Themes > Science > Chemistry > Inorganic Chemistry > Acids and Bases > Acids and Bases Index > Indicators


An indicator is a weak acid that changes color when it loses a proton. As for any weak acid,

  • HIn(aq) < = > H+(aq) + In-(aq)
  • Ka = [H+][I-]/[HIn]
The weak acid HIn has a different color than the weak conjugate base In-. As the solution pH changes, the ratio of HIn to In- changes. As a rule of thumb
  • If [HIn]/[In-] > 10, you see the acid color
  • If [HIn]/[In-] < 10, you see the base color
  • If [HIn]/[In-] ~ 1, you see a color inbetween the two.

Since [HIn]/[In-] = [H+]/Ka, it's obvious that the pH of the solution will change the HIn to In- ratio, changing color. Indicators generally change color over a range of about two pH units, with the midpoint pH ~ pKa for the indicator. This rule lets you choose the proper indicator: if you want to titrate a mixture with an endpoint around pH 9.0, you should choose an indicator with a pKa around 9.0, such as phenolphthalein.


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