Fixing pH with HNO3 (Nitric Acid)

[awillou4]

Hello,

new to the forum here.  I've been working on a surface complexation model for my Master's Thesis for the past year.

I'm trying to fix pH using HNO3; however I'm unable to get the results described in the subtext of Example 8.  pH isn't effected at all when I use HNO3; however fixing pH with HCl works just fine.  When using HNO3 I receive the warning "Pure phase with add formula has not converged."

Here's a simplified version of my input.  Please let me know if you see where my issue lies.  Thank you!

SURFACE 1
    Hfo_wOH    1.13E-05    600       0.00504


PHASES
NaNO3
NaNO3 = Na+ + NO3-
log_K -20

pe_FIX
    e- = e-
    log_k     0

Fix_H+
    H+ = H+
    log_k     0

SOLUTION 1
    temp      25
    pH        7
    pe        1
    redox     pe
    units     ppm
    density   1
    As(5)     0.365
    Ca        20
    Mg        5
    S(6)      25
    Na        10
    C(4)      10
    -water    0.2 # kg

EQUILIBRIUM_PHASES 1
    Fix_H+    -4 HNO3      10
    pe_FIX    0.1 O2(g)     10

USE solution 1
USE surface 1

end

[dlparkhurst]

Why are you fixing the pe? I think that trying to fix the pH and pe may not be possible in this case. I almost never encounter a case where the pe should be fixed. Let the reactions determine the pe.

You may fix the partial pressure of O2(g) if you wish.

[awillou4]

Thank you for your quick response.  You were right, when I removed the fix_pe equilibrium phase my HNO3 pH fix worked.  Unfortunately the HNO3 addition seems to skyrocket my pe levels (up to 10+) while HCl does not (pe between 0 to -1).  Does nitric acid generally increase pe?

I'm interested in fixing pe to fit my modeled Arsenic batch isotherms to lab isotherms.  When fixing pe lower to make an As(3) dominant equilibrium solution I tend to see much improved fits.  Do you recommend a different fitting parameter besides pe?

[dlparkhurst]

I think the problem is that equilibrium in a nitric acid solution would force As to be in the +5 valence state.

You have afew o ptions:

You could define a new "element" for nitrate that cannot be reduced.

Alternatively, you could define As(3) to be a separate "element" that cannot be oxidized.

or,

You could use HCl in place of HNO3, which will not oxidize or reduce, at least with the phreeqc.dat or wateq4f.dat database. I don't think the calculations with Cl- should differ very much from using NO3- more rigorously.

[awillou4]

HCl does give a much better fit and with more reasonable pe values, I may choose that route.

I'm also interested in exploring your 2nd option of fixing As(3) as a separate element; however I'm not exactly sure how to go about rewriting the database.  Could you point me in the right direction?

[dlparkhurst]

Example 9 redefines Fe(2) and Fe(3) as separate elements.

The attached file treats all redox states in phreeqc.dat as separate elements, but does not include As. I generated this redox.dat database, but I present it as is. It has had little evaluation.