pH ajustment

[waterterre]

Hi there,

I am a begginer with Phreeqc, so I'm little bit confused with its utilisation.

I need to ajuts a groundwater pH from 8.92 to 7.00 with citric acid, C6H8O7. the citric acid will be pure, in solid form. I want to know what is the amount of acid needed to ajust the pH for 1 lier of groundwater.

Other useful information:
As 0.004 ppm
Bicarbonate (HCO3-) 259 ppm
Hardness 5470 ug/L - CaCO3
B 1.28 ppm
Cl- 14 ppm
F- 1.09 ppm
Pb 0.02 ppm
Na 195 ppm
S- 45.7 ppm
Zn 0.0038 ppm
Mn 0.008 ppm

So far, I have tried to do a EQUILIBRIUM_PHASE with Fix_H+, but I'm not sure of my calculation. The results seems to give me as little as 141mg/L of citric acid... I want to be sure!

Thanks for your help!

[Charlie]

Hi,
If I understand the question properly, I would suggest treating the simulation like a titration reaction, adding moles of the acid and using the selected output to assess the pH of the resulting solution. i.e..

SOLUTION 1
your data here

REACTION 1
C6H8O7 1.0
10 moles in 100 steps  #for example

SELECTED_OUTPUT
-file acid.xls
-pH true
-step true
-total C6H8O7

END

you could also add a USER_PUNCH block to do the conversions of the moles of acid into the mass (or just do in excel..)

Hope that helps.  Please feel free to post your working input script once complete!
Cheers

[dlparkhurst]

Here is my version of the calculation, along the lines you originally outlined. I am using minteq.dat, which has a definition of Citrate. It is appropriate if the Citrate is expected to remain intact during the course of the simulation, i.e., not decompose into other carbon aqueous species. (Adding as C6H8O7 will cause some perhaps unwanted redox reactions.)

I have made a couple of assumptions about the analysis: (1) I added all of the hardness as Ca. Certainly some of the hardness is Mg, but for the purpose of this calculation, it should not have much affect on the result. (2) I assumed that it was a sulfate analysis, but perhaps it was total sulfide. You can make changes if necessary.

I have used USER_PRINT to print the results as moles of Citrate, and mg of CitrateH3 added. Note this calculation uses pure citric acid, it would need some adjustment to consider a solution that is a mixture of citric acid and water, or to consider the volume change associated with the addition of the acid.

----------------------------------User print-----------------------------------

Moles of CitrateH3:       3.2089e-04
mg of CitrateH3 added:    6.1638e+01




SOLUTION 1
    temp      25
    pH        8.92
    pe        4
    redox     pe
    units     ppm
    density   1
    Alkalinity 259 as HCO3
    As        0.004
    B         1.28
    Ca        5470 ppb        as CaCO3  # assumes all hardness is Ca
    Cl        14
    F         1.09
    Mn        0.008
    Na        195
    Pb        0.02
    S(6)     45.7        # SO4 or S-2?, assumes SO4
    Zn        0.0038
    -water    1 # kg
END
PHASES
Fix_pH
    H+ = H+
    log_k     0
END
USER_PRINT
10 PRINT "Moles of CitrateH3:    ", TOT("Citrate")
20 PRINT "mg of CitrateH3 added: ", TOT("Citrate") * GFW("CitrateH3") * 1000.0
USE solution 1
EQUILIBRIUM_PHASES 1
    Fix_pH    -7 CitrateH3 10
END

[waterterre]

Thanks for your help, it worked perfectly!

[brunagr]

Hello there,

I am trying to model a solution with 10mM of citric acid in water, and then another two solutions with 10mM of citric acid and 50mM of sodium citrate and 10mM of citric acid and 100mM of sodium citrate.
For citric acid I am using the definition below, from minteq.dat:

SOLUTION_MASTER_SPECIES

 Citrate Citrate-3 2 189.06 189.06

SOLUTION_SPECIES
Citrate-3 = Citrate-3
        log_k   0
        delta_h 0       kcal

3H+ + Citrate-3 = CitrateH3
        log_k   14.18
        delta_h 0       kcal

Solution 1
pH 2.5
        pH      2.5
        pe      4.0               
        units   mol/kgw
        Citrate 0.01

I did not find any definition that could be used for sodium citrate though, if anyone knows or has an idea and could help me, I would really appreciate! Thank you very much!

[dlparkhurst]

I think you also need definitions of CitrateH-2 and CitrateH2-. The database minteq.v4.dat includes Citrate definitions, including Na-Citrate complexes (although I do not think they are important).

[brunagr]

Thank you very much Mr. Parkhurst, the addition of those definitions were very helpful, it is looking a lot more reasonable now. Thank you!