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Use of Density Gradients for Size Distribution Analysis¹

ABSTRACT

A linear-density gradient tube, sensitive to 0.00001 g./ml., was prepared from mixtures of kerosene and bromobenzene and used to measure the specific gravity of small drops of clay and/or soil suspensions.

A small drop of suspension (0.005 ml.) was transferred with a micropipette at the required depth from the soil or clay suspension and placed in the density gradient. The drop seeks a level in the tube according to its density which depends upon the concentration of solids. The level is read with a cathetometer and compared to the level of standards of known density and weights.

The method is especially suited for the size distribution analysis of material < 0.002 mm. in diameter. Since only 0.005 ml. of suspension is required for a determination, a high-speed centrifuge that accommodates small tubes can be employed.

The density gradient technique is more reproducible than the standard pipette method and lends itself more readily for the size distribution analysis of the clay size materials. It is as accurate for the 0.002-mm.-diameter clay and up to the 0.02-mm.-diameter silt as the standard pipette analysis, and it requires less time for a determination since only a cathetometer reading is necessary.

¹ Contribution from the Department of Agronomy, Illinois Agr. Exp. Sta., Urbana, Illinois. Presented before Div. I, Soil Science Society of America, Nov. 15, 1956 at Cincinnati, Ohio.

² Assistant Professor of Soil Physics, Department of Agronomy, University of Illinois. The author is indebted to Mr. J. M. Parker who made the pipette analysis and to Dr. Arnold Klute who offered many valuable suggestions.

Received for publication August 12, 1957. Accepted for publication October 15, 1957.