An acid-base titration is a quantitative analysis technique used to estimate the concentration of an acid/base by neutralizing it with a known concentration standard solution of acid or base. It is classified as a strong acid-strong base, weak acid-strong base, strong acid-weak base, and weak acid-weak base.
From the point of view of chemical analysis, the method of titrating a weak base with a weak acid is not useful. Because of their low Ka/Kb values, any dissociation would be minor, necessitating the use of a large amount of titrant as well as consideration of chemical kinetics.
As well as when titrating a weak acid with a weak base, no indicator is suitable. Since the pH shift is very slow and close to the equivalence point, a weak acid solution cannot be titrated with a weak base using an indicator to find the endpoint.
At the equivalence point of a titration between a weak acid and a weak base, the pH does not change sharply. The pH varies gradually throughout a range of pH values. Therefore, no usable indicator exists for this type of titration.
Below is the titration curve for the 0.1 M ethanoic acid (CH3COOH) with 0.1 M ammonia solution (NH3) titration.
In the vicinity of the equivalence point, the slope of the titration curve is quite large for the strong acid/base case and much less for the weak acid/strong base case. In a weak acid/weak base titration, the slope of the titration curve would be substantially less. In this example, potentiometric techniques would be needed to find the endpoint. Chemical indicators, which typically show a color change at a pH change of two units, would be ineffective.
Assertion:
The titration curve for weak acid and the weak base has no sharp changes. For this titration, no suitable indicator is present.
Reason:
The indicator should change its color sharply for the indication of the reaction to be complete, so its pH range lies perfectly within the sharp change to avoid an experimental error.
- A. Both assertion and reason are correct and reason is the correct explanation for assertion
- B. Both assertion and reason are correct but reason is not the explanation of assertion
- C. Assertion is correct but reason is incorrect
- D. Assertion is incorrect but reason is correct
Correct answer is option A.
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