Capillary tube rise

The surface tension of a liquid can be measured in a variety of ways, including capillary tube rise, du Noiiy ring method, bubble pressure, and drop weight. These methods vary greatly in their applications. 

The simplest and most common method of determining the surface tension of a liquid is the capillary tube rise method. If a glass capillary tube is immersed in a liquid such as water, the liquid in the capillary tube will rise above the outside level of the liquid. This is due to the greater liquid-solid force than the liquid-liquid intermolecular forces, and the liquid tends to wet as much of the solid... 

A liquid of density 2.0 g/cm forms a meniscus of shape corresponding to /3 = 80 in a metal capillary tube with which the contact angle is 30°. The capillary rise is 0.063 cm. Calculate the surface tension of the liquid and the radius of the capillary, using Table II-l. 

Fig. 32). Using a fine pipette insert about i cm. length of the liquid into the bottom of the tube. Now place in the tube A a fine inverted melting-point tube B of about i mm. diameter, sealed at the upper end. Fasten the capillary tube to the ther- Fio. 32. mometer by means of a rubber band and place in a melting-point apparatus. Heat slowly until a stream of bubbles rises from the bottom... 

The capillary tube method initially involves packing a powdered sample into a glass capillary tube of uniform diameter and length, carefully sealed at one end so that it forms a rounded tube of uniform thickness. The tube is then attached to a standardized thermometer so that the end of the tube reaches the middle of the thermometer reservoir bulb. This assembly is then inserted into a vessel contg a suitable liq which is uniformly heated so that the temp rises at a rate of about 1° per minute. Ref 1 discusses in detail equipment design and thermometer calibration. It should be noted that this technique is the method most widely used by organic chemists... 

The height, h, of a column of liquid in a capillary tube can be estimated by using h = lylgdr, where y is the surface tension, d is the density of the liquid, g is the acceleration of free fall, and r is the radius of the tube. Which will rise higher in a tube that is 0.15 mm in diameter at 25°C, water or ethanol The density of water is 0.997 g-cm-3 and that of ethanol is 0.79 g-cm-3. See Table 5.3. 

Unlike at adiabatic conditions, the height of the liquid level in a heated capillary tube depends not only on cr, r, pl and 6, but also on the viscosities and thermal conductivities of the two phases, the wall heat flux and the heat loss at the inlet. The latter affects the rate of liquid evaporation and hydraulic resistance of the capillary tube. The process becomes much more complicated when the flow undergoes small perturbations triggering unsteady flow of both phases. The rising velocity, pressure and temperature fluctuations are the cause for oscillations of the position of the meniscus, its shape and, accordingly, the fluctuations of the capillary pressure. Under constant wall temperature, the velocity and temperature fluctuations promote oscillations of the wall heat flux. 

That boiling produces bubbles of vapor creates an additional problem for performing the experiment. If a bubble of gas forms at the bottom of a capillary tube, its expansion and rise to the top of the capillary will expel the rest of the liquid. This is due to the fact that the surface tension of most liquids combined with the narrow bore of the capillary will not allow fluid to drain around the bubble as it rises. The solution is that a larger sample tube and sample is required for the experiment. One advantage of the boiling point experiment is that the thermal conductivity of a liquid is higher than its solid because of the mobility of the molecules. 

When it is necessary to remove completely such solvents as alcohol or benzene it is not sufficient to use the steam or water bath alone, because the boiling point rises higher and higher as the concentration of the solution increases even ether causes difficulties. In this case, when the solvent ceases to distil, an oil bath or, more frequently, a vacuum is used. It is sufficient to mount a capillary tube, place the flask in a deep porcelain basin or in an enamelled basin maintained at a moderate temperature, and connect directly... 

When the sum (1 IRi) + (1 /R2) is positive, as in a drop, the vapor pressure is greater than above a plane surface and when this sum is negative, as for a meniscus of a water column rising in a capillary tube, then px [Pg.44]

It is common observation that a liquid takes the shape of a container that surrounds or contains it. However, it is also found that, in many cases, there are other subtle properties that arise at the interface of liquids. The most common behavior is bubble and foam formation. Another phenomena is that, when a glass capillary tube is dipped in water, the fluid rises to a given height. It is observed that the narrower the tube, the higher the water rises. The role of liquids and liquid surfaces is important in many everyday natural processes (e.g., oceans, lakes, rivers, raindrops, etc.). Therefore, in these systems, one will expect the surface forces to be important, considering that the oceans cover some 75% of the surface of the earth. Accordingly, there is a need to study surface tension and its effect on surface phenomena in these different systems. This means that the structures of molecules in the bulk phase need to be considered in comparison to those at the surface. 

The rise or fall of liquid in a capillary tube or between plates inclined at a small angle. 

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