pH curve in calcite dissolution and Fe precipitation

[pia said]

Hi i'm modelling calcite dissolution and Fe precipitation. I plotted calcite vs pH and i don't understand the shape of the pH curve (there is a inflexion point at pH = 4 aprox. that i can`t explain). Help me please.

this is my code:

Code: [Select]

SOLUTION 1
    temp      25
    pH        2
    pe        6.33 O2(g) -0.68
    redox     pe
    units     ppm
    density   1
    As(5)     4
    B         30
    Fe(2)     50
    S(6)      173 charge
    -water    1 # kg
END

REACTION 1
    Calcite 1
    0.015 moles in 100 steps #verificar concentracion disuelta de calcita

EQUILIBRIUM_PHASES 1
    Calcite 0 0
    Goethite 0 0
END

USE equilibrium_phases 1
USE reaction 1
USE solution 1
USER_GRAPH 1
    -headings               Reaction pH Fe
    -axis_titles            "CaCO3 added, moles" "pH" "Fe, kg"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x RXN
20 pH = -LA("H+")
30 graph_y pH
40 graph_sy TOT("Fe(3)")
  -end
    -active                 true
END

USE solution 1
USE reaction 1
USE equilibrium_phases 1
USER_GRAPH 2
    -headings               Reaction pH Goethite
    -axis_titles            "CaCO3 added, moles" "pH" "Geothite, mol"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x RXN
20 pH = -LA("H+")
30 graph_y pH
50 graph_sy EQUI("Goethite")
  -end
    -active                 true
END


[dlparkhurst]

It is basic aqueous carbonate chemistry and is the reverse of an alkalinity titration.

In an alkalinity titration, H+ is added to convert HCO3- to CO2:

HCO3- + H+ = CO2(aq) + H2O

In your simulation, you are adding CO3-2, which is converted to CO2(aq) and ultimately HCO3-.

pH is determined by the ratio of CO2(aq)/HCO3- (and HCO3-/CO3-2).

log K = log[CO2(aq)/HCO3-] - log[H+]

pH = log K - log[CO2(aq)/HCO3-]

[pia said]

Thank you !