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Prediction of Diffusion Coefficients from the Electrical Conductance of Soil
Author(s) -
Palmer C. J.,
Blanchar R. W.
Publication year - 1980
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1980.03615995004400050009x
Subject(s) - conductance , diffusion , electrical resistivity and conductivity , electrolyte , conductivity , chemistry , analytical chemistry (journal) , electrical resistance and conductance , tortuosity , soil water , thermodynamics , electrode , materials science , soil science , chromatography , physics , condensed matter physics , composite material , porosity , organic chemistry , quantum mechanics , environmental science
Electrical conductance may be used to predict salt movement through soil by diffusion. A two‐component model was used which considers electrical conductance in soil to be a combination of a solution and a surface process. It has the form: EC a = EC w θ T 8 + EC 8 where EC a is the specific conductance of the soil, EC w is the conductance of the solution, θ is the volumetric moisture content, EC r is the surface conductance, and T 8 is the tortuosity. It was assumed that T 8 and EC 8 did not vary with changes in solution concentration, and these two parameters were calculated by measuring EC a and EC w on two soil samples which differed only in electrolyte concentrations. These values were used to calculate the equivalent conductance and the diffusion coefficients for Na + and K + . The alternating current measurement of conductivity was frequency ( f ) dependent due to electrode‐electrolyte interactions known as the Warburg impedance. The specific conductance was found by plotting the measured conductance against f ‐0.5 and extrapolating to zero. At low electrolyte concentrations, the conductivity of the soil varied as a linear function of the logarithm of frequency and this factor was evaluated before predicting diffusion coefficients from the conductance of the soil. The diffusion coefficient for K + in four soils with 0.0133 M KCl varied from 1.64 × 10 −6 to 0.50 × 10 −6 cm 2 /sec when computed from electrical conductivity but varied from 1.84 × 10 −6 to 0.33 × 10 −6 cm 2 /sec when computed from self‐diffusion. The fraction of the electrical current carried by the 0.0133 M KCl in the soil solution varied from 0.85 to 0.13 and was inversely related to the cation exchange capacity. A comparison of diffusion coefficients estimated from the self‐diffusion of 42 K and 24 Na in concentrated and dilute systems indicates good agreement except for 0.0133 M K + in the Mexico Ap soil. The data suggest that diffusion measurements made by electrical conductance are more reproducible than those made by self‐diffusion, but give estimates of the diffusion coefficient that are of similar magnitude.