# What is the pH of carbon dioxide? What is the pH of carbon dioxide?

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Dec 25, 2017

Carbon dioxide does not have a pH, because it is a gas.

#### Explanation:

However, an aqueous solution of carbon dioxide has a pH, because the carbon dioxide reacts with the water to form carbonic acid.

There is a complex series of equilibria:

$\boldsymbol{\left(1\right)} \textcolor{w h i t e}{m} \text{CO"_2"(g)" ⇌ "CO"_2"(aq)"; k_text(H) = "29.76 L·atm·mol"^"-1}$

$\boldsymbol{\left(2\right)} \textcolor{w h i t e}{m} \text{CO"_2"(aq)" ⇌ "H"_2"CO"_3"(aq)"; K_text(h) = 1.7 ×10^"-3}$

$\boldsymbol{\left(3\right)} \textcolor{w h i t e}{m} \text{H"_2"CO"_3"(aq)" + "H"_2"O(l)" ⇌ "H"_3"O"^"+""(aq)" +"HCO"_3^"-""(aq)"; K_text(a₁) = 2.5 ×10^"-4}$

$\boldsymbol{\left(4\right)} \textcolor{w h i t e}{m} \text{HCO"_3^"-""(aq)" + "H"_2"O(l)" ⇌ "H"_3"O"^"+" + "CO"_3^"2-""(aq)"; K_text(a₂) = 4.47 ×10^"-7}$

Thus, the pH of a pure ${\text{CO}}_{2}$ solution depends on the partial pressure of ${\text{CO}}_{2}$above the solution.

Here's a table of the pH at three important partial pressures:

bbul(color(white)(mml)p_text(CO₂)"/atm"color(white)(ml)"pH"color(white)(m))
bb((1))color(white)(ml)3.5 × 10^"-4"color(white)(ml)5.65
bb((2))color(white)(ml)1.0 ×10^0color(white)(mll)3.92
bb((3))color(white)(ml)2.5 × 10^0color(white)(mll)3.72

$\boldsymbol{\left(1\right)} =$ the normal atmospheric partial pressure
$\boldsymbol{\left(2\right)} =$ a partial pressure of 1 atm
$\boldsymbol{\left(3\right)} =$ the partial pressure in bottled carbonated drinks

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Dec 26, 2017

Carbon dioxide exists as a gas at room temperature, I have never heard gases having some pH . This is the same reason why litmus test of dry HCl gas doesn't yield any result .
If you perform a combination reaction of $C {O}_{2}$ and ${H}_{2} O$ to form ${H}_{2} C {O}_{3}$ . H2CO3 is a diprotic acid. The pKa for H2CO3 is 6.35, and the pKa for HCO3- is 10.33. pH depends on molarity now .

pH of ${H}_{2} C {O}_{3}$

10mM $\implies$ $4.68$
100mM $\implies 4.18$
1000mM $\implies 3.68$

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Dec 25, 2017

depends on what it becomes, but cannot be below 6.

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