Electrolytic coloration below 0 °C and spectral properties of ammonium alum crystals using a pointed anode

•We expanded the traditional electrolysis method.•Electrolysis experiments can be performed below 0 °C for the first time.•Ammonium alum crystal is colored electrolytically below 0 °C for the first time.•The crystal is colored electrolytically with a pointed anode for the first time.•SO2−, O3−, O2− and O− hole-trapped centers are produced in colored crystal.

Ammonium alum crystals are colored electrolytically below 0 °C and under various voltages using a pointed anode and a flat cathode. SO2−, O3−, O2−, O− hole-trapped centers and O0, O2−, H+ radicals are produced in colored ammonium alum crystals. The absorption bands of the SO2− and O3− hole-trapped centers are observed in the absorption spectra of the colored ammonium alum crystals. The hole-trapped centers and radicals come mainly from the electric-field-induced decomposition of the SO42− radicals and crystalline water molecules. Current is measured during electrolytic coloration. Electron exchanges from electrons and small charged radicals to electrodes induce complete current.

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