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Physical & Chemical properties

Vapour pressure

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Reference
Endpoint:
vapour pressure
Type of information:
experimental study
Adequacy of study:
key study
Study period:
1968
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
test procedure in accordance with generally accepted scientific standards and described in sufficient detail
Qualifier:
no guideline followed
Principles of method if other than guideline:
A static capsule method was used to measure the vapor pressure of potassium from 945 to 2170 °K.
The vapor pressure of potassium was determined by heating a quantity of potassium in a closed capsule and measuring the pressure produced at various temperatures.
GLP compliance:
not specified
Type of method:
static method
Remarks:
capsule method
Temp.:
25 °C
Vapour pressure:
0 Pa
Remarks on result:
other: calculated by using the Kirchhoff equation, which was derived from the experimental results of the study (Table I)
Transition / decomposition:
no

RESULTS AND DISCUSSION

During the series of five vapour-pressure runs, a total of 51 data points were recorded (see attached file: " Vapour pressure_potassium_Table I_NASA_1968.png"). The data were fitted to the three-constant Kirchhoff equation by the method of least squares and are represented by the equation:

lg (p/p0) = 7.74887 - (4812.30 / T) - 1.02160 lg T

where p is the vapour pressure in newtons per square meter p0 is atmospheric pressure at sea level (1. 01325 x 105 N/m2), and T is the absolute temperature in K. The standard deviation in pressure p of the data from the equation is calculated to be ±1.23 percent. A semilog plot of this equation as p/p0 against T-1 is shown in figure 3 (attached).

Prior to the start of run 4, 25 percent more potassium was injected into the capsule to verify the existence of true liquid-vapour equilibrium over the entire range of this investigation. There is no noticeable deviation between the vapour-pressure curve calculated from runs 1, 2, and 3 and the data points of run 4.

The depression of the vapour pressure of potassium due to the impurities in the potassium, assuming that Raoult's law is obeyed, is calculated to be negligible. Dissolution of the container material, mostly tantalum, is dependent on the amount of oxygen in the potassium metal and in the container material. The maximum depression of the vapour pressure of potassium, caused by a potassium-oxygen-tantalum reaction, is calculated to be less than 0. 25 percent at the maximum temperature of this study.

An analysis was made to determine the effect of the instrumentation inaccuracies on the deviation of the experimental data from absolute or true values of the vapour-pressure data. Of the 51 data points obtained, only 5 fell a significant distance outside these calculated deviations built into the experiment by the instrumentation. Two other data points fell approximately at the limits of the calculated deviations, and all the remaining data points were well within these limits.

As a means of comparing the vapour-pressure equations determined from the data of this study with vapour-pressure equations of other investigators, data calculated from the equation of this investigation were pooled with calculated data from the equations of references 1 to 4 to determine a single median curve. Each vapour-pressure equation was solved for p at integral values of T-1 x 10-4 K-1 over its experimental temperature range of study.

Figure 4 (attached) shows a graphical comparison of the various computed vapour-pressure values. The abscissa is shown as temperature in K, and the ordinate is shown as [ (pp - p) / pp] x 100, where pp denotes the pooled vapour-pressure value. The solid line curve for each investigator is shown to extend over the range of the data from which the mathematical vapour-pressure relation was derived. The data of reference 5 are not shown since they fall outside the ordinate limits of this figure.

A low-temperature extrapolation of data points calculated from the vapour-pressure equation developed in this investigation is shown as a dashed line in figure 4. The pooled equation is also extrapolated to T-1 = 13 x l0-4 K-1 (770 K). These extrapolated data points are compared with low-temperature data calculated from a thermodynamic study (ref. 8) of the experimental data of several investigators.

Data calculated from the smoothed vapour-pressure curve of reference 9 are also shown in figure 4. In reference 9 the vapour-pressure equation was developed by combining the vapour-pressure data of selected investigators in a least squares fit to a three-constant equation. This vapour-pressure equation of reference 9 differs only slightly from the pooled equation of this report. A divergence at the low temperatures is apparent, although the maximum is only about 2 percent at 770° K.

