# Evaporation rates

## Introduction

The evaporation rate of solvents is given either as the rate relative to n-butyl acetate, RateBuOAc, (1) or the time relative to ether, TimeEther (VZ - Verdunstungszahl), (2).
The two scales are approximately inversely proportional but unfortunately both are commonly referred to as "evaporation rates". Both are unitless.
Note that absolute evaporation rates are sensitive to temperature, pressure, humidity, air flow et c. and values may vary substantially. For that reason absolute evaporation rates are rarely used.

If RateBuOAc or TimeEther is known then the other can be calculated.

RateBuOAc(solvent) = TimeEther(BuOAc)/TimeEther(solvent)
TimeEther(solvent) = RateBuOAc(Ether)/RateBuOAc(solvent)

The relation between the evaporation rate and vapour pressure is not a straight line but is curved . This is not unexpected since the vapor pressure is an equilibrium property whereas evaporation is a kinetic process. Volatile solvents may experience cooling reducing the rate of evaporation.

RateBuOAc ≈ 0.47 + 0.1593 vp0.77

## Experimental data

Experimental data for the evaporation rates and vapour pressures of pure solvents can be found →here.
Solvent suppliers often publish evaporation rates for their solvents and solvent mixtures on their sites.

## Approximations

These approximations have been obtained from fits and therefore deviate slightly from the relations above.
Enter the known data and the other two fields will be adjusted accordingly.

 RateBuOAc = (range 0.01-14.5) TimeEther = (range 1-6000) Vapour pressure = Torr (range 0.1-450)

Some sense of the reliability of the above equations can be gained by entering the experimental values for n-butyl acetate (RateBuOAc=1, TimeEther=11.8, Vapour pressure=7.8 Torr) or diethyl ether (RateBuOAc=11.8, TimeEther=1, Vapour pressure=442 Torr).
Note that the error is not independent of the evaporation rate! The relations are accurate enough for ranking of solvents but not much more. They will not work for solvent mixtures.

## References

1. ASTM D3539-87(2004), "Standard Test Methods for Evaporation Rates of Volatile Liquids by Shell Thin-Film Evaporometer"
2. DIN 53170, "Lösemittel für Lacke und ähnliche Beschichtungsstoffe; Bestimmung der Verdunstungszahl".