Ultrasonic monitoring of powder dissolution

Sucrose (20 wt%) and lactose (10 wt%) powders were mixed with water (at 2, 20, and 40°C) and allowed to dissolve. The proportion of an ultrasonic pulse reflecting at the interface between a polymer delay line and the dissolving sugars decreased during the dissolution process, and the magnitude of this signal, normalized to the echo from pure water, was proportional to only the dissolved portion. Ultrasonic reflectance is thus an appropriate sensor to monitor powder dissolution since it can distinguish dissolved and undissolved fractions without prior separation. The kinetics of sucrose dissolution were modeled using a cube-root equation and the rate constant of the process increased with temperature.