Grasas y aceites

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  • Publicado : 13 de agosto de 2010
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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 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 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 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 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 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 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 fromresults produced by trichlorotrifluoroethane.
1,2 Trichlorotrifluoroethane may at some point be prohibited
entirely by regulatory authorities. EPA currently recommends
use of n-hexane in place of trichlorotrifluoroethane.3
The methods presented here are suitable for biological lipids
and mineral hydrocarbons. They also may be suitable for most
industrial wastewaters or treated effluentscontaining these materials,
although sample complexity may result in either low or
high results because of lack of analytical specificity. The method
is not applicable to measurement of low-boiling fractions that
volatilize at temperatures below 85°C.
1. Significance
Certain constituents measured by the oil and grease analysis
may influence wastewater treatment systems. If present in excessiveamounts, they may interfere with aerobic and anaerobic
biological processes and lead to decreased wastewater treatment
efficiency. When discharged in wastewater or treated effluents,
they may cause surface films and shoreline deposits leading to
environmental degradation.
A knowledge of the quantity of oil and grease present is helpful
in proper design and operation of wastewater treatmentsystems and
also may call attention to certain treatment difficulties.
In the absence of specially modified industrial products, oil
and grease has two primary components: fatty matter from
animal and vegetable sources and hydrocarbons of petroleum
origin. The portion of oil and grease from each of these two
major components can be determined with Method 5520F. A
knowledge of the relativecomposition of a sample minimizes the
difficulty in determining the major source of the material and
simplifies the correction of oil and grease problems in wastewater
treatment plant operation and stream pollution abatement.
2. Selection of Method
For liquid samples, four methods are presented: the liquid/
liquid partition-gravimetric method (B), the partition-infrared
method (C), the...
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