# Would sodium carbonate from carbon dioxide plus hydroxide be formed in solution or as a solid?

If normal breathable atmosphere were to be reacted with aqueous sodium hydroxide, what form would the resulting sodium carbonate take, would it be dissolved in the solution or solid?

Note:

I'm defining air here as the atmosphere of earth at between sea level and 200m.

(Not a homework question, just interested.)

• What d'you mean by "reacted with"? All NaOH solutions in contact with air "react" but in more or less negligible way. – Mithoron Dec 15 '16 at 22:12
• I had in mind a water pipe percolator, but the most efficient reaction method would be preferable, I'm aiming for the maximum CO2 extraction. – Transmission Dec 15 '16 at 22:19

This depends on a lot of things: the volume of solution, the time carbon dioxide has to get dissolved and the temperature are probably the most important.

The actual reaction taking place is happening in solution. Thus, carbon dioxide must first dissolve in the solution (1) before dissolved carbon dioxide can react with the hydroxide base (2). You should view these steps as distinct even though the second is much faster than the first.

\begin{align}\ce{CO2 (g) &-> CO2 (aq)}\tag{1}\\ \ce{CO2 (aq) + OH- (aq) & -> HCO3- (aq)}\tag{2}\end{align}

Since these happen in solution, the counterion — a spectator ion — is irrelevant and the same would happen with $\ce{KOH, Ca(OH)2}$ and others.

At some point you will reach saturation, because either too much carbon dioxide dissolved or (much more likely) too much water evaporated. Thenceforth, sodium hydrogencarbonate or sodium carbonate (I never performed a formal analysis) will precipiate as a white crust. Incidentally in Munich in the inorganic teaching labs, there were 5 litre reservoirs of $6~\mathrm{M}\ \ce{NaOH}$ solution from which smaller bottles could be filled. If there was a spill (and there always was — it was undergrads in that lab after all) it would show up days later as a white crust. When I myself was a TA, part of my job preparing the labs for the next student generation was to clean those areas. Diluted hydrochloric acid worked wonders.

• What do you think would be the optimum parameters to maximise the amount of CO2 converted into HCO3- (aq)? (Great answer, by the way) – Transmission Dec 15 '16 at 22:29
• @Transmission higher concentration of $\ce{OH-}$, lower temperature (increases solubility of $\ce{CO2}$). – Jan Dec 15 '16 at 22:31
• Are you suggesting that Na2CO3 could be a dominant species in this solution? Since it is highly alkaline, I could agree with that. I'd be interested in knowing whether the carbonate or bicarbonate salt predominates for a given temperature and pH. – electronpusher Dec 16 '16 at 3:12
• @electronpusher I didn’t think it through whether hydrogencarbonate or carbonate are the dominant species at timepoint X. All I’m sure of is that either of them is the one that gives the crust upon drying. Of course, you could calculate it given concentrations, and $\mathrm{p}K_\mathrm{a}$ values. – Jan Dec 16 '16 at 3:43
• Ah yes, you could use the van't Hoff equation to find Ka at a given temperature, and it's basically ICE tables the rest of the way. – electronpusher Dec 16 '16 at 4:08