Azeotropic Distillation

The extraction of water from soil and brines can be performed by azeotropic distillation for δ2H, δ18O and tritium analyses. The method, developed by Dewar and McDonald (1961) for the measurement of water content in silage, was adapted by R. M. Brown and G. B. Allison in the 1970's for isotopic analyses of brines and is described by Revesz and Woods (1990). Precisions of ±2.0 ‰ and ±0.2 ‰ for δ2H and δ18O respectively were reported (EIL has similar results).

An azeotrope is a mixture of two liquids that boils without any change in composition at a significantly lower temperature than its constituents. This property provides the basis for this method. In the case of toluene, the azeotrope has a boiling point of 84.1°C while water boils at 100°C and toluene at 110°C. At room temperature, the toluene separates and floats on top of the water. The technique may employ several different solvents such as kerosene, xylene, petroleum ether or toluene.

DEWAR, W. A., and McDonald, P., 1961. Determination of dry matter in silage by distillation with toluene. Journal of the Science of Food and Agriculture; 12,790-795.

REVESZ, K., and Woods, P. H., 1990. A Method to Extract Soil Water for Stable Isotope Analysis. Journal of Hydrology; 115,397-406.