Pyknometer

Quick Answer: A pyknometer is a small glass flask of fixed volume, usually closed by a ground stopper with a fine capillary bore, used to measure the density or specific gravity of a liquid by weighing it. Filling it until liquid rises through the capillary gives the same volume every time.

Density by Weighing a Fixed Volume

The name comes from the Greek word for dense, and the principle is simple. The dry, empty pyknometer is weighed; it is then filled with distilled water, the stopper is inserted so excess water overflows through the capillary, the outside is wiped dry, and it is weighed again. Repeating the fill with the sample liquid gives a third mass. Because the volume is the same each time, the specific gravity of the sample is its mass divided by the mass of water, and density follows by multiplying by the density of water at that temperature.

Temperature control matters more than any other factor, because liquids expand when warm. Filling and weighing at a stated temperature, often after standing the bottle in a water bath, and handling it with tissue rather than bare fingers, keeps repeat results close together when an analytical balance is used for the weighings.

The same bottle also measures solids: a powder of known mass is added, the bottle is topped up with a liquid it does not dissolve in, and the displaced volume gives the solid’s density. The terms pyknometer, density bottle and relative density bottle overlap in catalogues; the listing does not state the pattern, nominal volume or whether a thermometer is fitted.

Care & Handling

• Hold the bottle by its neck with tissue or tongs, as body heat from fingers warms the liquid and changes its volume.
• Between samples, rinse with the next liquid or with propanone and dry thoroughly, so no water or previous sample remains.
• Keep each stopper with its own bottle; they are matched as a pair and are not interchangeable.
• Wipe the capillary tip level with the top of the stopper in one stroke, without drawing liquid out of the bore.

Applications

• Specific gravity of oils, syrups and solvents in pharmacy and food laboratories
• Checking the concentration of sugar or alcohol solutions against density tables
• Density of powders and granules by liquid displacement
• Physics and chemistry practicals on density and relative density

Specifications

Why Choose LabEquip

Pharmacy, food and petroleum testing laboratories, along with college physics and chemistry practicals, use these bottles wherever a density must be found by weighing. LabEquip supplies the item as part of its chemistry lab equipment; send the volume and quantity you need through the contact page.

Frequently Asked Questions

What is the difference between density and specific gravity?

Density is mass per unit volume, for example grams per millilitre. Specific gravity, or relative density, is the ratio of a liquid’s density to that of water at a stated temperature, so it has no units.

Is a pyknometer the same as a specific gravity bottle?

They work on the same principle and the names are often used interchangeably. Some catalogues reserve pyknometer for patterns with a thermometer or a particular stopper design, so compare the pattern with your method before ordering.

Why must the temperature be controlled?

Liquids expand when they warm, so the same volume holds less mass at a higher temperature. Filling at one fixed temperature for both water and sample, ideally in a water bath, removes this error.

How do I avoid air bubbles when filling?

Fill slowly down the side of the neck, tap the bottle gently to release any bubbles, and insert the stopper smoothly so excess liquid rises through the capillary. Refill if a bubble remains under the stopper.

Can it measure the density of a solid?

Yes, if the solid does not dissolve in the filling liquid. Weigh the solid, add it to the bottle, fill with the liquid and weigh again. The volume of liquid it displaces gives the solid’s volume and hence its density.

How accurate is the method?

Accuracy depends mainly on the balance and on temperature control rather than on the glass. With an analytical balance and careful thermostatting, it is one of the more precise simple methods for liquid density.

Last Updated: September 2026