Weighing the Grease: Gravimetric Oil and Grease Testing in Water

Oil and grease in water come from many sources: industrial discharge, food processing, workshops, domestic wastewater, and urban runoff. Because these substances can harm aquatic life, clog treatment systems, and create unsightly films, reliable measurement is important. 

The gravimetric method is one of the classic ways to do it. Its principle is simple: extract the oil and grease from water with an organic solvent, evaporate the solvent, and weigh what remains.

The first step is sampling. A representative water sample is collected in a clean glass container, kept cool, and usually acidified to preserve it. Plastic containers are avoided because oils can stick to them. The sample should be analyzed as soon as possible to prevent biological or chemical changes.

Next comes extraction. A measured volume of water is placed in a separatory funnel. A solvent such as n-hexane or petroleum ether is added. The funnel is shaken vigorously, then allowed to stand so the water and solvent separate. The solvent layer, which now holds the dissolved and dispersed oil and grease, is collected. This extraction is repeated two or three times with fresh solvent to recover as much as possible. The extracts are then combined.

The combined extract is washed to remove acids and inorganic matter, then dried with anhydrous sodium sulfate. It is filtered if necessary and transferred into a clean, dry flask that has already been weighed. The solvent is carefully evaporated using gentle heat, a water bath, or a rotary evaporator. After evaporation, the flask is dried at a controlled temperature, cooled in a desiccator, and weighed. The increase in mass is the oil and grease residue.

The concentration is calculated from the weight difference and the original sample volume. A method blank, duplicate samples, and a spiked sample are normally run to check accuracy and precision. A solvent blank helps correct for any residue left by the solvent itself.

Safety matters throughout. Organic solvents are flammable and toxic, so the work should be done in a fume hood with gloves and eye protection. When performed carefully, the gravimetric method remains a practical and dependable tool for monitoring oil and grease in water.