Chemicals that change colour with pH are frequently used as indictors for acid-base titrations. The aim of this experiment it to determine the concentration of an unknown base using garlic powder as an olfactory indicator.

Chemistry stinks – olfactory indicators experiment

Class practical

Chemicals that change colour with pH are frequently used as indictors for acid base titrations; however it is also possible to use chemicals that change odour with pH – termed ‘olfactory indicators’. The aim of this experiment it to determine the concentration of an unknown base using garlic powder as an olfactory indicator, and to analyse the accuracy and precision of the results.

Lesson organisation

You have to decide if this experiment is suitable to use with different classes, and look at the need for preliminary training in using techniques involved in tritration. What follows here is an experiment were titration techniques can be practised and polished as it allows for an interesting comparison between indicators and and assessment if accuracy at determining the end-point of this strong acid-strong base titration. If is expected that this class practical will take 60 minute to complete.

Apparatus Chemicals

Eye protection

Beaker (500cm3), 2

Parafilm or watch glass

Each working group requires:

Burette (25 cm3)

Pipette (10cm3)

Small funnel

Conical flasks (50cm3 or 100cm3), 3

Hydrochloric acid solution, HCl(aq) (1 M) (IRRITANT) 250cm3

Sodium hydroxide solution, NaOH(aq) (approx. 1 M) (IRRITANT) 100cm3

Garlic powder (May cause allergic response and skin/eye irritation), approx. 0.4g

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Hydrochloric acid, HCl(aq) (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book

Sodium hydroxide solution, NaOH(aq) (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

The chemical quantities suggested in the table will be enough to run 10 titration experiments. Depending on the size of the class and the number of trials you want each group to complete you many need to adjust the quantities to suit.

The preparation of garlic infused sodium hydroxide is probably best completed in advance of the lesson

  • The sodium hydroxide (the unknown concentration) (approx. 1 mol dm-3) should be poured into a beaker. Add enough olfactory indicator (garlic/onion powder or cloves), such that the smell can just be detected (2-3 small spatula tips per 100 ml of solution).
  • Cover the beaker (Parafilm or watch glass) and allow to infuse for approx. 15 minutes
  • Decant the solutions into a clean vessel, ensuring no solid indicator is transferred

NB: The smell is detectable in basic conditions but at the end point there is a sudden change of intensity whereby the smell seems to get richer and can easily be detected by wafting the odour towards oneself. It may be beneficial to provide enough glassware such that the students can retain a neutralised solution of the garlic-infused sodium hydroxide to use as a reference.

Procedure

1. Using a small funnel, pour a few cubic centimetres hydrochloric acid, HCl (aq) (1 mol dm-3) into the burette, with the tap open and a beaker under the open tap. Once the tip of the burette is full of solution, close the tap and add more solution up to the zero mark. (Do not re-use the acid in the beaker – this should be rinsed down the sink.)

2. Use a pipette with pipette filler to transfer 10 cm3 of the unknown concentration garlic infused sodium hydroxide solution to the conical flask.

3. Add the hydrochloric acid to the garlic infused sodium hydroxide solution in small volumes, swirling gently after each addition. A sudden change in intensity to the smell is detectable at the end-point of the titration.

4. Repeat until concordant results are obtained

NB: The smell is detectable in basic conditions but at the end point there is a sudden change of intensity whereby the smell seems to get richer and can easily be detected by wafting the odour towards oneself. It may be useful to retain a neutralised solution of the garlic-infused sodium hydroxide to use as a reference.

5. The experiment can be repeated and compared to a standard methyl orange indicator based titration experiment.

Discussion: How did using an olfactory indicatory compare with using a colour-based indicator? How would you rate the accuracy of the different methods?

Teaching notes

The method is somewhat subjective, and while it is possible to obtain concordant results, there is likely to be small variation person-to-person and a marginally different value than that obtained using phenolphthalein as an indicator. The precision and accuracy of these experiments should prove and interesting discussion point with pupils.

Health & Safety checked, October 2013

Credits

This experiment was inspired by an experiment described in the Journal of Chemical Education (J. Neppel, M. T. Oliver-Hoyo, C. Queen and N. Reed, J. Chem. Edu, 2005, 82, 607). In this article a set-up for a visually impaired student is described, where a fan is placed behind the titration apparatus to enable the student to smell without the need for wafting manually. A scheme to show the production of the thiosulfonates and disulfides (responsible for the smell at neutral and acidic pH) from the garlic precursor allicin (diallyl thiosulfinate) is also provided in this article.

This practical chemistry resource was developed by the ETG group for the Royal Society of Chemistry.

Page last updated June 2015

Additional information

The Royal Society of Chemistry would like to thank Dr Jacob Cox, Dr Simon Rees, Rebecca Edwards and Kate Madden as part of the Educational Techniques Group (ETG) for their involvement in the development of this resource.