Overview

A thistle tube, often called a thistle funnel, is a piece of laboratory glassware consisting of a small reservoir or funnel leading to a long, narrow stem. Liquid poured into the top flows by gravity down the stem and can be delivered to a specific point inside another vessel, often beneath the surface of a liquid already present. The simple form and low cost make it common in teaching laboratories and in preparative work where precise metering is not required.

Design and materials

Most thistle tubes are made from borosilicate glass for chemical resistance and thermal stability. The funnel portion may be quite small and calibrated only by sight; the stem varies in length and bore to suit different uses. Some thistle tubes are plain glass tubes, while others are fitted with ground-glass joints or rubber adapters so they can be inserted through stoppers or connected to tubing. Variants include very long stems for reaching into narrow flasks and short stems for surface additions.

Common uses

  • Introducing reagents into a reaction flask without splashing, especially when a localized addition is needed.
  • Adding liquid into a closed or stoppered system by passing the stem through a stopper or septum.
  • Filling narrow-necked vessels or placing a liquid underneath another layer for layering experiments.

Operation and safety

There is no built-in stopcock, so flow control depends on the pour rate and the tube diameter; small tubes give finer flow but still lack precise volumetric control. Users must avoid creating pressure in closed systems, take care with volatile or corrosive liquids, and wear appropriate personal protective equipment such as safety glasses and gloves. Ensure any rubber adapters are compatible with the chemicals used and inspect glassware for chips or cracks before use.

Alternatives and selection

For controlled, measured additions a dropping funnel, burette or micropipette is preferred. A thistle tube is chosen when simple, quick additions or physical placement of liquid are the priority rather than accurate metering. When additions must be performed into sealed apparatus with flow control, a dropping funnel with a stopcock or a syringe through a septum will usually be safer and more precise.

Care and maintenance

Clean thistle tubes promptly after use with suitable solvents and laboratory detergent; rinse thoroughly and allow to dry. Avoid sudden temperature changes to prevent thermal shock. Store upright or cushioned to prevent breakage. If rubber or plastic parts are used, replace them if they show wear or chemical attack.