Practical activity

Titration

There are a number of ways that you could carry out a titration in Chemistry. This is an outline of the required steps to undertake one of these methods.

It is important in this practical to use appropriate apparatus to make and record a range of volume measurements accurately.

Aims

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

Titration is a practical technique used to determine the amount or concentration of a substance in a sample. It is an example of quantitative analysis. An acid-alkali titration can be used to find out what volume of acid (or alkali) of known concentration exactly neutralises a known volume of alkali (or acid) of unknown concentration. This concentration can then be calculated.

To obtain valid results, it is important that measurements are precise and accurate. This can be achieved by using a standard procedure for carrying out a titration.

Method

  1. Use a pipette and pipette filler to add 25 cm3 of alkali solution to a clean conical flask.
  2. Add a few drops of a suitable indicator and put the conical flask on a white tile.
  3. Fill the burette with dilute acid. Flush the tap through to remove any air bubbles. Ensure the burette is vertical.
  4. Slowly add the acid from the burette to the conical flask, swirling to mix. (The mixture may at first change colour, and then back again when swirled.)
  5. Stop adding the acid when the end-point is reached (when the colour first permanently changes). Note the final volume reading.
  6. Repeat steps 1 to 5 until three results are repeatable (in close agreement). Ideally these should lie within 0.10 cm3 of each other.
Burette dripped acid into a known volume of alkali containing a few drops of indicatorThe solution of known concentration is placed in the burette and the solution of unknown concentration is added to the flask

Results

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

RunRough1234
End reading (cm3)25.4524.8047.9023.7023.90
Start reading (cm3)0.001.0023.800.000.00
Titre (cm3)25.4523.8024.1023.70 23.90

The titre is the volume added (the difference between the end and start readings). The burette is marked in 0.10 cm3 graduations. It is possible to record results to the nearest 0.05 cm3 by noting if the meniscus lies between markings.

Analysis

Example

Calculate the mean titre. In the calculation, ignore the rough run and any results that are not in close agreement (24.10 cm3 in the table above).

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

mean\ titre = \frac{23.80 + 23.70 + 23.90}{3}

= 23.80 cm3

Evaluation

Question

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

A pipette is more precise than a measuring cylinder. Adding slightly different volumes of alkali to the flask will result in a systematic error. The pipette allows the same volume of acid to be added each time, helping to make the results repeatable.

Question

Describe two steps needed to obtain accurate results.

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.

Question

Explain the importance of a suitable indicator in obtaining accurate results.

The indicator must change colour sharply when the solution in the flask is neutralised. This means the volume of acid measured is very close to the true value. The white tile makes it easier to see the colour change. Misjudging the colour change could result in a random error.

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:

HazardPossible harmPossible precaution
Dilute sodium hydroxide solutionCauses skin and serious eye irritationWear gloves and eye protection, and use a pipette filler
Spilling hydrochloric acid while filling the buretteCauses eye irritationFill the burette slowly below eye level, using a funnel