Evaporation

Evaporation Rate

Definition

  • MSDS sheets: An evaporation rate is the rate at which a material will vaporize (evaporate, change from liquid to vapor) compared to the rate of vaporization of a specific known material. This quantity is a ratio, therefore it is unitless.
  • General usage: The mass of material that evaporates from a surface per unit time (examples: 3 grams per square meter per hour, 1 inch per acre per month).

MSDS Relevance

    Evaporation rate can be useful in evaluating the health and fire hazards of a material. For example, a substance with a high evaporation rate will readily form a vapor which can be inhaled or explode.Evaporation rates generally have an inverse relationship to boiling points; i.e. the higher the boiling point, the lower the rate of evaporation.

    The general reference material for evaporation rates is n-butyl acetate (commonly abbreviated BuAc) which has the chemical structure shown below. Whenever a relative evaporation rate is given, the reference material must be stated.

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    a chemical structure

    The relative evaporation rate of butyl acetate is 1.0. Other materials are then classified as:

    Speed Evaporation Rate
    (BuAc = 1.0)
    Examples
    Fast > 3.0 Methyl Ethyl Ketone = 3.8
    Acetone = 5.6
    Hexane = 8.3.
    Medium 0.8 to 3.0 95% Ethyl Alcohol = 1.4
    Naphtha = 1.4
    Slow < 0.8 Xylene = 0.6
    Isobutyl Alcohol = 0.6
    Water = 0.3
    Mineral Spirits = 0.1

    We are not aware of a specific number for the absolute evaporation rate (i.e. in mass/time units) of butyl acetate. Presumably, such a number would depend on myriad variables such as temperature, atmospheric pressure, humidity, air flow, viscosity etc. The ASTM has developed a standard test method, D3539-87(2004) Standard Test Methods for Evaporation Rates of Volatile Liquids by Shell Thin-Film Evaporometer. We don’t own a copy so we can’t give you a synopsis of the variables involved.

    In the absence of evaporation rate data, you can roughly assess the volatility using the vapor pressure of the material.

Further Reading

See also: alcohol, boiling point, vapor pressure, volatility.

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