Entrainment distillation as a reference method for the determination of the water content of foods: I. Theoretical aspects of analytical dehydration with special reference to entrainment desiccation

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Abstract
Thermodynamic considerations indicate that in a reference method for water determination it is imperative to choose the water vapour pressure (p), prevailing during the analysis, very low (order: 1 mm Hg), to prevent a retention of water, which surpasses the precision of the method (order: 0.1% abs.).To reach the desiccation equilibrium in a reasonable time (≤ 8h) an elevated temperature is necessary; the upper limit of temperature is determined by the thermostability of the least stable component of the actual substrate. Out of the existing methods, entrainment distillation with isooctane (bpt 99° C) was chosen, since in a comparison, incorporating thermodynamic, kinetic, chemical and experimental aspects, it showed the highest overall suitability. For this technique p was found to be directly proportional to the solubility of water in the entrainer at the temperature of the receiver. For isooctane as an entrainer, p was calculated from solubility data and found to be of the order of 1 or a few mm Hg, involving a total retention of water equalling 10‐30 mg for an intake of 10‐20 grams. The retention of water by the boiling entrainer was calculated and turned out to be negligible; the amount of water dissolved by the entrainer in the receiver was calculated and found not to surpass 10 mg. Kinetic considerations revealed that under “ideal” conditions dehydration of the normal intake in entrainment distillation can be completed in about ½ hour. The additional time of operation (∼ 5 h), noticed in practice, must be attributed to slow sedimentation and collection of small water droplets and to internal transport phenomena in the substrate.

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