How can I make an \underbrace without a brace?

Question

I'm trying to write the line diagram of the glass electrodo to measure pH. I've got a working code:

\documentclass{article}
\usepackage{xcolor}
\usepackage{amsmath}
\usepackage[version=4]{mhchem}
\begin{document}
\begin{equation*}
\overbrace{\underbrace{\ce{Ag (\textit{s}) | AgCl (\textit{s}) | KCl  (\textit{aq, sat}) }}_{E_{ref1}} \underbrace{||}_{E_j}}^{\text{Electrodo de referencia 1}} \overbrace{\ce{H^+ (\textit{aq}, a_1)}}^{\text{Analito}} \overbrace{\underbrace{|}_{E_1} \text{Membrana de vidrio} \underbrace{|}_{E_2} \textcolor{blue}{\ce{H^+ (\textit{aq}, 0,1M), }} \underbrace{\textcolor{blue}{\ce{Cl^- (\textit{aq}, 0,1 M)}}\ce{|AgCl (\textit{s})|Ag (\textit{s})}}_{\text{Electrodo de referencia 2, }E_{ref2}}}^{\text{Electrodo de vidrio}}
\end{equation*}
\end{document}

and the result look like this: I need to make it a little more narrow without losing information, and my best option is to save the empty space in the brackets for the interfaces (Ej, E1 and E2).

Is there a way to do that?

Answer 1

To really economize on space, I'd ditch the \underbrace machinery and instead resort to vertical arrows as pointers that direct explanatory stuff towards the items being explained.

\documentclass{article}
\usepackage[margin=2.5cm]{geometry} % set page parameters suitably
\usepackage{xcolor}
\usepackage{mathtools}
\usepackage[version=4]{mhchem}

\begin{document}
\small % so that formula fits within the textblock
\begin{equation*}
\overbrace{%
  \underbrace{\ce{Ag (\textit{s})} \mid 
              \ce{AgCl (\textit{s})} \mid 
              \ce{KCl^{\vphantom{+}}  (\textit{aq, sat})}
             }_{E_{\textrm{ref1}}} 
  %\underbrace{||}_{E_j}
  \underset{\substack{\uparrow\\\mathclap{E_j}}}{\mathrel{\Vert}}
}^{\text{Electrodo de referencia 1}} 
\overbrace{\ce{H^+ (\textit{aq}, a_1)}}^{\text{Analito}} 
\overbrace{%
  %\underbrace{|}_{E_1} 
  \underset{\substack{\uparrow\\\mathclap{E_1}}}{\mid}
  \text{Membrana de vidrio} 
  %\underbrace{|}_{E_2} 
  \underset{\substack{\uparrow\\\mathclap{E_2}}}{\mid}
  \textcolor{blue}{\ce{H^+ (\textit{aq}, 0,1M), }} 
  \underbrace{%
    \textcolor{blue}{\ce{Cl^- (\textit{aq}, 0,1 M)}} \mid 
    \ce{AgCl (\textit{s})}\mid
    \ce{Ag (\textit{s})}
  }_{\text{Electrodo de referencia 2, $E_{\textrm{ref2}}$}}
}^{\text{Electrodo de vidrio}}
\end{equation*}
\end{document}

Answer 2

Try \hspace*{-3cm} just before the \overbrace: ...

\begin{equation*}
\hspace*{-3cm}\overbrace{\underbrace{\ce{Ag (\textit{s}) | AgCl (\textit{s}) | KCl  (\textit{aq, sat}) ...
\end{equation*}

The result is: