How to get text above and below double harpoon (reversible reaction) symbols in an equation

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

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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