Titrations with a pH meter

Introduction and teacher's notes

Both Standard and Higher Level students need to understand about strong and weak acids and bases, (sub-topic 8.4) and the pH scale (sub-topic 8.3). They also need to be able to calculate the concentration of an acid or base by titration with a standard solution of a base or acid. As part of the mandatory laboratory requirement for Topic 8.3 pH scale they are required to be familiar with the use of a pH meter and universal indicator.

This practical will give students good experience of using a pH meter. It can also be used to fulfil some of the ICT in practical work requirements as a data logger with a pH probe can be used and also the graphs can be plotted using graph plotting software. Although pH curves as such are not required by Standard Level students l think that it gives students good practical experience of handling solutions of weak acids and bases. Often in exams Standard Level students (and some HL students!) state that one of the ways to distinguish a weak acid from a strong acid is by titration as it will take less sodium hydroxide solution to neutralise it. It is true that you could use a use a titration curve to distinguish them but they will see that it takes just as much sodium hydroxide to neutralise a 0.100 mol dm−3 solution of ethanoic acid as it does for a 0.100 mol dm-3 solution of hydrochloric acid. There are fewer hydrogen ions in the solution of the weak acid but they forget that once the hydrogen ions have reacted with hydroxide ions to form water more of the weak acid dissociates according to Le Chatelier’s Principle. You could mention here that because extra bond breaking (and hydration) is involved the ∆H value for the enthalpy of neutralisation will be different for weak acids or weak bases whereas all strong acids and bases give the same value (∆H⦵ = − 57.3 kJ mol−1 ) as the only reaction taking place is the reaction between H+(aq) ions and OH﹣(aq) ions to form water.

The obvious acid and alkali solutions to use for this practical are 0.100 mol dm−3 solutions of hydrochloric acid, sodium hydroxide, ethanoic acid and ammonia but you can really use any common monoprotic acids.

After they have done their own titration(s) and drawn the graphs for the other systems I usually then introduce them to the Phet acid-base titration simulator from the University of Colorado. They should then have a good understanding of pH curves. and can also see that different indicators (with different pH ranges) are required depending upon whether the acids and alkalis used are strong or weak.

Student worksheet

ACID/BASE TITRATIONS USING A pH METER

In this practical you will gain direct experience of using a pH meter and will plot a pH curve for a particular acid/base titration. From your own results and those of others in the group you will be asked to interpret the curves obtained for different acid/base pairs. This experiment provides a good opportunity to use a data logger with the pH probe.

ENVIRONMENTAL CARE:

The products will be neutralised solutions of common salts which occur in sea-water and can be disposed of safely down the sink.

SAFETY:

Although not strictly a safety point, be particularly careful when you handle the electrode as it is very fragile.

PROCEDURE:

Rinse the electrode with distilled water. Check the pH meter reading, making any necessary adjustments, by means of a standard buffer solution of pH 4 or 9, depending on the range likely to be used in the acid/base titration. Pipette 25.0 cm3 of the chosen acid (or base) with a concentration of approximately 0.1 mol dm−3 into a tall 150 cm3 beaker. Introduce the pH electrode and, if necessary, add distilled water until the electrode is properly immersed. Take a reading of the pH. From a burette, run in either a standard 0.100 mol dm−3 solution of a base (or acid) in about 2 cm3 additions, recording the pH each time. About 5 cm3 from the approximate equivalence point reduce the additions of alkali to 1 cm3 and finally to 0.1 cm3 nearer the end point. Continue to add alkali and take the pH readings for a further 5 cm3. Plot the graph of pH against volume of alkali added either mIB Docs (2) Teamally or using 'Excel' on the computer.

QUESTIONS

1. Make brief sketches of curves obtained by the rest of the class for other acid/base systems.

2. Explain the different curves in terms of whether the acid and bases involved are strong or weak.

3. For your particular experiment what is the accurate concentration (in mol dm−3) of the acid (or base) used?

4. What advantage(s) (if any) has the pH meter over the use of an ordinary indicator?

The next two questions are only for HL students

5. If your experiment involved a weak acid or weak base what is the pKa of the acid (i.e. NH4+ if the base was NH3)?

6. What is meant by a 'buffer solution'? Give the composition of an example of a typical acidic buffer.

This worksheet can also be downloaded from:

  Acid-base titrations with a pH meter

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