Olfactory Titration Experiment |

Background

The most common use for titration experiments is to determine the concentration and pH of an analyte by adding a standard solution to it in small increments until there is an indication that it has been neutralised. For example, an indicator can be added to an alkali solution with an unknown concentration and an acid with a known concentration can be titrated into it. The volume of the acid that has been added is recorded when there is an indication that the solution is neutral. From this, the concentration of the alkali can be calculated. We will go into more detail about how this is done later on.

Normally the indicator is a weak acid, which changes colour depending on whether it is in acidic or basic conditions. In our previous example, the indicator would change colour when the acid had neutralised the alkali.

Diagram showing the equilibrium between a protonated and de-protonated indicator molecule, each of which are different colours. — A diagram illustrating the indicator’s colour change at the endpoint.

Some common colour indicators for titration experiments are:

Phenolphthalein

Methyl orange

Litmus

Despite 1 in 12 men and 1 in 200 women in the UK having colour vision deficiency, titration experiments in schools currently rely solely colour change observations.

Experiment

This experiment uses onion as a multi-sensory indicator (displays a change in smell and colour) to assess the titration endpoint.

All instructions and questions for students are available to download at the top of the page and can be edited as appropriate for the VI student/cohort. This can include being translated into braille, being enlarged or implementing a screen reader.

Aim

To work in pairs to determine the NaOH concentration in analyte solutions containing brown onion or red onion indicators by titrating them with 0.1M HCl.

Safety

A lab coat, goggles and gloves must be worn throughout this experiment.

Teachers/technicians should make the notched syringes and the analyte solutions before pupils start the experiment (instructions are available in the downloads at the top of the page).

You will need:

3 x onion indicator solution

1 x 5mL notched syringe.

1x 1mL notched syringe.

1 x 0.5mL notched syringe.

1 x stirrer bar

1 x stirrer bar magnet

1 x stirrer plate

1 x fan

1 x 200mL beaker filled with HCl (Ask a teacher to top up this HCl throughout the experiment if you are finding it hard to draw liquid out of the beaker).

Method

Procedure for the student making observations:

Place one of the beakers containing the analyte solution on the magnetic stirrer plate and put the stirrer bar in the solution.
Turn the stirrer plate and fan on, sit about a meter away from the solution and check that the airflow of the fan is in line with your nose.
Observe the changes in smell as your partner transfers HCl into the analyte solution, telling your partner every time the intense smell appears or disappears.
When the intense smell lingers and does not disappear the reaction is over.

Procedure for the student putting the HCl into the solution:

See instruction sheet on how to use the notched syringes (available in the downloads at the top of the page).
First run: Transfer the HCl in 5mL increments until your partner observes a lingering smell change. Let’s call the value of HCl added on this run XmL.
Second run: Add 5mL increments until you have added X-5mL, then add 1mL increments until your partner observes a lingering smell change. Let’s call the value of HCl added on this run YmL.
Third run: Add 5mL increments until you have added X-5mL. Then added 1mL increments until you have added Y-1mL then add 0.5mL increments until your partner observes a lingering smell change.

The value obtained in run three is the total volume of HCl added at the endpoint to the nearest 1mL.

The setup of the experiment is shown: a fan blows scent into a students nose-line and a notched syringe is used to add HCl instead of a burette. — A diagram of the experiment setup.

Questions

Volumes of 0.1M HCl needed to neutralise 10mL of NaOH analyte solution:

Run 1:

Run 2:

Run 3:

Using your result from run 3, work out the concentration of NaOH in the analyte solution.
Observations at the endpoint:
The actual concentration of NaOH is 0.2M, what do your results tell you about the indicators being used?

Answer to question 3 – discussion about the accuracy of the experiment (extension work):

Students should not worry if they don’t understand all the details of this explanation as it is beyond the scope of the course.

At the equivalence point in a titration there is a sudden drop in pH:

Curve showing the pH change when a strong acid is titrated into a strong base. Th pH starts high and consistent, there is a sudden drop when the solution is neutralised and then levels off at a much lower pH — A typical strong acid-strong alkali titration curve.

In order for the solution to be neutral there must be an equal number of moles of NaOH and HCl. Assuming we are titrating 0.1M HCl into 10mL 0.2M NaOH, the steep region of the graph should occur when 20mL of HCl has been added (as seen when phenolphthalein is the indicator):

A triangle showing how to perform titration calculations — Formula triangle for titration calculations

However, this is not what we observed when the olfactory indicators were in solution!

Titration curves seen when red onion, brown onion, garlic and phenolphthalein are used as indicators. The steep region of the titration curves occur at lower HCl volumes when onion or garlic are used as the indicators compared to when phenolphthalein is used — Real titration curves obtained when phenolphthalein, red onion, brown onion and garlic were used as indicators.

The fact that we need less acid than expected to neutralise the NaOH solution must mean that the onion/garlic indicators are significantly acidic, making the analyte solution less alkali than we thought!

This means that they cannot be used to produce accurate concentration values but are a good way for students with and without vision impairment to learn about titrations.