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I'm working on the Chemwiki Dynamic Textbook Project at UC Davis, California and I've run into a problem. I need to formulate a symbol containing double harpoons (as in a reversible reaction, e.g. \rightleftharpoons); however, I need to have text both above and below the harpoons. I have an image I can email, but I can't post with an image because I'm new to this.

I can find code for \overleftrightarrow and \underleftrightarrow, but that's not quite what I need. I need both, on the same expression. If anyone has a minute to help me, I would greatly appreciate it!

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If you don't mind using TikZ, a quite similar (if not the same) problem was solved in this question. –  sgbmyr Apr 4 '11 at 23:26
    
possible duplicate of Extending arrows with overset text –  Caramdir Apr 5 '11 at 2:11
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1 Answer

You could use the features provided by the mhchem package. A little example, taken from the package documentation:

\documentclass{article}
\usepackage{mhchem}

\begin{document}

\ce{Zn^2+
<=>[\ce{+ 2OH-}][\ce{+ 2H+}]
$\underset{\text{amphoteres Hydroxid}}{\ce{Zn(OH)2 v}}$
<=>C[+2OH-][{+ 2H+}]
$\underset{\text{Hydroxozikat}}{\cf{[Zn(OH)4]^2-}}$
}

\end{document}

The result:

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