Comparison of three methods to find the vapor activity of a hydration step

The formation of a stochiometric salt hydrate takes place at a well defined vapor activity. We have compared three methods to measure this vapor activity. In two of the methods we used a sorption balance in step mode and in ramp mode, respectively, and in one method we used a newly developed sorption microcalorimeter. The tests were made with the formation of morphine sulphate pentahydrate from its dihydrate at 25 degrees C. With all three methods this transition was found to take place at a vapor activity close to 0.21.