Measuring Surface Tension

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Measuring Surface Tension - Capillary Method
 
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You will need to construct a device such as shown on the left. The main component of the device is a capillary tube in which your liquids will rise a significant distance (at least a couple of centimeters for precision measurement). As shown here, the container is a large test tube. This is good because the parallel walls of the test tube allow good viewing of the two meniscuses that need to be seen. Surface tension, γ (gamma), is in units of dynes/cm

Procedure:

  1. Determine the specific gravity of your experimental liquid (specific gravity is unit-less as it is a proportion of densities: how much more dense is your liquid than is water?). The easiest way to do this is to weigh 100 ml of your liquid. If it weighs 105 grams, it has a specific gravity of 1.05.
     
  2. You will need to set up a small aquarium or large beaker of water to as close to 25°C as possible.
     
  3. Before assembling your apparatus, the capillary tube must be scrupulously clean INSIDE. It should be washed in hot cleaning solution, and repeatedly rinsed in the liquid to be tested. the first liquid that you will test will be pure water, which has a surface tension of 71.8 dynes/cm. Those directions come next, but first you should know that your final calculations regarding the experimental liquid(s) will be proportional to that number. After doing water, rinse the tube with your experimental liquid.
     
  4. You should see that the liquid inside the capillary is elevated over that of the surrounding liquid. First, very lightly blow a puff of air at the side tube to make the inner liquid rise slightly in the capillary tube. The inner liquid should settle back to its equilibrium level. Read the BOTTOMS of the meniscuses of the liquid inside the capillary and outside the capillary. Now apply a momentary and slight suction to the side tube, which will drop the inner liquid slightly, and it will then rise to its equilibrium position. Take the readings of the two meniscuses again. Make readings of five puffings and five suctions, and average them.
     
  5. Determine the average height differential for your liquid, and for water.
     
  6. You must now correct for the density of your experimental liquid. Simply multiply your average height by the specific gravity of your solution. (Remember that water's specific gravity is 1.000.)
     
  7. Your liquid's surface tension is proportionate to its height relative to that of pure water. Thus is your experimental liquid rises higher than water, its surface tension will generally be higher than that of water, and vice versa.
     
  8. Hence: γ = ρ (hx / hw), where
      hx is the height of the experimental, and hw is the height of the water sample.


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