# Dissolved Inorganic Carbon in a closed system

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## WHAT IS IT?

Carbonic acid (H2CO3), Bicarbonate ion (HCO3-), and carbonate ion (CO3 (2-))are three species of dissolved inorganic carbon (DIC) that coexist in water. In a closed system, assuming that no carbon leaves or enters the system, the proportions of these species are determined by many factors. This model presents the proportions of three DIC species at different pH levels, ranging from 0 to 14, holding other factors constant. This model may serve as the foundation for students to explore the phenomenon of ocean acidification.

## HOW IT WORKS

In the model, a total DIC concentration of 0.002 (mol kg-1) is used. The values of two equilibrium constants, Ka1 = 10 ^ (-5.903) and Ka2 = 10 ^ (-9.702), obtained from Millero et al. (2006), are used to calculate the concentrations of three species in a closed system where salinity is 3.3758% and temperature is 20 degree celsius.

The following equations are used in calculations:

1) [H2CO3]+[HCO3-]+[CO3--]= Total Carbon (TC) 2) [HCO3-][H+]/[H2CO3]=Ka1 3) [CO3--][H+]/[HCO3]=Ka2

Equations 2-3 can be rearranged: 4) [H2CO3]=[HCO3-][H+]/Ka1 5) [CO3--]=Ka2*[HCO3]/[H+]

Equations 4-5 can be substituted in equation 1 to solve [HCO3-]:

6) [HCO3-] = (TC * Ka1 * [H+])/([H+]^2 + Ka1 * [H+] + Ka1 * Ka2)

Then [HCO3-] can be substituted in equations 4-5 to solve corresponding [H2CO3] and [CO3--] at the same pH level.

The percentage of each species is calculated using equation 7:

7) Percentage of [species] = [species] * 100 / TC

## HOW TO USE IT

Choose a visual style. The icon visual is more visually friendly, and the molecule visual provides more authentic molecular properties of the species.

Click on the

**set up/restart**button the confirm the setting.You may

**run**the model and then drag the pH slider to observe how the proportions of three species vary at the pH ranging from 0 to 14.You may also let the model

** auto plot** the changes for you.

## THINGS TO TRY AND NOTICE

The main observation is to see how the concentration of three species changes at different ph levels by dragging the pH slider or using the auto plot button. The teacher may also guide students to see that the current ph change in the ocean concerns a small range (8-8.2) in the spectrum.

Use switches to hide some ions to focus your observation. For example, only show H2CO3 and hide the rest ions.

## RELATED MODELS

See more chemistry models at https://sites.google.com/view/3d-science-abm/physics-chemistry

## CREDITS AND REFERENCES

Dr. Lin Xiang created this module at the University of Kentucky in 2022. If you mention this model in a publication, we ask that you include the citations below.

Xiang, L. (2022). Dissolved Inorganic Carbon in a closed system. Department of STEM Education, University of Kentucky, Lexington, KY.

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/4.0/.

References: Millero, F. J.; Graham, T. B.; Huang, F.; Bustos-Serrano, H.; Pierrot, D. Dissociation constants of carbonic acid as a function of salinity and temperature. (2006). Marine Chemistry, 100 (1-2), 80-94.

This model is inspired by the KCVS applets from the 2100 C.E. RCP projections. https://applets.kcvs.ca/OceanAcidification/oceanAcid.html?rcp=2.6

