Problems in measuring the density of a gelling system with an oscillating tube

Abstract

In this paper some evidence is reported for the change of the bending modulus of a hollow tube oscillator, filled with a solution, during the gelation of the mixture which makes exact density measurements impossible.The principle of the method to measure densities with a density oscillator was founded by Kratky, Leopold and Stabinger'**). It depends on the stimulation of the resonance frequency of a U-shaped quartz tube. If the tube is filled with a fluid, the resonance frequency is a function of the density of the fluid at constant pressure and temperature. The time t for one period of oscillation is given by Eq. (1): where m and V are the mass and the inner volume of the oscillator tube, respectively, and p is the density of the fluid inside the tube; k is the elastic constant of the oscillator. If t is measured for two different fluid substances (indices 1 and 2) with known density, a constant A can be calculated from Eq. (1)under the assumption that the densities are not very differentwhich is given by:It is known that A depends on the temperature. The density p, of an unknown substance is obtained from Eq. (3): 1;t;with i = 1, 2. It is the advantage of this method that the density of fluids can be determined with high precision which corresponds to & 1,s * g . ~m -~. The temperature dependence of the elastic coefficient can be reduced by measuring densities in a reference mode using two oscillators, one of which is filled by a liquid of known density, the other one with the liquid of unknown density. Because of the high accuracy the time dependence of the density of gelatin solutions with concentrations above the critical one for gel formation was measured. The gelatin was a pig skin gelatin, that has been freed from low-molecular-weight compounds by dialysis. For these investigations the system DMA 60/DMA 602 from A. E a r , Graz,