Required Practical 9
Required Practical 9: Investigating pH changes
The experimental set up for measuring pH changes in titrations
Steps in the procedure
- Measure out 50 cm3 of 1.0 mol dm-3 ethanoic acid using a measuring cylinder and transfer it to a 200 cm3 beaker
- Place the beaker onto a magnetic stirrer and add a magnetic stirrer bar
- Position the pH probe so that it does not interfere with the movement of the stirrer bar
- Take an initial pH reading
- Fill the burette with 1.0 mol dm-3 sodium hydroxide
- Add 2 cm3 of sodium hydroxide from the burette into the beaker and take the pH reading
- Continue adding 2 cm3 portions until you get to 20 cm3 then switch to 1 cm3 portions
- The pH begins to rise rapidly- when it begins to slow down then you can switch back to 2 cm3 portions
- Plot a graph of pH versus volume of base added
- The procedure can then be repeated with 1.0 mol dm-3 hydrochloric acid and 1.0 mol dm-3 ammonia solution
Practical tips
- You may need to wait a few seconds until the pH stabilises before you take a reading
- Make sure the stirrer is not moving too fast as a rapid spin generates bubbles in the solution which gives an unsteady pH reading
Specimen Results
- The pH curves for a weak acid with a strong base, and a strong acid with a weak base are shown below:
pH curves from weak acid + strong base, and strong acid + weak base
Analysis
- The pH curves show a characteristic s-shape curve and the midpoint of the inflection is called the equivalence or stochiometric point
- From the curves you can
- Determine the pH of the acid by looking where the curve starts on the y-axis
- Find the pH at the equivalence point
- Find the volume of base at the equivalence point
- Obtain the range of pH at the vertical section of the curve
