# Expressing capital M for molar in siunitx package

The following code isn't valid but conveys my intention:

``````\SI{}{\micro\molar}
``````

as in micro followed by a big M. I don't want to use the explicit form of micro moles per L raised to the negative one in this case.

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See groups.google.com/group/comp.text.tex/browse_thread/thread/… for a near-identical question posed earlier today on comp.text.tex and an answer by Joseph Wright. – Mico Sep 6 '11 at 0:10
Which I quoted in my answer. :-) – Alan Munn Sep 6 '11 at 0:42
The timing is truly a remarkable coincidence! Given this is a relatively obscure question too. – ptrcao Sep 6 '11 at 3:24

Joseph Wright just answered a similar question on comp.text.tex:

Take a look at the section 'loading additional units' in the manual. In version 1, I did include `\molar` and `\Molar`, but tightened up on sticking to SI units (almost) exclusively for version 2. The suggested definitions are

``````\DeclareSIUnit\molar{\mole\per\cubic\deci\metre}
\DeclareSIUnit\Molar{\textsc{m}}
``````
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So do I have to declare M myself, is that what this means? – ptrcao Sep 6 '11 at 0:59
Yes. Using the second definition will then provide a `\Molar` command that can be used like any other regular SI units. So in your example, you would write `\SI{}{\micro\Molar}` to get the output you asked for. – Alan Munn Sep 6 '11 at 1:03
Where do I put the custom file siunitx.cfg? – ptrcao Sep 6 '11 at 1:05
I would put it in `<path-to-local-texmf>/tex/latex/siunitx`. (i.e. your local `texmf` directory.) – Alan Munn Sep 6 '11 at 1:09
@ptrcao Take a look at this question: Where do I place my own .sty files. You need to put them in your local texmf folder, not in `C:\texlive...` – Alan Munn Sep 6 '11 at 2:53

According to the `siunitx` manual (page 14), you can create your own units easily by putting the following command in the preamble:

``````\DeclareSIUnit{\molar}{M}
``````
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Whenever I put that command in the preamble, I get an ``undefined control sequence'' error. Do you know what could be the problem? – Nick Thompson Jan 12 '13 at 23:09
@NickThompson: Are you including the `siunitx` package? – You Jan 13 '13 at 8:56