Fixing pH and solution conductance while saturating in Cu(OH)2

[Luca Peruzzo]

Hello everyone,

I'm trying to simulate my lab experiments, solution preparation which is a first step of the next adsorption experiments.
For my experiments I'm using:

DI water (water 0.5 , pH 7 , temp 20)
Chalcanthite (CuSO4 5H2O) ;
Halite (NaCl) ; for SC regulation
NaOH and HCl ; for pH regulation
CO2(g) and O2(g) ; equilibrium with atmosphere
 
pH range 4.5 to 6
Chalcanthite range 0 to 70 [mg/500ml]

1) I would need to fix the pH: I've read the topic referring to the example 8 footnote. There is showed how you can allow the use of either NaOH or HNO3. In my experiments I'm using HCl and NaOH, I guess it is possible to to something similar but as I'm writing here for the other difficulties I take the opportunity to ask you about this as well. To make sure the final instructions are corrected.
The pH should be fixed while changing the amount of chalcanthite added, which of course is affecting the pH.

2) Fix the SC at a specific value, let's say 500 µS/cm, by adding NaCl. Different chalcanthite concentration implies different concentration of NaCl to add, I'm looking for an automatic way to do this.
The conductivity should also be fixed together with the pH if possible. If it is not, I'll neglect the pH correction contribution to the SC, considering the pH range just a small amount of NaOH or HCl should be needed.

3) For a specific pH I would like to estimate the maximum amount of Cu (added as chalcanthite always) before saturation of cupric hydroxide (Cu(OH)2), always fixing the pH and SC.

I'm using reaction to add the chalcanthite, while EQUILIBRIUM_PHASES for pH control, CO2(g) and O2(g). You can see in the attached file my try.

Thank you very much, I'm conscious that the request is articulate and may take time to answer but I think it could be useful also for other users.

Luca Peruzzo,
Bordeaux university / Berkeley Lab

[dlparkhurst]

Let's start with item 3, ignoring the specific conductance. Here is a file that calculates about 0.5 mol of Chalcanthite dissolves at pH 5 to produce equilibrium with Cu(OH)2 (using phreeqc.dat). However, the spcific conductance is about 29000 uS/cm (at 20 C). I can't reconcile that with 500 uS/cm that you estimated, unless your log K or the aqueous model of phreeqc.dat is wrong, which is possible.

PHASES
Chalcanthite
    CuSO4:5H2O = Cu+2 + 5H2O + SO4-2
    log_k     -2.64
    delta_h   1.44 kcal
Cu(OH)2
    Cu(OH)2 + 2H+ = Cu+2 + 2H2O
    log_k     8.674
    delta_h   -56.42 kJ
Fix_H+
    H+ = H+
    log_k     0
END
SOLUTION 1
   pH 7
   temp 20
   density 1
   water 0.5
END
USE solution 1
EQUILIBRIUM_PHASES
Cu(OH)2 0.0 Chalcanthite 10
Fix_H+  -5 NaOH 10
END

[Luca Peruzzo]

Thanks,
I wasn't clear sorry, the part of fixing the SC is due to the fact the I'm using 0 ; 10 ; 20 ; 50 ; 70 [mg/500ml] of chalcanthite.
Then the SC will always be lower then 500 µS/cm, so the NaCl. 

While, checking the max amount of chalcanthite at each pH is to make sure the experimental values will be always lower than the precipitation limit, in particular if I decide to increase the maximum concentration of Cu. In this case I'll also increase the desired value of conductivity, reasonable with the new range of chalcanthite concentrations.

Thanks again for replying, I hope this will not take too much of your time.
If you have any other doubts about my script, I remind you that it was attached at the first post.

Luca

[dlparkhurst]

I think the attached file does what you want in terms of fixing the pH and SC. Addition of 0.0003 mol of Chalcanthite (~70 mg) results in undersaturation to a pH of over 6.0 according to the calculations.

A kinetic reaction is used to fix the specific conductance. Note that the specific conductance is at 20 C. If your measurement is giving you SC at 25 C, then you need to correct the PHREEQC SC. The following formula can be used:

SC_25 = SC/(1 + 0.021 * (TC - 25))

The file uses KOH and HBr to adjust the pH; however, I redefined the K and Br diffusion coefficients to be the same as Na and Cl, which should have the effect of making K and Na and Cl and Br equivalent in the SC calculation. To compare PHREEQC to experiment, the total Na in solution would be K + Na, and the total Cl in solution would be Cl + Br.

[Luca Peruzzo]

Thank you very much. I was busy yesterday and I wanted to understand the part where you managed to fix SC using RATES and KINETICS, I'm not familiar with them jet.

I've learnt something, thanks again.

best,
Luca