Any of various substances, such as litmus or phenolphthalein, that indicate the presence, absence, or concentration of another substance or the degree of reaction between two or more substances by means of a characteristic change, especially in color.

• 指示剂任何一种具有下列功能的物质，即能通过自身性质的变化，特别是颜色的变化来表明另一种物质的存在、消失或聚集，或可以表明两种以上的物质相互反应的程度；例如石蕊、酚酞