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sodium hydroxide turning into sodium carbonate


scotchlady

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Hello,

 

My background is not chemistry, but I have been on a mission to learn...since my significant had his drain cleaning incident...

 

From my understanding sodium hydroxide will turn into sodium carbonate in time as it absorbs carbon dioxide from the air. How long will this take?? As I understand this depends on how much surface area of the sodium hydroxide is exposed. So for instance if you had some NaOH that was exposed to the air for 6 months...would it that be sufficient time for it to have changed into sodium carbonate?

 

Thanks,

Jennifer

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more that likely yes, 6 months would be quite sufficient I would imagine if it had a reasonable surface area and humidity level.

 

there are Plenty of other factors that could make an Exception to this sure, but on the whole, you can more or less say there will be a Very significant amount of the carbonate present :)

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more that likely yes, 6 months would be quite sufficient I would imagine if it had a reasonable surface area and humidity level.

 

there are Plenty of other factors that could make an Exception to this sure, but on the whole, you can more or less say there will be a Very significant amount of the carbonate present :)

And what factors would make this an exception? Just wondering...

 

Jennifer

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no no, what happens is, is that NaOH will take moisture from the air and make itself into a Liquid (Hygroscopic).

but when That happens CO2 in the air can react with it quite easily and ultimately convert it all the the carbonate as all the ions are motile and free to move around.

solids can`t do this.

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YT2095 If solids can't absorb gases like that please let me know how things like soda-lime have been doing it for years.

Anyway, if you leave NaOH exposed to air it picks up water to form a solution. (Strictly, deliquescense rather than hygroscopy.) That solution picks up CO2 to make a solution of Na2CO3. Then that solution dries out to give, initially hydrated Na2CO3.10H2O (I'm not sure, but I think it's the decahydrate). Whatever, this loses water to give the powdery monohydrate.

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YT2095 If solids can't absorb gases like that please let me know how things like soda-lime have been doing it for years.

 

certainly,

"In soda lime absorption, the carbon dioxide first reacts with water to form carbonic acid, which then reacts with sodium hydroxide to form a soluble carbonate. The soluble sodium carbonate then reacts with calcium hydroxide to form an insoluble carbonate and replenishes the sodium hydroxide. Heat and water are produced during the reaction. Exhaustion of its activity is indicated by dyes; the most common one changes from pink to white."

 

taken from: http://www.anaesthesiauk.com/article.aspx?articleid=247

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NaOH is also very basic, when shown to air, expect it to absorb A LOT of water and take the concentraton down immensly.

Yup, it is one of the strongest bases. One of its features is also that it is hygroscopic, which means that it has the ability of absorbing water from air. If left in air NaOH acts with CO2 as in the following equation:

 

[ce]2NaOH + CO2 -> Na2CO3 + H2O[/ce]

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"Would it be at all possible that the sodium carbonate would react with the calcium hydroxide in the concrete thus forming sodium hydroxide again? "

Yes, and no.

If there wre any exposed (CaOH)2 it would have been converted to CaCO3 by exposure to CO2 in the air.

With very fresh lime mortar there might be enough Ca(OH)2 to convert sodium carbonate to the hydroxide but it wouldn't last anyway.

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So, I have this straight. The reaction of sodium hydroide with the carbon dioxide in the air would be 100%-in the end there would be only sodium carbonate. And the sodium carbonate can potentially absorb the carbon dioxide and you will get bicarbonate of soda. So what was once very corrosive and harmful does break down into things that are less harmful. This is good to have this clarified.

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yup, that about sums it up nicely :)

 

did you know this can be done with Calcium carbonate also? if you bubble CO2 through Lime water you get a white precipitate of Calcium Carbonate, if you Continue bubbling the CO2 through it will go Clear again, that also forms an interesting species that can only exist in solution called Calcium hydrogen carbonate (or calcium bicarbonate) ;)

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