PREDICTION OF PH ERRORS IN SOIL-WATER EXTRACTORS DUE TO DEGASSING

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 CO//2 degassing and the resulting pH error. Using this model measured pH values can be corrected back to in situ soil water pH. 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.