I have already defined a new lenght \co as the height of H+ but it doesn't work with \makebox even though \makebox can take \height or \totalheight as an optional argument.
Code: Select all
\documentclass{article}
\usepackage[version=3]{mhchem}
\newlength{\co}
\settoheight{\co}{\ce{H+}}
\begin{document}
\fbox{\makebox[\co]{C}} \fbox{\ce{H+}}
\end{document}