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5520 OIL AND GREASE*

* Approved by Standard Methods Committee, 1997.

5520 B. Partition-Gravimetric Method

1. General Discussion

 

    Dissolved or emulsified oil and grease is extracted from water by intimate contact with an extracting solvent. Some extractables, especially unsaturated fats and fatty acids, oxidize readily; hence, special precautions regarding temperature and solvent vapor displacement are included to minimize this effect. Organic solvents shaken with some samples may form an emulsion that is very difficult to break. This method includes a means for handling such emulsions. Recovery of solvents is discussed. Solvent recovery can reduce both vapor emissions to the atmosphere and costs.

 

2. Apparatus

 

    a. Separatory funnel, 2-L, with TFE* stopcock.

    b. Distilling flask, 125-mL.

    c. Liquid funnel, glass.

    d. Filter paper, 11-cm diam.

    e. Centrifuge, capable of spinning at least four 100-mL glass centrifuge tubes at 2400 rpm or more.

    f. Centrifuge tubes, 100-mL, glass.

    g. Water bath, capable of maintaining 85C.

    h. Vacuum pump or other source of vacuum.

    i. Distilling adapter with drip tip. Setup of distillate recovery apparatus is shown in Figure 5520:1. Alternatively, use commercially available solvent recovery equipment.

    j. Ice bath.

    k. Waste receptacle, for used solvent.

    l. Desiccator.

____________

*Teflon or equivalent.

Whatman No. 40 or equivalent.

 

 

3. Reagents

 

     a. Hydrochloric or sulfuric acid, 1:1: Mix equal volumes of either acid and reagent water.

    b. n-Hexane, boiling point 69C. The solvent should leave no measurable residue on evaporation; distill if necessary. Do not use any plastic tubing to transfer solvent between containers.

    c. Methyl-tert-butyl ether (MTBE), boiling point 55C to 56C. The solvent should leave no

measurable residue on evaporation; distill if necessary. Do not use any plastic tubing to transfer solvent between containers.

    d. Sodium sulfate, Na2SO4, anhydrous crystal.

    e. Solvent mixture, 80% n-hexane/20% MTBE, v/v.

 

4. Procedure

 

    When a sample is brought into the laboratory, either mark sample bottle at the water meniscus or weigh the bottle, for later determination of sample volume. If sample has not been acidified previously (see Section 5520A.3), acidify with either 1:1 HCl or 1:1 H2SO4 to pH 2 or lower (generally, 5 mL is sufficient for 1 L sample). Using liquid funnel, transfer sample to a separatory funnel. Carefully rinse sample bottle with 30 mL extracting solvent (either 100% n-hexane, 3b, or solvent mixture, 3e) and add solvent washings to separatory funnel. Shake vigorously for 2 min. Let layers separate. Drain aqueous layer and small amount of organic layer into original sample container. Drain solvent layer through a funnel containing a filter paper and 10 g Na2SO4, both of which have been solvent-rinsed, into a clean, tared distilling flask. If a clear solvent layer cannot be obtained and an emulsion of more than about 5 mL exists, drain emulsion and solvent layers into a glass centrifuge tube and centrifuge for 5 min at approximately 2400 rpm. Transfer centrifuged material to an appropriate separatory funnel and drain solvent layer through a funnel with a filter paper and 10 g Na2SO4, both of which have been prerinsed, into a clean, tared distilling flask. Recombine aqueous layers and any remaining emulsion or solids in separatory funnel. For samples with <5 mL of emulsion, drain only the clear solvent through a funnel with pre-moistened filter paper and 10 g Na2SO4. Recombine aqueous layers and any remaining emulsion or solids in separatory funnel. Extract twice more with 30 mL solvent each time, but first rinse sample container with each solvent portion. Repeat centrifugation step if emulsion persists in subsequent extraction steps. Combine extracts in tared distilling flask, and include in flask a final rinsing of filter and Na2SO4 with an additional 10 to 20 mL solvent. Distill solvent from flask in a water bath at 85C for either solvent system. To maximize solvent recovery, fit distillation flask with a distillation adapter equipped with a drip tip and collect solvent in an ice-bath-cooled receiver (Figure 5520:1). When visible solvent condensation stops, remove flask from water bath. Cover water bath and dry flasks on top of cover, with water bath still at 85C, for 15 min. Draw air through flask with an applied vacuum for the final 1 min. Cool in desiccator for at least 30 min and weigh. To determine initial sample volume, either fill sample bottle to mark with water and then pour water into a 1-L graduated cylinder, or weigh empty container and cap and calculate the sample volume by difference from the initial weight (assuming a sample density of 1.00).

 

                   

 

5. Calculation

 

    If the organic solvent is free of residue, the gain in weight of the tared distilling flask is due to oil and grease. Total gain in weight, A, of tared flask, less calculated residue from solvent blank, B, is the amount of oil and grease in the sample:

 

                                                                (A-B) X 1000

                    mg oil and grease/L =                                        

                                                                  mL sample

 

6. Precision and Bias

 

    Method B with 80:20 hexane/MTBE mixture was tested by a single laboratory on a raw wastewater sample. The oil and grease concentration was 22.4 mg/L. When samples were dosed with 30 mg Fisher Heavy Mineral Oil, recovery of added oil was 84.2% with a standard deviation of 1.2 mg/L. Method B was tested with n-hexane as solvent. The method detection limit was determined to be 1.4 mg/L.1 When reagent water was fortified with hexadecane and stearic acid each at approximately 20 mg/L, initial precision and recovery limit standards were 10% and 83 to 101%, respectively. Acceptable recovery limits for laboratory-fortified matrix/laboratory-fortified matrix duplicate and ongoing laboratory control standards are 79 to 114%, with a relative percent difference limit of 18%.

 

 

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7. References

  1. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1995. Report of the Method 1664 Validation Studies. EPA-821-R-95-036, U.S. Environmental Protection Agency, Washington, D.C.

  2. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1995. Method 1664. EPA-821-B-94-004B, U.S. Environmental Protection Agency, Washington, D.C.

8. Bibliography

 

KIRSCHMAN, H.D. & R. POMEROY. 1949. Determination of oil in oil field waste waters. Anal. Chem. 21:793.

 

 

Standard Methods for the Examination of Water and Wastewater. 20th Ed. American Public Health Association, American Water Works Association, Water Environment Federation.