PhreeqcUsers Discussion Forum

Mixing => Mixing Solutions => Topic started by: waynehwm on July 04, 2018, 06:34:52 AM

Title: Base titration of acetic acid model result vs reality
Post by: waynehwm on July 04, 2018, 06:34:52 AM
Hi David,

I am trying to model a base titration curve of acetic acid to match the published titration curve (https://users.humboldt.edu/rpaselk/C109.S11/C109_Notes/C109_lec41.htm).

As we know the acetic acid has a buffering effect around pH 4.5 that delays the rise of pH. But the output curve from my model does not have that same trend as the published data. I wonder if it is because my model is set up incorrectly, or if you think the model is correct what do you think could be the cause of the difference between model and actual data?

As always, thank you David.

Code: [Select]
SOLUTION_MASTER_SPECIES
Ac     Ac-         0.0 60.00 60.00

SOLUTION_SPECIES
#Acetic acid
Ac- = Ac-
        log_k   0.0;    -gamma  1e7   0.0
Ac- + H2O = HAc + OH-
        log_k   4.75;    -gamma  1e7   0.0

SOLUTION 1
    temp      20
    pH 2.5
    redox     pe
    units     mol/l
    density   1
    water    0.050 # kg
            Ac 1
SELECTED_OUTPUT; -file case2.prn; -reset false; -high_p true
USER_PUNCH
-headings pH
 -start
 10 PUNCH -LA("H+")
 -end

USE SOLUTION 1
REACTION 2
    H2O        1
    NaOH       0.00180
0.2 moles in 40 steps
END
Title: Re: Base titration of acetic acid model result vs reality
Post by: dlparkhurst on July 04, 2018, 07:07:56 AM
You have the wrong K for the reaction, or you have the wrong equation for the K:

Ac- + H+ = HAc
        log_k   4.75; 
Title: Re: Base titration of acetic acid model result vs reality
Post by: waynehwm on July 04, 2018, 05:25:44 PM
Hi David,

Yes, you are correct, now the titration curve fits data perfectly.

Thank you very much!