What does the "! Incomplete \ifx" error mean?

Question

\documentclass[12pt,a4paper]{article}
\usepackage[version=3]{mhchem}
\usepackage{siunitx}

\begin{document}

\begin{align*}
 \ce{PH &= -\log  \ce{[H+]}} \\
 -\ce{PH} &= \log \ce{[H+]} \\
 10^{-\ce{PH}} &= \ce{[H+]} \\
\end{align*}
\end{document}

gives

(/home/peter/texlive/2011/texmf-dist/tex/latex/siunitx/config/siunitx-abbreviat
ions.cfg)
(/home/peter/texlive/2011/texmf-dist/tex/latex/siunitx/config/siunitx-binary.cf
g)
! Incomplete \ifx; all text was ignored after line 11.
<inserted text> 
                \fi 
l.11 \end{align*}

What is wrong?

this question was closed as too localized, but it actually helped me — simona, Apr 19 '13 at 15:36
well there are other maybe more general questions e.g. see "related questions" list in https://tex.stackexchange.com/questions/387380/incomplete-iftrue-all-text-was-ignored-after-line. Looks like this particular one is popular (good SEO optimization) though — user202729, May 19 '22 at 00:39

score 9 · Accepted Answer · edited Jul 18 '17 at 07:05

9

The first \ce is missing a }, while the second one has one too much. So you should have

\begin{align*}
 \ce{PH} &= -\log  \ce{[H+]} \\
 -\ce{PH} &= \log \ce{[H+]} \\
 10^{-\ce{PH}} &= \ce{[H+]} \\
\end{align*}

edited Jul 18 '17 at 07:05

David Carlisle

757,742

answered Sep 30 '11 at 05:12

Caramdir

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What does the "! Incomplete \ifx" error mean?

1 Answers1