The amount of indicator used in an acid-base titration must be small. Why?

An indicator in acid-base titration acts as a chemical detective, signaling the moment when the acid and base in the solution have neutralized each other. This crucial juncture is known as the equivalence point. With just a few drops, the indicator will show a clear color change, making it evident when the equivalence point is reached. Importantly, indicators are selected based on the pH range over which they change color, which should align closely with the equivalence point of the particular titration.

For example, phenolphthalein, a common indicator, is colorless in acidic solutions and turns pink in basic ones, making it ideal for titrations with an equivalence point in the slightly basic pH range. The key is using just enough to observe the color change, but not so much that the indicator's own weak acid or base properties alter the solution's pH, which would compromise the accuracy of the experiment.

The equivalence point of a titration marks the exact moment when the amount of acid equals the amount of base in the solution. At this point, the number of moles of hydrogen ions, H+, from the acid, matches the number of moles of hydroxide ions, OH-, from the base, resulting in a neutral solution — at least theoretically. It's the target that chemists aim for in a titration, and it can be confirmed by the indicator’s color change or by monitoring the solution's pH.

In practice, the equivalence point is where the titration curve shows the steepest rise or fall in pH. It's important to understand that the equivalence point and the end point (where the indicator changes color) might not be exactly the same due to the properties of the indicator used. Therefore, selecting an appropriate indicator, whose pH range of color change flanks the pH of the equivalence point, is critical for accurate titration results.

Accuracy in titration hinges on several factors, such as the precise measurement of volumes, the purity of reagents, the skill of the experimenter, and the correct use of indicators. Each drop can make a difference; thus, using calibrated burettes and pipettes for measurements is essential. Impurities in reagents can introduce errors, so chemists must use substances of known high purity.

Technique matters greatly: the way in which the titrant is added, the mixing of the solution, and the detection of the end point all play roles in achieving an accurate titration. Careless addition of titrant can overshoot the equivalence point, and improper mixing might lead to a delayed indicator response. When it comes to indicators, using the optimal amount and type for the given reaction ensures that the end point coincides with the equivalence point as closely as possible. Minor deviations between the end point and the equivalence point are known as titration errors and will affect the accuracy of the titration. Standardizing solutions against primary standards and performing replicate titrations can help in minimizing these errors and refining the accuracy.