What is the polarity index (P′)?

The polarity index, written P′, is a scale introduced by L. R. Snyder that summarizes a solvent's polarity as one number. It's derived experimentally from how strongly the solvent interacts with three test solutes (ethanol, dioxane, and nitromethane), so it captures real solvation behavior rather than a single physical constant.

The scale

P′ runs from about 0.0 for pentane and 0.1 for hexane (essentially non-polar) up to 10.2 for water (the most polar common solvent). Familiar mobile-phase solvents sit in between: methanol ≈ 5.1, acetonitrile ≈ 5.8, THF ≈ 4.0, ethyl acetate ≈ 4.4, dichloromethane ≈ 3.1, toluene ≈ 2.4. See the full polarity index table.

How to use it

P′ predicts eluent strength — but in opposite directions for the two HPLC modes:

• Reversed-phase (non-polar C18 column, polar mobile phase): a lower-P′ organic (e.g. acetonitrile, THF) is the stronger solvent — it elutes analytes faster. Adding water (high P′) makes the mobile phase weaker.
• Normal-phase (polar silica column, non-polar mobile phase): a higher-P′ solvent is the stronger solvent. You start non-polar (hexane) and add a polar modifier to increase strength.

To move retention, you can estimate the polarity of a binary mixture roughly as the volume-weighted average of the two P′ values — a handy first guess before you run the gradient.

Its limits

P′ tells you about strength but not selectivity. Two solvents with the same P′ can separate a mixture differently because they interact through different mechanisms — that's what the selectivity triangle and selectivity groups describe. Use P′ to set strength, then switch solvent type to change which peaks move.