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Description of experiment
Below follows a plain text transcript of the selected
experiment.
Needed compounds: ----------------- hydrochloric acid : HCl ascorbic acid : C6H8O6 sodium hydroxide : NaOH potassium dichromate : K2Cr2O7
Class: ------ elem=Cr coordination redox
Summary: -------- Basic chromium (VI), chromate, is not capable of oxidizing ascorbate. On acidification, immediate oxidation occurs and a coordination complex is formed.
Description: ------------ Add an excess amount of a solution of ascorbic acid to a solution of K2Cr2O7 in a solution of NaOH (in fact, a chromate solution [CrO4]2- is prepared this way): The liquid remains bright yellow. The chromate is not capable of oxidizing the ascorbic acid (or better, ascorbate in the strongly alkaline liquid).
Add a small excess amount of hydrochloric acid (10% by weight): Almost instantaneously, the liquid changes from bright yellow to grey. The chromate is converted to chromium (III), which apparently forms a grey coordination complex with ascorbic acid or its oxidation product.
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