Randy Holmes-Farley
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I did some calculations. The overall reaction is 2NaOH + CO2 = Na2CO3 + H2O. The molecular weight of NaOH is 40, Na2CO3 is 106. So For each 100g of Na2CO3 will come from 75g of NaOH.
Solubility of Na2CO3 around room temperature is 18~20% weight. So it's like 250g of it to 1L of water. That equals to 187.5g of NaOH. So if we dissolve more than that, it will Na2CO3 will start to precipitate when NaOH close to deplete. Given some margin of error, we can make 200~250g sodium hydroxide per liter of water, and change it when precipitation start to appear. Sounds about right?
The calculation is not technically perfect because sodium carbonate is less soluble in a sodium hydroxide solution than in pure water (due to the common ion effect of the sodium present), but it sounds like a fine plan to try.