Determination of Oil and Grease

Aim
To determine the quantity of oil and grease present in the given sample of water by partition gravimetric method.

Principle
The oil and grease contents of domestic and certain industrial wastes and the sludge, is of an important consideration in the handling and treatment of these material for ultimate disposal. Knowledge of the quality of the oil and grease present is helpful in proper design and operation of wastewater treatment system. The term grease applies to wide variety of organic substance that is extracted from aqueous solution or suspension by hexane. Hydrocarbons, esters, oils, fats, waxes and high molecular weight fatty acids are the major materials dissolved by hexane. All these material have a greasy feel and are associated with the problems in wastewater treatment related to grease.

Three methods by which oil and grease are estimated are (i) the partition-gravimetric method (ii) partition infrared method, and (iii) the solvent extraction method. Though method (i) does not provide needed precision, it is widely used for routine analysis of samples with high oil and grease contents because of its simplicity and no need of special instrumentation. In method (ii) adequate instrumentation allows for the measurements of as little as 0.2 mg oil and grease. Method (iii) is identical to gravimetric method but it is designed for the samples that might contain volatile hydrocarbons that otherwise would be lost in the solvent removal operation of gravimetric method.

In the Partition-Gravimetric method, dissolved or emulsified oil and grease is extracted from water by intimate contact with trichlorotrifluoroethane; petroleum ether (40/60) or hexane.

Apparatus

1. Beaker
2. Separating funnel
3. Distilling flask
4. Desiccators
5. Vacuum pump.

Reagents (click to check the preparation of reagents)

1. HCl
2. Trichlorotrifluosoethane (freon).

Procedure

1. Collect about 1 litre of sample and mark sample level in bottle for latter determination of sample volume. Acidity to pH 2 or lower; generally, 5 ml HCl is sufficient. Transfer to a separating funnel. Carefully rinse sample bottle with 30 ml trichlorotrifluoroethane and solvent washing to separating funnel.
2. Preferably shake vigorously for 2 minute. However, if it is suspected for stable emulsion shakes gently 5 to 10 minute.
3. Let layer separate out, drain solvent layer through a funnel containing solvent– moistened filter paper into a clean, evacuated distilling flask. If a clear solvent layer cannot be obtained, add 1g Na₂SO₄, if necessary.
4. Extract twice more with 30 ml solvent each time but first rinse sample container with solvent. Combine extracts in evacuated distilling flask and mash filter paper with an additional 10 ml to 20 ml solvent.
5. Distill solvent from distilling flask in a water bath at 70°C. Place flask on water bath at 70°C for 15 minute and draw air through it with an applied vacuum for final 1 minute after the solvent has evaporated. If the residue contains visible water, add 2 ml acetone evaporates on a water bath and repeat the addition and evaporation until all visible water has been removed. Cool in a desiccators for 30 minute and weigh it.

Precautions
No known solvent will dissolve effectively only oil and grease. Solvent removal results in the loss of short chain hydrocarbon and simple aromatic compound by volatilisation, and heavier residual of some effluents may contain significant portion of material that are not extracted with the solvent.

Observation
Temperature of measurement = .............°C
Mass of evacuated flask = .............g
Mass of evacuated flask + residue = .............g
Mass of residue = .............mg
Volume of sample = .............mL

Calculation
The amount of oil and grease in the sample can be calculated as,
Oil and Grease (mg/L) = (A - B) 1000/volume of the sample
where,

A = mass of evacuated flask and residue (g)
B = mass of evacuated flask (g)

Result
The amount of oil and grease in given water sample = .............mg/L.