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Petroleum Analysis
point is a test (ASTM D87 and ASTM D127) that is widely used by suppliers of wax and by the wax
consumers; it is particularly applied to the highly paraffinic or crystalline waxes. Quantitative prediction of the melting point of pure hydrocarbons is difficult, but the melting point tends to increase
qualitatively with the molecular weight and with symmetry of the molecule.
Unsubstituted and symmetrically substituted compounds (e.g., benzene, cyclohexane, p-xylene,
and naphthalene) melt at higher temperatures relative to the paraffin compounds of similar molecular weight: The unsymmetrical isomers generally melt at lower temperatures than the aliphatic
hydrocarbons of the same molecular weight.
Unsaturation affects the melting point principally by its alteration of symmetry; thus ethane
(−172°C, −278°F) and ethylene (−169.5°C, −273°F) differ only slightly, but the melting points of
cyclohexane (6.2°C, 21°F) and cyclohexane (−104°C, −155°F) contrast strongly. All types of highly
unsymmetrical hydrocarbons are difficult to crystallize; asymmetrically branched aliphatic hydrocarbons as low as octane and most substituted cyclic hydrocarbons comprise the greater part of the
lubricating fractions of petroleum, crystallize slowly, if at all, and on cooling merely take the form
of glasslike solids.
Although the melting points of petroleum and petroleum products are of limited usefulness,
except to estimate the purity or perhaps the composition of waxes, the reverse process, solidification, has received attention in petroleum chemistry. In fact, solidification of petroleum and petroleum products has been differentiated into four categories, namely, freezing point, congealing point,
cloud point, and pour point.
Petroleum becomes more or less a plastic solid when cooled to sufficiently low temperatures.
This is due to the congealing of the various hydrocarbons that constitute the oil. The cloud point of
a petroleum oil is the temperature at which paraffin wax or other solidifiable compounds present in
the oil appear as a haze when the oil is chilled under definitely prescribed conditions (ASTM D2500
and ASTM D3117). As cooling is continued, all petroleum oils become more and more viscous and
flow becomes slower and slower. The pour point of a petroleum oil is the lowest temperature at
which the oil pours or flows under definitely prescribed conditions when it is chilled without disturbance at a standard rate (ASTM D97).
The solidification characteristics of a petroleum product depend on its grade or kind. For grease,
the temperature of interest is that at which fluidity occurs, commonly known as the dropping point.
The dropping point of grease is the temperature at which the grease passes from a plastic solid to
a liquid state and begins to flow under the conditions of the test (ASTM D566 and ASTM D2265).
For another type of plastic solid, including petrolatum and microcrystalline wax, both melting point
and congealing point are of interest.
The melting point of wax is the temperature at which the wax becomes sufficiently fluid to drop
from the thermometer; the congealing point is the temperature at which melted petrolatum ceases
to flow when allowed to cool under definitely prescribed conditions (ASTM D938).
For paraffin wax, the solidification temperature is of interest. For such purposes, the melting point is the temperature at which the melted paraffin wax begins to solidify, as shown
by the minimum rate of temperature change, when cooled under prescribed conditions. For
pure or essentially pure hydrocarbons, the solidification temperature is the freezing point, the
temperature at which a hydrocarbon passes from a liquid to a solid state (ASTM D1015 and
ASTM D1016).
The relationship of cloud point, pour point, melting point, and freezing point to one another varies widely from one petroleum product to another. Hence, their significance for different types of
product also varies. In general, cloud, melting, and freezing points are of more limited value and
each is of narrower range of application than the pour point.
The cloud point of petroleum or a petroleum product is the temperature at which paraffin wax
or other solidifiable compounds present in the oil appear as a haze when the sample is chilled under
definitely prescribed conditions (ASTM D2500, ASTM D3117, IP 219, IP 444, IP 445, IP 446).
Petroleum Analysis
point is a test (ASTM D87 and ASTM D127) that is widely used by suppliers of wax and by the wax
consumers; it is particularly applied to the highly paraffinic or crystalline waxes. Quantitative prediction of the melting point of pure hydrocarbons is difficult, but the melting point tends to increase
qualitatively with the molecular weight and with symmetry of the molecule.
Unsubstituted and symmetrically substituted compounds (e.g., benzene, cyclohexane, p-xylene,
and naphthalene) melt at higher temperatures relative to the paraffin compounds of similar molecular weight: The unsymmetrical isomers generally melt at lower temperatures than the aliphatic
hydrocarbons of the same molecular weight.
Unsaturation affects the melting point principally by its alteration of symmetry; thus ethane
(−172°C, −278°F) and ethylene (−169.5°C, −273°F) differ only slightly, but the melting points of
cyclohexane (6.2°C, 21°F) and cyclohexane (−104°C, −155°F) contrast strongly. All types of highly
unsymmetrical hydrocarbons are difficult to crystallize; asymmetrically branched aliphatic hydrocarbons as low as octane and most substituted cyclic hydrocarbons comprise the greater part of the
lubricating fractions of petroleum, crystallize slowly, if at all, and on cooling merely take the form
of glasslike solids.
Although the melting points of petroleum and petroleum products are of limited usefulness,
except to estimate the purity or perhaps the composition of waxes, the reverse process, solidification, has received attention in petroleum chemistry. In fact, solidification of petroleum and petroleum products has been differentiated into four categories, namely, freezing point, congealing point,
cloud point, and pour point.
Petroleum becomes more or less a plastic solid when cooled to sufficiently low temperatures.
This is due to the congealing of the various hydrocarbons that constitute the oil. The cloud point of
a petroleum oil is the temperature at which paraffin wax or other solidifiable compounds present in
the oil appear as a haze when the oil is chilled under definitely prescribed conditions (ASTM D2500
and ASTM D3117). As cooling is continued, all petroleum oils become more and more viscous and
flow becomes slower and slower. The pour point of a petroleum oil is the lowest temperature at
which the oil pours or flows under definitely prescribed conditions when it is chilled without disturbance at a standard rate (ASTM D97).
The solidification characteristics of a petroleum product depend on its grade or kind. For grease,
the temperature of interest is that at which fluidity occurs, commonly known as the dropping point.
The dropping point of grease is the temperature at which the grease passes from a plastic solid to
a liquid state and begins to flow under the conditions of the test (ASTM D566 and ASTM D2265).
For another type of plastic solid, including petrolatum and microcrystalline wax, both melting point
and congealing point are of interest.
The melting point of wax is the temperature at which the wax becomes sufficiently fluid to drop
from the thermometer; the congealing point is the temperature at which melted petrolatum ceases
to flow when allowed to cool under definitely prescribed conditions (ASTM D938).
For paraffin wax, the solidification temperature is of interest. For such purposes, the melting point is the temperature at which the melted paraffin wax begins to solidify, as shown
by the minimum rate of temperature change, when cooled under prescribed conditions. For
pure or essentially pure hydrocarbons, the solidification temperature is the freezing point, the
temperature at which a hydrocarbon passes from a liquid to a solid state (ASTM D1015 and
ASTM D1016).
The relationship of cloud point, pour point, melting point, and freezing point to one another varies widely from one petroleum product to another. Hence, their significance for different types of
product also varies. In general, cloud, melting, and freezing points are of more limited value and
each is of narrower range of application than the pour point.
The cloud point of petroleum or a petroleum product is the temperature at which paraffin wax
or other solidifiable compounds present in the oil appear as a haze when the sample is chilled under
definitely prescribed conditions (ASTM D2500, ASTM D3117, IP 219, IP 444, IP 445, IP 446).