Ice freezing temperatures and hydrate formation temperatures have been measured by the dynamic cooling method for aqueous ethylene oxide (EO) solutions containing from 0 to 95 mol% EO. The ice and the congruent hydrate freezing temperatures exhibited standard errors on a single measurement of 0.004 degrees C and 0.013 degrees C, respectively. The ice-hydrate eutectic temperature was observed at -2.107 degrees C with standard error 0.001 degrees C and composition 1.991 mol% EO with standard error 0.008 mol% EO. The congruent hydrate was found to freeze at 11.083 degrees C with standard error 0.002 degrees C and composition 12.64 mol% EO with standard error 0.02 mol% EO. The formula of the congruent hydrate was EO-6.91H(2)O with standard error 0.013 mol water/ mol EO. Only a single hydrate was found over the whole composition range down to -26 degrees C: the shoulder of the hydrate freezing curve above 40 mol% EO resulted from the high activity coefficients to dilute water in concentrated EO solutions. Equations and best values for the ice freezing temperatures and the hydrate formation temperatures together with their standard errors were evaluated by the method of least squares.
