# Latex: How to draw an equilibrium arrow with overpic?

Does anybody know how to draw an equilibrium arrow with overpic (into a picture)?

With equilibrium arrow I mean the arrows in a chemical equilibrium equation:

In the code I would like to put an equlibrium arrow, where %\put(30,11)\ce{<=>} stands.

Thanks!

\documentclass[12pt,a4paper,headsepline,liststotoc,bibtotoc,numbers=noenddot]{scrreprt}

\usepackage{amsmath}
\usepackage{graphicx}

\usepackage{chemgreek}
\usepackage[version=4]{mhchem}
\usepackage{chemfig}

\usepackage[top=3cm, left=3.0cm,right=2cm,bottom=2.5cm]{geometry}

\setcapwidth[c]{.8\textwidth}
\setcapindent{0pt}
\usepackage{caption}
\usepackage{subcaption}

\usepackage{units}

\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}

%Schriftbild
\setlength{\parindent}{0pt} % ein neuer Paragraph hat keinen Einzug in der ersten Zeile
\setlength{\parskip}{6 pt}

% Overpic dient der nachträglichen Beschriftung von Grafiken in der gewählten Schriftart von Latex
\usepackage[percent]{overpic}

\usepackage{etex}
\usepackage{pgfplots}
\usepgfplotslibrary{ternary}
\usepgfplotslibrary{groupplots}
\usetikzlibrary{pgfplots.groupplots}

\usepackage{rotating}

\usepackage[
pdftitle={Titel},
pdfsubject={Thema},
pdfauthor={Autor},
pdfkeywords={Schluesselwoerter}
]{hyperref}

\newcommand{\C}{$^\circ \text{C}$}

\begin{document}

\begin{figure}[H]
\centering
\begin{overpic}[width=0.4\textwidth, grid,tics=5
]
{GEH_Oberfl_neu.png}
\put(11,10){OH\textsubscript{2}\textsuperscript{+}}
%\put(30,11)\ce{<=>}
\put(44,10){OH}
%   \put(11,10){p}
\put(77,10){O\textsuperscript{-}}
\end{overpic}
\end{figure}

\end{document}

• Welcome to TeX.SX! Can you please expand the code snippet that you have posted to a full minimal working example. A MWE should compile and be as small as possible to demonstrate your problem. it's much easier to help you if we have full working code to start from. Also, what is an "equilibrium arrow"? – Andrew Dec 6 '17 at 13:24
• Hi Andrew, thanks for the tipps! I changed it! – Sophie Dec 6 '17 at 14:19

I would suggest something like the following:

\documentclass[12pt,a4paper,headsepline,listof=totoc,bibliography=totoc,numbers=noenddot]{scrreprt}
\usepackage{graphicx}
\usepackage[version=4]{mhchem}
\usepackage[utf8]{inputenc}
\usepackage[percent]{overpic}

\begin{document}

\begin{figure}
\centering
\begin{overpic}[width=0.4\textwidth, grid,tics=5]{example-image}
\put(11,10){\parbox{0.4\textwidth }{\ce{OH2+ <=> OH}}}
\put(77,10){O\textsuperscript{-}}
\end{overpic}
\end{figure}

\end{document}


I have enclosed the \ce{} command from mhchem in a parbox, so it is displayed correctly using overpic. The result is shown in the following image:

Please note that I have deleted packages and commands that are not nessesary for the MWE. Additionally, I have replaced the deprecated class options liststotoc and bibtotoc by their up to date replacements listof=totoc and bibliography=totoc respectively.