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 5520 C, 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 5520 B, D, and E, and an alternative to the liquid/liquid extraction procedure using solid-phase adsorbent disks is included.
Oils and greases are defined by the method used for their determination.1–3
The methods presented here are suitable for biological lipids and mineral hydrocarbons. They also may be suitable for most industrial wastewaters or treated effluents containing 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.