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Particles Size Separation by Pipette Method

 

The purpose of this method is to determine the quantity of each of the main sand, silt, and clay fractions in samples of soil form each horizon of the soil profile. A 2-m (#10) sieve will be used to separate the gravel (particles coarser than 2-mm) from the grains less than 2-mm in diameter and the percent sand will be isolated by wet sieving through a set of nested sieves. The silts and clays in each sample will be determine by using a (1) pipette that measures weight percent of sample or (2) a hydrometer that measure the density of a solution of silt and clay suspended in water.

Rationale

The settling method is based on Stokes’ Law. This law states that denser (larger, usually) particles sink farther than less dense (smaller) particles when suspended in a liquid. There are two critical assumptions: (1) the particles all have the same density and (2) the particles are spherical. Actually, neither of these assumptions can be perfectly satisfied.

The pipette method measures the actual percent by weight of each particle size class in your sample. The hydrometer method uses the density of the soil/water mixture. The more particles that are in suspension at any one time, the more dense the soil/water mixture will be; the more dense the mixture, the better it will support an object placed in the mixture. That is, the more dense the mixture, the higher the hydrometer will float in the soil/water mixture. As larger mineral particles fall from suspension, the density of the soil/water mixture decreases. As the density decreases, the hydrometer sinks farther into the mixture. The stem of the hydrometer is marked in grams of sediment remaining in suspension.

In each method, after n seconds have elapsed from the time the soil/water mixture is thoroughly mixed (usually 40 seconds), all particles of one size will have fallen below a certain level in the suspension. After 6 hours and 52 minutes, all the silt (.05-. 002mm) will have fallen below this level, and only clay (less than .002mm) remains in suspension. The hydrometer reading (corrected) at 6 hours and 52 minutes will be a measure of the amount of clay remaining in suspension. From this hydrometer reading, and the initial total weigh of the sample, you will be able to calculate the proportion of the sample that is clay. The proportion of silt is the difference between the calculated percent clay and remaining silt & clay percent after the sands were removed.

 

Source: http://www.geology.iupui.edu/research/SoilsLab/procedures/psd/

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3 Comments

  1. Shishir Kumar Sarker says:

    good job Tania…thanks

  2. Shahanur Shakil says:

    Thanks.

  3. Labiba Farhana says:

    Great .. it’s helpful for class work ..

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