Draw diagrams by changing a parameter

A follow-up answer to the request for varying color.

This requires a slightly different approach, as it's well known, that \foreach doesn't allways work in pgfplots as we are used to. Basically proposals are:

• use \pgfplotsinvokeforeach or \pgfplotsforeachungrouped, see here, there or here
• use a \edef\temp{...}\temp approach, e.g. here

I go with the latter, reworking the other solution. The first part \edef\temp{...} defines and expands a command, while the second \temp just calls it.

In the code I once demonstrated the changed call, together with the color from the loop ... color=\xx, ...:

    % ~~~ recommended code ~~~~~~~~~~~~~~~~~~~
    %frontiere horiz Fe/FeII
    \edef\temp{
        \noexpand\addplot expression[domain=0:\phPrecipitationII, color=\xx, solid, very thin]{\potFeIIFe};
        }
    \temp

and for the rest used refactoring, i.e. introduced this new command:

...
% ~~~ from refactoring ~~~~~~~
\newcommand\loopclr[1]{\edef\temp{\noexpand#1}\temp }
...
    % ~~~ using refactoring, i.e. the new command ~~~~~~~~~~~~~~~~~~
    %frontiere horiz FeII/FeIII     
    \loopclr{\addplot expression[domain=0:\phPrecipitationIII, color=\xx, solid, very thin]{\potFeIIIFeII};}

Though it works, I'm pretty sure it's not an optimal approach.

\documentclass[border=0.2cm,11pt]{standalone}%conversion en png avec ghostscript

%COMPILé AVEC XeLaTeX

\usepackage[T1]{fontenc}
\usepackage[dvipsnames,svgnames]{xcolor}
\usepackage{lmodern}
\mathcode`\.="013B%virgule décimale en mode math
%\usepackage{tikz}% chargé automatiquement par pgfplots
\usepackage{pgfplots}\pgfplotsset{compat=newest}
\pgfplotsset{/pgf/number format/.cd,1000 sep={~},use comma}
\usepackage{siunitx}% unités et écriture de valeurs
\usepackage[modules={all}]{chemmacros}%le module orbital a été commenté (lignes 3816 à 3824
\usetikzlibrary{spy}

% ~~~ from refactoring ~~~~~~~
\newcommand\loopclr[1]{\edef\temp{\noexpand#1}\temp }

% ~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{document}

\footnotesize
 \begin{tikzpicture}

    \begin{axis}[% mise en forme du graphique :
    xmin=0,xmax=14,
    ymin=-1,ymax=2,
    xlabel=pH,
    ylabel={$E (\unit{\V})$},
    ytick distance=0.5,
    minor y tick num=4,
    minor x tick num=4,
    grid style={black!10},
    grid=both,% grilles primaire et secondaire
    axis line shift=1pt,
    xtick align=outside,
    ytick align=outside,
    tickpos=left,
    legend pos=north west,% place de la légende,
    axis x line=bottom,
    axis y line = left,
    samples = 200,
    no markers,
    ]
    

\foreach \c / \xx in {0.1/red, 0.01/blue, 0.001/cyan}{  % <<<

\def\concentration{\c}      % <<<
\def\pKe{14}
\def\pKsII{15.1}
\def\pKsIII{38}
\def\potstandFeIIIFeII{0.77}
\def\potstandFeIIFe{-0.44}

\pgfmathsetmacro{\phPrecipitationII}{\pKe-0.5*\pKsII-0.5*log10(\concentration)}
\pgfmathsetmacro{\phPrecipitationIII}{\pKe-1/3*\pKsIII-1/3*log10(\concentration)}


\pgfmathsetmacro{\potFeIIFe}{\potstandFeIIFe +0.03*log10(\concentration)}
\pgfmathsetmacro{\potFeIIIFeII}{\potstandFeIIIFeII}
\pgfmathsetmacro{\potstandFeOHIIIFeII}{\potstandFeIIIFeII+0.18*\phPrecipitationIII+0.06*log10(\concentration)}
\pgfmathsetmacro{\potstandFeOHIIIFeOHII}{\potstandFeOHIIIFeII-0.18*\phPrecipitationII-0.06*log10(\concentration)+0.06*\phPrecipitationII}
\pgfmathsetmacro{\potFeOHIIhaut}{\potstandFeOHIIIFeOHII-0.06*\phPrecipitationII}
\pgfmathsetmacro{\potstandFeOHIIFe}{\potFeIIFe+0.06*\phPrecipitationII} 

    % ~~~ recommended code ~~~~~~~~~~~~~~~~~~~
    %frontiere horiz Fe/FeII
    \edef\temp{
        \noexpand\addplot expression[domain=0:\phPrecipitationII, color=\xx, solid, very thin]{\potFeIIFe};
        }
    \temp
    
    % ~~~ using refactoring, i.e. the new command ~~~~~~~~~~~~~~~~~~
    %frontiere horiz FeII/FeIII     
    \loopclr{\addplot expression[domain=0:\phPrecipitationIII, color=\xx, solid, very thin]{\potFeIIIFeII};}
      
    
    %frontiere verticale FeII
    \loopclr{\addplot [color=\xx, very thin]coordinates{(\phPrecipitationII,\potFeIIFe) (\phPrecipitationII,\potFeOHIIhaut)};} 


    %frontiere vericale FeIII
    \loopclr{\addplot [color=\xx, very thin]coordinates{(\phPrecipitationIII,\potFeIIIFeII) (\phPrecipitationIII,2)}; }


    %frontiere oblique Fe(OH)3 et Fe2+
    \loopclr{\addplot expression[domain=\phPrecipitationII:\phPrecipitationIII, color=\xx, solid, very thin]{\potstandFeOHIIIFeII-0.18*x-0.06*log10(\concentration)};}


    %frontiere oblique Fe(OH)3 et Fe(OH)2
    \loopclr{\addplot expression[domain=\phPrecipitationII:14, color=\xx, solid, very thin]{\potstandFeOHIIIFeOHII-0.06*x};}

    %frontiere oblique Fe(OH)2 et Fe
    \loopclr{\addplot expression[domain=\phPrecipitationII:14, color=\xx, solid, very thin]{\potstandFeOHIIFe-0.06*x};}

        \node at (4,-0.75){\ch{Fe\sld}};
        \node at (4,0){\ch{Fe\pch[2]}};
        \node at (1,1.2){\ch{Fe\pch[3]}};
        \node at (8,0.6){\ch{Fe(OH)3\sld}};
        \node[rotate=-12] at (11,-0.6){\ch{Fe(OH)2\sld}};

%\spy on (6.82,3.65) in node at (8.5,3.8);
 }  % end of foreach    <<<
    \end{axis}
 \end{tikzpicture}
\end{document}