To Determine the Concentration of an Unknown Dilution of Blue Food Colouring by Use of a Spectrophotometer and a Calibration Curve

Notes for those delivering the outreach session

1. This investigation has been delivered in both school laboratory setting and in university teaching laboratories with good effect.
2. Timing: The minimum time for delivery is 60 mins (and then only with gifted and talented students who have previously handled a burette) or with Post 16 (pre-university) students. Far better 90 minutes for younger students Year 6 and Key Stage 3 or 4 students (11-16) who can graph or 120 minutes for mixed ability groups to allow for skills teaching with volumetric flask, burettes and for graphing.
3. Younger students, or those with difficulty in graphing may appreciate a calibration curve prepared earlier and they can sue this to produce their own graph OR use the graph provided to interpolate to get their own results for the unknown.
4. The use of the colorimeter is easily understood by showing students in small groups as they have prepared their first solution. Don’t overload them with this information at the beginning.
5. If there is sufficient time it is interesting to show the students an ‘Oscillating Clock Reaction’ to demonstrate depth of colour. A torch can be shone behind the bottle holding the reaction to emphasis the absorbance of light as the concentration changes.
6. If there is time the entire exercise can be used to discuss errors in experimental work.

Oscillating Clock Reaction

This may be used as part of the starter activity

Take the usual safety precautions of lab coat and safety glasses.

Pre measure:

• 800 cm³ of 2.0 molar sulfuric acid, [Harmful, Corrosive],
• 9g of malonic acid (propandioic acid), [Harmful],
• 8g of potassium bromate (v), [Toxic and oxidising]
• 1.6g of hydrated manganese (II) sulfate. [Harmful]

Equipment:

• Scott Bottle
• Magnetic stirrer
• Follower
• Torch or other light source.

Procedure
The acid should be placed in a Schott bottle with a magnetic follower onto a stirring mantle. Add the malonic acid and potassium bromate (v) solids and allow to dissolve. This takes a few minutes and should be done well ahead of time. Add the manganese sulfate solid and allow the reaction to settle. This takes about 5 minutes before the oscillating reaction occurs convincingly. It then proceeds for around 60 minutes.

Disposal
Wash contents down the sink in a fumehood with plenty of water.

Technician List
For a class of 20 students (and accompanying teacher)

Equipment

• 4x Spektra^TM visible light spectrometers with mains transformers
• x20 blister trays (and some spares)
• 1x box of Pasteur pipettes
• 40x teats for Pasteur pipettes
• 20x 50mL burettes
• 20x burette funnels
• 20x 100mL beakers
• 20x clamps and stands
• 4x 250 mL beakers (for used pipettes)
• 20x 50mL volumetric flasks and stoppers
• 2x lab roll
• 3x boxes disposable gloves (various sizes)
• Lots of lab coats (various sizes)
• 25 x Safety glasses (including over-glasses)
• 20x sheets of graph paper
• 20 x Pencils
• 20 x Rulers
• Dustpan and brush (in case of breakages)

Chemicals

• 6x 250cm³ bottles labelled ‘Stock Solution’
• 6 x 50 cm³ Schott bottles labelled ‘Unknown Solution’
• 1x 2.5 litre Winchester bottle with ‘Stock Solution’
• Tap water access (no need for distilled water)

Stock Solution
This is a solution of 20 g of commercial blue food colouring solution added to 100 cm³ of water.

i.e. for the Winchester of ‘Stock Solution’ dilute 200g of food colouring solution with 2 litres of water.

Unknown Solution
This is a 30% solution of the original ‘Stock Solution’.

Risk Assessment
The blue food colouring has no chemical hazards stated on the packaging.
The risks present are mainly due to the possibility of broken glass.

Notes for Demonstrators for Outreach Use
of Spektra Colorimeters

In the trials run so far there is no student script: instructions are given verbally.
This can be delivered by one experienced demonstrator but it is far easier with two!

Outline

Go though Health and Safety ⇒ Define problem ⇒ Show used of volumetric flasks ⇒ make some dilutions ⇒ show use of colorimeter in small groups ⇒ complete task and solve problem ⇒ Feedback

Show the students the Stock Solution and the Unknown Solution and set the scene.

Explain that the unknown concentration can be calculated from a calibration curve. To construct the calibration curve a series of measurements of the colour depth needs to be made for different concentrations. Students need to prepare 80%, 60%, 40% and 20% solutions by diluting the Stock Solution. Bright students need little guidance in the calculation of the volumes of Stock Solution to be measured by the burette. Other students need a table of volume of the Stock Solution to be measured out and made up to 50 cm³ by the addition of water.

Students to work in pairs as students find it less intimidating to work in pairs. If students wish they may work individually BUT this does make the exercise a little longer.

Students are shown how to use the burettes and the volumetric flasks (if they have not come across this equipment before) and are set off in preparing the first of the dilutions. After the students have made at least one dilution take small groups of them aside and show them how to use the colorimeter. This is not done at the beginning as students would need to hold too much new information in the heads and so would confuse them. Show the students the zeroing using water and then for the Stock Solution. Explain the need for the consistent depth of solution to be used. This has been found to be in the order 8 - 10 drops.

The students are then given time to complete their readings but may need to be reminded to take the reading of the Unknown Solution too!

For students whose graphing skills are still developing have a completed graph to show them as an example.

Ages of Students
This task has already been trialed with 14 to 18 year olds at present (October 2009)

Worksheet for experiment

Prepared by: Bristol ChemLabS, University ob Bristol, UK