AWM's Carbon Dioxide page

CO₂ in air/water:

Solubility of CO₂ in water - dependence on temperature

Ionization constants: pK_a1 and pK_a2 and pK_a1=f(T)

Ionization of CO₂ in water - composition and pH

Titration of acidified electrolyte with NaOH: effect of CO₂-contaminated NaOH and CO₂ in electrolyte (equlibrated in contact with air).

R: solubility as CO₂ [mol] / [dm³] of pure water
q: solubility as mass of CO₂ [g] dissolved in 100g of pure water
a: Bunsen solubility coeff. as CO₂ volume [dm³] / [dm³] of pure water
Ionization constants:: (1) relative to total CO₂ content (CO₂ + H₂CO₃) at 25°C:
pK_a1=6.352, pK_a2=10.329 (or 10.24 - Ref. dependent)
(2) relative to actual H₂CO₃ at 25°C: pK_a1=3.76

**Solubility and dissociation of CO₂ in water:**
Solubility: Normal curves: R, q, a = f(T)	Solubility: Linearized log(R) = f(1/T)	pK_a1 = f(T) (relative to total CO₂)

**Composition and pH of CO₂ equilibrated water:**
**NOTE** For simplicity, activity coefficients were ignored (usually titration is performed at constant ionic strength - relative pH changes are only weakly affected.
Equilibrium composition = f(pH): 0.03% of CO₂ in air at 25°C	Zoomed view of Equilibrium composition = f(pH): 0.03% of CO₂ in air at 25°C	pH of water equilibrated against atmospheric CO₂ at 25°C (0.03% in air is marked)

Titration of HCl with NaOH:
NOTE For simplicity, activity coefficients were ignored (usually titration is performed at constant ionic strength - relative pH changes are only weakly affected.

Model lines:
30ml of electrolyte solution (e.g. 0.1M NaCl - see NOTE above) is acidified with 0.3ml of 0.2M HCl. Then solution is titrated with 0.1M NaOH at T=298K.

(---o--- red + circles) - no CO₂ is present (solution is properly degassed and NaOH is free of carbonates).
(----- green) - electrolyte solution (30ml) is saturated with CO₂ by contact with air containing 0.12% of CO₂ (atmosphere average content is 0.035% but in laboratory this must be higher because of people present in the lab, building or industrial/city environment) - solution is left as-is (no degassing); NaOH is free of carbonates.
(----- light blue) - electrolyte solution is properly degassed, but NaOH contains some carbonates (2% of NaOH as Na₂CO₃).
(---o--- dark blue + circles) - electrolyte is equilibrated against air with 0.035% of CO₂ and titrated with NaOH containing 2% of carbonates (as above) - no degassing is performed.

**Titration of HCl with NaOH (model):**
Full range curve	Enlarged range (close to pH=7)

Compare general potentiometric titration page.

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