Grasas Y Aceites

5520 OIL AND GREASE*
5520 A. Introduction
In the determination of oil and grease, an absolute quantity of
a specific substance is not measured. Rather, groups of substances
with similar physical characteristics are determined
quantitatively on the basis of their common solubility in an
organic extracting solvent. “Oil and grease” is defined as any
material recovered as a substance soluble in the solvent. It
includes other material extracted by the solvent from an acidified
sample (such as sulfur compounds, certain organic dyes, and
chlorophyll) and not volatilized during the test. The 12th edition
of Standard Methods prescribed the use of petroleum ether as the
solvent for natural and treated waters and n-hexane for polluted
waters. The 13th edition added trichlorotrifluoroethane as an
optional solvent for all sample types. In the 14th through the 17th
editions, only trichlorotrifluoroethane was specified. However,
because of environmental problems associated with chlorofluorocarbons,
an alternative solvent (80% n-hexane and 20% methyl-
tert-butyl ether) was included for gravimetric methods in the
19th edition. In the 20th edition, trichlorotrifluoroethane was
dropped from all gravimetric procedures (retained for 5520C, an
infrared method), and replaced with n-hexane. Solvent-recovery
techniques were included and solvent recycling was strongly
recommended. In the methods given below, the 80% n-hexane
and 20% methyl-tert-butyl ether solvent mix has been dropped
from 5520B, D, and E, and an alternative to the liquid/liquid
extraction procedure using solid-phase adsorbent disks is included.
It is important to understand that, unlike some constituents that
represent distinct chemical elements, ions, compounds, or groups
of compounds, oils and greases are defined by the method used
for their determination. In detailed studies involving wastewaters
and solid matrices, it was shown that n-hexane produced results
statistically different... [continua]

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