Handbook of Solvents - George Wypych - ChemTech - Ventech!

15.1 Standard methods of solvent analysis 1057
15.1.9 CORROSION (EFFECT OF SOLVENTS)
The corrosiveness of perchloroethylene to copper is determined using Soxhlet apparatus. 27
Three pre-weighed strips of copper are used, one placed in the bottom flask, the second in
the bottom of the Soxhlet attachment, and the third below the condenser. The specimens are
exposed to refluxing solvent for 72 h after which the entire apparatus is flushed with distilled
water to wash all acidic substances back to the flask. The water layer is titrated with
0.01 N NaOH to determine its acidity and the strips are weighed to determine weight loss.
The results indicate quality of solvent. A different method is used to test copper corrosion
by aromatic hydrocarbons. 28 Here, a copper strip is immersed in a flask containing solvent
and the flask is placed in boiling water for 30 min. Next, the copper strip is compared with
ASTM standard corroded copper strips.
If 1,1,1-trichloroethane is not properly stabilized it forms hydrochloric acid in the
presence of aluminum. HCl corrodes aluminum. The presence of free water invalidates the
result of this test. 29 An aluminum coupon is scratched beneath the surface of a solvent. The
coupon is observed for 10 min and 1 h and the degree of corrosion is recorded in form of
pass (no reaction) or fail (gas bubbles, color formation, or metal corrosion). The test is important
to cleaning operations because aluminum should not be used for parts of machines
(pumps, tanks, valves, spray equipment) in contact with corrosive solvent.
15.1.10 DENSITY
Density and specific gravity of solvents are discussed together. The difference in their definitions
is that specific gravity is the density of material relative to the density of water
whereas the density is the weight in vacuo of a unit volume. The density of liquids (including
solvents) can be measured by a Bingham pycnometer. 30 The determination includes introduction
of the liquid to the tared pycnometer, equilibration of temperature, then
weighing. Other standardized method 31 determines the specific density of liquid industrial
chemicals by two methods: hydrometer and pycnometer. The pycnometric method is essentially
similar to the previously described. It differs in that the water and then solvent are
weighed. Thus the density determination error may only be due to an imprecise weighing
(the pycnometer calibrated volume does not enter calculations). In the hydrometer method,
the calibrated hydrometer is immersed in controlled temperature liquid and direct readings
are obtained.
The standard method for determining the specific gravity of halogenated organic solvents
32 involves the use of both a pycnometer and a hydrometer as described above but, in
addition, an electronic densitometer is also used. Here, a liquid is placed in U-shaped tube
and subjected to electronic excitations. The density changes the mass of tube and frequency
of oscillations which is the basis for measurement and display of specific gravity readings.
Two standard tables 33,34 (American and metric system of units) are used to calculate
weight and volume of benzene, toluene, xylenes mixture and isomers, styrene, cumene, and
ethylbenzene as well as aromatic hydrocarbons and cyclohexane. Tables provide volume
corrections for these solvents in a temperature range from -5 to 109 o F (-20.5 to 43 o C).
15.1.11 DILUTION RATIO
The dilution ratio is the maximum number of units of diluent that can be added to unit
volume of solvent before precipitation occurs. Cellulose nitrate dissolved in an oxygenated
solvent (8 wt% resin) is the most classical method to determine dilution ratio used to
evaluate toluene as a standard diluent and to compare different diluents and solvents with a
standard solvent (n-butyl acetate). The standard dilution ratio of toluene by n-butyl acetate