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Prediction of pH Errors in Soil-water Extractors Due to Degassing¹

ABSTRACT

Moisture samples taken from the unsaturated zone with soil water extractors undergo degassing and an upward shift in pH. The measured pH values from commercially available extractors are usually sufficiently in error that they cannot be used in a quantitative manner. A model has been developed that predicts the extent of CO₂ degassing and the resulting pH error. Using this model measured pH values can be corrected back to in situ soil water pH provided that precipitation has not occurred in the extractor. Extractors are classified into two groups—single chamber and multichambered. The extractors are evaluated for both operation under constant vacuum (open to the source) and decreasing vacuum (evacuated and then sealed). Analysis of the data and model predictions indicates that the major factor controlling the pH error is the ratio of liquid volume to total extractor volume. Additional factors exerting major influence are the initial extractor gas composition and the total pressure in the extractor when sampled. Variations in soil solution composition and differences in soil CO₂ concentrations in carbonate buffered systems had a major effect on pH values but a negligible effect on the extractor induced pH error. Under typical field conditions the multichambered extractor is predicted to give the most satisfactory results; the pH errors were sufficiently small that no corrections for degassing were necessary.

¹ Contribution from the U.S. Salinity Laboratory, USDA-ARS, Riverside, CA 92501.

² Geochemist, U.S. Salinity Laboratory, USDA-ARS 4500 Glenwood Dr., Riverside, CA 92501.

Received for publication February 18, 1986.

This article has been cited by other articles:

				L. Weihermuller, J. Siemens, M. Deurer, S. Knoblauch, H. Rupp, A. Gottlein, and T. Putz In Situ Soil Water Extraction: A Review J. Environ. Qual., October 24, 2007; 36(6): 1735 - 1748. [Abstract] [Full Text] [PDF]