Determining critical relative humidity by measuring air humidity in equilibrium directly.

A method that simply, accurately and directly determines the critical relative humidity (H(cr)) of drugs by the measurement of the relative air humidity in equilibrium with saturated solution of drugs is presented. A new pocket humidity meter with a calibrated humidity sensor has been devised. The work of the humidity meter including the sensor is described. The principle for the proposed method is more rational and much simpler than that for both conventional methods and dynamic vapour sorption (DVS) methods. Compared with conventional methods, the proposed method is simple, prompt and can save sample; compared with the DVS system, the pocket humidity meter is significantly simpler and cheaper. The H(cr) of eleven salts and drugs measured by the proposed method and conventional methods serves as a model. The results of two-tailed two samples t-test indicate that there is no significant difference between the new method and the conventional ones.