Question

how do the synthetic acid-base indicator works?

Answer 1

An acid-base indicator is either a weak acid or weak base that exhibits a color change as the concentration of hydrogen (H+) or hydroxide (OH-) ions changes in an aqueous solution. Acid-base indicators are most often used in a titration to identify the endpoint of an acid-base reaction. They are also used to gauge pH values and for interesting color-change science demonstrations.

Answer 2

The most common indicator I'd found on litmus paper. its is red below pH 4.5and blue above pH 8.2.