High Sensitivity Enthalpimetric Determination of Olefins

Abstract

We have investigated the use of an enforced linear resistance bridge to increase the sensitivity of direct injection enthalpimetry of olefins. Enthalpimetry of olefins, as we have developed it, depends on the exothermic nature of the addition of hydrogen across a double bond in the presence of 5% palladium catalyst on charcoal. The term “direct injection enthalpimetry” indicates that the unknown sample is injected directly into the calorimeter at constant pressure, making the observed temperature rise proportional to the enthalpy change for hydrogenation. If the olefin in question is one component of a mixture containing only saturated or aromatic compounds as the remaining components, the temperature rise, as measured with a thermistor-Wheatstone bridge circuit, is also proportional to the amount of olefin present in the mixture. A knowledge of the molar heat of hydrogenation for the compound, or comparison with a calibration curve of bridge output as a function of olefin concentration, leads directly to the amount of olefin present in an unknown mixture. As implied above, aromatic compounds are not hy-drogenated under the mild conditions described here.