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I am a journal typesetter, and recently I came to a problem that I'm not sure I know to solve. The question is how to properly typeset the "NOx" abbreviation for nitrogen oxides. Which is typographically correct:

  1. $\mathrm{NO}_x$ (because $x$ is a numeric variable); or
  2. $\mathrm{NO}_{\mathrm{x}}$ (because all names of chemical compounds are strictly upright)?
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I quote the Green Book by IUPAC, 2nd printing (2008), section 1.6, enumeration item 2:

The overall rule is that symbols representing physical quantities or variables are italic, but symbols representing units, mathematical constants, or labels, are roman. [...]

As such, the correct way to write it is $\ce{NO}_x$, because $x$ is a variable. Please note that it is only the sum formula for the compound and not its name. Since the element names still are in roman (upright) font, everything is fine.

The same goes for a formula like $\ce{C_{$n$}H_{$2n+2$}}$, which is typeset exactly like that in the Gold Book (hat tip to @Martin).

Here at StackExchange, you can type it as $\ce{NO_$x$}$ ($\ce{NO_$x$}$) or $\ce{NO_x}$ ($\ce{NO_x}$).

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    $\begingroup$ Note that I haven't been able to find a direct reference or instruction by IUPAC, so I might well be wrong here. I'm just combining some rules in a logical way. $\endgroup$ – tschoppi Dec 4 '14 at 16:31
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    $\begingroup$ Well, the analogy to $\mathrm{C}_n\mathrm{H}_{2n+2}$ is convincing me quite. $\endgroup$ – yo' Dec 4 '14 at 16:33
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    $\begingroup$ @tohecz The IUPAC goldbook typesets it exactly this way. $\endgroup$ – Martin - マーチン Dec 5 '14 at 2:08
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The American Chemical Society recommends $\ce{NO}_x$: page 191 of Appendix 10-2, Abbreviations, Acronyms, and Symbols of their style guide (PDF).

Unfortunately, it's just in a list of abbreviations and I've not managed to find any definitive explanation of why it should be typeset that way, elsewhere in the style guide. However, tschoppi's explanation is what I would have said, too.

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The IUPAC Nomenclature of Inorganic Chemistry (‘Red Book’) Recommendations 2005 reflects on this subject.

IR-2.9 ITALIC LETTERS

(…)

(g) Other uses of italicized capital letters are as locants in substitutive nomenclature (see, for example, Section IR-6.2.4.1), and the letter H for indicated hydrogen (see, for example, Section IR-6.2.3.4). Italic lower case letters are used to represent numbers, especially in formulae where the numbers are undefined.

Examples:

  1. $\ce{(HBO2)_n}$
  2. $\ce{Fe^{n+}}$

$\ce{NO_x}$ is analogous to the examples mentioned above.

Some loosely related examples with ‘x’ variable:

IR-2.2.3 Parentheses

(…)

(f) To indicate the composition of a non-stoichiometric compound.

Examples:

  1. $\ce{Fe_{3x}Li_{4-x}Ti_{2(1-x)}O_6} ~ (x=0.35)$
  2. $\ce{LaNi5H_x} ~ (0<x<66.7)$

or

IR-11.3.2 Phases with variable composition

(…)

Examples:

  1. $\ce{Cu_xNi_{1-x}} ~ (0 \le x \le 1)$ is equivalent to $\ce{(Cu,Ni)}$.
  2. $\ce{KBr_xCl_{1-x}} ~ (0 \le x \le 1)$ is equivalent to $\ce{K(Br,Cl)}$.
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