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

Dec 25, 2017

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

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

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 $\implies 3.68$