Determinación De Área Específica

SURFACE AREA Many physical and most chemical properties of soils are attributed to surface phenomena that take place at the interface between the liquid and the solid phases. These propertiesare correlated to the specific surface area (a) of the solid phase. Following are three formulae for expressing the specific surface area of a particle: am = As/Ms av = As/Vs ab = As/Vt Totalsurface area of particles, As, per unit mass, Ms. Total surface area of particles, As, per unit of volume, Vs. Total surface area of particles, As, per unit bulk volume, Vt.

According toHillel “The specific surface of a soil material is a fundamental and intrinsic property which has been found to correlate with important phenomena such as cation exchange, retention and releaseof various chemicals (including nutrients and certain potential pollutants of the environment), swelling, retention of water, and such mechanical properties as plasticity, cohesion, andstrength.”

The specific surface area (a) can be measured but it is informative to calculate it. To make this calculation it is estimated from the sizes and shapes of particles: CUBED SOILPARTICLE As = 6 l2 Assuming this cube has a density, Ds (M/L3): Ds = 2.65 g/cm3 or 2,650 kg/m3 and Vs = l3 Ms = DsVs = Dsl 3

l l
(NOTE: L is used in the textbook, not l)

am = As/Ms = 6l2/Dsl3 = 6/Dsl (cm2/g; m2/kg; L2/M) = 6/l av = As/Vs = 6 (cm2/cm3; m2/m3; L2/L3) l2/l3

SPHERICAL SOIL PARTICLE

As = 4Br2 = Bd2 Vs = (1/6)Bd3

av = Bd2/(Bd3/6) = 6/d am = 6/Dsd

dTable 1. Relationship of particle size of soil separates to surface area, am. __________________________________________________________________ Size Diameter No. of particles Surface area*separate (mm) per gram (cm2/g) Gravel >2.00 9.0x101 11 Very coarse sand Coarse sand Medium sand Fine sand Very fine sand Silt Clay 2.00-1.00 1.00-0.50 0.50-0.25 0.25-0.10 0.10-0.05 0.05-0.002