In the temperature range above 1000 K the vapour-pressure curves of reference 4 and of this investigation agree within 2.0 percent. Above 1100° K the vapour-pressure equation of reference 1 also falls within this 2-percent range. Above 1100 K there may be a systematic error, possibly caused by the difference in experimental equipment, between these three investigations. The low-temperature extrapolation of the vapour-pressure curve from this investigation agrees well with the calculated data of reference 8 and differs by an approximately constant percentage. The data of references 2 and 3 vary as much as 3 percent from the pooled equation of this investigation at temperatures near the normal boiling point of potassium.

REFERENCES

1) M. M. Makansi, M. Madsen, W. A. Selke, C. F. Bonilla, Vapor Pressure of Potassium, J. Phys. Chem. 1956, 60, 128.

2) J. F. Walling, H. K. Nuzum, A. W. Lemmon jr., The Vapor Pressure and Heat of Vaporization of Potassium from 480° to 1150° C, Rep. No. BATT-4673 -T3 (NASA CR-52425), Battelle Memorial Institue, 1963-04-30.

3) D. V. Rigney, S. M. Kapelner, R. E. Cleary, The Vapor Pressure of Potassium Between 1065° and 1500° K, Rep. No. TIM-810, Pratt and Whitney Aircraft, 1965-06-29.

4) J. P. Stone, C. T. Ewing, J. R. Spann, E. W. Steinkuller, D. D. Williams, R. R. Miller, High Temperature Vapor Pressures of Sodium, Potassium, and Cesium, J. Chem. Eng. Data 1966, 11, 315-320.

8) R. Hultgreen, R. L. Orr, P. D. Anderson, K. E. Kelley, Selected Values of Thermodynamic Properties of Metals and Alloys. John Wiley and Sons, Inc., 1963.

9) S. Heimel, Thermodynamic Properties of Potassium to 2100° K. NASA TN D-4165, 1967.

Conclusions:
Calculated low-temperature vapour-pressure at 25 °C is 0.00000122 N/m2.
Executive summary:

A static capsule method (no guideline followed, GLP not specified, test procedure in accordance with generally accepted scientific standards and described in sufficient detail) was used to measure the vapour pressure of potassium at temperatures of 945° to 2170° K. The resulting data are represented by the following Kirchhoff-equation

lg (p/p0) = 7.74887 - (4812.30 / T) - 1.02160 lg T

where p is the vapour pressure in newtons per square meter p0is atmospheric pressure at sea level (1. 01325 x 105N/m2), and T is the temperature in K. The standard deviation of the experimental data from this equation is ±1. 23 percent. Calculated points from the equation, including extrapolated low-temperature vapour-pressure points, are in acceptable agreement with those of other investigators.

The results of this investigation have demonstrated the usefulness of the developed apparatus as a research tool for obtaining very high temperature vapour-pressure data for liquid alkali metals.

Calculated low-temperature vapour-pressure at 25 °C (room conditions) is 0.00000122 N/m2.

Description of key information

A static capsule method (no guideline followed, GLP not specified, test procedure in accordance with generally accepted scientific standards and described in sufficient detail) was used to measure the vapour pressure of potassium at temperatures of 945° to 2170° K. The resulting data are represented by the following Kirchhoff-equation

lg (p/p0) = 7.74887 - (4812.30 / T) - 1.02160 lg T

where p is the vapour pressure in newtons per square meter p0is atmospheric pressure at sea level (1. 01325 x 105N/m2), and T is the temperature in K. The standard deviation of the experimental data from this equation is ±1. 23 percent. Calculated points from the equation, including extrapolated low-temperature vapour-pressure points, are in acceptable agreement with those of other investigators.

The results of this investigation have demonstrated the usefulness of the developed apparatus as a research tool for obtaining very high temperature vapour-pressure data for liquid alkali metals.

Calculated low-temperature vapour-pressure at 25 °C (room conditions) is 0.00000122 N/m2.

Key value for chemical safety assessment

Vapour pressure:
0 Pa
at the temperature of:
25 °C

Additional information