Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: vwyw on 03/07/2007 09:51:43
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"Why is NaOH(aq) stored in glass reagent bottle with a plastic stopper?"
This is a question from my school exam?
The given answer is that NaOH reacts with CO2 to form Na2CO3 and is a glass stopper is used, the glass stopper will get stuck tightly. By the reaction 2NaOH(aq) + CO2(g) -> Na2CO3(s) + H2O(l). [???]
But is this correct? Or is it the reaction between NaOH with SiO2? 2OH- + SiO2 -> SiO3- + H2O
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"Why is NaOH(aq) stored in glass reagent bottle with a plastic stopper?"
This is a question from my school exam?
The given answer is that NaOH reacts with CO2 to form Na2CO3 and is a glass stopper is used, the glass stopper will get stuck tightly. By the reaction 2NaOH(aq) + CO2(g) -> Na2CO3(s) + H2O(l). [???]
But is this correct? Or is it the reaction between NaOH with SiO2? 2OH- + SiO2 -> SiO3- + H2O
It is correct the first. NaOH reacts with the CO2 present in the air over the solution inside the bottle. A glass stopper has a conical shape and to remove it with low pressure inside you should make a tremendous force (breaking it or the bottle) to pull it out. With a plastic stopper you can simply unscrew it (not possible to make a glass stopper of that kind).
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But does the pressure inside the bottle really matters much, does it change that much. [???]
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One reason is that Na2CO3 is also a component of glass. So the Na2CO3 formed by the reaction of NaOH and CO2 in the air can start crystallizing on the Na2CO3 in the glass, thus creating a bonding between stopper and bottle.
There are Teflon sleeves for standardized conical glass stoppers that prevent this, and there are also conical plastic stoppers. You do not need to use a screw stopper.
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Eric has the right answer, the base attacks the glass.
As for "A glass stopper has a conical shape and to remove it with low pressure inside you should make a tremendous force (breaking it or the bottle) to pull it out. "
As far as I can see the "temendous pressure" would be roughly 300 millionths of an atmosphere, a lot less than day to day variation in atmospheric pressure.
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One reason is that Na2CO3 is also a component of glass. So the Na2CO3 formed by the reaction of NaOH and CO2 in the air can start crystallizing on the Na2CO3 in the glass, thus creating a bonding between stopper and bottle.
There are Teflon sleeves for standardized conical glass stoppers that prevent this, and there are also conical plastic stoppers. You do not need to use a screw stopper.
That's right, this must be considered together with what I said and is probably the main reason.
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Eric has the right answer, the base attacks the glass.
As for "A glass stopper has a conical shape and to remove it with low pressure inside you should make a tremendous force (breaking it or the bottle) to pull it out. "
As far as I can see the "temendous pressure" would be roughly 300 millionths of an atmosphere, a lot less than day to day variation in atmospheric pressure.
I said "tremendous force" not "tremendous pressure", actually, because I had in mind the low pressure combined with a glass stopper "glued" to the bottle by the carbonate (personal experience).
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I think the glass stopper and the glass bottle stuck together because the is some think form between them, not the pressure problem. But anyway, thanks for lightarrow's help and suggesting a possible but may be not thought of reason.
Anyone, please share your view.
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Eric has the right answer, the base attacks the glass.
What's the equation for the reaction between sodium hydroxide and glass?
Chris
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I never spoke about a chemical reaction, just wrote that the Na2CO3 from the reaction of NaOH and CO2 could combine with the Na2CO3component of the glass into big crystals, welding the glass stopper and the glass flask together.
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What's the equation for the reaction between sodium hydroxide and glass?
Chris
2NaOH + SiO2 --> Na2SiO3 + H2O.
The solutions of NaOH stored in a glass container must be dilute infact, to avoid this reaction. Better in a plastic container.
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I never spoke about a chemical reaction, just wrote that the Na2CO3 from the reaction of NaOH and CO2 could combine with the Na2CO3 component of the glass into big crystals, welding the glass stopper and the glass flask together.
It's difficult for me to think of "the Na2CO3 component of the glass". Glass is a solid solution of SiO2 and other oxides and/or silicates.
When Na2CO3 or other carbonates reacts with SiO2 at high temperature, it releases CO2:
Na2CO3 + SiO2 --> Heat --> Na2SiO3 + CO2