Pycnometer

A pycnometer (or pyknometer, specific gravity bottle) is a device used to determine the density of a liquid or solid, depending on the type.

General

The pycnometer is usually made of glass, preferably one with a very low coefficient of thermal expansion, to limit the effects of ambient temperature on the flask. It has a close-fitting ground glass stopper with a capillary tube through it, so that air bubbles may escape from the apparatus. This device enables a liquid's density to be measured accurately by reference to an appropriate working fluid, such as water or another solvent, as well as mercury, using an analytical balance.

Some pycnometers have a thermometer, which allows you to determine the density of a liquid at a specific temperature.

How to use a pycnometer

To determine the volume of the pycnometer, you must do the following:

• Make sure the pycnometer is dry;
• Weight the empty pycnometer and note down its weight;
• Fill the pycnometer with water at room temperature until it's slightly above the lower level of the ground glass joint, then carefully place the stopper with the capillary, and make sure the liquid fills up the capillary;
• Wipe the outside of the pycnometer to remove any traces of water and place it on the analytical balance;
• Note down the weight of the pycnometer filled with water;
• Since the density of water at room temperature is known, you can calculate the volume of the pycnometer by subtracting the weight value of the empty pycnometer from the weight value of the filled pycnometer, which gives the weight value of the pycnometer water, which is divided to the density of water at room temperature;

To determine the density of a liquid, you must do the following:

• Dry the pycnometer and let it cool to room temperature;
• Weight the empty pycnometer and note down its weight;
• Fill the pycnometer with your desired liquid, and make sure the liquid level is just slightly above the lower end of the ground glass joint, then carefully place the stopper with the capillary and make sure the excess liquid fills up the capillary;
• Wipe the outside of the pycnometer to remove any traces of liquid and place it on the analytical balance;
• Note down the weight of the pycnometer filled with liquid;
• Calculate the weight of the liquid from the pycnometer, then divide it to the volume calculated using water, which will give you the density of the liquid;
• If your pycnometer has a thermometer, it can be used to indicate the temperature where the liquid has the determined density;

To determine the density of a solid, you must do the following:

• Dry the pycnometer and let it cool to room temperature;
• Weight the empty pycnometer and note down its weight;
• Place the solid in the empty pycnometer, close the stopper with the capillary then measure and note down their weight;
• Calculate the mass of the solid, by subtracting the weight value of the empty pycnometer from the weight value of the pycnometer with solid
• Fill the pycnometer with solid, with water or another suitable liquid;
• Carefully place the stopper with the capillary, then wipe it to remove all traces of water;
• Place the pycnometer with solid filled with liquid on the analytical balance and note down its weight;
• By subtracting the weight value of the pycnometer filled with water from the weight value of the pycnometer with solid filled with liquid, you can obtain the volume of solid material added in the empty pycnometer;
• Divide the weight value of the solid to the volume value of solid to obtain its density;

Notes:

• The liquid you use must not dissolve the solid or react with it in any way. Make sure you use the most inert liquids if possible.
• Avoid using volatile liquids for density determination;

Availability

Pycnometers can be bought from lab suppliers and online. Good ones are not cheap though.

See also

• Volumetric flask