# Equivalence point in acid-base titration

• Tony Stark
In summary, pH at equivalence point is a function of the equilibrium constant for the reaction between the weak acid and the weak base.

#### Tony Stark

In acid base titration, we try to neutralize the analyte by adding sufficient titrant and observing the color change in the indicator.
But if the analyte is neutralized, then it must always be at ph 7. Why is it that we use indicator of different ranges to check equivalence point?

We can and many laboratory practitioners do use a pH meter to watch for the endpoint.

pH 7 endpoint would be for titrating strong acid or base, with strong base or acid.

Why to pH 7?
Look at the water dissociation constant.
(H+)(OH-)=1.0*10-14

For titration of ...
Strong Acid + Strong Base => Equiv Pt pH = 7 b/c the ions of the salt formed do not hydrolyze. Example: HCl + NaOH => NaCl + H2O ... NaCl => Na+ + Cl- ... Na+ + H2O => No Rxn and Cl- + H2 => No Rxn. Consequently the pH depends solely upon the autoionization of water;i.e. H2O => H3O+ + OH- and Kw = [H3O+][OH-]. [H3O+] = [OH-] = 1 X 10-7M => Kw = [10-7][10-7] = 1x10-14 ... and, pH = -log[H3+] = -log(10-7) = 7

A Weak Acid + Strong Base => Equiv Pt pH > 7 b/c the anion of the salt formed hydrolyzes leaving an excess OH- in solution at equivalence point. Example: HOAc + NaOH => NaOAc + H2O ... NaOAc => Na+ + OAc- ... Na+ + H2O => no rxn ... OAc- + 2H2O => HOAc + OH- ... The excess OH- leaves the pH at equivalence point ~ 8.90 for a mix of equal volumes of a 0.10M HOAc + 0.10M NaOH reaction.

A Strong Acid + Weak Base => Equiv Pt pH < 7 b/c the cation of the salt formed hydrolyzes leaving an excess of H3O+ ions in solution at the equivalence point. Example: NH4OH + HCl => NH4Cl + H2O ... NH4Cl => NH4+ + Cl- ... NH4+ + 2H2O => NH4OH + H3O+ ... The excess H3O+ => pH ~ 5.0 for a mix of equal volumes of 0.10M NH4OH + 0.10M HCl.

A Weak Acid + Weak Base at Equiv Pt is dominated by the electrolyte with the larger Keq value. That is, if Kb > Ka => pH > 7 at Equiv Pt, and if Kb < Ka => pH < 7 at Equiv Pt.

## 1. What is an equivalence point in acid-base titration?

The equivalence point in acid-base titration is the point at which the number of moles of acid is equal to the number of moles of base in a solution. This is when the reaction between the acid and base is complete and the solution is neutral.

## 2. How is the equivalence point determined in acid-base titration?

The equivalence point is determined by using an indicator or pH meter to track the change in pH as the titrant (either the acid or base) is added to the solution being titrated. The equivalence point is reached when the pH stops changing.

## 3. What is the significance of the equivalence point in acid-base titration?

The equivalence point is significant because it marks the point at which the reaction between the acid and base is complete. This information can be used to calculate the concentration of the unknown solution or to determine the molar ratio between the acid and base.

## 4. How does the strength of the acid or base affect the equivalence point?

The strength of the acid or base does not have a direct effect on the equivalence point. However, if a strong acid or base is used, the equivalence point will be reached more quickly and at a lower pH compared to using a weak acid or base.

## 5. What factors can affect the accuracy of determining the equivalence point?

Factors that can affect the accuracy of determining the equivalence point include human error in reading the indicator or pH meter, impurities in the solutions being titrated, and improper calibration of equipment. It is important to carefully follow the titration procedure and use high quality chemicals to minimize these potential sources of error.