The Static Method

The Static Method.—In the static method for vapour-pressure measurement, a sample of the substance is confined in a vessel attached to a manometer, and the pressure exerted by the sample, when it is maintained at a fixed temperature, is measured. The key points about the method are as follows  

The last paragraph has been written on the assumption that no azeotrope is formed with impurities present. If one is formed and the sample being examined is of the azeotropic composition (as is possible if purification has been carried out by fractional distillation) the presence of impurity will not be revealed by this test. Although azeotropic composition changes with temperature, and consequently the sample might not pass the test if it were 

In another apparatus the sample bulb incorporates a condenser, and immediately above this a tap, so that the sample may be degassed by boiling under reflux at atmospheric pressure if the condenser is then allowed to warm, the rising vapour expels the remaining air from the sample container, and the tap is shut (with simultaneous discontinuance of the heating) at the moment the condensing ring reaches it. However, it does not seem that this method will lead to elimination of the last trace of air as easily as the other method just described, and it does not avoid the need for vacuum pumps in the apparatus. 

An example has already been given of the use of the quartz helix gauge, not as a null instrument but for direct measurement of vapour pressures of samples at temperatures below ambient. Singh and Benson and Fowler, Trump, and Vogler have used this type for samples at temperatures above 

For measurements on aniline at the low end of the pressure range Rock has described a simple apparatus. The degassed sample was sealed in a U-tube with two vertical limbs 12 cm high joined by a horizontal tube of 2 mm bore. The two limbs were surrounded by jackets through which liquids were circulated, one at the temperature at which the vapour pressure was required and one at a temperature at which the vapour pressure was very low. The vapour-pressure difference between the two temperatures was therefore obtained in terms of the height of a column of the aniline itself. The vapour pressure at the lower temperature, which was a small correction to be added to the difference, had in this investigation been measured by another method. 

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In the static method the powder is isolated under high vacuum and surface gases driven off by heating the container. The container is next immersed in hquid nitrogen and known amounts of nitrogen vapor are admitted into the container at measured increasing pressures in the relative pressure range 0.05 to 0.35. 

The static methods simply imply that they are fundamentally static and are essentially observational in character. An extremely popular method to metallurgists, microscopy, may be cited as one of the leading examples of static methods. 

Pyroelectricity of several kinds of alternating LB films consisting of phenylpyrazine derivatives and stearic acid was measured by the static method at various temperatures. Effects of thermal expansion and molecular packing density of the film on pyroelectricity were also examined. The following conclusions were derived. 

When adsorption and condensation can be avoided, both dynamic and static sampling methods can be used. Often rinsing the sampling apparatus or even the whole olfactometer with odorous air is necessary to reduce adsorption. Before using the static method a comparative study should be carried out if possible. On the other hand extreme fluctuating emissions can only be sampled statically. 

Whilst certain of these methods involve measurements only at the liquid-vapour or liquid-liquid interfaces involved in the static methods we must usually consider besides the interfacial energy of the two fluids, that between each of them and a solid... 

Having performed the yield stress test, each category is then divided into static or dynamic methods. Dynamic methods indicate an actual flow test of a certain type, whereas the static methods indicate tests such as rotational flow between two cylinders. 

What are some of the dynamic methods for measuring surface tension What are the differences between these and the static methods ... 

This is easy to explain because values derived from the elution partial isotherm only pay regard to the amount adsorbed in the mesopores and the outer surface area. By contrast, the static method is not able to distinguish between these contributions and the micropore part of adsorption. Therefore the obtained values are higher but have no physical meaning whereas the elution values give a more realistic picture. The results for the standard carbon are very similar to the static values. This means that there are almost no micropores and the sorption processes take place in the mesopores and the outer surface area. This is confirmed by the huge difference in the thermodesorption peak of both materials. 

The recirculation methods were developed from the static methods. The main part of the recirculation apparatus is a thermostated equilibrium cell. The equilibrium cell has facilities for mechanically driven circulation through external loop(s) of either the lighter (i.e., the top) phase or the heavier (the bottom) phase, or of both phases. Magnetically operated high-pressure pumps usually achieve the circulation. 

Both the static and the flow methods of experimentation have been used. It is more difficult to obtain evidence for the formation of peroxides by the static method,... 

Of the static methods of hardness measurement, based on point measurement of deformations, those developed by Brinell, Rockwell and Vickers are now in use. 

The choice between the static methods (Wilhelmy plate method and the du Noiiy ring method) should primarily be based on the properties of the system being studied, in particular, the surfactant. As mentioned in UNITD3.5, the transport of surfactant molecules from the bulk to the surface requires a finite amount of time. Since static interfacial tension measurements do not yield information about the true age of the interface, it is conceivable that the measured interfacial tension values may not correspond to equilibrium interfacial tension values (i.e., the exchange of molecules between the bulk and the interface has not yet reached full equilibrium and the interfacial tension values are therefore not static). If the surfactant used in the experiment adsorbs within a few seconds, which is the case for small-molecule surfactants, then both the Wilhelmy plate method and the du Noiiy ring method are adequate. If the adsorption of a surfactant requires more time to reach full equilibrium, then the measurement should not be conducted until the interfacial tension values have stabilized. Since interfacial tension values are continuously displayed with... 

