather than one colour rule.

Summary

Acid favours orange dichromate-related chromium(VI) species; base favours yellow chromate. The visible response can be reversible because chromium remains +6. Green Cr³⁺ arises from a separate reduction and must not be conflated with the pH shift.

Practice questions

1. Predict the colour direction when acid is added to a yellow chromate-rich solution. Answer: Toward orange as dichromate-related species become more favoured. 2. Calculate chromium's oxidation state in Cr₂O₇²⁻. Answer: Seven oxygens total −14; two chromium atoms total +12, so each Cr is +6. 3. Why is a reversible yellow–orange change different from orange-to-green dichromate reduction? Answer: The first changes chromium(VI) speciation without electron transfer; the second produces chromium(III) through redox.