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Thermometers—What Kind to Buy?
Routine Student Use
Select between digital, spirit-filled (n-hexane plus a red dye) or Ever-Safe® thermometers. Digital thermometers are the safest, most convenient to use
and easiest to read. For calorimetry experiments involving heat of solution or heat of reaction calculations, the greater precision afforded by digital
thermometers permits more accurate determinations than other thermometers. Both spirit-filled and Ever-Safe thermometers contain solvents, which
may cling to the walls of the thermometer column, making them a slightly less accurate than digital or mercury thermometers. Mercury thermometers
present an ongoing and serious chemical hazard if broken and spilled and are thus no longer available. At least seven states have banned the use of
mercury thermometers. Digital thermometers are the overall best option when selecting a thermometer.
Non-Routine Student Use
For those few labs where the highest accuracy is required, digital thermometers are the best choice. We have tested the accuracy of melting point
determinations using both digital and mercury thermometers. The digital thermometers gave comparable results and excellent agreement with literature
values.
For Ovens and Incubators
Do not use mercury-filled thermometers in hot ovens or incubators. A broken mercury thermometer in a hot appliance is an invitation to fill the lab with
mercury vapor.
Hazard Alert
Mercury has a Threshold Limit Value (TLV) of 0.025 mg per cubic meter of air. In a room with approximate dimensions of 40' x 30' x 9', four broken thermometers
could raise the mercury vapor level to the established TLV on a hot day. Review the extensive discussion of TLVs on pages 690–691.
Total or Partial Immersion—What’s the Difference?
Total immersion thermometers should be submerged in the medium up to the reading point. For example, if you are measuring the temperature of boiling
water (100 °C), then the thermometer should be immersed in the water to the 100 °C mark.
Partial immersion thermometers need only to be immersed to the immersion line. The immersion line is generally just above or just below the first
graduation mark.
Total immersion thermometers produce more accurate results since the thermometer stem and bulb are both at the same temperature; however, they
are not always convenient. For more accurate temperature readings, make sure you know if the thermometer you are using is calibrated for partial or
total immersion and follow the above guidelines.
Thermometer Quality
Know what you’re getting before you buy! The market is full of thermometers with a wide variety of quality. Flinn carries only high-quality, non-mercury
thermometers. Other supply houses may be supplying so-called “best quality” thermometers; however, they are often less accurate and have markings
that easily wear off. Flinn guarantees you’ll be pleased with the accuracy and overall quality of our thermometers.
Why Do Spirit-Filled Thermometer Columns Separate?
• Improper handling
• Thermal or mechanical shock
• Improper storage; always store thermometer in an upright (vertical) position
• Bore diameter is 15–20 times larger in a spirit ther mometer than a mercury thermometer
How to Reunite Separated Thermometer Columns
A common problem in school laboratories is a separated spirit-filled or mercury column in a thermometer. It may be possible
to reunite the liquid column using the following procedure.
Cooling Method: Place a small piece (about 1 in3) of dry ice in a 150-mL borosilicate beaker. Over the dry ice, pour about 75
mL of one of the following organic solvents: acetone, isopropyl alcohol or ethyl alcohol. 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 column (broken column parts and all) will retreat into
the bulb. Continue to immerse the bulb only until all parts of the liquid 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 3–5 minutes and repeat the
process. If the column will still not reunite, abandon this method.
Alerts: Do not touch the supercooled bulb until it warms for several minutes 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 the entire procedure so 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.
Do not swing the thermometer to create centrifugal force to reunite a broken mercury column. This practice is dangerous,
could fill the room with mercury or vapor droplets if the thermometer breaks and should not be done.
Expansion
Chamber
Separated
Column
Bulb