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

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Dec 25, 2017

Answer:

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:

#bb((1))color(white)(m)"CO"_2"(g)" ⇌ "CO"_2"(aq)"; k_text(H) = "29.76 L·atm·mol"^"-1"#

#bb((2))color(white)(m)"CO"_2"(aq)" ⇌ "H"_2"CO"_3"(aq)"; K_text(h) = 1.7 ×10^"-3"#

#bb((3))color(white)(m)"H"_2"CO"_3"(aq)" + "H"_2"O(l)" ⇌ "H"_3"O"^"+""(aq)" +"HCO"_3^"-""(aq)"; K_text(a₁) = 2.5 ×10^"-4"#

#bb((4))color(white)(m)"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 #"CO"_2# solution depends on the partial pressure of #"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#

#bb((1)) = # the normal atmospheric partial pressure
#bb((2)) = # a partial pressure of 1 atm
#bb((3)) = # 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 #CO_2# and #H_2O# to form #H_2CO_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_2CO_3#

10mM #=># #4.68#
100mM #=> 4.18#
1000mM #=> 3.68#

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Dec 25, 2017

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

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