Solvent Polarity Index Calculator
Calculate empirical polarity index and dielectric constant relationship for solvents.
What the Polarity Index Measures
Snyder's polarity index P′ ranks solvents by their measured ability to interact with polar test solutes. It is empirical rather than derived, which makes it practical for chromatography where what matters is observed behaviour.
| Solvent | P′ | Common use |
|---|---|---|
| Hexane | 0.1 | Normal phase, weakest eluent |
| Toluene | 2.4 | Intermediate |
| Ethyl acetate | 4.4 | Normal phase, stronger |
| Acetonitrile | 5.8 | Reversed phase standard |
| Methanol | 5.1 | Reversed phase, different selectivity |
| Water | 10.2 | Reversed phase, weakest eluent |
Mixtures blend approximately linearly by volume fraction, which is what makes the index useful for method development — you can predict the polarity of a gradient without measuring it.
Eluent Strength Reverses Between Modes
This is the point that confuses newcomers. In normal phase, the stationary phase is polar silica, so more polar solvent elutes faster. In reversed phase, the stationary phase is non-polar C18, so less polar solvent elutes faster.
| Mode | Stationary phase | Strong eluent | Weak eluent |
|---|---|---|---|
| Normal | Polar (silica) | Polar — methanol | Non-polar — hexane |
| Reversed | Non-polar (C18) | Non-polar — acetonitrile | Polar — water |
Note that methanol and acetonitrile have similar polarity indices but give different selectivity, because they interact through different mechanisms — methanol as a hydrogen bond donor, acetonitrile as a dipole and weak acceptor. Swapping between them is a standard way to change peak order when polarity alone is not the problem.
Worked Examples
Common Mistakes
Only in normal phase. In reversed phase the stationary phase is non-polar, so less polar solvent is the stronger eluent.
Methanol and acetonitrile have similar polarity but different selectivity, because they interact through different mechanisms. Swapping them changes peak order.
The index mixes approximately linearly by volume fraction. Using mass fractions gives incorrect predictions.
Hexane and water do not mix. Gradient methods need solvents miscible across the whole composition range, or a bridging solvent.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This relationship uses electronic structure, bonding or molecular geometry to predict a chemical property or structural descriptor. Assumption: The model may be an approximation; resonance, solvent, coordination environment, conformation and experimental conditions can affect real molecules.