Policeman, Rubber

Quick Answer: A rubber policeman is a flexible rubber tip fitted to the end of a glass stirring rod. The Policeman, Rubber is used to wipe the last traces of a precipitate from the walls and base of a beaker so that the whole solid reaches the filter.

Quantitative Transfer, Explained

Gravimetric results depend on every particle of precipitate reaching the filter. A plain glass rod scratches the beaker and leaves a film of solid behind; a rubber tip is soft enough to wipe across glass without scratching and flexible enough to follow the curve of the base. The usual routine is to pour off most of the liquid, rinse the beaker with a stream from a wash bottle, then scrub the wall with the policeman and rinse both the solid and the tip into the filter.

The name is an old laboratory nickname for a tool that rounds up particles which would otherwise stay behind. Tips are usually flat or wedge-shaped, with a socket that slides over the rod. The rubber version grips glass better than a plastic policeman and conforms more closely to the surface, while plastic tips are easier to clean after sticky or coloured precipitates and cope with some solvents better.

Specifications

Care & Handling

• Rinse the tip into the filter after every scrape so no precipitate leaves on the rubber.
• Wash the policeman in warm water after use and dry it before storage.
• Keep the tip out of flames and hot concentrated acids, which degrade rubber.
• Replace tips that have split, hardened or become sticky.

Applications

• Transferring barium sulfate or silver chloride precipitates in gravimetric practicals
• Recovering crystals left on the walls of an evaporating basin or beaker
• Cleaning out a mortar after grinding a weighed sample, so that nothing is lost
• Loosening cells or films grown on a glass surface in biology teaching labs

Why Choose LabEquip

Rubber policemen are ordered in class sets by colleges and senior schools teaching gravimetric and preparative chemistry. LabEquip lists them in General Lab Products; ask about rods and tip sizes through the contact page.

Frequently Asked Questions

Why is it called a policeman?

It is a long-standing laboratory nickname. The tool rounds up the last particles of precipitate that would otherwise be left behind in the beaker, much as a constable rounds up stragglers.

Can I use a glass rod alone instead?

A bare glass rod can scratch the beaker and cannot wipe the curved base cleanly, so a thin film of solid is often left behind. For quantitative work, the rubber tip gives far more complete recovery.

Does the rubber tip hold on to any precipitate?

Rubber is non-porous, but solid clings to its surface. Always rinse the tip into the filter with the wash bottle after scraping, so the rinsings pass through the same filter.

Should I choose a rubber or a plastic policeman?

Rubber conforms more closely to glass and wipes more thoroughly; plastic tips are easier to clean and tolerate some organic solvents better. Many labs keep both.

Can the policeman be used in hot solutions?

Brief contact with warm solutions is fine, but prolonged use in hot or concentrated acids and oxidisers damages rubber. Let the mixture cool before the final scraping where possible.

How do I fit a new tip onto a rod?

Wet the end of the rod and the socket in the tip, then push the rod in with a slight twist until it is held firmly. The rod should not poke through the rubber edge used for scraping.

Last Updated: September 2026