Chromates and dichromates are salts of chromic and dichromic acid. Salts have an intense yellow or orange color, respectively.
When solid potassium dichromate (K2Cr2O7) is dissolved in water the resulting solution is orange. The dichromate ion in aqueous solution is in equilibrium with the chromate ion, and this can be shown with the following equation:
This is a dynamic equilibrium and as such is sensitive to the acidity or basicity of the solution. Shifting the equilibrium with pH changes is a classic example of Le Chatelier’s principle at work.
Le Chatelier's principle states that if a chemical dynamic equilibrium is disturbed by changing the conditions (concentration, temperature, volume or pressure), the position of equilibrium moves to counteract the imposed change. So if more reactant is added, the equilibrium shifts to the right in order to consume that extra reactant, which results in more product; also if the product is removed from the system, the equilibrium shifts to the right completely increasing the yield.
Yellow chromate and orange dichromate are in equilibrium with each other in aqueous solution. The more acidic the solution, the more the equilibrium is shifted to the left towards the dichromate ion. As hydrochloric acid is added to the chromate solution, the yellow color turns to orange. Increasing the hydrogen ion concentration is shifting the equilibrium to the left in accordance with Le Chatelier's principle, where we expect the reaction to try remove some of the H+ we have added by reacting with the CrO42-, and yielding more Cr2O72- which we observe as color change.
When sodium hydroxide is added to the dichromate solution, the orange color turns back to yellow, hydroxide ions react with hydrogen ions forming water, driving the equilibrium to the right (OH- removes H+ ions by neutralizing them and the system acts to counteract the change) and further shifting the color. We can observe that the addition of hydroxide ions promotes the conversion of dichromate to chromate.
Acids and bases are added to a system so as to shift the position of a chemical equilibrium. The ions have different colors, so that changes are detected visually. Yellow chromate ion turns orange by addition of acid, while the orange dichromate in reaction with bases turns yellow. The equilibrium depends on the acidity of the solution, so the color in this case is pH dependent.
Successive addition of sodium hydroxide and hydrochloric acid causes alternative changes in solution color, during which the color intensity fades due to dilution.
The addition of concentrated acids, such as sulfuric acid into chromate/dichromate solution causes further shifting of the equilibrium, and more intense colors, turning the solution to carmine-red.
If we add some barium nitrate (Ba(NO3)2) to chromate/dichromate solution, NO3- acts as a spectator ion, not effecting the reaction. On the other hand Ba2+ reacts with the chromate CrO42- forming an insoluble salt BaCrO4, while BaCr2O7 does not precipitate. So the more CrO42- there is, or the higher the pH is, the greater the amount of precipitate would form.