In most areas of chemistry, standard units are adopted - we never would think to use ounces (even American chemists resist) BUT even in modern journals or textbooks, we commonly still see the unit mmHg used. Why is this the case and why have people not moved to adopt SI units in this situation?

To give an example, consider the "Purification of Laboratory Chemicals" book that is often referenced here. Many distillations are given using pressures in mmHg.


closed as primarily opinion-based by Mithoron, airhuff, Zhe, Jon Custer, bon Jul 25 '17 at 7:08

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  • $\begingroup$ In comparison with inHg and psi, mmHg looks pretty decent. Also, habits and arguably convenience when interpreting information from the resources as dusty as the first Merck's Index. $\endgroup$ – andselisk Jul 24 '17 at 18:45
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    $\begingroup$ Predominantly habit, I suppose. Most people of a certain age remember ACTUAL manometers that used millimetres of mercury. $\endgroup$ – NotEvans. Jul 24 '17 at 18:46
  • $\begingroup$ Actual millimeters of actual mercury, I would stress. I remember them myself, and I'm not that old. Once in a while they would break and spill mercury all around, which was a peculiar kind of fun. $\endgroup$ – Ivan Neretin Jul 24 '17 at 19:05
  • $\begingroup$ Unfortunately the same reason why the modern HPLC I use registers pressure in Megapascals on the hardware (which is fine that is SI) and psi on the software (pisses me off:). Convention, and stubbornness. $\endgroup$ – Shawn CoteBurk Jul 24 '17 at 20:58
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    $\begingroup$ I just graduated from grad school last year and I exclusively used Schlenk lines with actual mercury bubblers with (metric) yard sticks attached for distillations. It was very convenient to measure the pressure in mmHg. $\endgroup$ – levineds Jul 24 '17 at 23:28

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