I never did find an answer from professors, or even see an acknowledgement in textbooks, on why capital-letter-r is invariably used to represent the constant 0.08206 L-atm/mol-K seen in chemistry everywhere. The only variation I've seen is to use subscripts or overbars to denote incorporation of a particular circumstances.

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    $\begingroup$ Try this che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/… "Why is the universal gas constant in PV = nRT represented by the letter R?" His answer: Clapeyron first used this, and proposes possible reasons why. $\endgroup$ Dec 29, 2018 at 16:06
  • $\begingroup$ @GeraldEdgar Perhaps you could convert this into an answer, with the quote about the ratio and Regnault ? $\endgroup$
    – Conifold
    Dec 30, 2018 at 3:55

1 Answer 1


I quote from Ask the Historian "The Universal Gas Constant" by William B. Jensen, Department of Chemistry, University of Cincinnati, published in J. Chem. Educ., 2003, 80, 731-732

Why is the universal gas constant in $PV = nRT$ represented by the letter $R$?
This is best answered by tracing the origins of the ideal gas law itself. One of the first persons to combine Boyle’s law (1662) relating volume and pressure and Gay-Lussac’s law (1802) relating volume and temperature in a single equation appears to have been the French engineer, Benoit-Paul Emile Clapeyron (1799- 1864). In his famous memoir of 1834 on the Carnot cycle, he wrote the combined equation as $$ pv = R(267 + t) \tag{1}$$ where $t$ is the temperature in degrees centigrade.

Later versions of the law kept the letter $R$.

Later in the same reference, Jensen guesses why "$R$" was chosen by Clapeyron.

So why did Clapeyron choose the letter $R$ for the constant in his gas law? The fact is that he doesn’t explicitly tell us why and we are left with two speculative answers: (a) it was arbitrary or (b) it stood for ratio or one of its French equivalents: raison or rapport, since Clapeyron noted that the value of R for each gas was obtained by evaluating the constancy of the ratio $pv/(267 + t)$ over a range of pressures and temperatures, a point also emphasized by Clausius using the revised ratio $pv/(273 + t)$.

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    $\begingroup$ Exasperating, but I can't force him (Clapeyron) to change now. $\endgroup$
    – K.A.Monica
    Dec 30, 2018 at 13:51

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