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Teacher Resources

How do I reunite separated mercury thermometers?

A common problem in school laboratories is the separated mercury column in a thermometer. The instrument(s) can be made whole if you rigidly follow the procedures suggested. Other methods may be employed but could cause irreparable damage to these expensive instruments.

Cooling Method: Place a small piece (about 1 in3) of dry ice in a 150-mL Pyrex® beaker. Over the dry ice pour about 75 mL of one of the following organic solvents: acetone, iso-propyl alcohol, ethyl alcohol or methylene chloride. There will be a lot of gas evolution (CO2) as the dry ice evaporates, but soon it will subside as the dry ice (-­78 °C) cools off the organic liquid. This mixture will be very cold so don't put your fingers in it.

Hold your separated column thermometer in a vertical position with the bulb portion down. Gradually immerse the bulb only into your cold dry ice/alcohol bath. The mercury column (broken column parts and all) will retreat into the bulb. Continue to immerse the bulb only until all parts of the column are in the bulb. If all the broken column parts will not retreat into the bulb, remove the bulb from the cooling bath, allow the bulb to warm for 3­5 minutes and repeat the process. If the column will still not reunite abandon this method. Do not-we repeat-do not swing the thermometer to create centrifugal force to reunite the broken mercury column. This practice is dangerous, could fill the room with mercury droplets if the thermometer breaks, and should not be done.

Some additional alerts: Do not touch the supercooled bulb until it warms for several moments at room temperature. Do not immerse the very cold bulb in another liquid until it warms. Locate a tray of some kind in which to do this whole procedure so that if a mercury thermometer breaks you have a tray beneath the thermometer to control the spilled mercury. Remember: immerse the bulb only and never immerse the stem or column. Freezing the column will cause the bulb to fracture.

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