Solubility curve of tenorite

[tyler_n]

I am trying to create a solubility curve of tenorite against pH and have looked at been looking at some previous examples posted on this forum.  I’ve tried to convert the existing examples to my own purpose and I keep running into the warning “Fix_H+, Pure phase with add formula has not converged”.  This message pops up for 8 of 23 simulations, does that mean that the formula does converge by the 9th simulation and that I am good to go?
Also, when I enter the alkalinity that I’ve measured in my waters the results change dramatically, even if I change the alkalinity to be very low, the results change just as much.

Any help would be greatly appreciated.  The file I have is:

PHASES
Fix_H+
H+ = H+
log_k 0
END
# pH-dependent Cu concentration in equilibrium with tenorite
#
SOLUTION 1
    temp      10
    pH        3
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Ca 5.56
    Mg 1.728
    Na 1.783
    K  .1
        -water    1 # kg

SELECTED_OUTPUT; -reset false; -file tenorite.prn
USER_PUNCH
20 sc$ = chr$(59)
30 for i = 3.5 to 12 step 0.5
40   punch "USE solution 1",EOL$
50   PUNCH "EQUILIBRIUM_PHASES",EOL$,"Tenorite",EOL$,"Fix_H+ ",-i," NaOH 10",EOL$
60   punch "END" + eol$
70 next i
-end
END
USER_GRAPH 1
    -axis_titles            "pH" "( Cu_tot / uM )"     -chart_title            "Tenorite Equilibrium"
-headings x Cu
-connect_simulations
 -initial_solutions true
 -start
10 graph_x -la("H+")
20 graph_y (tot"Cu")
  -end
    -active                 true
END

PRINT; -reset false; -selected_out false
INCLUDE$ tenorite.prn
END

Thank you,
Tyler

[dlparkhurst]

You have a couple things to consider. To reach low pH, you need to add an acid instead of a base. (You can effectively add acid to the point that you have Na in the system by removing NaOH, but beyond that limit, you need to add acid.)

Further, at low pH the solubility of tenorite is many moles. The applicability of the ion-association model is suspect beyond ionic strength of ~1.

In the file below, I have added a trick (footnote in example 8) to add either HCl or NaOH as necessary to reach the desired pH. I've increased the amount of HCl or NaOH added to 30 moles. It is way past the applicability of the model, but the calculation shows that at pH 3.5, all 10 moles of tenorite dissolves (molality of Cu is less than 10 because the reactions have generated water). At higher pH the effect of the reactions is to add NaOH.


PHASES
Fix_H+
H+ = H+
log_k 0

NaCl
NaCl = Na+ + Cl-
log_K -20
END
# pH-dependent Cu concentration in equilibrium with tenorite
#
SOLUTION 1
    temp      10
    pH        3
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Ca 5.56
    Mg 1.728
    Na 1.783
    K  .1
        -water    1 # kg

SELECTED_OUTPUT; -reset false; -file tenorite.prn
USER_PUNCH
20 sc$ = chr$(59)
30 for i = 3.5 to 12 step 0.5
40   punch "USE solution 1",EOL$
50   PUNCH "EQUILIBRIUM_PHASES",EOL$,"NaCl 0 30",EOL$,"Tenorite",EOL$,"Fix_H+ ",-i," HCl 30",EOL$
60   punch "END" + eol$
70 next i
-end
END
USER_GRAPH 1
    -axis_titles            "pH" "( Cu_tot / uM )"     -chart_title            "Tenorite Equilibrium"
-headings x Cu
-connect_simulations
 -initial_solutions true
 -start
10 graph_x -la("H+")
20 graph_y (tot"Cu")
  -end
    -active                 true
END

#PRINT; -reset false; -selected_out false
PRINT; -selected_out false
INCLUDE$ tenorite.prn
END

[tyler_n]

Dave, thank you very much for your quick reply. The solution works perfectly.