Carbonic acid equilibrium¶

Carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) form in water through:

CO₂ + H₂O ⇌ H₂CO₃
H₂CO₃ ⇌ HCO₃⁻ + H⁺
HCO₃⁻ ⇌ CO₃²⁻ + H⁺

The distribution depends on pH. This example titrates 1 mmol NaHCO₃ from pH 0 to 14.

You can run these examples

Click Run (or Ctrl+Enter), or Run all to execute every editor in order. Editors on this page share a session. The loop has many pH steps and can take a short while in the browser.

Editor (session: carbonic)Run

from phreeqpython import PhreeqPython
import numpy as np
import matplotlib.pyplot as plt

pp = PhreeqPython()
solution = pp.add_solution_simple({'NaHCO3': 1.0})
print('pH {:.2f}, SC {:.2f} uS/cm'.format(solution.pH, solution.sc))

OutputClear

Changes are additive

change_ph doses acid or base into the same solution. Run the loop once. To start over, rerun the first editor.

Editor (session: carbonic)Run

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

for pH in np.arange(0, 14.1, 0.2):
    solution.change_ph(pH)
    phs.append(pH)
    co2.append(solution.total('CO2') * 1000)
    hco3.append(solution.total('HCO3') * 1000)
    co3.append(solution.total('CO3') * 1000)

fig = plt.figure(figsize=[10, 5])
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')
plt.legend()
show_plot(fig)

OutputClear