CO2, pH, and pressure changes

[ACH]

Hi all,
I have worked on a simulation that tracks the changes in pH and pressure as I add CO2. It seems to be working fine, however, despite having a closed system (I think?) adding CO2 results in 0 pressure. This surely can't be correct. The code is as follows:

Code: [Select]

SOLUTION 1 Seawater Composition, Nordstrom et al., 1979
    units ppm
    temp 25
    pH 8.22
    pe 8.451
    redox O(0)/O(-2)
    units umol/kgw
    density 1.023
    -water 1 # kg
    Ca 412.3
    Mg 1291.8
    Na 10768.0
    K 399.1
    Si 4.28
    Cl 19353.0
    Alkalinity 141.682 as HCO3
    S(6) 2712.0
    O(0) 1.0 O2(g) -0.7

GAS_PHASE 1
    -fixed_volume
    -volume 0.1
    -pressure 1
    -temperature 25
    CO2(g) 0.01

REACTION 1 CO2 addition
    CO2(g) 0.1 # moles
    0.01 moles in 10 steps

SELECTED_OUTPUT
    -reset false

USER_PUNCH 1
    -start
    10 PUNCH STEP_NO, GAS("CO2(g)"), -LA("H+"), TOTAL_PRESSURE
    -end

USER_GRAPH 1
    -headings Step CO2 pH Pressure
    -axis_titles "Step Number" "Values" "pH"
    -initial_solutions true
    -connect_simulations true
    -start
    10 GRAPH_X STEP_NO
    20 GRAPH_Y GAS("CO2(g)")
    30 GRAPH_Y -LA("H+")
    40 GRAPH_Y TOTAL_PRESSURE
    -end
I would appreciate if anyone have any ideas for the missing pressure. Thank you

[dlparkhurst]

The Basic function is PRESSURE, not TOTAL_PRESSURE.

You have only CO2(g) in your gas phase, so the pressure is quite small for the amounts of CO2 that you are adding. You may need to include the effects of unreactive Ntg(g) (N2 gas) and Oxg (O2 gas) in your calculations.