Flash-point

The temperature at which the vapour of a liquid becomes inflammable in air when ignited is usually termed its flashpoint. For legal purposes it is often necessary to determine this temperature with accuracy, and carefully standardised apparatus is employed for the purpose. The flash-point of an oil is frequently a useful index of its purity. Thus, for example, the presence of rosin oil, flash-point 155° to 160° C., is readily detected in this manner, in linseed oil, flash-point 250° C. The following flash-points refer to common oils and spirits:

	Flash point ° C
Linseed oil	250
Rosin oil	155-160
Paraffin illuminating oils	38-50
Turpentine	35-40
Rosin spirit	35-40
Naphtha	16-21
Methylated spirit	14-16
Ethyl alcohol, 99.6 per cent	11
Ether	-47

The minimum legal flash-point for illuminating oils in this country is 73° F., i.e. 22.8° C.

For binary mixtures of ethyl alcohol and water the following values have been obtained:

Ethyl alcohol, per cent, by weight	99.6	97.2	94.9	91.9	90.0	85.0	80.1	75.4
Flash-point °C	11	13	15	16	16.5	17.5	18.5	20.5

The flash-point cannot be calculated by simple proportion from the flash-points of the constituents. Increase of pressure tends to raise the flash-point. Thus, in the case of a kerosene, the following data have been obtained:

Pressure in mm	760	811	836	862
Flash-point, °C	31	33	34	35.5

Flash-points determined in an oxygen atmosphere are appreciably lower than in air.

The foregoing results refer to what may be termed the lower flashpoint, that is, the temperature of inflammation when the combustible vapour is present in sufficient quantity to reach the lower limit. There is a corresponding upper flash-point, which is seldom referred to and which may be determined by sparking in a more or less confined space. It represents the temperature of inflammation of the vapour at its higher limit.

Several attempts have been made to connect the flash-point with certain other physical constants of the substances concerned, such as the vapour pressure and boiling-point, but the most successful is that of Ormandy and Craven, who point out that, at their flash-points, different hydrocarbons exert approximately the same vapour pressure. The following relationship is found to hold to a rough approximation:

Flash-point = k×boiling-point,

where k is a constant which varies according to the type of combustible and the nature of the flash-point, namely, whether higher or lower. In the case of the hydrocarbons a mean value of 0.736 has been obtained for the lower flash-point constant, and 0.800 for the higher. A few values are given in the table.

The temperatures at which solids, in the compact form, will ignite in air without the application of any spark or other local high temperature, have occasionally been determined. The following are a few of the better-known results.