Pipette Analysis

Objective

  • To determine the silt and clay content of the <63m m fraction of the sediment

Equipment and Reagents

Eight measuring cylinders, ~50mm internal diameter, 500ml capacity

Rubber bung to fit the cylinders.

A constant temperature water bath capable of maintaining temperatures within +0.5oC, capable of immersing the cylinders up to the 500ml mark, vibration-free.

10ml pipettes with suction bulbs, marked 100mm above the inlet.

25 x Glass vials capable of holding 15ml of water, weighed to 0.0001g.

0.5litre Pyrex beakers

Hotplate

Magnetic stirrer plus eight magnets

Temperature-controlled oven, preset to 105oC + 5

Balance capable of weighing to 0.0001g

A desiccator containing anhydrous silica gel.

Digital watch

Washbottle/spray filled with tap water

Log sheets

Supply of tap water

Distilled water

Hydrogen Peroxide solution (20 volume solution)

Sodium Hexametaphosphate solution (dissolve 33g of sodium hexametaphosphate and 7g of sodium carbonate in distilled water and make up to 1litre of solution. Should be freshly prepared monthly).

Methodology

  1. Wash subsample (iii)  into a 0.5litre Pyrex beaker

  2. Add 60ml of hydrogen peroxide and allow to stand overnight.

  3. Gently heat on the hotplate, agitating occasionally and being careful to avoid excessive frothing. With washbottle wash down sediment drying on beaker sides. As soon as all the vigorous frothing has subsided reduce the water volume slightly by boiling until the sample has the consistency of a thick slurry. NOTE: up to six samples can be batch-prepared overnight in this manner.

**** VERY IMPORTANT .. ….. DO NOT ALLOW TO DRY OUT …… MAINTAIN A SLURRY CONSISTENCY ********

  1. Subsample into a 500ml beaker two teaspoonfuls of the slurry ( about 2-5g of sediment), mixing and stirring thoroughly before subsampling. Use a washbottle sparingly to clean the spoon each time. Add 25ml of sodium hexametaphosphate solution, and swirl vigorously for a few minutes. Wash the subsample into a 500ml measuring cylinder. Add tap water to make 500ml of suspension.

  2. Place the measuring cylinder full of suspension into the water bath. Set the ambient temperature to just above the expected daily maximum ambient temperature.

  3. Repeat steps 4 & 5 for all the prepared batch of samples.

  4. Wait until all samples are at the bath temperature (~1 hour).

  5. Remove a cylinder from the bath. Add a magnet/stirrer and stir for five minutes. Invert the cylinder end over end 10-20 times, observing there are no clots of sediment remaining, so that a uniform suspension is achieved.

  6. Quickly replace the cylinder in the bath and IMMEDIATELY insert a pipette into the centre of the tube (away from the walls) to about the 10cm depth mark. Quickly withdraw exactly 10ml of suspension using the fitted suction bulb. Remove the pipette. Note the time that the sample was collected (nearest 30s). THIS NEEDS PRACTICE.

  7. Drain the pipette sample into a pre-weighed labelled glass weighing tube. Suck 3-5ml clean water into the pipette, shake (with finger over bottom inlet) and flush this too into the tube. Place tube in rack in the oven.

  8. At about ten to fifteen minute intervals repeat process with each of the measuring cylinders full of suspension.

  9. At intervals of x and y minutes, (see example in Table 1), repeat the withdrawal process being very careful not to ‘stir’ the settling suspension, and being very careful to withdraw at exactly the 10cm depth mark. Place all labelled tubes of suspension in the oven at 105oC.

  10. When all the weighing tubes have been in the oven for sufficient time for weights to be constant, place tubes in a desiccator and allow to cool. Weigh to 0.0001g.

  11. Enter the three sets of weights for each sample and the bath temperature into the EXCEL file for calculating the clay content of the samples on the basis of Stokes Law.

NOTE: The above analysis can be efficiently conducted in batches of six samples.

  Table showing settling times