RP9 - pH curves
On this page
In AQA Required Practical 9 – pH curves, the purpose is to investigate how pH changes during the gradual addition of a strong base (sodium hydroxide) to a weak acid (ethanoic acid), and to use this data to construct a pH curve.
The initial calibration of the pH meter improves the accuracy of pH measurements. This allows for the identification of the equivalence point and the pH behaviour of weak acid–strong base titrations.
In Required Practical 9: pH curves, as outlined by AQA, there are several important safety requirements to be aware of (beyond normal lab practice, such as wearing goggles).
- Ethanoic acid is an irritant – avoid skin contact and rinse spills immediately with water.
- Sodium hydroxide is a mild irritant – wipe up spills promptly to prevent skin or eye exposure.
- pH buffers (pH 4.00 and 9.00) are weak acid/alkali solutions – handle carefully to avoid skin irritation.
- pH probe is fragile – avoid knocking the glass bulb against the beaker or with the stirrer to prevent breakage.
Experimental method for AQA Required Practical 9 – pH curves
To prevent incorrect additions.
To record a clear titration curve with enough data around the equivalence point.
Prevent contamination on the pH probe that would affect your calibration and titration data.
To ensure accurate pH readings from the pH meter.
To produce a pH curve from the results collected in AQA Required Practical 9: pH curves.
- Adjust the pH readings obtained using the calibration graph. Ensure all pH values are corrected based on the calibration curve.
- Plot a graph of the corrected pH values (Y-axis) against the volume of sodium hydroxide added (X-axis).
- Join the points with a smooth pencil line, tracking the moving average.
The equivalence point occurs during the vertical line of this pH curve, this is vertical line represents the end point of the titration.