The Carbonic Acid/Bicarbonate/Carbonate Equilibrium $\require{mhchem}$

Carbonic Acid ($\ce{H2CO3}$), Bicarbonate ($\ce{HCO3-}$) and Carbonate ($\ce{CO3^{2-}}$) form in water through the following equilibrium reactions:

$$ \ce{CO2 + H2O <=> H2CO3} $$$$ \ce{H2CO3 <=> HCO3- + H+} $$$$ \ce{HCO3- <=> CO3^{2-} + H+} $$

The distribution of carbonic acid, bicarbonate and carbonate is dependent on the pH of the water, and is easily simulated using PhreeqPython.

Importing Modules

We start by importing phreeqpython package and creating a new PhreeqPython instance



In [1]:

    
%pylab inline
from phreeqpython import PhreeqPython
# create new PhreeqPython instance
pp = PhreeqPython()









    



Populating the interactive namespace from numpy and matplotlib

Solution Definition

We define a simple solution that contains 1 mmol of Sodium Bicarbondate ($\ce{NaHCO3}$)



In [2]:

    
solution = pp.add_solution_simple({'NaHCO3':1.0})
print("This solution has a pH of: {0:.2f} and a conductivity of: {1:.2f} uS/cm".format(solution.pH,solution.sc))









    



This solution has a pH of: 8.27 and a conductivity of: 92.97 uS/cm

List Definition

We initialize four arrays, one for the pH and one for each of the different carbonate species.



In [3]:

    
phs = []
co2 = []
hco3 = []
co3 = []

Calculation Loop

We now iteratively change the pH to the desired value, using the change_ph function to dose either hydrochloric acid ($\ce{HCl}$) or lye ($\ce{NaOH}$). Using the total function we can find the total amount of carbon dioxide, bicarbonate and carbonate.



In [4]:

    
for pH in arange(0,14.1,0.1):
    # change the solution pH
    solution.change_ph(pH)
    # get and store the ph, CO2, HCO3 and CO3
    phs.append(pH)
    co2.append(solution.total('CO2')*1000)
    co3.append(solution.total('CO3')*1000)
    hco3.append(solution.total('HCO3')*1000)

Display Results

Using matplotlib we can display the results:



In [5]:

    
fig = plt.figure(figsize=[14,6])
plt.plot(phs,co2,label='CO2')
plt.plot(phs,hco3,label='HCO3-')
plt.plot(phs,co3,label='CO3-2')
plt.xlabel("pH")
plt.ylabel("Concentration (mmol)")
plt.title("Carbonic Acid, Bicarbonate, Carbonate distribution")
lgnd = plt.legend()