Frequently asked questions

A miscibility chart is a grid that tells you whether two solvents will mix into one uniform phase (miscible), form two layers (immiscible), or mix only within a limited range (partially miscible). It is used to avoid phase separation when mixing mobile phases, doing extractions, or switching solvents.

Pick one solvent on the left and one across the top; the cell where they meet shows miscible, partially miscible, or immiscible. On this site you can also just select two solvents and read the answer plus a short note.

The UV cutoff is the wavelength at which the pure solvent absorbs so strongly (an absorbance of 1 in a 1 cm cell) that it interferes with UV detection. Below that wavelength the solvent’s own absorbance swamps your analyte, so you should run UV detection above the cutoff. Acetonitrile (~190 nm) and methanol (~205 nm) are low; acetone (~330 nm) is unusable for most UV work.

The polarity index P′ is a number (about 0 for hexane up to 10.2 for water) that ranks how polar a solvent is. It helps you estimate eluent strength in both reversed-phase and normal-phase HPLC and compare solvents on one scale.

Polar solvents and non-polar alkanes follow the "like dissolves like" rule only weakly. At room temperature methanol and acetonitrile form two layers with hexane, heptane and other alkanes over part of the composition range — a common surprise when switching between reversed-phase and normal-phase solvents.

Acetonitrile has a lower UV cutoff (~190 vs ~205 nm) and much lower viscosity (less backpressure), and it is aprotic. Methanol is cheaper, is a proton donor (different selectivity), but is more viscous and absorbs at lower wavelengths less cleanly. Many methods try both because they give different selectivity.

USP General Chapter <467> (which adopts ICH Q3C) sorts solvents by toxicity: Class 1 solvents should be avoided, Class 2 are limited, and Class 3 have low toxic potential. It matters for pharmaceutical solvent selection; this site shows each solvent’s class as a reference.

Treat it as general guidance for method development, not a guarantee. Miscibility depends on temperature, water content and proportions, and property values vary slightly between sources. Always consult the Safety Data Sheet (SDS) and confirm critical mixtures at your working conditions.