Sodium Oxalate solubility

[Sinaatalebi]

Hi

I am new to PHREEQC
I wanted to know if is it possible to calculate the sodium oxalate solubility in various temperatures.
For the beginning and learning how to work with PHREEQC, I am simulating the data of a published paper about the solubility of soidum oxalate in concentrated electrolyte solutions.
In the paper, 200 mL of 1 molar NaOH is added to a Parr reactor alongside 10 gr sodium oxalate salt.
The calculated solubility for oxalate is about 0.07 mol/kg.

Can somebody help with the code for this to start learning PHREEQC?

Thanks in advance.

[dlparkhurst]

Here is a script for you to learn from. The density corrections are not exact, but it should give you the basic idea. Use sit.dat, which has definitions for a few oxalate species. If Na+ complexes with oxalate, you need to find log K for the complexation reaction. If you have data for different temperatures, you need to fit a delta H of reaction, or an analytical expression for the log K of Na2Ox (and NaOx- if the complex is important).

Code: [Select]

SOLUTION 1
-units mol/L
-density 1.03
pH 14 charge
Na 1
END
# Calculate ion activity product = log K
USE solution 1
REACTION
Na2Ox 1
0.07
USER_PRINT
10 PRINT LA("Ox-2") + 2*LA("Na+")
END
# Use log K to simulate solubility in 1 M NaOH
PHASES
Na2Ox
Na2Ox = 2Na+ + Ox-2
log_k -2.33
END
USE solution 1
EQUILIBRIUM_PHASES
Na2Ox 0 1
END


[Sinaatalebi]

Dear dlparkhurst,

Thanks for your help. I read your code and searched for the keywords. Now I know a lot more.

Yes. I have data for other temperatures (25, 50, 125C) and NaOH molar concentrations (0.5, 1, 2, 5M).
The oxalate solubility data for example are:

Temp 25C, 1M  NaOH: 0.07 mol/kgw
Temp 50C, 1M NaOH: 0.13 mol/kgw
Temp 125C, 1M  NaOH: 0.23 mol/kgw
Temp 25C, 2M NaOH: 0.03 mol/kgw
Temp 25C, 0.5M NaOH: 0.17 mol/kgw


The code is as you wrote. I tried the code with other temperatures and NaOH molar concentrations but since I dont know how to obtain delta_h or an analytical expression for the log K of Na2Ox, the results are not correct. Would you please help me with this?

# Calculate Sodium Oxalate (Na2C2O4) solubility

SOLUTION 1
-units mol/L
-density 1.03
pH 14 charge
Na 1
#OH 1
END



# Calculate ion activity product = log K
USE solution 1
REACTION
Na2Ox 1
1 moles
USER_PRINT
10 PRINT LA("Ox-2") + 2*LA("Na+")
END



# Use log K to simulate solubility in 1 M NaOH
PHASES
Na2Ox
Na2Ox = 2Na+ + Ox-2
log_k -2.33
END




USE solution 1

REACTION_TEMPERATURE
25 50 125

REACTION_PRESSURE
7

EQUILIBRIUM_PHASES
Na2Ox 0 1

USER_GRAPH
 -chart_title     "OX Solubility in 1M NaOH"
 -axis_titles   "Temperature, in Celcius" \
         "Oxalate Solubility, in moles per kilogram water"
# -axis_scale x_axis 20 130

10 plot_xy TC, tot("Ox"), color = Red, line_width=3

END

[dlparkhurst]

Here is a script that runs the 5 solutions and plots them in a sequence. You can compare results to your analyical data. To fit better, you can adjust the delta_h of Na2Ox. I started arbitrarily with delta_h of 10.

Code: [Select]

END
EQUILIBRIUM_PHASES
Na2Ox 0 10
END
USE solution 1
USE equilibrium_phases 1
REACTION_TEMPERATURE
25 50 125
REACTION_PRESSURE
7
USER_GRAPH
 -chart_title     "OX Solubility"
 -axis_titles   "Sequence" \
         "Oxalate Solubility, in moles per kilogram water"
10 sequence = GET(1) + 1
20 PUT(GET(1) + 1, 1)
30 graph_x sequence
40 graph_y tot("Ox")
END
USE solution 1
USE equilibrium_phases 1
REACTION
NaOH 1
1
END
USE solution 1
USE equilibrium_phases 1
NaOH 0.5
-1
END


[Sinaatalebi]

Hello

I think there might be a problem with the code. it's not complete and I am having some warnings. Also, the results do not make sense. Where did you define the enthalpy?
Thanks in advance

[dlparkhurst]

Sorry, I must not have copied the whole input file.

Code: [Select]

# Use log K to simulate solubility in 1 M NaOH
PHASES
Na2Ox
Na2Ox = 2Na+ + Ox-2
log_k -2.33
delta_h 10
END
SOLUTION 1
-units mol/L
-density 1.03
pH 14 charge
Na 1
END
EQUILIBRIUM_PHASES
Na2Ox 0 10
END
USE solution 1
USE equilibrium_phases 1
REACTION_TEMPERATURE
25 50 125
REACTION_PRESSURE
7
USER_GRAPH
 -chart_title     "OX Solubility"
 -axis_titles   "Sequence" \
         "Oxalate Solubility, in moles per kilogram water"

#10 plot_xy TC, tot("Ox"), color = Red, line_width=3
10 sequence = GET(1) + 1
20 PUT(GET(1) + 1, 1)
30 graph_x sequence
40 graph_y tot("Ox")
END
USE solution 1
USE equilibrium_phases 1
REACTION
NaOH 1
1
END
USE solution 1
USE equilibrium_phases 1
NaOH 0.5
-1
END