Titration of HNO3 by NaHCO3; problem with CO2

[jungsdao]

I'm trying to simulate neutralizing 100mL of 3 M nitric acid using NaHCO3.
As NaHCO3 is added, it will react and produce CO2 which will evolve.
So I wrote code as follows.
As partial pressure of CO2 will remain as 10^-3.5, I wrote "CO2(g) -3.5" in equilibrium phases.
By including this keyword, final pH is around 10.
But when I don't include this keyword, final pH is around 5.
I also have an experimental result that indicates final pH value is way lower than 10..
As I spotted this discrepancy, There might be some flaws in the code I wrote.. but seems too hard for me to find it.
Any suggestion or comment would be appreciated.

Thank you.

===================================================
DATABASE C:\phreeqc\database\llnl.dat

USER_PUNCH
-head Spec_Cond
PUNCH SC
SELECTED_OUTPUT
-activities H+ Ca+2
-ionic_strength true
-pH
-charge_balance true
-saturation_indices
-equilibrium_phases

SOLUTION 1
pH 7 charge
temp 25
units mol/L
water   0.1 # kg
N(5) 3

PHASES 1
NaNO3
NaNO3 = Na+ + NO3-
log_k   3.69
delta_h 20.4
# NaHCO3
# NaHCO3 = Na+ + HCO3-
# log_k  -0.25
# delta_h  16.7

equilibrium_phases 1
Nahcolite 0 0
CO2(g) -3.5
NaNO3 0 0

REACTION 1 
Nahcolite 1
0.6 moles in 100 steps

USER_GRAPH 1
-headings Addition "NO3-" "Na+" "pH"
-axis_titles "Added NaHCO3 (mol)","MOLALITY PER Kg" ,"pH"
-chart_title  "Titration Curve"
     -axis_scale x_axis 0 auto
     -axis_scale y_axis 1e-2 auto auto auto
     -axis_scale sy_axis -1 14 1
-start
10 pH=-LA("H+")
20 GRAPH_X RXN
30 GRAPH_Y MOL("NO3-"), MOL("Na+")
40 GRAPH_SY pH
-end
-active                 true

USER_GRAPH 2
-axis_titles "Added NaHCO3 (mol)", "Saturation Index"
-headings Addition "NaHCO3" "NaNO3"
-chart_title  "Saturation index"
-axis_scale x_axis  0 auto
-axis_scale y_axis  auto auto 1
-start
10 GRAPH_X RXN
20 GRAPH_Y SI("Nahcolite") SI("NaNO3")   
-end
-active                 true

USER_GRAPH 3
-axis_titles "Added NaHCO3 (mol)", "Water mass (g)"
-headings Addition
-chart_title  "Water mass change"
-axis_scale x_axis  0 auto
-axis_scale y_axis  auto auto 10
-start
10 GRAPH_X RXN
20 GRAPH_Y TOT("water")*1000 
-end

END

[dlparkhurst]

I don't see anything wrong with your calculation. Degassing of CO2 is not instantaneous, so perhaps your experimental results did not achieve complete equilibrium with the atmosphere. In that case, any pH between 5 and 10 would be possible, depending on the extent of CO2 evolution.

[jungsdao]

Thank you very much, Dr. Parkhurst.

Your answer is always helpful and insightful.