Ask A Scientist Chemistry Archive

name         Sujith
status       educator
age          30s

Question -   What is the upper limit (maximum measurable temperature) of
(1) alcohol thermometer?
(2) mercury thermometer?
----------------------------------------------------------------
There is no "sharp" cutoff where any liquid can be used as a capillary
thermometer . At the 'high' end, one just begins to get less accuracy
because the thermal expansion is no longer a linear function of temperature,
and the increasing vapor pressure causes the liquid to evaporate from the
'hotter' bulb end into the 'colder' stem end. At the 'low' end, increasing
viscosity and increasing surface tension as the temperature decreases makes
the readings increasingly sluggish. Of course the freezing point of the
liquid is the ultimate lower limit.
     Having said that, there is an additional IMPORTANT consideration. There
is no justification any longer for using mercury thermometers in a lab. They
have pretty much been regulated out of pharmacies and over the counter
sales. The chemical industry/academia have been negligently (some of us would
say criminally) slow in removing mercury thermometers from the laboratory
environment. Metallic mercury is very toxic. A single broken thermometer can
contaminate an entire lab. I know, I know -- there are those of us who used
to play with metallic mercury for fun. That, it turns out, was foolhardiness
based upon ignorance. Just because "We did it and look at us, we are OK." is
no justification. Fifty years ago lead was used in house paints too. We now
know that the use of lead was based on the ignorance of how toxic it is and
how pervasive its spread in a building could be. And radium was used on the
hands of watches to make them glow in the dark. And nobody wore seat belts.
And the list of "we always did it's" goes on and on...
     There are a variety of inexpensive digital thermometers available.
Mercury thermometers should be removed from high school and university
chemistry labs and disposed of properly -- that means DON'T PUT THEM IN THE
TRASH.

Vince Calder
=====================================================