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Carry out an accurate acid-alkali titration

There are a number of ways that you could carry out an acid-alkali titration in Chemistry. This is an outline of the required steps to undertake one of these methods. It is important in this core practical to use appropriate apparatus to make and record a range of volume measurements accurately. This includes the safe use and handling of liquids, and monitoring chemical changes.

Aims

To carry out an accurate titration using dilute hydrochloric acid, dilute sodium hydroxide solution, and phenolphthalein indicator.

Method

1. Use a pipette and pipette filler to add 25 cm³ of dilute sodium hydroxide solution to a clean conical flask.
2. Add a few drops of phenolphthalein indicator and put the conical flask on a white tile.
3. Fill the burette with dilute hydrochloric acid and note the starting volume.
4. Slowly add the acid from the burette to the conical flask, swirling to mix.
5. Stop adding the acid when the end-point is reached (when the colour first permanently changes from pink to colourless). Note the final volume reading.
6. Repeat steps 1 to 5 until you get concordant titres (see step 1 in the Analysis).

Results

Record the results in a suitable table. The one here also shows some sample readings.

Readings should be recorded to two or three decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette). The titre is the volume added (the difference between the end and start readings).

Analysis

Tick (✔) at least two concordant titres. These are titres within 0.20 cm³ (or sometimes 0.10 cm³) of each other.

Worked example

Calculate the mean titre.

Ignoring the rough run, and run 2 (because it is not concordant):

= 23.75 cm³

Evaluation

Worked example

Describe four steps needed to obtain accurate results.

(Make sure the burette is vertical, and take the readings from the bottom of the meniscus. Near to the end-point, rinse the inside of the flask with distilled water and add the acid drop by drop.)

Worked example

Explain why a pipette is used to measure the acid, rather than a measuring cylinder.

The pipette allows the same volume of acid to be added each time, helping to make the results repeatable.

Hazards, risks and precautions

It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example: