Applied Science A3 Scientific detection Technicians notes
Further information for technicians
Activity AA3.10 Colour matching
Students are unlikely to detect the presence of iron in water sample A as described in the Guidance note, using the thiocyanate test. We suggest that you increase the concentration of iron(III) nitrate-9-water - say to 0.05 g/dm3 (0.00012 M or 1 in 150 000), or 0.1 g/dm3, depending on how obvious you want the colour to be.
stirrer in beaker Activity AA3.14 Detecting with colour
Some schools have had difficulty in extracting iron from breakfast cereal in this practical. Technician Mark Beech of Sir John Deane's College Northwich suggests using a magnetic stirrer. In his photos, you can see the dark patches of iron at both ends of the white stirrer.
Mark Beech's photos of the magnetic stirrer before and after
Activity AA3.19 Find the forgery
We are grateful to CLEAPSS for the following note. See the CLEAPSS health & safety warnings given here.
This activity involves chromatography of various metal salts, separately and mixed, to demonstrate how forged coins might be identified. The locating agent used is dithizone in methyl benzene.
Dithizone is available from Sigma Aldrich or VWR. The solid carries Risk Phrases R36/38 IRRITATING to eyes and skin, but the reagent is made up at 0.05% at which concentration it carries no hazard classification. The solvent, methylbenzene is more hazardous than the dithizone, and while unlikely to exceed the Workplace Exposure Level it is unpleasant and can cause dizziness and drowsiness.
One of the salts, nickel(II) nitrate is OXIDISING (R8), HARMFUL if swallowed (R22) and May cause sensitisation by skin contact (R34). At the 2% concentration used it is IRRITANT, so those with sensitive skins should use care to avoid spilling it or getting it on the skin. For some pupils disposable gloves might be a good idea.
The chromatography solvent contains a high proportion of propanone (HIGHLY FLAMMABLE and IRRITANT), so heed the advice given in the OUP text to keep a lid on the chromatography tank.
Spraying or dipping of the locating agent should be done in fume cupboard, either by the teacher or the pupils, depending on the teacher’s risk assessment. The chromatograms should be dried on a sand tray heated on a hotplate in the fume cupboard.
Activity 3.23 Crime scene investigation
Some schools have had problems with the mounting medium for pollen grains. Technician Gill Halton at MMU Didsbury decided to see what happened if she did without the mounting medium and found that the slides were fine, and still good after a weekend.
Gill also tried setting the centrifuge on a speed (much lower than the recommended 3000 rpm) to match those of the smaller and cheaper centrifuges which schools are more likely to possess. Gill reports: ‘I have just finished the trial on the honey. It is possible to see pollen without spinning the honey down or staining it. If you centrifuge at 1800 rpm for 5 minutes, however, the results are much better. Procedure step d: I don't think it is necessary to mix residual pellets from the two tubes together: there should be sufficient material left in a single tube after the supernatant has been decanted off.’
Later Gill reported further: 'I have now made up the pollen mounting media. The only "nasty" part is making up the 80% phenol. As only very small amounts of media are required it may be possible for schools to share, with a college providing the solution for them. I also used the method shown on the SAPS website and it worked beautifully.'
All through these supplementary notes, remember that you must carry out your own risk assessment and take suitable precautions.
