SOIL WATER CONTENT BY KARL FISCHER TITRATION
|L. Prunty, M.K. Zellis, and J. S. Bell, Department of Soil Science, North Dakota State University, Fargo, ND 58105-5638||
Specific analytical determination of water content is often desirable in contaminated soils. For instance, when soil simultaneously contains water and a nonaqueous phase liquid (NAPL), conductivities for each liquid are functions of the concentrations of both.
Similarly, vapor phase diffusivities depend on both. It follows that the effectiveness of decontamination technologies such as vapor extraction also depend on the same variables. Volatile solutes can influence the same parameters, but through constitutive relations that may be of different form. Overall, though, there is a general need to determine soil water content independent of volatile soil contaminants.
Karl Fischer (KF) titration employs a specific reaction to consume H2O, independent of the presence of other volatile substances. Thus, we felt that it would be useful to add KF to the suite of readily available and tested soil analytical methods. Although it is commonly used for water content determination in a wide variety of products and substances, KF has been virtually ignored in soil and environmental science.
Oven drying (OD), conversely, is considered the standard method of soil water determination. But, since OD eliminates all volatiles from soil samples, it is clearly inadequate when volatile substances other than water are present. Using KF, OD, and other methods together should yield a more accurate and useful analysis of soil.
Our objective was to experimentally examine KF titration as a technique for soil water content determination. Two soil types (sand and loam) were spiked with varying amounts of water (0 to 240 g kg-l) and a NAPL, either octane or toluene (0 to 128 g kg-l). The prepared samples were analyzed for volatile/moisture content using OD/KF analysis. The experimental matrix included six combinations of the two soils and three liquid compositions.
The liquids were water only, water with octane, and water with toluene. Soil moisture values determined by the calibrated KF method (wc) compared favorably to those determined by OD (wo) in terms of regression slope, intercept, correlation coefficient, and t. Evaluation of wo was possible in this experimental situation because the samples contained predetermined amounts of octane or toluene. Slopes for six wc vs. wo regressions ranged from 0.980 to 1.009 while intercepts ranged from -0.6 to 7.8 g kg-l. Sand with octane and sand with toluene mixtures had intercept values of 6.1 ( 2.5 and 7.8 ( 5.4 g kg-l (95% confidence intervals), respectively.
All other intercept confidence intervals included zero. Plots of the data show essentially a 1:1 correspondence of wc to wo. Variability was greater when toluene was present. Soil water content can be accurately measured using KF.
Key words: extraction, methanol, toluene, octane, drying
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