## Comments and Questions

;Dr. Lin Xiang created this module at the University of Kentucky in 2022. If you mention this model in a publication, we ask that you include the citations below. ; ;Xiang, L. (2022). Dissolved Inorganic Carbon in a closed system. Department of STEM Education, University of Kentucky, Lexington, KY. ; ;![CC BY-NC-SA 4.0](http://ccl.northwestern.edu/images/creativecommons/byncsa.png) ; ;This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/4.0/. breed [H2CO3s H2CO3] breed [HCO3s HCO3] breed [CO3s CO3] breed [H3OS H30] breed [OHs OH] breed [legends legend] turtles-own[ ] globals [ Num-H Num-OH Num-H2CO3 Num-HCO3 Num-CO3 current-ph current-pOH current-conc-H current-conc-OH current-conc-H2CO3s current-conc-HCO3s current-conc-CO3s Ka1 Ka2 TC denom num ] to setup ca set-bk set-legends set-shapes set-tt plot-ion-concentrations-log reset-ticks end to set-bk ask patches [set pcolor 96.5] ask patches with [ pycor = max-pycor or pycor < 6] [set pcolor 101] ask patches with [ pxcor = min-pxcor or pxcor = max-pxcor] [set pcolor 101] end to set-tt ;set constants set TC 0.002 set Ka1 10 ^ (-5.903) ;from Millero et al. (2006) set Ka2 10 ^ (-9.702) ;;from Millero et al. (2006) ;set initial carbon species set ph 0 set-carbon-concentrations ; set particles set num-H2CO3 round (5000 * 3 ^ (log current-conc-H2CO3s 10)) set num-HCO3 round (5000 * 3 ^ (log current-conc-HCO3s 10)) set num-CO3 round (5000 * 3 ^ (log current-conc-CO3s 10)) create-H2CO3s Num-H2CO3 [set-tt-properties-aq] create-HCO3s Num-HCO3 [set-tt-properties-aq] create-CO3s Num-CO3 [set-tt-properties-aq] set num-H round (500 * 1.3895 ^ (-1 * current-ph)) set num-OH round (500 * 1.3895 ^ (-1 * (14 - current-ph))) create-h3os num-H [set-tt-properties-aq ] create-ohs num-OH [set-tt-properties-aq ] ;hiding ions if Hide-H2CO3? = true [if any? H2CO3s [ask H2CO3s [die]]] if hide-HCO3? = true [if any? HCO3s [ask HCO3s [die]]] if hide-CO3? = true [if any? CO3s [ask CO3s [die]]] if Hide-H3Os-and-OHs? = true [if any? H3Os [ask H3Os [die]] if any? OHs [ask OHs [die]]] end to set-carbon-concentrations set current-ph pH set current-conc-H 10 ^ (- current-ph) set denom (current-conc-H * current-conc-H + Ka1 * Ka2 + current-conc-H * Ka1) set current-conc-H2CO3s (current-conc-H * current-conc-H * TC) / denom set current-conc-HCO3s (Ka1 * current-conc-H * TC) / denom set current-conc-CO3s (Ka1 * Ka2 * TC) / denom end to set-tt-properties-aq setxy 1 + random-float (max-pxcor - 2) 6 + random-float (max-pycor - 7) (ifelse choose-a-visual = "Molecule visual" [set size 1.25] choose-a-visual = "Icon visual" [set size 1.75]) end to set-legends create-legends 10 [set label-color white] ask legend 0[setxy 2 3.5] ask legend 1 [set shape "blank" set size 1 set label "Hydroxide(OH-)" setxy 10 3.9] ask legend 2 [setxy 2 1] ask legend 3 [set shape "blank" set size 1 set label "Hydronium (H3O+)" setxy 11 1.3] ask legend 4[setxy 28 3.5] ask legend 5 [set shape "blank" set size 1.5 set label "Carbonate (CO3--)" setxy 37.4 3.9] ask legend 6 [setxy 14 3.5] ask legend 7 [set shape "blank" set size 1.5 set label "Carbonic acid (H2CO3)" setxy 25 3.9] ask legend 8 [setxy 14 1] ask legend 9 [set shape "blank" set size 1.5 set label "Bicarbonate (HCO3-)" setxy 24 1.3] end to go set-shapes-1 aq-move set-carbon-concentrations display-particles plot-ion-concentrations-log tick end to display-particles set num-H2CO3 round (5000 * 3 ^ (log current-conc-H2CO3s 10)) set num-HCO3 round (5000 * 3 ^ (log current-conc-HCO3s 10)) set num-CO3 round (5000 * 3 ^ (log current-conc-CO3s 10)) ifelse Hide-H2CO3? = false [ let current-H2CO3 count H2CO3s (ifelse current-H2CO3 > num-H2CO3 [ask up-to-n-of (current-H2CO3 - num-H2CO3) H2CO3s [die]] current-H2CO3 < num-H2CO3 [create-H2CO3s (num-H2CO3 - current-H2CO3) [set-tt-properties-aq]]) ] [if any? H2CO3s [ask H2CO3s [die]]] ifelse hide-HCO3? = false [let current-HCO3 count HCO3s (ifelse current-HCO3 > num-HCO3 [ask up-to-n-of (current-HCO3 - num-HCO3) HCO3s [die]] current-HCO3 < num-HCO3 [create-HCO3s (num-HCO3 - current-HCO3) [set-tt-properties-aq]])] [if