Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 38

1.2.5 T ESTING S OLVENT S TABILITY AND P ERFORMANCE
The balancing act of maximizing solvent utility while controlling and preventing solvent deteriora-
tion requires early detection of changes in solvent characteristics. Visual inspection of the solvent is
not a reliable means for determining its condition. A solvent that is perfectly clear can be depleted
of its stabilizers, whereas one that is badly discolored and cloudy can still be well stabilized. The
most reliable method for coni rming the presence and quantity of stabilizers is laboratory analysis
by gas chromatography-mass spectrometry; however, the associated delay and expense are too
impractical for most operations, so surrogate tests were developed (Howell and Tarrer, 1994).
Operators of equipment using chlorinated solvents—such as degreasers, phosphatizing lines, and
dry cleaners—routinely test solvents for the presence of acid. A variety of testing approaches have
been developed since 1970.
The main approaches for testing solvents check for acids by titrating a base [the acid acceptance
value (AAV) test] and test for reaction with alkali metals (the aluminum scratch test). Tests with
more specii c endpoints—such as reaction with copper (ASTM D 3316), zinc, or brass, and dye-bleed
tests—have also been developed for specii c applications. Light transmittance has been used in dry-
cleaning operations as an indicator of when a solvent is due for replacement. Solvents usually trans-
mit light in the range from 450 to 600 nm; a 50% reduction in transmittance in solvent that has been
i ltered is often used as the rule of thumb for when the solvent must be replaced (Tarrer et al., 1989).
Transmittance is measured by using a visible light spectrometer or a colorimeter. Color, odor, pres-
ence of dirt and grease, and boiling temperature have also been used by dry-cleaner operators to
gauge when the solvent is due for i ltration, distillation, or replacement. All the solvent performance
tests are temperature sensitive; a water bath is usually used to hold solvent temperatures constant to
produce comparable results.
Solvent power is a measure of the solvent's effectiveness at dissolving oils, soldering l ux, and buff-
ing compounds. The most common test for judging changes to solvent power yields the Kauri-butanol
value (KBV) (ASTM D 1133). The test uses Kauri gum, * which is very soluble in butanol but less
soluble when butanol is diluted with a solvent that does not dissolve the resin. The test uses a burette,
a l ask, and a precision balance and provides a relative ranking of solvency. The solvent tested is added
in small amounts until the solution becomes cloudy because of the precipitation of Kauri gum from
the butanol solution. The more the solvent added before the solution becomes cloudy, the greater the
solvent power and the higher the corresponding KBV. When tested as new solvent, uncontaminated by
cleaning waste, water, or other impurities, the major chlorinated solvents have the following lower to
higher order of solvent power and KBVs: perchloroethylene (KBV = 93) < methyl chloroform

(KBV 178) † (Tarrer et al., 1989; Dow Chemical

Company, 2002, 2006c; Solvay SA, 2002b). The solvent power of a solvent decreases with continued
use and an increasing fraction of oily waste, grease, or soil.
Another test used to determine solvent stability involves the Acid Number. The Acid Number
establishes the amount of fatty acid in a solvent, determined as the number of milligrams of potas-
sium hydroxide to neutralize 1.28 mL of the solvent, by using a burette, a pipette, and a l ask with
potassium hydroxide, methanol, and phenolphthalein (Tarrer et al., 1989). Fatty acids such as stearic
acid and oleic acid are introduced to the solvent when the work is coated with bufi ng compounds
and drawing oils (Klabunde, 1949).

= 124) < TCE (KBV = 130) < dichloromethane (KBV =

* Kauri gum is formed when resin exudes from a crack in the bark of the kauri tree ( Agathis australis ), found in New
Zealand, and hardens on exposure to air. It also exists in fossil form. Its uses range from chewing gum to industrial
applications.
† Dichloromethane's Kauri Butanol Value (KBV) is listed as 87 for unstabilized methylene chloride and 178 for Solvaclene,
metal-stabilized dichloromethane, both sold by Solvay SA. Dow lists a KBV for Dow Methylene Chloride (dichlo-
romethane) as 136 expressed as cubic centimeters of solvent per 20 g of Kauri-Butanol solution. Dow's KBV value for
perchloroethylene is 90, the value for methyl chloroform is 124, and that for NEU-TRI trichloroethylene is 129 (Dow
Chemical Company, 2002, 2006c).