The Chemistry of Liquids, Part 3 - Measuring Properties

There are several interesting properties of liquids which can be measured quantitatively.  Among the most interesting are viscosity, refractive index, and the colligative properties - freezing point depression and boiling point elevation.  Viscosity and refractive index are described below.

Viscosity

Viscosity is a measure of the "thickness" of a liquid.  More specifically, the force against a smooth object (such as a metal sphere or cylinder) moving a certain distance at a particular rate is measured.  A common viscosity unit is Stokes.  The instrument for viscosity determination, shown in the diagram (below), uses a rotating spindle placed into a sample of liquid.  The resistance to motion of the spindle is detected and a signal is sent to the computer.  There, a program computes the viscosity based on the spindle size and turning rate (i.e. angular momentum).

Refractive Index

The refractivity of a substance is a measure of its ability to bend a ray of light.  Transparent liquids bend light because the speed of light is slightly lower going through a liquid than it is going through the surrounding air.  The refractive index is determined by a simple application of Snell's Law, which states that, for a particular material, a constant ratio equal to sin(incoming light angle)/sin(angle of refraction) exists.  The constant ratio, just described, is the refractive index.  A refractometer is an instrument used to determine the refractive index of a liquid sample.  A beam of light is allowed to go through a liquid sample and the incoming light angle and angle of refraction are sensed by a detector.  This information is then used to calculate the refractive index for the liquid sample.  A diagram of a typical refractometer is provided in the diagram below.

 

 

The properties, viscosity and refractive index, are easily determined and are used in research and quality control laboratories around the world.

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