The simplest of the dicarboxylic acids, oxalic acid is a colorless crystalline solid that is also colorless in aqueous solutions. Due to its greater acidic strength, oxalic acid is often used as a reducing agent, with a conjugate base known as oxalate (also a chelating agent for metal cations). Oxalic acid is generally found as a dihydrate, and ingesting the substance in quantity can prove highly toxic, and prolonged/extended skin contact can be similarly problematic. For centuries, oxalic acid was extracted from the sorrel plant as a salt, then by reacting high concentrations of nitric acid with sugar, until the modern method was obtained in the 19th century by reacting cyanogen with ammonia in an aqueous solution. The primary production method in use today is the oxidation of carbohydrates or glucose using nitric acid. Precursors include glycolic acid and ethylene glycol. The primary use of oxalic acid industrially is as a mordant in dyeing textiles, and in various industrial bleaching agents.