Titrating with H2SO4 - figuring amount added

[JVillinski]

I have a problem where groundwater is pumped, and we want to keep iron in solution after it equilibrates with oxygen. I found an older topic where David provided a way to titrate in H2SO4. So I added that to my input file, and was able to titrate any number of step I wanted prior to O2 equilibration. So then I chose an amount of H2SO4 that would get me to about pH 3 or a bit below, and then equilibrated with O2 (and CO2) and checked ferrihydrite SI. Once I had that I decided to try and check the output to make sure I was putting in the amount that I thought I was.

I have included the first 3 simulations through the acidification step below.

Code: [Select]

SOLUTION 1 LPC Lagoon 1
    temp      20
    pH        6.1
    pe        4
    redox     pe
    units     mg/l
    density   1
    Al        0.035
    Alkalinity 94.8 gfw 50.4
    Be        0.00067
    Ca        161
    Cd        0.00059
    Cl        32.4
    Cr        0.00025
    Cu        0.0164
    Fe        6
    K         4.05
    Mg        36.2
    Mn        6.11
    Na        45
    Ni        0.0859
    S(6)      612 gfw 96.06 charge
    Zn        0.136
    -water    1 # kg
PRINT
    -alkalinity            true
    -warnings              -1
SELECTED_OUTPUT 1 Lagoon 1 Water Chemistry
    -file                 Lagoon 1 0.1 mmol 36 Fe3.out
    -simulation           true
    -solution             true
    -step                 true
    -pH                   true
    -pe                   true
    -reaction             true
    -alkalinity           true
    -ionic_strength       true
    -water                true
    -charge_balance       true
    -percent_error        true
    -totals               Al  Alkalinity  Be  Ca  Cd  Cl  Cr
                          Cu  Fe  Fe(2)  Fe(3)  Hfo_s  Hfo_w  K
                          Mg  Mn  Na  Ni  S(6)  Zn
END
TITLE Add 0.1 mmol H2SO4
REACTION 1 Acidify with 0.1 mmol H2SO4
# Pure H2SO4, 18.65 mol/L = 1.83 kg/L / (98.08 g/mol * 0.001 kg/g)
    H2SO4      18.65
    0.1 millimoles in 1 steps


END
TITLE This simulation runs the reaction
INCREMENTAL_REACTIONS True
RUN_CELLS
    -cells 1
SAVE solution 2
SELECTED_OUTPUT 2 Titrated with 0.1 mmol H2SO4 in 1 step
    -file                 Lagoon 1 titrate 0.1 mmol H2SO4 36 Fe3.out
    -reset                true
    -distance             false
    -time                 false
    -totals               Al  Alkalinity  Be  Ca  Cd  Cl  Cr
                          Cu  Fe  Fe(2)  Fe(3)  Hfo_s  Hfo_w  K
                          Mg  Mn  Na  Ni  S(6)  Zn
END

The portion of the input that I got from David's post was the Reaction 1 and defined H2SO4 as 18.65 moles per liter of water, and the Incremental_Reaction and Run_Cells commands.

I thought I told the program to add 1 mmol of H2SO4 in one step. However, when I run the file, I have S(6) increase from 5.37E-03 moles to 7.23E-03 moles, which is 1.864E-03 moles. What am I missing here.

Thank you,
John

[dlparkhurst]

This REACTION adds 0.1*18.65 moles of H2SO4.

Code: [Select]

REACTION 1 Acidify with 0.1 mmol H2SO4
# Pure H2SO4, 18.65 mol/L = 1.83 kg/L / (98.08 g/mol * 0.001 kg/g)
    H2SO4      18.65
    0.1 millimoles in 1 steps
If you want to add 1 millimole, then

Code: [Select]

REACTION
H2SO4 1
0.1 millimoles


[JVillinski]

Thanks. I figured it had to be something like that, but I guess I was uncertain about the logic. And I meant I thought I asked it to add 0.1 mmol, not 1 mmol, so that was a typo.

[dlparkhurst]

Sorry, mine was a typo too. Should have been "1 millimole" to add 1 millimole. What I wrote would only have added 0.1 millimoles.

[JVillinski]

Understood. Now that I see what I was missing, it should be easy going now! Thanks again!