# PhreeqcUsers Discussion Forum

## Processes => Dissolution and precipitation => Topic started by: pia said on August 12, 2019, 07:31:08 PM

Title: pH curve in calcite dissolution and Fe precipitation
Post by: pia said on August 12, 2019, 07:31:08 PM
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 # kgENDREACTION 1    Calcite 1    0.015 moles in 100 steps #verificar concentracion disuelta de calcitaEQUILIBRIUM_PHASES 1    Calcite 0 0    Goethite 0 0ENDUSE equilibrium_phases 1USE reaction 1USE solution 1USER_GRAPH 1    -headings               Reaction pH Fe    -axis_titles            "CaCO3 added, moles" "pH" "Fe, kg"    -initial_solutions      false    -connect_simulations    true    -plot_concentration_vs  x  -start10 graph_x RXN20 pH = -LA("H+")30 graph_y pH40 graph_sy TOT("Fe(3)")  -end    -active                 trueENDUSE solution 1USE reaction 1USE equilibrium_phases 1USER_GRAPH 2    -headings               Reaction pH Goethite    -axis_titles            "CaCO3 added, moles" "pH" "Geothite, mol"    -initial_solutions      false    -connect_simulations    true    -plot_concentration_vs  x  -start10 graph_x RXN20 pH = -LA("H+")30 graph_y pH50 graph_sy EQUI("Goethite")  -end    -active                 trueEND`
Title: Re: pH curve in calcite dissolution and Fe precipitation
Post by: dlparkhurst on August 12, 2019, 09:14:01 PM
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-]
Title: Re: pH curve in calcite dissolution and Fe precipitation
Post by: pia said on August 12, 2019, 10:05:32 PM
Thank you !