A study on the equilibration processes in the isopiestic experiment

Abstract

The temperature factor and various individual equilibration processes in the isopiestic experiment on low molality solutions have been theoretically analyzed. The theoretical analysis gives the relation between the temperature difference inside the apparatus and the error of the determined isopiestic equilibrium molality. The equations of the rates of all individual equilibration processes are also obtained, and those equations give that the magnitudes of the rates should be in the following sequence: the water vapor diffusion in the gas phase < the water vapor transfer via the gas circulation < the heat conduction in the sample solution < the heat conduction in the heat transfer block and the heat transfer in the sample solution caused by stirring < the solutes diffusion in the sample solution < the equilibration between the gas phase and sample solution and the water vapor flow caused by pressure difference. Those equations also show that the whole equilibration process may be approximately described by d ln | Δ m e , b / m ∘ | / d t = - k · m b 2 / ( m a · w a ) , where “ m” is the molality of the test solution, and “ w” is the mass of the water, and “ k” is the equilibration rate parameter, and “ t” is the time, and the subscripts, “a” and “b” and “e”, are the initial state and the end state and the equilibrium state, respectively. Also, it is necessary to clearly eliminate the air in the gas phase so as to intensify the water vapor flow process through the gas phase and consequently to speed up the whole rate of vapor transfer. Also more, good fit and low friction between the sample container and the heat transfer block are important to isopiestic experiments. The equilibration isopiestic experiments with our specially designed apparatus show that the whole equilibration process is essentially determined by the change processes of the reference standard solutions, and the whole equilibration process can be described approximately by the equation mentioned above. The experimental results indicate that the whole equilibration process should be determined neither by gas-phase equilibration process only nor by heat conduction process only. In addition, the equilibration process in our isopiestic apparatus possesses a fairly fast rate, which is practical even for the samples of the molality less than 0.05 mol · kg −1.