Question

In a titration (one that doesnt measure pH), is it possible to tell if an acid is polyprotic? Indicator used was phenolphthalein. Titration ended when unknown acid turned light pink.

Answer 1

The polyprotic acids have more than one dissociation constant called K. For diprotic acids having K_{​​​​​​1} and K_{​​​​​​2} close enough, it is not possible to differentiate between two stages of neutralization while titrating. All the replaceable H⁺ ions tend to titrate in the single stage and the value of K_{​​​​​​2} becomes very low. So when it is titrated, the sharp change in pH occures near the 1st equivalent point. But the pH range in this point will be 4-6 i.e. acidic in nature. When phenolphthalein is used as the indicator, there will be no change in color, as the pH range of Phenolphthalein is 8-10. The 2nd equivalent point will be near the range of 8-10 and the color of Phenolphthalein will change from colorless to light pink in that range. But it will be unable to show the two different equivalent point separately. So it is not possible to tell if the acid is monoprotic or polyprotic. Similarly, when titrating the Tri or Tetraprotic acids or so on, the same problem will occur. Thus, Phenolphthalein is not a good choice as the indicator in this type of thing titration.