Practice Assignment 08
A better bit of butter?
Between them, the class should test samples of unsalted, slightly salted and salted butter, covering several brands of each. (Students could bring their own small samples from home to provide a range of brands.)
The procedure involves a back-titration. Since this is likely to be a new concept to students, it should be explained. As an extension activity, (which could be run parallel by part of the class), conventional titration of the chloride content against standardised silver nitrate using potassium chromate as indicator could be attempted. The method does not work because silver chromate, which should precipitate out to indicate the end point, fails to do so, since it is soluble in the nitric acid added to digest the butter.
A milligram balance is specified in the standard procedure. If not available, weighing to 2 decimal places is acceptable, bearing in mind the likely margin of error in students’ titrations.
Health and safety note
Concentrated nitric acid is corrosive. Teachers may wish to dispense it themselves directly into students’ conical flasks. Goggles with chemical splash protection are required.
Silver nitrate solution stains skin and clothing. Protective clothing and gloves are advised.
Apparatus and Reagents
- Standard Procedure:
- SP 0008:2005 Method for determining the salt content of butter
- balance capable of weighing to 0.001 g
- 250 cm3 conical flask
- 1 cm3, 10 cm3 and 50 cm3 measuring cylinders
- 25 cm3 pipette and pipette filler
- 50 cm3 burette
- standardised 0.025 mol dm-3 potassium thiocyanate (See SP 0007:2005)(approximately 100 cm3 per pair)
- access to:
- 0.025 mol dm-3 silver nitrate solution (100 cm3 per pair)
Caution: stains clothing and skin black - concentrated nitric acid (40 cm3 per pair)
Caution: corrosive - 50% ammonium iron(III) sulfate solution (50g in 95 cm3 water + 5 cm3 5 mol dm-3 nitric acid) (10 cm3 per pair)
- 0.025 mol dm-3 silver nitrate solution (100 cm3 per pair)
Test Specimens
- •approximately 2 g of butter (2 x 1 g samples)