Question

1-Why, in the standardization of H₂SO₄ with Na₂CO₃ solution, is titration carried out to pH 4.3 (methyl orange endpoint) rather than to pH 8.3 (phenolphthalein endpoint)?

2-For each of the solutions titrated, why are the concentrations of OH^-, CO₃^2-, and HCO₃^- somewhat different from those which were indicated on the sample designations?

3-At what pH do the phenolphthalein and methyl orange endpoints occur for the sample for which the titration curve was determined? Compare the equivalence points (inflection points of curves) with the commonly used endpoints of pH 8.3 and 4.3 and explain any difference.

Answer 1

1)

Titration equations:

Step-1: H₂SO₄ (aq) + Na₂CO₃ (aq)   ------> NaHSO₄ (aq) + NaHCO₃ (aq)  

Step-2: NaHSO₄ (aq) + NaHCO₃ (aq) -----> Na₂SO₄ (aq) + H₂CO₃ (aq)

In the first step, carbonate is converted into bicarbonate. So, the phenolphthalein is suitable for this step.

In second step, bicarbonate is converted into carbonic acid. So, the methyl orange is suitable for this step.

Phenolphthalein is a basic indicator that acts at a pH range of 8-10 whereas methyl orange is an acidic indicator that acts at a pH rang of 4-5.

In the standardization of H₂SO₄, we known the required volume of sodium carbonate to complete the two steps.

So, the methyl orange is used instead of phenolphthalein.