Question

When I see demonstrations of titration of acid solutions of unknown concentration with phenolphthalein as the indicator, when the last drop of NaOH went in the analyte solution became permanently pink. This means that more than enough NaOH was used and the recorded endpoint has some error (by 1 drop?). If we use a diluted titrant, the error is still 1 drop but it is diluted so could the margin of error be lower?

Answer 1

Using a diluted titrant in an acid-base titration can reduce the margin of error in volume measurement but may not improve the precision of visually detecting the endpoint. Selecting the correct indicator, such as phenolphthalein for titrations that have a sharp pH change at the equivalence point, is critical for indicating a clear endpoint and accurately calculating the concentration of the unknown acid solution.

Step-by-step explanation:

When titrating an acid solution with a base, using a dilute solution of titrant can indeed lower the margin of error in terms of the concentration of the excess titrant added. This is because each drop contains fewer moles of titrant, and thus the point at which the endpoint is exceeded by one drop represents a smaller absolute quantity of titrant. However, although it may affect the precision of the volume measurement, this does not necessarily improve the accuracy with which the endpoint is detected visually.

It is also essential to choose an appropriate indicator for the titration, which should have a color change interval that closely matches the pH change at the equivalence point. For example, phenolphthalein is a good choice for a titration that has a sharp pH change at the equivalence point, because it changes color within a narrow pH range, indicating a clear endpoint.

The concentration of the unknown acid solution can be calculated by analyzing the volume and molarity of the NaOH used to reach the endpoint, with an accurate phenolphthalein color change providing a close approximation to the true equivalence point.