Question

Experiment to measure surface tension

Answer 1

 Measure the ambient temperature and fill the burette with the liquid to be analyzed.
 The drop formation time is regulated slowly enough to avoid hydrodynamics effects. For the liquids considered in this paper 30 seconds per drop are enough.
 Wait until 30 drops are fully formed and deposited in a vessel. The weight of the vessel and the radius of the burette tip must be known at this stage.
 The vessel containing the 30 drops is weighted with an analytical balance and the average drop weight is calculated.
 The correction factor F is calculated by applying equation (4) once the mean drop volume V is calculated from the mean mass m and the density of the liquid, which can been obtained from the technical literature.
 Finally, the surface tension is calculated by applying equation (2). To measure the surface tension two recycled and chipped 50 ml burettes have been used. The lower damaged tip of the burettes was cut and polished with P600 sandpaper, ensuring no sharp edges. Burettes were thoroughly cleaned before use. Table 1 summarized the material required to measure the surface tension of the sample liquids. Table 1. List of material required to measure the surface tension Analytical balance Micrometer or x20 stereoscopic microscope Used glass burette P600 sandpaper Sample liquids: distilled water, 1-butanol and glycerol Vessel 25ml Fig. 4 shows the two burettes dealt with. Each burette was later filled with water, 1-butanol and glycerol to measure the surface tension of such liquids. This paper deals with distilled water, 1-butanol and glycerol because they are inexpensive and commercially available liquids, and it is easy to find data in the bibliography of their physicochemical variables at different temperatures. 5 Figure 4. a) The two burettes used to measure the surface tension of distilled water, 1-butanol and glycerol. b) Detail of the tip of burette A. c) Detail of the tip of burette B. To determine the surface tension by means of the drop weight method it is required to know the diameter of the dripping tip with high accuracy. Experimental results prove that water, 1-butanol and glycerol drops are formed at the outer perimeter of the dripping tip, so r is the outer radius of the glass capillarity. Therefore, the outer diameter of the burettes tip was measured by using a x20 Jeulin stereoscopic microscope equipped with a reticule in which one millimeter is divided in 100 parts, thus resulting in a resolution of 10-5 m. Figure 5 shows the photographs of the two burette tips dealt with in this work, which were taken with a 5 megapixels BMS digital camera coupled to the stereoscopic microscope. Figure 5. Photographs of the burette tips dealt with in this work taken with a x20 stereoscopic microscope. a) Burette A. b) Burette B. After performing the abovementioned measurements, the outer tip radius of burettes A and B were found to be 1.71 and 2.95 mm, respectively. Note that very similar results were obtained when measuring both tip radiuses with a standard micrometer.