Question

An indicator is udeful in determining the equivalence point of a reaction if 1) the color change occurs on the vertical portion of the titration curve, and 2) the color change is sharp i.e, occurs over a small volume of added titrant (NaOH).

On the basis of the pH/color changes observed, would you be able to carry out an acetic acid-NaOH titration by using an bromocresol green indicator and why?

Would phenolphthaleinm colorless at pH 7.8 and pink at pH 8.2 serve as a useful indicator for the acetic acid-NaOH titration and why?

Answer 1

Bromocresol can not be used as an indicator for acetic acid-NaOH titration.

Titration of acetic acid(pK_a = 4.756, weak acid) with NaOH (strong base) has equivalence point around pH 8. Hene, it shows color change around pH 8. Bromocresol changes its color in the pH range 4.0 to 5.6. If bromocresol is used as an indicator, it will change its color much before the equivalence point. Hence, its not useful for acetic acid-NaOH titration.

Since phenolphthalein changes its color in the pH range 8.0 to 10.0, it is useful as an indicator for titration of weak acid vs. strong base which has equivalence point around pH 8. Hence, phenolphthalein which is colorless at pH 7.8 and pink at pH 8.2 serve as a useful indicator for the acetic acid-NaOH titration.