Laboratory Funnels

Filter funnels are for gravity filtration with folded paper. Powder funnels have a wide stem for solids. Separating funnels split immiscible liquids, dropping funnels add a reagent slowly to a reaction, and Buchner funnels are for vacuum filtration. If a school lab buys only one type, make it a plain glass filter funnel.

Borosilicate 3.3 glass copes with hot liquids and most acids and solvents. Polypropylene costs less and won't break when dropped, which suits water-based solutions, but it doesn't like strong heat or some organic solvents. Porcelain Buchner funnels are tough and easy to scrub clean.

Choose a funnel that fits your filter paper once it's folded, with the paper sitting just below the rim. The stem should fit loosely inside the neck of the flask or bottle you're filling, so air can escape as the liquid runs in and nothing overflows.

Separating and dropping funnels come with glass or PTFE stopcocks. PTFE keys need no grease and won't seize, so they're easier for learners. Jointed funnels have ground-glass sizes such as 19/26 or 24/29, so match the joint to the flask you'll use.

A laboratory funnel guides liquids and powders into containers with narrow openings without spilling. Specialised types go further: filter funnels hold paper for gravity filtration, separating funnels divide two immiscible liquids, and Buchner funnels speed filtration up with a vacuum.

Fold a circle of filter paper in half, then in half again, open it into a cone and seat it in the funnel. Wet it with a little solvent so it clings to the sides. Stand the funnel in a retort ring over a flask and pour slowly down a glass rod, keeping the liquid below the top of the paper.

Glass is the better choice for hot liquids, organic solvents and anything you need to see clearly. Plastic is cheaper, lighter and won't shatter, which makes sense for younger learners and for moving water-based solutions. Most school labs keep a few of each.

Usually the paper is too fine for the job or the solid has clogged it. Let the solid settle and pour the liquid off first, adding the sludge last. If it's still slow, try a coarser grade of filter paper or switch to a Buchner funnel and filter flask under vacuum.

Rinse it straight after use, before residue dries on. Wash glass and plastic funnels with warm water, lab detergent and a soft brush, then rinse with distilled water and leave to drain. For a separating funnel, take the stopcock key out, clean it separately and store the funnel with the key loose so it doesn't stick.