Weak Acid

breed [waters water]            ;; water molecules
breed [hydroniums hydronium]    ;; hydronium ions (green)
breed [hydroxides hydroxide]    ;; base molecules (red)
breed [acids acid]              ;; parent acid molecules (yellow)
breed [conj-bases conj-base]    ;; conjugate base molecules of parent acid (orange)


globals [
  pH
  Ka             ;; variable for acid reactions
  base-added     ;; use to keep track of how much total base has been added
]

to setup
  clear-all
  set-default-shape waters "molecule2"
  set-default-shape hydroniums "molecule3"
  set-default-shape hydroxides "molecule1"
  set-default-shape acids "molecule2"
  set-default-shape conj-bases "molecule1"
  set Ka 1.9
  create-acids vol-acid                 ;; adds parent acid molecules
    [ set color yellow ]
  ask acids
    [ if random-float 100.0 < Ka              ;; creates initial dissociated acid
        [ hatch-hydroniums 1
            [ set color green
              fd 3 ]
         set breed conj-bases                 ;; parent turtle turns into a conj-base
         set color orange ] ]
  create-waters (100 - count hydroniums)
    [ set color blue ]
  ask turtles                   ;; randomize position and heading of turtles
    [ setxy random-xcor random-ycor ]
  set base-added 0
  calculate-pH
  reset-ticks
  plot-pH
end 

to go
  ask hydroxides [ react-hydroxide-hydronium ]
  ask hydroxides [ react-hydroxide-acid ]
  ask acids [ react-acid ]
  ask hydroniums [ react-hydronium ]
  ;; move turtles randomly around the world
  ask turtles
    [ fd 1
      rt random 10
      lt random 10 ]
  tick
  calculate-pH
  plot-pH
end 

;; adds base molecules to the solution

to add-base
  create-hydroxides vol-base
    [ fd 1
      set color red ]
  set base-added base-added + vol-base
end 

;; hydroxide procedure

to react-hydroxide-hydronium
  let partner one-of hydroniums-here     ;; tells hydroxide to recognize hydronium
  if partner != nobody              ;; if you are a hydroxide molecule and you encounter a hydronium
    [ ;; then turn yourself and the hydronium into water molecules
      set breed waters
      set color blue
      ask partner
        [ set breed waters
          set color blue ] ]
end 

;; hydroxide procedure

to react-hydroxide-acid
  if random-float 100.0 < Ka                  ;; react with acid molecules according to the acid strength
    [ let partner one-of acids-here
      if partner != nobody              ;; if you are a hydroxide molecule and you encounter an acid molecule
        ;; then turn the hydroxide into a conjugate base molecule
        ;; and turn yourself into a water molecule
        [ set breed waters
          set color blue
          ask partner
            [ set breed conj-bases
              set color orange ] ] ]
end 

;; acid procedure

to react-acid
  if random-float 100.0 < Ka                  ;; acid molecules may dissociate to hydronium depending on their Ka
    [ let partner one-of waters-here
      if partner != nobody              ;; if you are an acid molecule and you encounter a water molecule
        ;; then turn the water molecule into a hydronium molecule
        ;; and turn yourself into a conjugate base molecule
        [ set breed conj-bases
          set color orange
          ask partner
            [ set breed hydroniums
              set color green ] ] ]
end 

;; hydronium procedure

to react-hydronium
  if random-float 100.0 < Ka                  ;; hydronium molecules may recombine with conjugate base to form acid
    [ let partner one-of conj-bases-here
      if partner != nobody              ;; if you are a hydronium molecule and you encounter a conjugate base molecule
      ;; then turn the conjugate base molecule into an acid molecule
      ;; and turn yourself into a water molecule
        [ set breed waters
          set color blue
          ask partner
            [ set breed acids
              set color yellow ] ] ]
end 

;; calculates the pH from the amount of the various ions in solution;
;; note that for simplicity the calculations don't take the true molar
;; concentration of water into account, but instead use an arbitrarily
;; chosen factor of 1000 to produce numbers lying in a reasonable range

to calculate-pH
  let volume count turtles
  let concH (count hydroniums / volume)
  let concOH (count hydroxides / volume)
  ifelse (concH = concOH)
    [ set pH 7 ]
    [ ifelse (concH > concOH)
      [ set pH (- log (concH / 1000) 10) ]
      [ let pOH (- log (concOH / 1000) 10)
        set pH 14 - pOH ] ]
end 

;; plotting procedures

to plot-pH
  set-current-plot "pH Curve"
  plot pH
end 

to record-pH
  set-current-plot "Titration Curve"
  plotxy base-added pH
end 


; Copyright 2001 Uri Wilensky.
; See Info tab for full copyright and license.