Required practical 9 Investigate how pH changes when a weak acid reacts…

The idea

Required practical 9 produces the curves rather than assuming them. Calibrate the pH meter against buffer solutions of known pH first, because the absolute readings depend on it. Pipette 25.0 cm³ of the acid into a beaker, stand the electrode in it, and add the alkali from a burette. Add in 1.0 cm³ portions at first but in 0.1 or 0.2 cm³ portions near the equivalence point, where the pH changes fastest and where too coarse a step will simply miss the steep section. Stir before every reading and rinse the probe with distilled water when moving between solutions. Plot pH against total volume added and draw a smooth curve rather than joining the points with straight lines. Read the equivalence volume at the mid-point of the steepest part, and read the pKa at half that volume.

What earns the marks

• Calibrating the pH meter with buffer solutions of known pH before starting.
• Adding the alkali in smaller portions near the equivalence point, where the pH changes fastest.
• Stirring the mixture before each pH reading.
• Plotting pH against total volume added and drawing a smooth curve.
• Identifying the equivalence point at the mid-point of the steepest part of the curve.

Common mistakes and how to put them right

• Adding the alkali in 5 cm³ portions throughout, so the steep region is missed entirely.

• Joining the plotted points with straight line segments instead of a smooth curve.

• Rinsing the pH probe with the acid rather than with distilled water between readings.

• Leaving the meter uncalibrated and then treating the absolute pH values as accurate.

• Reading the equivalence point at pH 7 rather than at the mid-point of the steep section.

Exam tips

• If asked to improve the method, 'add smaller volumes near the equivalence point' is almost always one of the marks.
• A pH meter is the answer whenever a titration has no steep section, because no indicator will work.