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pH level of CO2 / Carbon Dioxide

January 13, 2008

Q. I am a student currently in the 11th grade enrolled in an AP Chemistry class. My question is what would be the pH of pure CO2/carbon dioxide? As well as the pH of CaO and SiO2. This information will be very helpful to me if answered as soon as possible.

Angela P [last name deleted for privacy by Editor]
AP Chemistry student - Palm Harbor, Florida, USA

January 22, 2008

A. This may be a trick question, Angela. Only water solutions can have the property of pH, whereas CaO as an example will not exist in a water solution. As soon as you add water you have Ca(OH)2 -- pH of 12.4 -- not CaO. The CO2 question is more complicated as it will become carbonic acid, sodium bicarbonate, or carbonate depending on the pH; plus there are solubility limits of how much you can get into the water and keep in the water. But I think a quick answer is you won't be able to drive the pH below about 6 with CO2.

An AP Chemistry student should be learning where to find pH charts, and other chemistry tables and charts in reference books, Angela. When you can't find the value for something in such a book, it usually indicates not that nobody hasn't thought of doing that one yet, but that there is something that renders the question not quite valid. Good luck.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

January 26, 2008

A. I have no idea what 11th grade means (or AP chemistry) but if you are discussing pH you should have been taught what it means.
pH is the reciprocal of the log (base 10) of the hydrogen ion (proton) concentration.
It follows that if there is no hydrogen, there can be no pH.

geoff smith
Geoff Smith
Hampshire, England

Ed. note: for our non-USA readers, 11th grade students are 16-17 years old, AP is advanced placement, i.e., "the geeks".

January 29, 2008

A. I totally agree with Geoff. However, you may also wish to investigate the concept of pCO2, which would be the CO2 equivalent of pH.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

January 31, 2008

A. I would only like to add that your statement "this information will be very helpful to me if answered as soon as possible" is also not so true. At least not for a student. The helpful information is the one that you find yourself, discuss and understand. Such info becomes knowledge, YOUR KNOWLEDGE. Not someone else's passed on you.
Hope you read this and understand it.

Guillermo Marrufo
Monterrey, NL, Mexico

May 19, 2011

A. CO2 is basically acidic in nature; when you tend to neutralize a basic solution so the range of pH for carbon dioxide would be probably 4-5

Haris Tanauli
- Karachi, Pakistan

March 12, 2012

A. An aqueous solution of Carbon dioxide is mildly acidic. Its pH can't be between 4-5. It has to be slightly more than 6.

Jaysheel Singh
- New Delhi, Delhi, India

September 18, 2014

A. Okay as a student I know what examiners and teachers want to see. CO2 is slightly acidic and has a pH of 2, although 3-4 is accepted but 2 is the answer. I know it sounds absurd but that is it. Hope I answered your query in time.

Sam S [last name deleted for privacy by Editor]
- Singapore

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