What is an acid indicator?

1 Answer

An acid-base indicator is a large organic molecule, which is a WEAK acid.....


And the indicator is chosen so that its acid form has a distinct and characteristic colour with respect to its conjugate base.


For the tritration of strong acid by a strong base, NEAR the ENDPOINT, the addition of 1 drop of titrant, approx. #0.01*mL# causes a rise in #pH# by 6-7 units.....and ALSO a distinct and observable colour change of the indicator, which is what we, the chemists, vizualize. Typically a chemist would titrate to the POINT of colour change, not the colour change itself.

The video below shows an experiment using an indicator derived from boiling red cabbage. A pigment from the cabbage called anthocyanin is what causes all of the different colors you see.

Other common indicators include:
bromothymol blue
thymol blue
methyl orange
bromocresol green
methyl red
phenol red

Hope this helps!