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Purify viologen by precipitation on adding excess of acetone to a concentrated solution of it in aqueous MeOH. It has also been recrystallised several times from MeOH or iso-propanol and dried at 70o under vacuum for 24hours [Prasad et al. J Am Chem Soc 108 5135 1986], and recrystallised three times from MeOH/isopropanol [Stramel & Thomas J Chem Soc, Faraday Trans 82 799 1986, Michaelis & Hill J Am Chem Soc 55 1481 1933, Tilford et al. J Am Chem Soc 70 4005 1948]. [Beilstein 23 I 49.]