any? HCO3s [ask HCO3s [die]]] ifelse hide-CO3? = false [let current-CO3 count CO3s (ifelse current-CO3 > num-CO3 [ask up-to-n-of (current-CO3 - num-CO3) CO3s [die]] current-CO3 < num-CO3 [create-CO3s (num-CO3 - current-CO3) [ set-tt-properties-aq]])] [if any? CO3s [ask CO3s [die]]] ifelse Hide-H3Os-and-OHs? = false [ H-OH-equilibrium] [if any? H3Os [ask H3Os [die]] if any? OHs [ask OHs [die]]] end to H-OH-equilibrium set num-H round (500 * 1.3895 ^ (-1 * current-ph)) set num-OH round (500 * 1.3895 ^ (-1 * (14 - current-ph))) let current-H count H3Os let current-OH count OHs (ifelse num-H > current-H [create-h3os abs (num-H - current-H) [set-tt-properties-aq ]] num-H < current-H [ask n-of abs (num-H - current-H) h3os [die]]) (ifelse num-OH > current-OH [create-ohs abs (num-OH - current-OH) [set-tt-properties-aq ]] num-OH < current-OH [ask n-of abs (num-OH - current-OH) OHs [die]]) end to aq-move ask turtles with [ breed != legends] ;ask aq turtles [(ifelse [pcolor] of patch-at dx dy = 101 or [pcolor] of patch-at dx dy = 89 [rt random 360] ;if it's boundary ahead, rotate [pcolor] of patch-at dx dy = 96.5 [fd 0.2 rt random 360 if ycor < 6 [set ycor 6]] ;if the patch-ahead has the proper color, move forward.If ycor is low, put ycor higher )] end to plot-ion-concentrations-log ;; this creates the bar graph set-current-plot "Relative Concentrations" clear-plot set-plot-x-range 0 2 set-plot-y-range 0 300 plot-pen-down set-current-plot-pen "H2CO3" plotxy 0 count H2CO3s set-current-plot-pen "HCO3-" plotxy 1 count HCO3s set-current-plot-pen "CO3(2-)" plotxy 2 count CO3s end to set-shapes (ifelse choose-a-visual = "Molecule visual" [set-default-shape ohs "ab-oh-1" set-default-shape H3Os "ab-h3o-1" set-default-shape H2CO3s "ab-H2CO3" set-default-shape HCO3s "ab-HCO3" set-default-shape CO3s "ab-CO3" ask legend 0 [set shape "ab-oh-1" set size 1.75] ask legend 2 [set shape "ab-h3o-1" set size 1.75] ask legend 6 [set shape "ab-H2CO3" set size 1.75] ask legend 8 [set shape "ab-HCO3" set size 1.75] ask legend 4 [set shape "ab-CO3" set size 1.75] ] choose-a-visual = "Icon visual" [set-default-shape ohs "ab-oh-2" set-default-shape H3Os "ab-h3o-2" set-default-shape H2CO3s "ab-H2CO3-1" set-default-shape HCO3s "ab-HCO3-1" set-default-shape CO3s "ab-CO3-1" ask ohs [set shape "ab-oh-2" set size 1.25] ask H3Os [set shape "ab-h3o-2" set size 1.25] ask legend 0 [set shape "ab-oh-2" set size 1.75] ask legend 2 [set shape "ab-h3o-2" set size 1.75] ask legend 6 [set shape "ab-H2CO3-1" set size 1.75] ask legend 8 [set shape "ab-HCO3-1" set size 1.75] ask legend 4 [set shape "ab-CO3-1" set size 1.75] ]) end to set-shapes-1 (ifelse choose-a-visual = "Molecule visual" [set-default-shape ohs "ab-oh-1" set-default-shape H3Os "ab-h3o-1" set-default-shape H2CO3s "ab-H2CO3" set-default-shape HCO3s "ab-HCO3" set-default-shape CO3s "ab-CO3" ask ohs [set shape "ab-oh-1" set size 1.25] ask H3Os [set shape "ab-h3o-1" set size 1.25] ask H2CO3s [set shape "ab-H2CO3" set size 1.25] ask HCO3s [set shape "ab-HCO3" set size 1.25] ask CO3s [set shape "ab-CO3" set size 1.25] ask legend 0 [set shape "ab-oh-1" set size 1.75] ask legend 2 [set shape "ab-h3o-1" set size 1.75] ask legend 6 [set shape "ab-H2CO3" set size 1.75] ask legend 8 [set shape "ab-HCO3" set size 1.75] ask legend 4 [set shape "ab-CO3" set size 1.75] ] choose-a-visual = "Icon visual" [set-default-shape ohs "ab-oh-2" set-default-shape H3Os "ab-h3o-2" set-default-shape H2CO3s "ab-H2CO3-1" set-default-shape HCO3s "ab-HCO3-1" set-default-shape CO3s "ab-CO3-1" ask ohs [set shape "ab-oh-2" set size 1.75] ask H3Os [set shape "ab-h3o-2" set size 1.75] ask H2CO3s [set shape "ab-H2CO3-1" set size 1.75] ask HCO3s [set shape "ab-HCO3-1" set size 1.75] ask CO3s [set shape "ab-CO3-1" set size 1.75] ask legend 0 [set shape "ab-oh-2" set size 1.75] ask legend 2 [set shape "ab-h3o-2" set size 1.75] ask legend 6 [set shape "ab-H2CO3-1" set size 1.75] ask legend 8 [set shape "ab-HCO3-1" set size 1.75] ask legend 4 [set shape "ab-CO3-1" set size 1.75] ]) end

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