Static vapor pressure measurements

Wartenberg and Albrecht (2 ) reported a boiling point of 1597 K for KOH from static vapor pressure measurements, while very... 

The most straightforward method for vapor-pressure measurement is the static method, in which the pressure of the vapor above a pure liquid is measured directly with a manometer, pressure gauge, or pressure transducer. All parts of the apparatus must be maintained at a temperature at least as high as that of the sample in order to avoid condensation. Static techniques may be used at high temperatures and pressures with appropriate apparatus construction, but they become difficult at low vapor pressures due to the difficulty of pressure measurement and the effects of impurities. With good equipment and procedures, the accuracy of static vapor pressure measurements can be on the order of 0.1%. 

Wolf and cowoikers" " applied an other GLC technique to VLE-measurements for polymer-solvent systems, the so-called head-space gas-chromatography (HSGC). This is practically a combination of static vapor pressure measurement with gas-chromatographic detection. HSGC experiments were carried out with an appa-... 

From the standpoint of physicochemical measurements, family and isothermal plots are useful for the determination of vapor pressures (and, in addition, recalling Equations 13 and 16, heats of vaporization as well) from the retention data obtained from just a few chromatographic runs (33). Furthermore, the GC technique is ideally suited to instances in which the solutes are available only in minute quantities or are substantially impure, where each of these constraints ordinarily precludes bulk vapor-pressure measurements by conventional static procedures. For example, Heath and Tumlinson (34) employed log(retention) plots to determine the vapor pressures of trace acetate ingredients used in pheromone formulations. An important aspect of their work was that family correlations were obtained with a chiral-nematic stationary phase, cholesteryl p-chlorocinnamate. 

Vapor pressure measurements can be carried out by static, dynamic, or isopiestic methods. Static methods are based on direct measurements with the help of manometers. They require thorough outgassing of the solution, prevention of vapor condensation in any part of the measuring device, and highly precise temperature constancy. Differential measurements on solution and pure solvent are advantageous. 

Closed Vessels. Liquid level can be measured by the static pressure method also at non atmospheric pressures. However, ia such cases the pressure above the Hquid must be subtracted from the total head measurement. Differential pressure measuriag instmments that measure only the difference ia pressure between the pressure tap at the bottom of the tank and the pressure ia the vapor space are used for this purpose. At each tap, the pressure detected equals the Hquid head pressure plus the vapor pressure above the Hquid. Siace the pressure above the Hquid is identical ia both cases, it cancels out. Therefore, the change ia differential pressure measured by the instmment is due only to the change ia head of Hquid ia the vessel. It is iadependent of the pressure within the tank and is an accurate measure of the level. 

The net head or pressure measured in ft. or m that causes a liquid to flow through the suction side of a pump, enter the pump chamber, and reach the impeller. When the source of liquid is above the pump, NPSH equals the barometric pressure plus the static head, less the entrance head, frictional losses in the suction piping and vapor pressure of the liquid. When the source of liquid is below the pump, NPSH equals the barometric pressure less the static head, entrance head, frictional losses in the suction piping and vapor pressure of the liquid. NPSH is specific for each pump design and application and must be supplied by the manufacturer. 

Sasse, K., Jose, J., Merlin, J.-C. (1988) A static apparatus for measurement of low vapor pressures. Experimental results on high molecular-weight hydrocarbons. Fluid Phase Equil. 42, 287-304. 

Absolute methods end group analysis, membrane osmometry, vapor pressure osmometry, static light scattering, mass spectrometry, sedimentation measurements. 

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

Vapor Pressure nf Explosives and Related Substances gives an idea as to their volatility and in some cases to their stability. Methods for measuring vapor pressure may be divided into static and dynamic. Their description can be found in the following Refs l)J.Reilly ft W.N.Rae, "Phyaico-Chemical Methods, VanNostrand.NY, v 1(1944),117 2)J.Sttong, "Procedures in Experimental Physics, Prentice-Hnll.NY... 

Other vapor pressures for CoCl2(cr, t) have been measured by static ( ) and transpiration (5, 6) methods. In all three studies the presence of a small amount of dimer in the saturated vapor was not considered. JANAF pressures based on (a) selected values of the heats of sublimation and vaporization for the monomer and dimer, (b) the mass spectrometric value of 2A j H(monomer)- Agy H(dimer) - -38.99 kcal mol" (see Co2Cl (g) table) at 298.15 K, and (c) adopted entropies for the condensed and gaseous species are compared with the observed total pressures in the table given below. 

Westrum and Pitzer (3) measured the dissociation vapor pressures of KHFj (cr, t) by a static method. Their pressure data have been analyzed by both 2nd law and 3rd law methods as given below. The enthalpy of formation derived from the 3rd law... 

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