Phenolphthalein (Phthalin) is an acid-base indicator which is colorless in acid solution, but turns pink to red as the solution becomes alkaline. It is used medicinally as a cathartic.

使用方法
Phenolphthalein 指示剂（0.5% Phenolphthalein 的乙醇溶液）配法：
取 0.5g Phenolphthalein ，用 95% 乙醇溶解，无需加水，稀释至 100mL 。
变色范围 pH8.3～10.0（无色→红），100mL 滴定液加 1～2 滴。