The drop methods

Since surface forces depend on the magnimde of the area, the drops tend to be as spherical as possible. Distortions due to gravitational forces depend on the volume of the drop. In principle, it is however possible to determine the surface tension by measurement of the shape of the drop, when gravitational and surface tension forces are comparable. Two principally different methods must be taken into account. There are methods based on the shape of a static drop lying on a solid surface or a bubble adhering underneath a solid plate, and dynamic methods, based on continuously forming and falling drops. It should be noted that all the principles described here for drops are valid also for bubbles. 

Using the Bashforth and Adams tables. Porter (1933) calculated the difference A between and where r is the equatorial radius. The variation of A with h r could be 

When this method is applied to liquids at room temperature with glass or metallic plates as bases, a precision of about 0.2-0.5% can be attained. However, at high temperatures, when the shape of the sessile drop is not as sharp from the photograph, the precision decreases very rapidly. A problem can also be to keep the base in horizontal position. When ceramic or graphite bases are used the precision can be also 10-20%. The reason is that ceramic and graphite materials are often porous and very hard to polish, which causes irregular wetting. 

Weighing drops falling from a tube into a container is fairly accurate and perhaps the most convenient laboratory method of surface or interfacial tension measurements. 

The principle of this method, proposed by Tate (1864), consists of the slow dripping of a liquid from a tube into a crucible. When enough drops are in the crucible, the average mass of the drops is calculated. The more the drops collected in the crucible, the more accurate is the result. The mass of one drop is given by the expression 

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Lewis, as already mentioned, used a solution of sodium glycocholate and determined the adsorption of the salt by a surface of paraffin oil. The interfacial tension solution—paraffin oil this was measured for a number of concentrations by the drop method just discussed, and the [Pg.42]

The sources of error indicated above were avoided in a series of experiments carried out by Donnan and Barker, which in principle resemble those made by Lewis, so that only a brief reference to them is necessary. The dissolved substance was nonylic acid, and a drop method. The results could be reproduced with very great accuracy, i.e., to a fraction of one drop in 300—500 drops. Adsorption was produced at a surface air-liquid, air being passed through the solution in bubbles of known size and number, so that the total active surface could be calculated. The bubbles, on reaching the surface, burst, hence the excess of solute carried by them remained in the surface very effective precautions were used to prevent diffusion backwards from this portion into... 

A marginal but very important application of the drop calorimetric method is that it also allows enthalpies of vaporization or sublimation [162,169] to be determined with very small samples. The procedure is similar to that described for the calibration with iodine—which indeed is a sublimation experiment. Other methods to determine vaporization or sublimation enthalpies using heat flow calorimeters have been described [170-172], Although they may provide more accurate data, the drop method is often preferred due to the simplicity of the experimental procedure and to the inexpensive additional hardware required. The drop method can also be used to measure heat capacities of solids or liquids above ambient temperature [1,173],... 

The "drop-method mentioned above as item d is very simple and much used for easily liquefiable gases as well as for... 

The following table gives values obtd by the "drop method ... 

It has been noted that some liquids are ignited at much lower temps in the "drop-method apparatus than when they are vaporized first and their vapors are let into the "concentric-tube app, described on p 967. The possible danger of expln from spilled liquid in an industrial plant is therefore better assessed by the "drop-method than by the "concentric-tube method, as well as being more easily determined (p 972)... 

Sharma, R. R. 1963. Determination of surface tension of milk by the drop method and the ring method. Ind. J. Dairy Sci. 16, 101-108. 

The organic substance and the ammonia were oxidized and the drop method (Shatenshtein et al., 1957b) was used to determine the concentration of deuterium in the resulting purified water (Shatenshtein and Antonchik, 1959 Shatenshtein and Yakovleva, 1958). The activity of tritium in the combustion water was determined as described by Yakushin and Shatenshtein (1960). 

Drop Softening Point Determine as directed in the Drop Method under Softening Point, Appendix IX, using a bath temperature of 100°. 

Yur ev et al. 2 prepared deuteriated selenophenes from the respective halogenated compounds by halogen-deuterium replacement. Thus, 2-deuterio-, 2,5-dideuterio-, tetradeuterioselenophene, 3-methyl-, and 5-methyl-2-deuterioselenophene were obtained from the corresponding iodoselenophenes on reduction by zinc in deuteriated acetic acid. 3-Deuterioselenophene was obtained from 3-selenienyl-lithium hydrolyzed with deuteriated acetic acid at — 7 0°C the lithium derivative was obtained from 3-bromoselenophene and ethyllithium, also at — 70°C. The deuterium content was determined by combustion, the water being analyzed by the drop method.43... 

In the laboratory, values are determined by measuring the change in heat content between room temperature (or calorimeter temperature - calorimeter is the measuring apparatus for heat) and a number of higher temperatures. The calorimeter commonly used for this purpose is the isothermal ( isoperiboF is a more appropriate term) room temperature calorimeter, which uses the drop method. The substance is heated to the desired temperature and is dropped into the calorimeter and the heat effect is measured. This technique is commonly referred to as the mixture method in calorimetry. 

Becker-Doiing nucleation hypothesis indicates a much larger number, of the order of 100, for the critical cluster. Klein and Driy, in nucleation studies combining the drop method and homogeneous precipitation, found the rate of nucleation of strontium sulfate to depend on the 27th power of the concentration, indicating a nucleus containing 52 ions. 

Start the glycerine into the nitrator in a fine, slow stream or even by the drop, depending on the temperature rise, and the speed and violence of the reaction. With plenty of time available the drop method is advisable. 

Mixing of liquids with solids. The simplest method to measure the heat at mixing a certain amount a of a liquid salt A, considered as the solvent and kept at the experimental temperature 7e, with the weighted amount b of a solid salt B kept at the room temperature To, is the drop method. This method is very easy to operate and has been described already by Kubashewski and Evans (1964) and was applied to micro-calorimetry. The schematic description of this method is shown in Eigure 4.7a. The measured enthalpy corresponds to the enthalpy of mixing according to the equation... 

An improvement of the drop method was achieved by the indirect drop method (Eigure 4.7b). In this variant of the drop method, the solid sample B is preheated and the mixing is carried out in two steps. First, the sample B is dropped into the... 

The drop methods, drop volume, drop shape and drop pressure, seem to be the most general ones, and they can usually be run, after small modification, with bubbles. These methods are applicable to both liquid-liquid and liquid-gas interfaces and need only small amoxmts of solvent and solute. In addition, the temperature control is easily arranged, even at higher temperatures. 

Two bioassay techniques, the capillary method [58], and the drop method [63], have already been developed to study the chemotaxis of phytopathogenic zoospores and gametes of marine brown algae. We have devised a new procedure (the "particle method") which is a qualitative, rather than a quantitative, method of detecting an attractant after it has been absorbed on to an inorganic particle. This method has been used to study attractants of A. cochlioides zoospores in the roots of both spinach and pigweed (Chenopodium album). 

Halides.—The kinetics of crystallization of vitreous Bep2 have been studied, using X-ray diffraction and calorimetric methods. An analysis of the data shows that the crystallization proceeds via spherulitic growth, with an activation energy of 222 kJ mor. The enthalpy increments of Bep2 have been measured by the drop method, in both the glassy and liquid states, from 450 to 914 K. ... 

A) Modified crucible used for temperature calibration by the drop method. (B)... 

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