MEASUREMENT AND CONTROL OF WATER ACTIVITY WITH AN ALUMINUM-OXIDE SENSOR IN ORGANIC 2-PHASE REACTION MIXTURES FOR ENZYMATIC CATALYSIS

An aluminium oxide film sensor (Endress & Hauser type DY20B) gives correct readings for the partial pressure of water dissolved in hexane or in the presence of various organic solvent vapors. This value, together with temperature, may be used to calculate thermodynamic water activity. The sensor must, however, be used with care, due to limitations of stability, sensitivity, and measurement range. Response time and lifetime are adequate. The sensor has been used for continuous monitoring and control of water activity during lipase-catalysed esterification in a hexane medium: water formed as a reaction product was removed by controlled recirculation of the headspace gases through a drying column. The rate of water production measured with the sensor agreed with the rate of ester synthesis determined by chemical analysis. © 1990.