METHOD FOR MEASURING THE VAPOR PRESSURE OF LIQUID AND SOLID SUBSTANCES

Abstract

In a method for measuring the vapor pressure of liquid and solid substances, in which a sample of the substance is arranged in a variable volume of a measuring cell, the measuring cell is gas-tightly closed. The volume of the measuring cell is increased until reaching a measuring volume, and at least one first value of the gas pressure prevailing after the increase in volume is measured. The volume of the measuring cell is additionally reduced until reaching the measuring volume, and at least one second value of the gas pressure prevailing after the reduction of the volume is measured. The vapor pressure is calculated from the at least one first and at least one second measured values.

Claims

1. A method for measuring the vapor pressure of liquid and solid substances, in which a sample of the substance is arranged in a variable volume of a measuring cell, the measuring cell is gas-tightly closed, the volume of the measuring cell is increased until reaching a measuring volume, and at least one first value of the gas pressure prevailing after the increase in volume is measured, wherein the volume of the measuring cell is reduced until reaching the measuring volume, and at least one second value of the gas pressure prevailing after the reduction of the volume is measured, and the vapor pressure is calculated from the at least one first and at least one second measured values.

2. The method according to claim 1, wherein at least two first measured values and at least two second measured values are measured, that a quantity characterizing the decreasing and the increasing pressure curve, respectively, is each determined from the at least two measured values, and that the vapor pressure is calculated from the characterizing quantities.

3. The method according to claim 2, wherein the characterizing quantity is each a straight line representing the local slope of the respective pressure curve in a time/pressure coordinate system, and the vapor pressure is calculated as the point of intersection of the straight lines.

4. The method according to claim 1, wherein the volume of the measuring cell is further increased to a transition volume after the determination of the first measured value and prior to the determination of the second measured value.

5. The method according to claim 1, wherein the volume of the measuring cell is reduced to a transition volume after the determination of the first measured value and prior to the determination of the second measured value.

6. The method according to claim 1, wherein the volume of the measuring cell is varied with the aid of a piston.

7. The method according to claim 1, wherein the at least one first value and the at least one second value are each determined after an identical period of time following the completion of the volume change.

8. The method according to claim 1, wherein the sample in the measuring cell is changed in temperature, in particular heated.

9. The method according to claim 6, wherein the sample is sucked into the measuring cell with the aid of the piston and the sample, upon completion of the measurement, is pressed out of the measuring cell.

10. The method according to claim 6, wherein the at least one first measured value and the at least one second measured value are measured in the measuring cell with the aid of a pressure sensor integrated in the piston.

11. The method according to claim 1, wherein a measuring cycle comprising the determination of the at least one first measured value and the at least one second measured value is repeated at least twice, wherein the measuring volume is each chosen to be different.

12. The method according to claim 1, wherein a measuring cycle comprising the determination of the at least one first measured value and the at least one second measured value is repeated three times, wherein the measuring volume is each chosen to be different.

Description

[0035] In the following, the invention will be explained in more detail by way of an exemplary embodiment schematically illustrated in the drawing. Therein,

[0036] FIG. 1 is a schematic illustration of a pressure/time diagram when carrying out the method according to the invention;

[0037] FIG. 2 is a schematic illustration of a first mode of calculation of the vapor pressure using the method according to the invention in a pressure/time diagram;

[0038] FIG. 3 is a schematic illustration of a second mode of calculation of the vapor pressure using the method according to the invention in a pressure/time diagram; and

[0039] FIG. 4 schematically depicts a measuring cell for carrying out the method according to the invention.

[0040] FIG. 1 depicts the pressure curve during a method according to the invention in a measuring cell. After having arranged the sample in the measuring cell, an initial pressure 1 exists in the volume of the measuring cell. In the first step 2, the volume of the measuring cell is increased to a measuring volume. The pressure in the measuring cell initially drops and, due to the gases escaping from the sample, subsequently rises again until the pressure in the volume of the measuring cell reaches the equilibrium pressure, which corresponds to the vapor pressure 3. The pressure curve follows curve 6. During this pressure increase, at least a first value of the gas pressure is measured. As soon as the at least one first value has been measured, the volume of the measuring cell can be further increased to a transition volume in the second step 4. In doing so, the pressure again initially drops and subsequently rises again, yet can never rise beyond the vapor pressure 3. After this, the volume in the measuring cell is reduced to the measuring volume in a third step 5. In doing so, the pressure initially increases and, due to a partial absorption of the gas in the sample, subsequently drops again until the vapor pressure 3 is reached. The pressure curve follows curve 7. During the pressure drop, at least a second value of the gas pressure is measured. As soon as the at least one second value has been measured, the method can be terminated. It is thus not necessary to wait for the respectively complete pressure equalization illustrated in FIG. 1, i.e. until the vapor pressure is reached, prior to beginning with the next step, or terminating the process. After having measured at least one first and at least one second value, these values can be used to calculate the vapor pressure as illustrated in FIG. 2.

[0041] FIG. 2 illustrates a first way of calculation of the vapor pressure according to the invention. To this end, the pressure curves after the increase of the volume to the measuring volume (curve 6, first step 2) and after the reduction of the volume to the measuring volume (curve 7, third step 5) are laid one above the other in a time/pressure diagram. It is apparent that the curves 6,7 are substantially symmetrical, i.e. that the vapor pressure is reached after the same period of time following the same pressure curve mirrored by the vapor pressure. In the case of the calculation illustrated in FIG. 2, the slope of the pressure curve is each determined, and the point of intersection 8 of the straight lines corresponding to the respective local pressure gradients is calculated, at the same point of time after reaching the measuring volume.

[0042] In an alternative mode of operation according to FIG. 3, two first pressure values and two second pressure values are each measured after identical time intervals so as to determine a slope in the form of a straight line each from the pressure difference determined in a defined time interval. After this, the point of intersection 8 of the two slopes is calculated. The vapor pressure is located in the point of intersection 8.

[0043] Alternatively, it is also possible to each determine, at the same time interval, only a first and a second value of curves 6, 7 of the maximum and minimum pressures, respectively, whereupon the mean value of these measured points is calculated, which constitutes the vapor pressure.

[0044] FIG. 4 depicts a measuring cell 9 comprising a piston 10, which defines a volume together with the measuring cell 9. The sample is arranged in the lower region 11 of the volume of the measuring cell, and a gas phase of the sample is formed in the upper region 12. By varying the position of the piston 10, the volume can be increased or reduced so that the method according to the invention can be performed.