what is an acid-base indicator
Click for larger image. Acid and Base Indicators. Acid-Base Indicators: The most common method to get an idea about the pH of solution is to use an acid base indicator. An indicator is a large organic molecule that works somewhat like a  color dye. Whereas most dyes do not change color with the amount of acid or base present, there are many molecules, known as acid-base indicators, which do respond to a change in the hydrogen ion concentration.