UCL Department of Geography


Miscellaneous procedures

Some methods cannot be categorised under "sediment" or "water" analysis although both may be involved. If you cannot find the method you are looking for in any of the other sections it may be here.

Digestion of Lens Tissue for diatom slide preparation


  • Concentrated Nitric Acid with bottle top dispenser
  • Saturated Sodium Carbonate Solution
  • 50 ml Centrifuge tubes with lids
  • Disposable pipettes
  • Distilled Water in a wash bottle
  • Water bath at 60 degrees C.
  • Metal rack for 50 ml tubes
  • Centrifuge to take 50 ml tubes


The procedure must be carried out in a fume cupboard. Where tubes are removed from the fume cupboard, they must have lids fitted securely.

Gloves, safety glasses and a lab coat must be worn at all times. All acid spillages must be neutralised with Sodium Carbonate and cleaned up immediatley.

Day One
  • Place lens tissue in a labelled 50 ml centrifuge tube
  • Carefully add 30 mls concentrated Nitric acid to each tube using the dispenser. Agitate gently and place them in the water bath at 60 degrees for 6 hours.
  • Turn off the water bath and leave it to cool overnight.
  • Ensure that the samples are labelled appropriately.
Day Two
  • Remove tubes from the water bath.
  • Carefully and slowly top up the tubes to 40 mls with distilled water. Adding water to acid is not usually recommended, but in this situation, it is necessary to dilute the acid and eventually wash it away completely. This procedure must be carried out very carefully in the fume cupboard with the window pulled down.
  • Screw the lids onto the tubes and centrifuge at 1200rpm for 4 minutes ensuring that the centrifuge is properly balanced.
  • Remove tubes from the centrifuge and return to the fume cupboard before taking off the lids.
  • Using a disposable pipette (a clean one for each sample) remove about 25 mls of the supernatant liquid being careful not to disturb the solids at the bottom of the tube and discard into a container of Sodium Carbonate solution to neutralise the acid before disposal.
  • Top up the tubes to 40mls with distilled water and repeat the centrifugation process.
  • Again remove the supernatant and discard.
  • Repeat these steps 7 to 10 until the supernatant liquid is neutral when tested with indicator paper indicating that all of the acid has been washed out of the sample.
  • The suspension can now be made into diatom slides using the standard procedures.