# volatility

Volatility (also volatility or evaporation number ) is a dimensionless relative number that describes the evaporation of a solvent .

## Evaporation rate

By definition , volatility is given as the ratio of the vapor pressure of the substance in question to the vapor pressure of the highly volatile reference substance diethyl ether (this at 20  ° C and 65 ± 5% relative humidity ):

${\ displaystyle {\ text {volatility}} = {\ frac {p _ {\ text {considered}}} {p _ {\ text {diethyl ether}}}}}$

In Germany, the evaporation rate (VD) is determined according to DIN 53170. The time it takes for a substance to completely evaporate (evaporation time = VDZ) is related to the time it takes diethyl ether to evaporate.

${\ displaystyle {\ text {VD}} = {\ frac {VDZ _ {\ text {considered}}} {VDZ _ {\ text {Diethylether}}}}}$

A high evaporation rate means relatively slow evaporation, i.e. low volatility. A small evaporation number means faster evaporation, i.e. a relatively high volatility.

Examples of the evaporation rates of solvents (for diethyl ether = 1)
solvent Evaporation rate
Butyl diglycol > 1200
2-butoxyethyl acetate 250
Butyl glycol 163
N -methyl pyrrolidone 095
water 080
Test petrol 145-200 (K30) 035
Xylene 017th
1-butanol 033
1-methoxy-2-propanol 028
Butyl acetate (acetic acid n- butyl ester) 011
Ethanol 008.3
Methyl isobutyl ketone 007th
acetone 002
Hexane 001.4

## Evaporation rate

In the USA, the evaporation rate (E) is determined over time and in relation to butyl acetate .

${\ displaystyle {\ text {volatility}} = {\ frac {t _ {\ text {90 (butyl acetate)}}} {t _ {\ text {90 (considered)}}}}}$

with = time, up to 90% of the sample has evaporated. ${\ displaystyle t _ {\ mathrm {90}}}$

Examples of the evaporation rates of solvents (for butyl acetate = 1)
solvent Evaporation rate
Methyl ethyl ketone 3.8
acetone 5.6
Hexane 8.3
Ethanol, 95% 3.8
naphtha 1.4
Xylene 0.6
2-methyl-1-propanol 0.6
water 0.3
White spirit 0.1

## Classification of solvents according to volatility

Comparison of different classifications
In Germany In the USA
Classification Evaporation rate Classification evaporation rate
very difficult to volatilize > 50 slow evaporation <0.8
not very volatile 35… 50 medium evaporation 0.8 ... 3.0
medium volatile 10 ... 35 rapid evaporation > 3.0
volatile <10