An superscript circle or Plimsoll is typically used to indicate that a thermodynamic property is that of a substance in its standard state. On the other hand, when writing a chemical equation we typically use abbreviations to designate the state or phase of the substance, e.g. "g" for gas, "aq" for aqueous. Is there a common or standard way of describing symbolically that a substance is in its standard state, e.g. $\ce{Cl2(g,^\circ)}$ or $\ce{Cl2(\mathrm{std})}$, or is it necessary to spell out those conditions explicitly and allow the reader to infer that those are standard conditions, e.g. $\ce{Cl2(g,\pu{1 bar})}$?

  • $\begingroup$ What about the notation $M^\circ(\ce{compound,state})$ where $M$ is the thermodynamic property? I wouldn't add the pressure since the $^\circ$ already implies $p^\circ \equiv \pu{1 bar}$. If I need the temperature going on during the algebra, I use $M^\circ(\ce{compound,state};T)$. E.g., $G^\circ(\ce{Cl2,g};T_2) - G^\circ(\ce{Cl2,g};T_1)$ $\endgroup$ Jan 7 at 18:42
  • $\begingroup$ @MetalStorm That would be the way to do this if I was referring to the property, but I am attempting to describe the substance. The thought occurred to write "$\ce{Cl2^\circ}$" but rather than invent my own notation I thought I'd ask around. There is likely a convention, I haven't seen this asked here before though. $\endgroup$
    – Buck Thorn
    Jan 7 at 18:58
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    $\begingroup$ You don't react Cl2 at 1 atm with H2 at 2 atm. The conditions refer to the reaction as a whole and should be marked as such. $\endgroup$ Jan 7 at 19:08
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    $\begingroup$ Ohh, my bad. I haven't either, but I would use something like $\{\ce{substance}\}^\circ$ (e.g., $\{\ce{MnO4^-}\}^\circ$) as not to conflate with symbols relating to charge, radical, transition state, and so on. $\endgroup$ Jan 7 at 19:08
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    $\begingroup$ I'm not aware of a standard notation, so in case of need I'd probably invent my own. $\endgroup$ Jan 7 at 19:25


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