Use of a buffer such as Sodium Acetate in Example 9

[sahilhaz1@gmail.com]

Can someone help me with defining Acetate as a buffer in Example 9? The database is insufficient. I am trying to model 100mM FeCl2 and 200mM CH3COONa.

[dlparkhurst]

You can run the following with llnl.dat. Note that there are definitions for quite a few Na and Fe Acetate complexes. If you want just Acetate and HAcetate, you can extract the SOLUTION_MASTER_SPECIES and SOLUTION_SPECIES definitions from llnl.dat and add them to an input file and run with phreeqc.dat.

Code: [Select]

SOLUTION
REACTION
FeCl2     0.1
NaAcetate 0.2
1 mol
END


[brunagr]

Hello,

I am also trying to model sodium acetate as a buffer for an acetic acid solution, but my results are not matching the experiment data. I tried to use the data from minteq.dat, but it seems to be considering only the addition of acetate and not considering the buffer, regardless of the change in the initial pH. I am going to send my input data in here, I would really appreciate if someone could help me correct it. Thank you very much!

SOLUTION_MASTER_SPECIES       

      Ac   Ac-   0.0  60 60

 SOLUTION_SPECIES

   #Acetic Acid
   Ac- = Ac-
   log_k    0.0;    -gamma 1e7 0.0

   Ac- + H+ = HAc
   log_k    4.75;    -gamma 1e7 0.0

   #sodiumacetate
   Na+ + Ac- = NaAc
   log_k   -0.18
   delta_h 0       kcal

END

SOLUTION 1          
        pH      4.68                   
        pe      4.0   
        temp    25                     
        units   mol/kgw                 
        Ac      0.02     
        Na      0.01                     

[dlparkhurst]

If you use SOLUTION, and set the pH, then that is the solution pH. The Ac-/HAc distribution will be set by the pH chosen.

 Here is a simulation that starts with 0.01 molal NaAc and calculates the pH of 8.4 from charge balance. unless you use charge balance to adjust it. Addition of 0.001 mol of HCl should lower the pH to about 3 in pure water, but with the acetate buffer, the pH decreases only to pH 5.7.

I don't know what you are trying to model, but you can start with a known solution composition and measured pH, or you can do as in this example and construct a solution and calculate the pH by charge balance, or you can add NaAc with REACTION to a known solution composition.

Code: [Select]


SOLUTION_MASTER_SPECIES       

      Ac   Ac-   0.0  60 60

 SOLUTION_SPECIES

   #Acetic Acid
   Ac- = Ac-
   log_k    0.0;    -gamma 1e7 0.0

   Ac- + H+ = HAc
   log_k    4.75;    -gamma 1e7 0.0

   #sodiumacetate
   Na+ + Ac- = NaAc
   log_k   -0.18
   delta_h 0       kcal

END
SOLUTION 1       0.01 molal NaAcetate   
        pH      7 charge                 
        pe      4.0   
        temp    25                     
        units   mol/kgw                 
        Ac      0.01     
        Na      0.01
REACTION 1
HCl 1
1 mmol
END


[brunagr]

Hello Mr. Parkhurst,

Thank you very much for your answer. I am studying the dissolution of calcite in an acetic acid solution, so I was trying to model a 10mM acetic acid solution with 10mM of sodium acetate in water.. In the experiment I prepared the solution with and without the sodium acetate buffer, measured the initial pH and used it as an input in the program.
When I include only acetic acid the results match my experiment data, but I am not succeeding in adding the sodium acetate as a buffer in this solution. I tried again as described below, but the result is still not looking good. Would there be another way to include the acetate buffer in the model?

SOLUTION_MASTER_SPECIES       

      Ac   Ac-   0.0  60 60

 SOLUTION_SPECIES

   #Acetic Acid
   Ac- = Ac-
   log_k    0.0;    -gamma 1e7 0.0

   Ac- + H+ = HAc
   log_k    4.75;    -gamma 1e7 0.0

   #sodiumacetate
   Na+ + Ac- = NaAc
   log_k   -0.18
   delta_h 0       kcal

END
SOLUTION 1       0.01 molal NaAcetate   
        pH      4.68               
        pe      4.0   
        temp    25                     
        units   mol/kgw                 
        Ac      0.01     
        Na      0.01

REACTION 1
HAc 1
10 mmol
END

[dlparkhurst]

I think your solution definition is wrong. The pH of a NaAc solution is not 4.88, which is what you specified.  The calculation I attached used pH to charge balance the solution and found a pH of 8.4. So here is one way to do the calculation using charge balance:

Code: [Select]

SOLUTION 1       0.01 molal NaAcetate   
        pH      4.68   charge           
        pe      4.0   
        temp    25                     
        units   mol/kgw                 
        Ac      0.01     
        Na      0.01

REACTION 1
HAc 1
10 mmol
END
[/code

Alternatively, you can add both NaAc and HAc in REACTION:

[code]
SOLUTION 1     
pH 7
REACTION 1
HAc  1
NaAc 1
10 mmol
END

If you want to use your measured pH, you can do this (which will include a small charge imbalance in the calculation):

Code: [Select]

SOLUTION 2       0.01 molal NaAcetate   + HAcetate
        pH      4.68   
        pe      4.0   
        temp    25                     
        units   mol/kgw                 
        Ac      0.02     
        Na      0.01


[brunagr]

Hello Mr. Parkhurst,

Thank you very much for your attention and help, I appreciate it! That was very helpful!