Solubility of hydroxides in water as a function of pH

[ChemInd74]

Hi,

I am new to PHREEQC, so please bear with me.

I am trying to extend the pH-range of the calculation example from Dr. Apello on Gibbsite (https://hydrochemistry.eu/exmpls/gibbsite.html) in water as a function of pH. The calculation does not converge below pH 2.0.

This is the code of the example:

Code: [Select]

# pH-dependent Al concentration in equilibrium with gibbsite
#
SOLUTION 1
 pH 3; Al 1e3 Gibbsite
USER_GRAPH
 -chart_title "Gibbsite Equilibrium"
 -axis_titles pH "log( Al_tot / M )"
 # -axis_scale x_axis 2 12 2
 #-axis_scale y_axis -8 0 2
 -headings pH  equilbrium
 -connect_simulations
 -initial_solutions true
 -start
 10 graph_x -la("H+")
 20 graph_y log10(tot"Al")
 -end

SELECTED_OUTPUT; -reset false; -file gibbsite.prn
USER_PUNCH
20 sc$ = chr$(59)
30 for i = 3.5 to 12 step 0.5
40   punch "SOLUTION 1" + sc$ + " pH " + str$(i) + sc$ + " Al 1 Gibbsite" + eol$
50   punch "END" + eol$
60 next i
-end
END

PRINT; -reset false; -selected_out false
INCLUDE$ gibbsite.prn
END
In the output file it says:
"Al solution phase boundary has not converged", "Mu Ionic strength has not converged" and "A(H2O) Activity of water has not converged"

Why does the calculation not converge?

[dlparkhurst]

Depends on which database you use. phreeqc.dat, wateq4f.dat, minteq.v4.dat, and llnl.dat converge.

sit.dat does not converge. I would say the parameterization is not reliable in sit.dat. If you scan across aluminum concentration at pH 3, the saturation index never reaches equilibrium, even at 50 mol/kgw.

The other databases may fail at lower pH as well. The ion-association models are not reliable at high ionic strength and the numerical method gets flaky at high concentrations. But gibbsite equilibrium at low pH is may not be physically reasonable anyway.

Code: [Select]

# pH-dependent Al concentration in equilibrium with gibbsite
#
KNOBS
#-step 1.2
#-pe   1.2
#-iter 2000
SOLUTION 1
 pH 3; Al 0.1e3 #Gibbsite
USER_GRAPH
 -chart_title "Gibbsite Equilibrium"
 -axis_titles log10(Al) SI("Gibbsite")
 # -axis_scale x_axis 2 12 2
 #-axis_scale y_axis -8 0 2
 -headings pH  equilbrium
 -connect_simulations
 -initial_solutions true
 -start
 10 graph_x log10(tot("Al"))
 20 graph_y SI("Gibbsite")
 -end
END
SOLUTION 1
 pH 3; Al 0.2e3 #Gibbsite
END
SOLUTION 1
 pH 3; Al 0.4e3 #Gibbsite
END
SOLUTION 1
 pH 3; Al 1e3 #Gibbsite
END
SOLUTION 1
 pH 3; Al 2e3 #Gibbsite
END
SOLUTION 1
 pH 3; Al 3e3 #Gibbsite
END
SOLUTION 1
 pH 3; Al 4e3 #Gibbsite
END
SOLUTION 1
 pH 3; Al 5e3 #Gibbsite
SOLUTION 1
 pH 3; Al 1e4 #Gibbsite
END
SOLUTION 1
 pH 3; Al 2e4 #Gibbsite
END
SOLUTION 1
 pH 3; Al 3e4 #Gibbsite
END
SOLUTION 1
 pH 3; Al 4e4 #Gibbsite
END
SOLUTION 1
 pH 3; Al 5e4 #Gibbsite