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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Particle Size Analysis  /  Sieving Method

Sieving Method

Before the mineral soil can be sieved, aggregates and organic matter must be broken down and removed. There is therefore a lengthy initial stage of sample preparation to be carried out first.

Sample Preparation

  1. Accurately weigh a 250 ml beaker.
  2. Into this weigh approximately 25 g of air dried soil, sieved finer than 2mm.
  3. Add 100 ml of 20 vol. hydrogen peroxide. Stir well and leave to stand overnight. The H2O2 attacks and destroys the organic matter.
  4. Place the beaker on a hotplate at 40oC to destroy the remaining organics. Care should be taken at this point as the liquid may froth. This can be washed down with a distilled water wash bottle.
  5. If necessary add a little more peroxide to ensure all the organic matter is removed.
  6. Boil down the contents of the beaker to decompose the remaining peroxide.
  7. Add enough distilled water to make the volume up to 150 ml and stir well. Cool and allow to stand overnight - any remaining large organic particles will float to the surface.
  8. Carefully decant off as much of the clear fluid as possible without disturbing the solid matter. This may be easier to achieve by removing the liquid with a pipette.
  9. Boil the remaining liquid to remove as much water as possible. Then place remainder in a fume cupboard to dry.
  10. When the liquid has completely evaporated, weigh, subtracting the weight of the beaker to give the weight of the mineral soil free of organic matter.
  11. It is necessary to disperse any aggregates which may exist before the sample can be sieved. This is done by vigorous stirring and by the addition of Calgon. The intention of the latter is to saturate the base exchange sites on the clays with sodium ions, thus giving the clays a net negative charge so that they repel one another.

Procedure 1

  1. Record the weight of the peroxidised soil sample in the beaker. Then moisten with a little water and transfer to a larger container.
  2. Add about 150 ml of distilled water and 10 ml of 5% Calgon solution. Stir for about 15 minutes This will disperse the aggregates in the soil.

Once the soil has been prepared to this stage, the particle size analysis can be carried out.

The first stage of the analysis is to separate the clay and silt from the sand, by passing the wet suspension through a 63 um sieve. The sand is then dried and weighed and the clay and the silt made up into a fresh suspension and allowed to settle out. Samples are taken at standard times from a standard depth below the surface and their sediment content determined.

Procedure 2

  1. Pour the soil suspension into a 63 um sieve, retaining the liquid and fine particles beneath the sieve in the tray. Make sure all the soil is transferred from the container and lightly wash the fine particles through with a jet of distilled water. Use as little water as possible and make sure that none of the suspension is lost.
  2. Wash the sand from the sieve into an evaporating basin and evaporate to dryness in an oven (105oC).
  3. Remove the sand fraction from the oven, cool and sieve through a sieve set -2mm; 1mm; 0.5mm; 0.25mm; 0.125mm; 0.05mm + base.
  4. Transfer the contents of each sieve to a weighed 50 ml beaker, weigh and calculate the weight of each fraction. Express as a percentage of the original weight of peroxidised soil, using the size classes given above.

This procedure establishes the distribution of particle size in the sand fraction, leaving the clay and silt fractions remaining. These are determined by sedimentation.

Pipette Method

  1. Add to the 500 ml measuring cylinder any fine dust which has collected in the base of the sieve set when sieving the sand fraction.
  2. Fill a deep sink with water and stand the measuring cylinder in it so as to maintain it's temperature as near constant as possible.
  3. Fill the cylinder with distilled water to the 500 ml mark. Mark the stem of a 25 ml pipette exactly 9 cm from it's tip.
  4. Vigorously stir the suspension so that the soil is evenly distributed throughout the cylinder, for about half a minute. Avoid introducing air bubbles into the suspension. Take the temperature of the suspension and then stir again for a further 30 seconds. On withdrawing the stirrer, start a stopwatch.
  5. At exact times (shown on the table below) take a sample from a depth of 9 cm below the surface using the 25 ml pipette. About 20 seconds before the time, lower the pipette slowly to the depth required and begin to take up the sample at the exact time shown.
  6. Transfer the pipetted sample to a weighed crucible and rinse out the pipette. Place the crucible in an oven, evaporate to dryness, cool and weigh.

 

pipette

 

Plot the results as a cumulative percentage curve and as a histogram.