Required Practical 9 (AQA A Level Chemistry): Revision Note

Required Practical 9

Required Practical 9: Investigating pH changes

The experimental setup 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, you can switch back to 2 cm³ portions

• Plot a graph of pH versus the 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 stoichiometric point

• From the curves, you can:

  • Determine the pH of the acid by looking at where the curve starts on the y-axis

  • Find the pH at the equivalence point

  • Find the volume of the base at the equivalence point

  • Obtain the range of pH at the vertical section of the curve

• To identify the buffer region in the graph, you need to find the flattest, most horizontal section of the graph

  • This is the section where the pH changes very little on the addition of acid or base

• To locate the half-equivalence point, you need to:

  • Identify the vertical section of the curve and find the exact midpoint of the vertical rise

  • Read the volume on the x-axis directly below this point

  • Divide the volume by 2

  • Find this new halved volume on the x-axis

  • Move vertically up to the curve and mark this spot

  • Read the corresponding pH

  • At this point pH= pK_a

• To select an appropriate indicator:

  • Pick one whose pH range falls within the vertical section of the pH curve

  • This ensures a sharp colour change exactly when the reaction is complete

  • Two main indicators used for titrations are

    • phenolphthalein (pH 8.3-10), colour change from colourless to pink

    • methyl orange (pH 3.1-4.4), colour change from red to yellow

Links to related notes

• pH Curves

• Choosing an Indicator

• Buffer Calculations