This reaction is catalyzed by the presence of MnO2, Mn2+, heat, light, and the presence of acids and bases. A moderately stable solution of permanganate can be prepared by boiling it for an hour and filtering through a sintered glass filter to remove any solid MnO2 that precipitates. Standardization is accomplished against a primary standard reducing agent such as Na2C2O4 or Fe2+ (prepared using iron wire), with the pink color of excess MnO4– signaling the end point. A solution of MnO4– prepared in this fashion is stable for 1–2 weeks, although the standardization should be rechecked periodically.