The main advantage of the static methods is cost. The equipment needed to conduct the dynamic measurements is approximately five times as expensive as the equipment required for static measurements (- 25,000 for a drop shape and drop volume analyzer versus - 5,000 for du Noiiy and Wilhelmy instruments). This is due to the additional capability of the former instruments to determine not only interfacial tension values but also the corresponding age of the interface. For more information on equipment, costs, and suppliers, see Internet Resources. 

The static method of estimating KLa for a fermenter is not always satisfactory. The use of pure water instead of the fermentation medium can lead to inaccuracies, and the deoxygenation of a complex broth with sodium sulphite can result in undefined changes to the broth and be expensive to carry out on a large scale. An... 

Whilst this method has its advantages over the static method it too presents some practical difficulties. One of these is that the dissolved oxygen concentration must not be allowed to fall below the critical value Ccri, as defined above, since this may... 

The leachate was distilled water. The iodine forms tested by the modified IAEA method were Agl, Ba(IO3)o, CaflOj) , and AgZI, all in Portland type III cement. In the static leacn test, the leachant was sampled on days 3, 7, 14, 56, 112, and 180. The sample was replaced with an equal volume of leachate preequilibrated with a pure cement sample. The Iodine forms tested by the static method were Agl, Cul, PbIo Ba(I03)2, Ca(I03)2, Hg(I0o)o PbXI, and AgZI all in Portland type III cement. Distilled water was the leachate. For Ba(I03)2 and Hg(I03)2, seawater and Columbia River water leachates were also tested. 

Two general methods are available. In the static method, the substance is confined by the strength of materials and the exposure times are long—seconds to months. In the dynamic method, the substance is confined by inertia and the exposure times are short, of the order of microseconds, due to the difficulty of maintaining large accelerations for long time periods. Nevertheless, the highest pressures are achieved by dynamic methods. 

Studies on the hydrodynamic properties of the same samples by dynamic light scattering and viscometry confirmed the conclusions of the static methods. Decreased diffusion coefficients and kD values, where kD is the coefficient of the concentration dependence of D (kD=2A2M-kfv), were observed for the zwitteri-... 

Adsorbed amounts can easily be measured with high accuracy by means of the conventional volumetric (49, 50) and gravimetric (50) techniques. For static measurements the desorption temperature is increased by certain increments and the desorption carried out at constant temperature. Thermogravi-metric analysis (TGA) is a dynamic method by which the weight loss is detected by a balance while heating the catalyst continuously. The same type of information can be obtained as with the static methods, provided the heating rate is sufficiently low to attain the characteristic irreversibly adsorbed amount at any temperature. 

The time in which a given molecule remained in the tube was estimated by dividing the volume of the tube by the rate of gas flow as determined with a flowmeter. The ratio of nitrites to total acidity gave the ratio N()2/(N2O5+NO2) and hence a measure of the percentage decomposition. The specific decomposition rate k was then calculated for various temperatures and for different pressures. Within the limit of experimental accuracy the specific decomposition rate of nitrogen pentoxide was the same as that determined by the static method, already described, and furthermore, this same constant was maintained down to a few millimeters and even to tenths of millimeters. 

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

The dynamic methods depend on the fact that certain vibrations of a liquid cause periodic extensions and contractions of its surface, which are resisted or assisted by the surface tension. Surface tension therefore forms an important part, or the whole, of the restoring force which is concerned in these vibrations, and may be calculated from observations of their periodicity. Dynamic methods include determination of the wave-length of ripples, of the oscillations of jets issuing from non-circular orifices, and of the oscillations of hanging drops. Dynamic methods may measure a different quantity from the static methods, in the case of solutions, as the surface is constantly being renewed in some of these methods, and may not be old enough for adsorption to have reached equilibrium. In the formation of ripples there is so little interchange of material between the surface and interior, and so little renewal of the surface, that the surface tension measured is the static tension ( 12. ... 

The various dynamic methods give the surface tension of more or less recently formed surfaces, and may yield results different from the static methods, if adsorption occurs, and is incomplete at the moment when the tension is actually measured. One factor in dynamic measurements, which cannot be satisfactorily measured at present, is the time which has elapsed between the formation of the surface from the homogeneous interior liquid, and the actual measurement of the surface tension. If this could be varied, and measured with an accuracy of say 10 4 second, a valuable new weapon would be available for investigating the progress of adsorption. Bohr s work on oscillating jets is probably the best on any dynamic method. 

The static methods arc volumetric or gravimetric. The volumetric method involves the use of a vacuum system comprising two sections, a dosing section which allows the introduction of accurately measured quantities of the adsorbate, and a sample section which contains the catalyst. The precision of the volumetric method depends on accurate calibrations of volumes. 

Preceding measurements by Swidersky on the same dyes were carried out with the static method [7, 8, 9, 14]. The differences between the measurements result from an incomplete purification of the same sample during preceding investigations. Recent experiments on purer dyes using the same static method agree very well with the measurements according to the dynamic method [15]. An adsorption effect of the stationary phase that has been used to precipitate the dyestuff on its surface is not found within the experimental accuracy. 

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