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Name:  Priya
Status: educator
Grade: other
Location: N/A
Country: N/A
Date: 7/19/2005


Question:
Hi there, Alcohol is commonly used as the thermometric liquid in Laboratory thermometers nowadays. Since alcohol has a boiling point of 78 degrees Celsius, how then can it measure the boiling point of say water which is 100 degrees Celsius without alcohol itself turning to vapour form?


Replies:
Two things, Priya:

The non-mercury liquid in your lab thermometer is probably not "alcohol" exactly. Laboratory thermometers with actual alcohol in them are probably hard to find. Easier to find alcohol in inexpensive thermometers for room-temperature and freezers. Many of the commonly used organic liquids in current lab thermometers boil at 150-250C. Look in any chemistry equipment catalog (Fisher, Cole-Parmer, others) and read the names of several of the liquids used. "Red Spirit" is usually Kerosene, "Blue Spirit" is often iso-amyl-acetate. I was surprised to see a freezer thermometer filled with Pentane. The guys who make thermometers usually do have their thinking caps on, and they are doing the best they can with the physical substances available to them. It is up to the user to choose the right kind of thermometer, and not let it get far off-scale.

The other thing is that, if the thermometer is sealed and strong, the text-book "boiling point" does not matter. Only "vapor pressure vs. temperature" of the liquid, compared with the bursting pressure of the bulb, matters. Putting an actual alcohol thermometer in boiling water or hotter might well cause it to burst a bit dangerously, or quietly crack and bleed out its contents. This bursting pressure might be 1.1, or 2, or 10 atmospheres, and so the bursting temperature will be an unknown distance above the boiling temperature of the liquid. Until bursting occurs, the liquid will expand and indicate temperature properly, because there is no room in the tube for the liquid to evaporate away or expand into large volumes of gas bubbles. The small amount of liquid that can evaporate at the top end of the column will pressurize the empty volume and push down on the liquid, making sure that the whole interior has the same high pressure.

A distant exception: the "critical temperature" of the liquid, above which this particular substance refuses to be liquid at any pressure. In a really thick strong tube with a really low-boiling (high-vapor-pressure) liquid, one might be able to reach the critical temperature of the fluid, without bursting the shell. Then the meniscus (the liquid-gas interface) would dissolve, disappear, and the entire interior would be evenly filled with a heavy, sloshy gas almost as dense as the liquid previously was. But you have to work exceptionally hard to design a thermometer this wrong, because the critical temperature for the alcohols is about 240C, and the pressure is 50-80 atmospheres. DiMethyl Ether would give this problem much sooner, at only 130C, but it is still 50 atmospheres. Pentane is probably even lower-temp than Ether. Don't put that one in boiling water...

Since the stem is usually at lower temperature than the bulb, If a thermometer had no gas in it but the vapor of the liquid, putting it in a hot liquid would often make it grow a vapor bubble near the bulb, and condense the cooler vapor at the top end. This could happen even below the boiling point. Or if there were no nucleation points for bubbles, it might not happen till above the boiling point. The thermometer would be completely useless until the lower void was eliminated. So I wonder if they put some inert gas in the thermometer, and what gas, and what pressure?

Jim Swenson


Priya,

The boiling point you quoted is true only for alcohol at 1atm (sea-level) pressure. Since the alcohol is in a sealed tube, its boiling point is mitigated by the -I would assume- increased pressure above the alcohol.

Greg (Roberto Gregorious)


You cannot use a "liquid capillary" type thermometer to measure temperatures near the boiling point of the liquid, or below the freezing point of the liquid. There is even a more subtle limitation. If the viscosity of the liquid increases, it may not be useful because the liquid will not change volume rapidly with changing temperatures. Alcohol (ethanol) thermometers are only useful below ~ 25 C. If you examine the scale etched on the thermometer it is a good indicator of the range of utility. Given: the limited range of utility of many liquids, the toxicity of mercury (long the standard of lab thermometry), and the increasing availability, decreasing cost, and improved sensitivity of various electronic/solid state thermometric devices, and the fragility of glass thermometers, I would guess that within a few years temperature-measuring devices will almost all be electronic-types of some sort, and the standard "liquid capillary" will become laboratory relics.

Vince Calder



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