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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. $\endgroup$ – Gerald Edgar Dec 29 '18 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 '18 at 3:55
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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

Question
Why is the universal gas constant in $PV = nRT$ represented by the letter $R$?
Answer
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 Dec 30 '18 at 13:51

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