Particle Size Determination (Pipette Method)

Introduction / Rationale / Methods / Data Sheets / Recommendations / References

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.

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.

1. Dry samples in oven at 100-105° C.
2. Gently break up sample fine enough to pass through a 2 mm sieve (use a wood mortar and pestle). If there are clay/silt agglomerates, repeat process several times.
3. If there is a significant amount of gravel-size material, follow the steps outlined below. If not, skip to step 4.

1. Remove any pebbles that are anomalously large for the overall size distribution. The presence of such should be noted in your descriptions, but their abundance can not be evaluated quantitatively by weight percentage (although, you can make an estimate of their percent volume if you desire.
2. Weigh out at least 100.00 g of dried sample on a pan balance (or analytical balance). Record weight to nearest .005g.
3. Pass the material through a 2mm sieve. Weigh the greater than 2.0-mm fraction and record both the weight and the percent gravel in the sample by weight.

1. Using the analytical balance, weigh out 20.0000 g of sample.
2. Transfer the sample to a baby bottle and add 10 ml of 30% H₂O₂ (WARNING!! H₂O₂ is a strong oxidizer and will readily react with hair, skin, eyeballs, clothes, etc.) Record the number of the baby bottle on data sheet.
3. When the reaction diminishes, add approximately 50-ml of distilled water and bring to a boil for 15 – 20 minutes. Watch carefully to prevent boil over.
4. Remove from heat source and let cool.
5. Add 20 ml of sodium hexametaphosphate (i.e. Calgon), put caps on bottles, check for leaks, and put on the shaker. Be sure that the bottles on the shaker are counterbalanced. Use water-filled blanks, if necessary. Leave samples on shaker overnight (or at least 6 hours).
6. Place a 62.5m m sieve over a large funnel and set in a 1000 ml cylinder (be careful, it may unsteady). Record the number of the cylinder on your data sheet. Remove the samples from the shaker and gently pour the sample through the sieve. Use caution not to lose any of the sample by spillage. Thoroughly wash all silt and clay through the sieve using distilled water. The entire sand fraction (very fine – very coarse) is now in the sieve. Carefully transfer all of the sand to a 50-ml beaker. Dry the sand and weigh. If necessary, the dried sand fraction can be run through a nest of sieves to determine sand size distribution.
7. The cylinder should now contain only the silt and clay fractions of the sample. Fill cylinders to the 1000-ml mark with distilled water.
8. Obtain 7 beakers (for each sample) and record their numbers and tare weights on the data sheet. These will be used for pipette "pulls" of the different size fractions …vcs silt, cs silt, med silt, fn silt, vf silt, cs clay, vf clay. These guidelines for a detailed particle size analysis. If doing basic particle size, that is measuring only the amounts of sand, silt, and clay…only 2 beakers are necessary for pipette withdraw of the silt and clay fraction.
9. Measure and record the temperature of the water in the cylinder. Consult the settling time chart to determine the time and depth oat which "pulls" must be made for the various size fractions.
10. Agitate the sample vigorously for 20 seconds. Immediately after you cease stirring the sample, begin the time count for the first settling time.
11. At the required time, "pull" the fraction form a depth of 10 cm (use depths as instructed on settling time chart) using a 20 ml pipette.
12. Dispense the sediment sample from the pipette into the 50-ml beaker designated for that size fraction.
13. Wash pipette into beaker with distilled water.
14. Place sample in drying oven.
15. When dried, place is dessicator to cool, and weigh immediately.
16. Repeat steps 12-18 for remaining size fraction.

To figure the calgon correction factor (CCF), pipette 20 ml of Calgon into three separate beakers. Place in oven until dry, cool in dessicator and weigh immediately. Use the following equation to figure the mean;

Calgon (sodium hexametaphosphate) ----75g/2Liters H₂O ---solve for the mean of the three weights, then;

Mean/50 = CCF

1. Depths for successive draws should be measured from the actual (declining) surface of the suspension; these depths are given by Stoke’s law without modification, regardless of whether a subsequent draw is made at lesser, equal, or greater depth than the preceding draw.
2. If you repeat a draw without restirring, you will have to correct your draw depth for the thickness of suspension removed during the original draw, as well as during any other intervening draws.
3. Restirring between draws does not affect subsequent draws, or a repeat of the draw that immediately preceded restirring. However, it does affect the weight of any other previous draws that may be repeated.
4. It is very important to record the times and depths of all draws, and any restirring. If errors occur, you can back-calculate to determine the limiting particle diameter that you actually sample.
5. Avoid blunders by allocating sufficient time to prepare for each group of draws, including the time to temporarily re-mark the pipette if you use non-standard depths, and to double-check that you have inserted the pipette to the correct depth.
6. Allow ~10 minutes to calculate and prepare if you must change any draw times. Draws may be taken at any convenient depth/time combination, however to eliminate confusion the above table should be employed.

Day, P.R., 1965, Particle fractionation and particle-size analysis, in Black, C.A., ed., Methods of soil analysis, Part 1: American Society of Agronomy, Inc., Madison, Wisconsin, p. 545-567.

Franzmeier, D.P., G.C. Steinhardt, J.F. Crum, and L.D. Norton, 1977, Soil Characterization in Indiana: I. Field and Laboratory Procedures: Research Bulletin No. 943, p. 13 –14.