Question

I am struggling with understanding the parameter called Relative Polarity (RP). I know it is supposed to be an arbitrary scale, but I fail to grasp what the values represent. I understand that higher values mean more polar, but nowhere have I found a proper definition.

As far as I know, the values are fixed between 0 and 1, 1 being the RP of water and 0 (I assume?) of some hydrocarbon (correct me please? I haven't found any compound for which RP = 0). In terms of physics I can think of several ways to describe polarity of a solvent:

Static dipole moment of the molecule
Spectral shift of the electronic spectral peaks of specified standards
(Arbitratily) normalized dielectric constant, i.e. how well does the solvent block electric field
Amount of specified standards that can be dissolved
etc.
Are any of the above the quantity by which RP is described?

Also, is the scale linear? Meaning, if I mix equal volumes of a solvent with RP 1 and a solvent with RP 0.5, will I get a mixture with RP 0.75? Or maybe it's not equal volumes, but weights/moles?

Answer 1

Relative Polarity (RP) is an empirical scale that compares the polarity of solvents with water as the standard (RP = 1) and hydrocarbons with values close to 0. The polarity scale reflects various factors and mixing solvents with different RPs does not always lead to a linear blend of their polarities.

Step-by-step explanation:

The concept of Relative Polarity (RP) is used to compare the polarity of various solvents relative to a standard, typically water, which is assigned an RP value of 1. Hydrocarbons, which are generally nonpolar, can be assigned RP values close to 0. Relative polarity is not defined by a single physical property but rather is an empirical scale that can reflect a number of factors, such as dielectric constant, dipole moment, and the ability of the solvent to dissolve polar compounds.

RP scales are not strictly linear, and mixing solvents with different RPs does not necessarily lead to a mixture with an RP that is the average of the two. The actual polarity of the mixture would be influenced by various interactions between the molecules of the solvents and could depend on factors such as volume ratios, molecular interactions, and temperature.