Question about Fix_pH

[Zhaoyang]

Dear all,

I have a question about Fix_pH in the attached example. Why can't I use NaOH to increase pH in here? Is this because Ferric will react with OH-? Any help will be appreciated.

Best regards,
Zhaoyang

Code: [Select]

DATABASE D:\Program Files\USGS\Phreeqc Interactive 3.6.2-15100\database\phreeqc.dat
SELECTED_OUTPUT
-high_precision true

SOLUTION_MASTER_SPECIES
Ferric  Ferric+3 0  Ferric   55.847
Ferrous  Ferrous+2  0  Ferrous    55.847

SOLUTION_SPECIES
Ferrous+2   = Ferrous+2 
    log_k     0
Ferric+3 = Ferric+3
    log_k     0

PHASES
   Fix_H+
   H+ = H+
   log_k 0

SOLUTION 1
units mg/l
Ferrous  10
pH  5
Cl  1  charge


EQUILIBRIUM_PHASES 1
Fix_H+   -7 HCl    10 # change pH
O2(g)      -0.679  10 # = log(20.946/100)


   
KINETICS
Iron_remove
-steps 1800 in 100 steps   # 900s = 15 min  # 1800 = 30 min
-formula Ferrous -1 O2 -0.25 H2O -2.5 Ferric 1 OH 3 H 2
-m0 100

RATES
Iron_remove
-start
10 k_plus1 = 10^-3.2/(60*60*24)  # Converted to seconds
20 k_plus2 = 1.2e-11/(60*60*24)  # Converted to seconds

30 Lang1 = k_plus1 * tot("Ferrous")               *  10^SI("O2(g)") #
40 Lang2 = k_plus2 * tot("Ferrous") / mol("H+")^2 *  10^SI("O2(g)") #

50 IF -la("H+") < 4 THEN Lang = Lang1 ELSE Lang = Lang2
90 moles = Lang * TIME
100 save moles
-end

USER_GRAPH 1
    -headings               time Ferric_(Fe3+) Ferrous_Fe(2+) Amount_of_acid_consumed
    -axis_titles            "Time (min)" "Ferric/Ferrous concentration (mg/L)" "Acid/base consumed (micromol)"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x total_time/60
20 graph_y TOT("Ferric")*55.847*1000, TOT("Ferrous")*55.847*1000
30 graph_sy (10-equi("Fix_H+"))*10^6
  -end

END

[dlparkhurst]

Short answer is that you need to add acid, not base.

With your simplified Ferric/Ferrous definitions, as Ferrous iron is converted to Ferric iron, the reaction is

Ferrous+2 + 1/4O2 + 1/2H2O = Ferric+3 + OH-

Initially, a little base (removal of HCl) is needed to increase the pH from 5 to 7, but ultimately, HCl is added to neutralize the base generated from the oxidation reaction. (Note that the -formula Ferrous -1 Ferric +1 is more appropriate. The H and O of your -formula cancel.)

Formation of Ferric(OH)n aqueous species or Ferric(OH)3 solid, changes the results drastically, in which case, base would be needed to maintain a pH of 7.

Code: [Select]

SELECTED_OUTPUT
-high_precision true

SOLUTION_MASTER_SPECIES
Ferric  Ferric+3 0  Ferric   55.847
Ferrous  Ferrous+2  0  Ferrous    55.847

SOLUTION_SPECIES
Ferrous+2   = Ferrous+2
    log_k     0
Ferric+3 = Ferric+3
    log_k     0

PHASES
   Fix_H+
   H+ = H+
   log_k 0

SOLUTION 1
units mg/l
Ferrous  10
pH  5
Cl  1  charge

EQUILIBRIUM_PHASES 1
Fix_H+   -7 HCl    10 # change pH
O2(g)      -0.679  10 # = log(20.946/100)
 
KINETICS
Iron_remove
-steps 180.0 in 100 steps   # 900s = 15 min  # 1800 = 30 min
#-formula Ferrous -1 O2 -0.25 H2O -2.5 Ferric 1 OH 3 H 2
-formula Ferrous -1 Ferric 1
-m0 100

RATES
Iron_remove
-start
10 k_plus1 = 10^-3.2/(60*60*24)  # Converted to seconds
20 k_plus2 = 1.2e-11/(60*60*24)  # Converted to seconds

30 Lang1 = k_plus1 * tot("Ferrous")               *  10^SI("O2(g)") #
40 Lang2 = k_plus2 * tot("Ferrous") / mol("H+")^2 *  10^SI("O2(g)") #

50 IF -la("H+") < 4 THEN Lang = Lang1 ELSE Lang = Lang2
90 moles = Lang * TIME
100 save moles
-end

USER_GRAPH 1
    -headings               time Ferric_Fe(3+) Ferrous_Fe(2+) Cl- HCl_added
    -axis_titles            "Time (min)" "mol/kg water" "HCl added, moles"
    -initial_solutions      true
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x total_time/60
20 graph_y TOT("Ferric"), TOT("Ferrous"), TOT("Cl")
30 graph_sy -EQUI_DELTA("Fix_H+")
  -end

END

[Zhaoyang]

Thanks for your reply. I have another two questions.

One is for the EQUILIBRIUM_PHASES.

Code: [Select]

EQUILIBRIUM_PHASES 1
Fix_H+   -7 HCl    10 # change pH
O2(g)      -0.679  10 # = log(20.946/100)Why can't I seperate them like below?

Code: [Select]

EQUILIBRIUM_PHASES 1
Fix_H+   -7 HCl    10 # change pH

Code: [Select]

EQUILIBRIUM_PHASES 2
O2(g)      -0.679  10 # = log(20.946/100)
Another one is about the EQUI_DELTA. I have checked PHREEQC Help. Why can't I get the same plot if replacing graph_sy 10-EQUI("Fix_H+") with graph_sy -EQUI_DELTA("Fix_H+")?

Thanks in advance

Best regards,
Zhaoyang

[dlparkhurst]

Only one EQUILIBRIUM_PHASE is included in a reaction calculation; I think the first encountered between END and END.

EQUI_DELTA("Fix_H+") and [10 - EQUI("Fix_H+")] are the same (provided the number of moles of Fix_H+ is set to 10 initially) and EQUILIBRIUM_PHASES is present in the calculation; however, the plot includes the initial solution, where EQUILIBRIUM_PHASES is not defined, so 10 - EQUI("Fix_H+") = 10 - 0 = 10 for the initial solution point.