The appearance is generally brown liquid, pH value (1% water solution) 2.0-4.0, anionic, odorless, slightly special smell, taste astringent, deeper color while exposed under light and in air. Soluble in water, soluble in ethanol, acetone and glycerin, almost insoluble in ether, benzene, chloroform and petroleum ether. It has strong hygroscopicity. Decompose in 210 ~ 215 ℃, It can produce insoluble salt with copper, aluminum, iron, tin, chromium and metal salts, and each has its own special color. It is best to use soft water or add 1 g/L 6 sodium phosphate when used.