Handbook of Solvents - George Wypych - ChemTech - Ventech!

46 George Wypych
Odor threshold, ppm
30
25
20
15
10
5
alcohols
aromatic CH
esters
ketones
0
50 100 150 200 250
Boiling point, o C
Figure 2.3.6. Odor threshold of solvent vs. its boiling
point.
-150
50 100 150 200 250 300
ferent groups of solvents and many experimental points are scattered. Figure 2.3.5 verifies
the origin of flash point which has strong correlation with boiling point. Here, all four chemical
groups of solvents have the same relationship.
Odor threshold is an approximate but quite unreliable method of detection of solvent
vapors. As the boiling point increases, the odor threshold (concentration in air when odor
becomes detectable) decreases (Figure 2.3.6). This may suggest that slower evaporating
solvents have longer residence time close to the source of contamination.
Figure 2.3.7 shows the relationship between boiling and freezing points. A general
rule is that the difference between boiling and freezing points for analyzed solvents is
190±30 o C. Relatively small fraction of solvents does not follow this rule. Natural solvent
mixtures such as aromatic or aliphatic hydrocarbons deviate from the rule (note that hydrocarbons
in Figure 2.3.7 depart from the general relationship). If more groups of solvents is
investigated, it will be seen that CFCs, amines, and acids tend to have a lower temperature
difference between boiling and freezing points whereas some aliphatic hydrocarbons and
glycol ethers have a tendency towards a larger difference.
2.3.3 SPECIFIC GRAVITY
The specific gravity of most solvents is lower than that of water. When solvent is selected
for extraction it is generally easy to find one which will float on the surface of water. Two
groups of solvents: halogenated solvents and polyhydric alcohols have specific gravity
greater than that of water. The specific gravity of alcohols and ketones increases with increasing
molecular weight whereas the specific gravity of esters and glycol ethers decreases
as their molecular weight decreases.
The specific gravity of solvents affects their industrial use in several ways. Solvents
with a lower density are more economical to use because solvents are purchased by weight
but many final products are sold by volume. The specific gravity of solvent should be considered
in the designs for storage systems and packaging. When switching the solvent types
in storage tanks one must determine the weight of new solvent which can be accommodated
in the tank. A container of CFC with a specific gravity twice that of most solvents, may be
Freezing point, o C
200
150
100
50
0
-50
-100
alcohols
aromatic CH
esters
ketones
Boiling point, o C
Figure 2.3.7. Relationship between boiling and
freezing points of solvents.