Chemistry of dentistry

This session should take four hours and 15 minutes. 

Suggested timings

• 10.45 Arrival and welcome, plus refreshments
• 11.15 Opening presentation, with questions
• 12.00 Practical session 1
• 12.30 Lunch
• 13.15 Practical session 2
• 13.45 Prepare report on first experiment
• 14.15 Presenting results and discussion
• 14.45 Closing remarks and feedback forms
• 15.00 Finish

Equipment

• Silicone impression putty
• Glass-ionomer dental cement or plaster of Paris
• Solutions of lactic acid
• Solutions of KF, Potassium Fluoride
• Toothpaste
• Ion-selective electrodes(2) x2
• pH meter

Health, safety and technical notes

• Read our standard health and safety guidance here. 
• Perform a safety assessment 
• The moulds are biohazards as they are contaminated with saliva, and they must be disposed of in the bags marked biohazard
• Although the light emits mainly visible light, it also emits a small amount of ultraviolet light. This damages the retina in your eye. The light must be used carefully and should always be viewed through the orange filter. 

Procedure

Practical session 1 - preparing a model tooth

1. Take equal – but small – amounts of each colour of the two-paste impression material and mix them with your fingers until they are fully mixed, and allow the putty to set slightly. This takes around 2-3 minutes.
2. Place the semi-set mould over one of your incisors and hold it there for around 5 minutes until it sets.
3. Weigh 6 g of cement powder in a measuring cup on a balance. Add 2 cm³ of water from a pipette. Use a stirrer to mix the cement in the cup until it becomes a paste.
4. Fill your tooth impression with the mixed cement and allow it to harden, this takes around 10 minutes.
5. Remove the model tooth from the mould.
6. Inspect your model tooth carefully. Is it a good representation of your teeth in terms of shape, colour and markings?

Practical session 2 - the pH of lactic acid solutions

1. Check that the pH meter is properly calibrated by using the pH 4 and pH 7 solutions provided.
2. Pour small amounts of each of the lactic acid solutions into beakers and check the pH of each solution.
3. Record your results
4. Either by calculating or estimating from your results, suggest the approximate concentration of lactic acid that causes the pH (4.5) of active tooth decay. What can you conclude about the amount of glucose needed to cause tooth decay?

Measuring the release of fluoride ions (F-) from toothpaste

1. Pour 25 cm³ of deionised water into a screw-capped test-tube.
2. Squeeze approximately 10 cm³ of toothpaste into a plastic cup and use a syringe to measure accurately 1 cm³ of toothpaste.
3. Syringe the toothpaste into the screw-capped test-tube containing the deionised water and replace the cap securely.
4. Shake the toothpaste solution vigorously for at least 3 minutes.
5. Use an electrode to measure the fluoride concentration
6. Record your results.

Fluoride uptake by hydroxyapatite

1. Check that the meter corresponds to 1000 ppm for the solution.
2. Weigh 0.6 g of hydroxyapatite powder in a beaker. Add 5 cm³ of 1000 ppm KF solution and note the time.
3. Using the meter, record the values at 2 minute intervals for 10 minutes.
4. Record the fluoride concentration values at the different time intervals.

Depth of cure of light-hardened composite resins

1. Fill the metal mould with composite resin.
2. Place a glass microscope slide over the end of the resin and expose to the cure light for an appropriate length of time – 10, 20, 30 and 40 seconds. The lamp runs for 20 seconds so must be moved away from the resin if necessary to make sure you use the correct cure time. 
3. After exposure to the light, remove the specimen from the mould and use a spatula to scrape any resin from the far end of the mould. Measure the length of the cured specimen.
4. Record the depth of cure values.