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Soil Science Society of America Journal Abstract -

Ion Activities and Osmotic Pressures in Clay-Water Systems1


This article in SSSAJ

  1. Vol. 18 No. 1, p. 19-21
    Received: June 1, 1953

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  1. N. T. Coleman and
  2. W. J. Jakobi2



Osmotic pressures of H-, Na-, and K-bentonite suspensions were measured with an Adair osmometer. Measurable osmotic pressures were observed, but the values were quite small, ranging from 0.6 cm H2O for 1% Na-bentonite to 4.2 cm H2O for 4% H-bentonite. The ion activity difference across the osmometer membrane, calculated from the membrane potential and the ion concentration on the solution side, was large. The ionic osmotic pressure difference across the membrane, from the van't Hoff approximation, was much larger for all systems than was the observed osmotic pressure, leading to the conclusion that the membrane potential cannot be regarded as a measure of an electrostatic potential difference between a clay suspension and its equilibrium dialysate.

Electrolyte mean activities of KCl solutions were consistently slightly smaller than in 2% K-bentonite suspensions of the same normality of KCl. Mean activities of KCl in the clay suspensions were much smaller than K+ activities measured with a membrane electrode-saturated calomel electrode cell.

The osmotic pressure data and potentiometric data are interpreted as indicative of a spurious potential at a clay-salt bridge junction.

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