Frederick R. Longo, Peter H. Daum, and Thomas Cachaza Drexel Institute of Technology Philadelphia, Pennsylvania 19104
In the study of the various physical properties of solutions, chemical homogeneity is of utmost importance. Therefore, it is essential to employ stirring or mixing techniques which insure uniform concentrations. While this is no difficulty for many room temperature measurements there may be problems when the experimental temperature is appreciably higher or lower. Also, mixing problems may arise when it is inconvenient to move the vessel which contains the solution. I n the course of the investigation of the physical properties of electrolyte solutions we have developed conductance cells and density cells which permit us to change the concentration easily and rapidly without removing our equipment from thermostatted oil baths. The feature essential for this is a novel stopcock depicted in Figure 1as part of a density cell. Many workers have used compressed gas mixing techniques but often the vessels possessed surfaces which could not be thermostatted and were exposed to air a t room temperature. Because of the stopcock which we have employed all surfaces of our cells can he brought to the temperature of a thermostatted oil bath. This is an important aspect when measurements are to be made above room temperature, for in these situations the components of a solution will condense on the relatively cold surfaces and the concentration of the solution becomes uncertain. Figure 1 illustrates the important features of the stop-
Figure 1.
Tho use of the stopcock with
0
Westphd density cell.
A Novel S ~ O ~ C for O C GUS ~ Mixing of Solutions cock. I n this figure it can be seen that a gas-carrying tube, B, is ring-sealed through the handle, A , of a hollow plug of a vacuum stopcock. The gas-carrying tube is brought to the surface of the plug at C and there is a hole in the plug at D, 180' from C. Gases can be led to either tube E or F by rotation of the plug. Two more holes, 180' apart, can be seen. Both of these are at an angle of 90' to C. Also, another part, exit tube G, is shown. G may communicate with a ballast or may be left open to the atmosphere. In any event, the pressure in tubes E, F, and G can easily be equalized. Figure 1 illustrates the use of the stopcock in combination with a Westphal density cell. Obviously, all surfaces which make contact with the solution or its vapor are thermostattable. The cell is charged by adding solvent or solute to compartment C. The hollow handle, A , is then turned so that nitrogen gas passes through the gas-carrying tube, R,and forces the liquid in compartment C up into compartment H; the gas above the solution in compartment H leaves the cell through exit tube, G. (In solutions of very low vapor pressure, we have allowed the nitrogen to bubble through the liquid for short periods). The process is reversed by turning the stopcock 180' and can be repeated until homogeneity is effected. The pressure in the compartments can be equalized and/or brought to atmospheric pressure immediately by bringing the stopcock to the proper position. In summary, the stopcock which we have employed permits us to immerse our entire system in a thermostatted bath to avoid the condensation problem and yet allows us to employ . r a.~ i d gas mixing techniques so that it is not necessar,v to remove the cell from the bath in order to change concentrations. Obviously, the stopcock can be incorporated into other kinds of apparatus used in the study of concentration effects in solution. Figure 2. Diogram showing importont features of the stopcock.
Volume 45, Number 8, August
1968
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541
