Acid-Base Indicators The most common acid-base indicators are complex molecules that are themselves weak acids (represented by HIn). They exhibit one color when the proton is attached to the molecule and another color when the proton is absent. Indicators are commonly used for titration to mark the end point of a titration (color change). The end point should be very close to the equivalence point (to reduce error).

For example, phenolphthalein is colorless in its HIn form and pink in its In- form.

HIn(aq) ⇌ H+(aq) + In-(aq) In solution, the indicator is involved in typical acid-base equilibrium: HIn(aq) ⇌ H+(aq) + In-(aq) Acid form (one color) Base form (another color) The corresponding equilibrium expression, Kin, is given by In a strongly acidic solution, when the H+ concentration is high, this equilibrium shifts to the left and most of the indicator exists in its “acid form”. We observe the color for HIn.

If the solution is made basic, the H+ concentration drops, and the equilibrium shifts to the right, toward In-, and the color we observe is that of the “base form” of the indicator. The observed change in color for an indicator actually occurs over a range of pH values. How much In- must be present for the human eye to detect that the color is different from the original? For most indicators, about a tenth of the initial form must be converted to the other form before a new color is apparent. In a titration of an acid with a base, the color change will occur at a pH where,

Choosing an Indicator In a typical acid-base titration, there is a sudden and large pH change around the equivalence point. The large swing in pH causes a sudden shift in the equilibrium position for the indicator. We go from the HIn form to the I- form and hence a color change. To pick an indicator best suited for a titration assumes that the midpoint point of the color change for the indicator corresponds to the pH at the eq. point. In other words, we want an indicator end point (where the color changes) and the titration equivalence point to be as close as possible.

At this midpoint there will be equal amounts of HIn and In-. This leads to So Kin = [H+], and taking the negative log of both sides gives us pKIn = pH (at eq. point). The titration curve of HCl and NaOH (strong acid/strong base). Note the endpoints of phenolphthalein and methyl red occur at virtually the same amount of added NaOH. Either would be a suitable choice for indicator.

The titration curve for HC2H3O2 and NaOH (weak acid/strong base). Recall that the equivalence point occurs at a pH greater than 7.00. Phenolphthalein will give a color change close to the equivalence point. Methyl red would change color well before the eq. point (so the end point would be different than the eq. point). Not a suitable indicator.