# How are the boiling points of Tungsten and other metals determined and proved?

The boiling point of Tungsten is 10,030 degrees Fahrenheit. How was this determined and proved? And more generally how are the boiling points of metals determined and proved?

Is it really so simple as "heat it up as high as you can and see if (and at what temperature) it evaporates"?

I feel like there is a more sophisticated process, but I can't really guess at what that might be, especially if the boiling point was determined years and years (100+?) ago.

The Internet is really good at telling me the boiling point of a lot of (or all, I haven't exhaustively checked) elements and alloys, unfortunately it seems it's not very good at telling me how these seemingly arbitrary temperatures were determined.

• The melting (and boiling) points of elements are certainly not arbitrary - they are well defined thermodynamically. To find them, yes, you heat things up until they melt, measuring the temperature along the way. Ther is nothing particularly hard about it. In fact, the temperature gets easier to measure as things get hotter. – Jon Custer Dec 8 '15 at 22:40
• I realize they're not completely arbitrary. I'm sure there are number of factors that affect them so many in fact that perhaps they may seem arbitrary to someone who hasn't studied chemistry, or physics in such depth. – Alex Favero Dec 8 '15 at 22:43
• But that seems too easy. I'd like to know details of the process as well. – Alex Favero Dec 8 '15 at 22:44
• You have to understand that there are dozens of ways to measure temperature. The particular technique used depends on many factors, including how accurate a determination is needed. You could easily earn a PhD refining an element and measuring the melting point more accurately. – MaxW Dec 8 '15 at 23:16

According to the 1913 paper THE VAPOR PRESSURE OF METALLIC TUNGSTEN, The Physical Review, vol. II, pp. 329-342:

The vapor pressure (p) of a pure substance is related to its evaporation rate (m) in a vacuum by the following relationship:

$$m= \sqrt{\frac{M}{2\pi RT}}p$$

Evaporation rates were measured at temperatures in the range from 2400K to 3100K, heating by electric resistance of a filament, and determining the evaporation rate by weighing the filament before and after the evaporation.

The temperature was determined by observing the color and intensity of the light emitted by the filament. (Paper says they will publish a separate paper about the details of the temperature determination technique).

Vapor pressure was extrapolated to a boiling point of 5110 K using the Clausius-Clapeyron relationship.

• That's the answer I was looking for! Thanks Dave. – Alex Favero Dec 9 '15 at 14:48

You certainly cannot measure >6000 K (Kelvin I am using SI units here) using a thermocouple because your probe would instantly evaporate itself. However, you can use the electromagnetic radiation spectrum to measure the temperature of the element. The radiation spectrum of a black body radiator is very well understood and can be measured using a spectrophotometer. Once you have the radiation specrum, you can use Planck's law of black-body radiation to calculate the temperature.

What you then need to take care of is that you melt/evaporate the element of interest and not the container and that you do not burn the element instead of vaporizing it. In case of tungsten, this can be achieved in a setup similar to a xenon lamp. A xenon lamp is nothing else than a glass container filled with inert xenon gas in which a tungsten wire is suspended and electrically connected. By carefully measuring the electromagnetic spectrum while increasing the voltage until the tungsten wire melts you should be able to measure the melting temperature. I am not sure if this also works to measure the evaporation temperature because the wire might disconnect from your power source once molten (commonly referred to the lamp being burnt out) It may be possible to shape the wire such that it does not disconnect from your power source while melting and that the heat dissipates fast enough at the electrical connections to avoid damaging those. Alternatively (with todays technical means) one could use a laser to heat the tungsten until it evaporates.