Particle Size (Hydrometer)
1
DATE: SEPT 2004
Particle Size Analysis (Hydrometer Method)
1. Application
The percentage of sand, silt and clay in the inorganic fraction of soil is measured in this
procedure. The method is based on Stoke’s law governing the rate of sedimentation of
particles suspended in water.
2. Summary of Methods
The sample is treated with sodium hexametaphosphate to complex Ca++, Al3+, Fe3+, and
other cations that bind clay and silt particles into aggregates. Organic matter is suspended
in this solution. The density of the soil suspension is determined with a hydrometer
calibrated to read in grams of solids per liter after the sand settles out and again after the
silt settles. Corrections are made for the density and temperature of the dispersing
solution.
3. Safety
Each chemical compound should be treated as a potential health hazard. The laboratory is
responsible for maintaining a current awareness file of OSHA regulations regarding the
safe handling of the chemicals specified in this method. A reference file of material
handling data sheets should be made available to all personnel involved in the chemical
analysis.
4. Interferences
The principal source of error in this procedure is the incomplete dispersion of soil clays.
These clays are cemented by various chemical agents and organic matter into aggregates
of larger size. Failure to effect complete dispersion results in low values for clay and high
values for silt and sand. The rate of sedimentation also is affected by temperature and the
density of the dispersing solution.
5. Apparatus and Materials
5.1 Glass cylinders, 1000-ml capacity
5.2 Thermometer, Fahrenheit
5.3 Hydrometer, Bouyoucos (Fisherbrand Model # 14-331-5c)
5.4 Electric mixer with dispersing cup
5.5 Plunger
5.6 Balance sensitive to ± 0.01g
