Beginners > PHREEQC basics

need help fixing pH

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wasylenki:
Hello! This beginner needs help speciating a very simple solution with fixed pH, and reviewing previous posts on this topic hasn't helped.

I made a solution in the lab with 7.57e-7 mol/L Ni. The solution was equilibrated with the atmosphere. I added small increments of KOH until the pH was 8.2. Now I would like to know the speciation of Ni in there.

I tried the following input, but the pH shifted to 5.96:
EQUILIBRIUM_PHASES

CO2(g)    -3.5
O2(g)   1.58
SOLUTION 1 Water Composition for low ionic strength

pH      8.3
temp    22.0
units mol/L
Ni  7.57e-5

After reading previous posts in this forum, I modified my input to the lines below, but this resulted in an error: "Phase fix_H+ not found in database. Stopping."  (I am using the wateq4f database, because it has Ni complexation data in it, and it will also work for my high ionic strength solution with Ni, with which I want to compare this low ionic strength speciation.)

PHASES
fix_H+
H+ = H+
logK 0

EQUILIBRIUM_PHASES

CO2(g)    -3.5
O2(g)   1.58
fix_pH -8.3. KOH 8.3

SOLUTION 1 Water Composition for low ionic strength

pH      8.3
temp    22.0
units mol/L
Ni  7.57e-5

Does anyone know how I can write appropriate input for this solution so that pH is fixed? Thank you!!

dlparkhurst:
You are inconsistent in the name given in the PHASES definition and the EQUILIBRIUM_PHASES definition. Use either fix_H+ or fix_pH.

Are you sure you want an O2(g) partial pressure of 10^1.58, about 40 atm? Atmospheric is 10^-0.7.

Another way to do the calculation without adding additional KOH to fix the pH is as follows:

SOLUTION 1 Water Composition for low ionic strength
temp      22
pH        8.3
pe        4
redox     pe
units     mol/l
density   1
C        1   CO2(g) -3.5
O(0)     1   O2(g)  1.58
Ni        7.57e-05

wasylenki:
Thank you, dlparkhurst. I apologize for a couple of careless errors in my post; I was sloppy and too fast while copying bits of input files from previous posts in the forum and pasting into my own input files, and I failed to notice that my student had put in O2 at 1.58! I have used your suggestion at low and high ionic strength now, with O2 = -0.7, and my output now makes a lot of sense.

wasylenki:
Ok, I have spent some time with the PHREEQC manual now, and I think I am getting somewhere, but would be so appreciative of a quick check of my work and happy to add an acknowledgement to my papers to David Parkhurst or anyone else willing to help a bit.

I have four papers in the works in which my students and postdocs have measured either Ni or Zn stable isotope fractionation during adsorption reactions. We observe different fractionations at low and high ionic strength, and we're interested in knowing how the speciation of Ni or Zn in our low versus high ionic strength solutions contributes to the differences in isotope behavior. So I want to use PHREEQC to determine the proportions of various Ni or Zn species in our starting solutions, and I have the theoretically predicted isotope fractionations among these species from the work of Fujii et al. (2014, GCA) with which to assess this.

Our low ionic strength solutions are simply deionized water, adjusted to pH 8 with a tiny bit of KOH, and amended with a bit of Ni or Zn. The solution is in equilibrium with the atmosphere. At high ionic strength, we've made a mixture of the six most abundant salts in seawater in the correct proportions.

What I've done is to use the wateq4f database for both low and high ionic strength, because I'm hypothesizing that it's best to use the same database for direct comparison of the two solutions. (Does this make sense?) For Zn, I've added under the keyword PHASE the log_k for hydrozincite, because this was not in the database.

Could you check to make sure that my input is specifying what I think it is specifying? I've learned in the last two days that small errors can make big differences in speciation of Zn or Ni, and our isotope stories depend on getting this right to the extent that's possible! Please let me know if I have remaining errors to correct. I will attach the two output files of interest for Zn here. Thank you very much!

dlparkhurst:
I won't check input files, but I will answer specific questions about input or output.