Chromium(II) chloride: properties, synthesis, and laboratory handling

Chromium(II) chloride is an inorganic chemical compound composed of chromium in the +2 oxidation state and chloride. Its empirical formula is CrCl₂. The anhydrous solid is typically crystalline, while its hydrated forms and aqueous solutions show the characteristic blue color of the Cr(II) ion.

Physical and chemical properties

CrCl₂ dissolves in water to give solutions dominated by the Cr2+ aquo complex, which is blue. The compound is readily oxidized by air and is sensitive to impurities; therefore aqueous preparations are normally handled under oxygen-free conditions. Chromium(II) chloride behaves as a strong reducing agent. In mildly acidic solution it can reduce protons to form hydrogen gas. Under basic conditions hydrolysis and subsequent reactions can produce precipitates of chromium(II) hydroxide or related chromium(II) oxides (CrO and related species may be involved), depending on conditions and the absence of air.

Preparation

One common laboratory route converts chromium(III) chloride into CrCl₂ by treatment with dry hydrogen at elevated temperatures, which reduces Cr(III) to Cr(II). Alternatively, CrCl₂ can be obtained by the chemical reduction of chromium(III) salts using metals such as zinc, or by other reductants under an inert atmosphere.

Applications and handling

• CrCl₂ is used mainly in research laboratories, for example as a reagent in synthetic inorganic and organic chemistry where a source of Cr(II) is required.
• Because Cr(II) is easily oxidized to Cr(III), solutions and solid samples are usually prepared and stored under inert gas and with clean, deoxygenated water to avoid decomposition.