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Author: Subject: Sodium fluoride attacks glass?
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Sodium fluoride attacks glass?

According to it's MSDS, sodium fluoride is incompatible with glass. Does this mean that solid sodium fluoride should not be stored in glass containers, or would the corrosive effects only be significant in solution?

Also, how would it attack glass? Would it attack the silica, or another component of the glass? If it is the silica, could sodium fluoride and finely divided silica be used to prepare silicon tetrafluoride, without having to use (insanely dangerous) hydrofluoric acid?

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vulture
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This is due to hydrolysis, F- is a weak base as HF is a weak acid:

F- + H2O <---> HF + OH-

The HF attacks the glass and is removed from the equilibrium, which then shifts..etc.

[Edited on 13-8-2004 by vulture]

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I would assume that such attack would only occur in large amounts in solution, however, powdered NaF could react with water vapor in the air to form HF which could also attack glass. I don't think it would be something to worry about too much, but the risk could still be there.
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Sodium fluoride is sold in poly bottles according to my Fisher catalog. I'm assuming that this isn't due to poly being cheaper than glass but to prevent attack from the HF that would be formed from water vapor in the air. My chemistry text says HF has the unusual property of attacking glass:

4HF + SiO2 -> 2H2O + SiF4

If you pumped moist air through the powdered reactants perhaps some SiF4 would form. You would still have HF present, just in smaller, more controllable amounts. And if the reaction was prompt it would all be consumed in situ. But I wouldn't want to try this without a lot more homework, and proper safeguards.

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Go to a pottery supply place and get yourself the finest-powdered silica and calcium fluoride they have. Mix, add sulfuric acid, and (optionally) warm, in an expendable or HF-resistant vessel. SiF4 for your chemical pleasure! Lungs? You don't need any steenkin' lungs.

Or you could do this with NaF, if you already have it on hand. But I'm pretty sure you need the acid to have a decent rate of production, and you want it to be a water-hungry acid to keep the SiF4 dry.

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Marvin
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I think the hydrolysis explanation is a little stretched. Particually when NaF is the salt of a strong base and a weak acid (making it basic) and SiO2 is an acidic oxide.

I would have though the first product would be a salt of fluorosilic acid. Its quite plausable heating this would produce SiF4.
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The way NaF could dissolve glass (although I think it would proceed at a reasonable rate at high t only) is like this:

6NaF + 3SiO2 => Na2SiF6 + 2Na2SiO3

Then (maybe):

Na2SiF6 + 2Na2SiO3 <=> 3Na2SiO2F2 (the complex anion SiO2F2--, akin to AlO2F2---, that froms when Al2O3 dissolves in cryolite, also NaF will corrode alumina in a similar way)

Or directly:

2NaF + SiO2 => Na2SiO2F2

GreenD
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I recently attemped this on borosilicate glass. NaF + H2O -> nothing after 10 minutes.
Endimion17
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Nothing? What do you consider as "anything"?

If you had a mass spectrometer, you'd see quite a bit of changes in the solution. It doesn't have to visibly gulp the glass in a speedy fashion. Dilluted hydrofluoric acid doesn't do that, either, yet it eats glass.

DJF90
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Not quite what this thread is about, but a saturated solution of KHF2 in a 1.75ml glass vial has shown very little corrosion/etching over the past couple of months (I made the solution to check whether a glass vessel should be used for the intended preparation of a potassium trifluoroboronate salt).
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No expected observations, smart ass.
Zan Divine
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I have a sample of NaF which has been stored in a glass bottle with a PE/Phenolic closure since 1997. The glass has no visible signs of attack.

The vial is borosilicate, however, not soft glass as found in most bottles.
Zan Divine
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 Quote: Originally posted by Marvin I think the hydrolysis explanation is a little stretched.

The equilibrium constant for HF + NaOH <--> NaF + H2O is obviously a large number.

Where that equilibrium lies isn't the only factor. Removal of HF by the glass guarantees that more HF will result.

The shifting of equilibriums is a very powerful force.

You could use a loosely analgous argument to say that you can't reduce cesium compounds with carbon. But, the fact is, you can.
Endimion17
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OH, let me get this right. You guys think soluble fluorides could be stored in glass containers? That the whole corrosion is an overblown myth?

Do you know why threy are stored in plastic? Because if something is a "reagent", it has a declaration on the bottle, saying something like "This sample contains a reagent X and impurities Y. The amount of impurities Y is Z(Y) and the manufacturer guarantees that until the date of expiration."

The salt doesn't have to chew the fuck up its container (or even etch it visibly) in order for the manufacturer to put it in another one, made from different materials. It only has to be altered beyond the manufacturer specification, and that's what happens with soluble fluorides in glass jars. They get contaminated.

The whole issue here is so... elementary school level or lower.

GreenD
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Read the title, lol. I really don't like you
 Quote: Originally posted by Endimion17 OH, let me get this right. You guys think soluble fluorides could be stored in glass containers? That the whole corrosion is an overblown myth? Do you know why threy are stored in plastic? Because if something is a "reagent", it has a declaration on the bottle, saying something like "This sample contains a reagent X and impurities Y. The amount of impurities Y is Z(Y) and the manufacturer guarantees that until the date of expiration." The salt doesn't have to chew the fuck up its container (or even etch it visibly) in order for the manufacturer to put it in another one, made from different materials. It only has to be altered beyond the manufacturer specification, and that's what happens with soluble fluorides in glass jars. They get contaminated. The whole issue here is so... elementary school level or lower.

 Sciencemadness Discussion Board » Fundamentals » Chemistry in General » Sodium fluoride attacks glass? Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication   » References Non-chemistry   » Forum Matters   » Legal and Societal Issues   » Whimsy   » Detritus   » The Moderators' Lounge