In biology, we commonly use $\ce{KOH}$ in respirometers to remove carbon dioxide. A common test in chemistry to detect the presence of carbon dioxide is to bubble it through limewater, turning it milky. This has led me to wonder which is more efficient in removing carbon dioxide—$\ce{KOH}$ or $\ce{Ca(OH)_2}$—when both solutions are sitting in the atmosphere (i.e. they’re just passively absorbing $\ce{CO_2}$ and $\ce{CO_2}$ is not bubbled through either solution). An explanation would be much appreciated.

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    $\begingroup$ KOH would be better since Ca(OH)2 has limited solubility. This of course infers a much more concentrated solution of KOH. $\endgroup$ – MaxW Apr 2 '16 at 3:31

$\ce{KOH}$ (and $\ce{NaOH}$) absorb $\ce{CO2}$ more quickly and completely than $\ce{Ca(OH)2}$, but they are much more dangerous to work with: caustic to skin, deliquescent (forming a highly concentrated solution that can pour out of the apparatus), etc. For that reason, "lime water" is preferred for classroom use to demonstrate the presence of $\ce{CO2}$.

  • $\begingroup$ Is KOH deliquescent? I know NaI is and KI isn't, so I thought maybe the $\ce{Na^+}$ may be the culprit with NaOH, also. $\endgroup$ – SendersReagent Apr 4 '16 at 0:45
  • $\begingroup$ The OP asked about solutions, not using the hydroxides as solid reactants. I still think capturing $\ce{CO2}$ with a concentrated $\ce{KOH}$ solution would be more efficient than a $\ce{Ca(OH)2}$ solution. $\ce{KOH}$ isn't very good for a demo since the solution would stay clear until a massive amount of $\ce{K2CO3}$ was created. Just a little $\ce{ CO2}$ would give a ppt with $\ce{Ca(OH)2}$. $\endgroup$ – MaxW Apr 4 '16 at 2:55
  • $\begingroup$ Agreed, Ca(OH)2 is a better CO2 indicator, but the alkali hydroxides are more effective absorbents. BTW, I've found it hard to get a sufficiently concentrated "lime water" so that a precipitate is immediately noticeable (perhaps the water was a bit acidic?). $\endgroup$ – DrMoishe Pippik Apr 4 '16 at 20:26
  • $\begingroup$ Limewater contains about a gram per liter of calcium hydroxide. When you figure in the stoichiometry of the reaction and the density of $\ce{CaCO3}$ you are likely to end up with something like one volume of precipitate per thousand volumes of solution. So you are not going to get copious white clouds of precipitate. Such is life in the world of alkaline earths. $\endgroup$ – Oscar Lanzi Jan 17 '19 at 19:12

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