Questions: Titrimetric Analysis: Principles and Terminology

Question 1

A chemist is standardizing a NaOH solution using potassium hydrogen phthalate (KHP). Which property makes KHP a suitable primary standard?

• A. KHP changes color at the equivalence point, eliminating the need for an indicator
• B. KHP is a pure, stable solid with a high molar mass that can be weighed accurately
• C. KHP reacts faster than other standards, reducing titration time
• D. KHP is less expensive than other available primary standards

Answer: (B)

A primary standard must be pure (≥99.9%), stable (not hygroscopic or prone to decomposition), and have a high molar mass so that weighing errors produce minimal uncertainty in moles. KHP (MW = 204.2 g/mol) satisfies all three criteria and has a single titratable proton, making stoichiometry straightforward. It does not serve as its own indicator.

Question 2

The endpoint and the equivalence point of a titration are the same thing.

• T. True
• F. False

Answer: False

The equivalence point is the theoretical point at which stoichiometrically equivalent amounts of titrant and analyte have reacted — a calculated quantity. The endpoint is the experimentally observed signal change (indicator color shift, potentiometric inflection) that the analyst uses to stop the titration. Titration error is the difference between these two points. A well-chosen indicator minimizes this error but cannot eliminate it entirely.

Question 3

Why might an analyst use a back titration instead of a direct titration?

Model answer: A back titration is used when the analyte reacts too slowly or incompletely with the titrant, when the analyte is insoluble in the titrant solution, or when no suitable indicator exists for the direct titration. A known excess of reagent is added to react completely with the analyte, and the unreacted excess is then titrated with a second standardized solution.

Back titrations extend titrimetry to problematic analytes. For example, calcium carbonate dissolves slowly in acid — a direct acid titration gives a sluggish endpoint. Adding excess standardized HCl, allowing complete reaction, then back-titrating the excess HCl with NaOH gives a sharp, accurate endpoint. Analyte concentration is calculated from the difference between added and recovered reagent.