A simple and sensitive densitometer for critical liquid mixtures

2014 A sealed volumeter for the study of liquid mixtures near their solution critical point is described The cell used allows optical investigations to be performed at the same time. Although the method is very simple, high accuracy can be obtained (d03C1/03C1 = 3 x 10-6 relative and 4 10-5 absolute) in a wide temperature range. Revue Phys. Appl. 19 (1984) 917-919 OCTOBRE 1984, PAGE Classification Physics Abstracts 06.30E 64. 70J For the special purpose of measuring the density of liquid mixtures near their critical solution point a densitometer must exhibit specific features. It must be : i) built in a chemically inert material, ii) sealed to ensure that the concentration remains constant and to avoid external contaminations, iii) easily thermalized with high accuracy. Moreover, in the present case it has to be designed so as to allow simultaneous optical investigations in the liquid mixture. The choice of a pycnometer (volumeter) rather than a mechanic oscillator [1] or a magnetic suspended buoy [2] was chiefly governed by this last requirement and also because a pycnometer has a high resolution while being much more simple to set up. A problem with pycnometers is how to reconcile the requirement of a high sensitivity and an extended thermal range (see Ref. [3]). Several stratagems have been used, which all reduce to a change of the liquid volume under study, either by confining the sample with mercury [4] or with a piston [5] .These methods however create some additional problems, so we have developed an alternative method. 1. Description. In figure 1 is shown the pycnometer. It was built in Pyrex glass which is chemically inert against most substances and it exhibits good optical properties. Moreover the thermal expansion coefficient is very low. Fig. 1. The pycnometer. Symbols are explained in the text The reservoir (A) which contains the liquid can be designed to allow use as an optical cell as well. Its volume at 0 °C is Vo. It is connected to a cylinder pipe of inner diameter ao (at 0 °C) ; the height (H) of the meniscus which separates the liquid from the vapour can be detected by optical means. C and D are other reservoirs and E is either a seal or a Teflon screw tab. The rôle of C is to maintain a nearly constant pressure in the cell, and that of D is explained in the following. 2. Filling procedure. We have adopted the following procédure : First fill C with a mass M of liquid under atmoArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:019840019010091700