Integrated Assignment 03
Soft, Absorbent and Strong?
Encourage students to select kitchen towel that has single colour printing and paper napkins that are self-coloured, rather than printed with a multicoloured pattern.
Before tackling the assignment, students should have experience of using microscopes, measuring tensile strength and performing chromatography, including measuring Rf values. They should also be familiar with the standard procedures that they are asked to adapt.
Tasks could be distributed amongst the students; for example making each pair of students responsible for testing one or two types of paper, or for conducting one or two types of testing, and pooling their results.
Practical tips
Microscopy
Students could investigate whether viewing paper with transmitted or incident light shows up more details of the fibres – and whether this depends on the thickness and/or colour of the paper.
Strength test
The specimens need to be wider than that stipulated in the Standard Procedure, to absorb enough water. The kitchen towel would have to be cut diagonally to obtain a 300 mm length. However, students can obtain adequate readings using a slightly shorter length.
A sheet of paper towel usually tears more easily in the direction of the roll than across its width. Students could investigate whether the strength differs significantly for samples cut along, across and diagonally – (technically, whether paper strength is an anisotropic property).
You may need to explain about using the same amount of water to dampen each of the specimens, so that the results are not affected by different absorbencies.
As it is difficult to tape wet paper, use a large bulldog clip instead of folding and taping the paper around the pointer.
Absorbency test
It needs to be stressed that surplus water should be removed by gently shaking the funnel.
Ideally, the students should have access to a balance weighing to 0.001 g.
Colour test
The use of 1 mol dm-3 ethanoic acid, white wool and ammonia in the Standard Procedure is not needed for this Assignment. It is used to separate the dye from the sugar. Consequently, it is not necessary to use a fume cupboard when concentrating the dye extract.
Extraction should be tried first with cold water, then hot. Cheaper brands of paper products tend to use water-soluble dyes, while others use more permanent dyes, that don’t leach out in contact with food, or when a drink is spilled.
If students bring in their own samples, it is likely that only some will contain extractable dyes. Teachers are advised to test some products in advance, to ensure availability of samples from which dyes can be extracted successfully.
It is not recommended that extraction should be attempted with organic solvents. However, low-hazard solvents, such as ethanol or propanone, could be tried as eluents for the chromatography, to see whether they give different separations. Such observations, caused by differences in solubilities, can help to explain how chromatography works.
Since kitchen paper comes into contact with food, any dyes used that are water- or fat-soluble must be harmless. Permitted food dyes (those with E-numbers) would meet this criterion. Students could run known food dyes alongside their extract on their chromatograms, to see whether any of the dyes match.
Health and safety note
If organic solvents are used to extract the dye, they must be low-hazard. Toxicity and flammability must be taken into account when evaporating the extract.
Apparatus and reagents
- Standard Procedures:
- SP 0001:2005 Methods for setting up and adjusting a light microscope
- SP 0002-1:2005 Mounting a hair or other fibre
- SP 0005:2005 Method for identifying food dyes in colour-coatedhocolate beans
- SP 0011-1:2005 Determining the density of regular-shaped objects
- SP 0012-1:2005 Comparing the tensile strength of plastics, paper and board
For microscopic examination of fibres:
- microscope
- light source (lamp or good daylight)
- paper specimen
- microscope slide and cover slip
- disposable dropper
- forceps
- mounted needle
- mountant (e.g. DPX Caution: contains xylene [dimethylbenzene] – harmful, irritant)
Available form BDH Chemicals (VWR International, Poole, BH15 1TD 01202 669700 www.vwr.com
For testing tensile strength:
- pair of scissors for cutting test pieces
- clear sticky tape
- bulldog clip
- metre rule
- clamp and stand
- wooden blocks
- strong thread
- dowel with a pin or pointer attached to one end
- 100 g mass hanger
- 100 g masses
- 10 g masses
For testing absorbency
- balance (preferably capable of weighing to 0.001 g)
- pair of scissors for cutting test pieces
- filter funnel (one per paper sample)
- 250 cm3 conical flask
- For testing dyes
- boiling tubes
- evaporating basin
- water bath (or beaker of hot water)
- capillary tubes, for spotting the chromatogram
- chromatography vessel (or large beaker with cling-film cover)
- chromatography paper
- pencil and ruler
- organic solvents (See notes above)
- selection of known dyes (approx 0.05% solutions)
(Details of food dye suppliers are given in Teachers’ Notes for PA 05)