Grasas y aceites

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 physicalcharacteristics 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 solublein 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 addedtrichlorotrifluoroethane 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 withchlorofluorocarbons,
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 wasdropped from all gravimetric procedures (retained for 5520C, an
infrared method), and replaced with n-hexane. Solvent-recovery
techniques were included and solvent recycling was stronglyrecommended. 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 usingsolid-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 andgreases 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...
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