# How can I calculate the polarity of a solvent?

Jan 8, 2015

Polarity scales are used when trying to determine how polar a solvent is. These scales rank a solvent's polarity according to the polarity index, which is a measure of a solvent's relative polarity.

The polarity index increases with polarity, water having the highest value at around 9.0. Other values for some common solvents include 6.6 for methanol, 6.2 for acetic acid, 5.2 for ethanol, and 2.3 for toluene.

One way you could actually calculate a solvent's polarity is by using its dielectrict constant, which is used to measure how well the solvent can partly cancel the field strenght of the electric field of a particle added to it.

The higher the value of a solvent's dielectric constant, the more polar it will be.

For example, water's very strong polar nature is indicated by the value of its dielectric constant, which at ${0}^{\circ} C$ is 88. By comparison, solvents with dielectric constants of less than 15 are considered to be non-polar.

As far sa I know, the dielectric constant can predict the solvent's ability to dissolve ionic compounds.

Here's a link to a polarity chart for some common solvents: