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Old 11-16-2008, 03:03 AM   #1
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Chemistry question: hydrogen fluoride and silicon tetrafluoride

I heard these two chemicals are a by-product of the phosphate fertilizer industry, and since the epa regulates them they are caught with a water spray instead of being blown out the smoke stack.

When they collect in the water they combine to make hydrofluorosilicic acid (or hexafluorosilicic acid?).

Can anyone confirm/deny or elaborate on this?
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Old 11-16-2008, 03:32 AM   #2
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Re: Chemistry question: hydrogen fluoride and silicon tetrafluoride

It looks like at a chemical level Silicon Tetrafluoride would ionize water which would in turn ionize Hydrogen Flouride, which would then allow for ions of SiF6(2-) to combine with ions of Hydrogen to create Hydrofluorosilicic Acid. This acid can only exist in as an equilibrium solution, meaning you won't find bottles of 100% Hydrofluorosilicic Acid. It can only exist in the presence of water and other ions.

What kind of information are you looking for?
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Old 11-16-2008, 07:06 AM   #3
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Re: Chemistry question: hydrogen fluoride and silicon tetrafluoride

Yes, hexafluorosilicic acid can be produced by hydrolysis of silicon tetrafluoride. I'm not sure why anyone would consider this a problem though. The same chemical is produced in huge quantities in volcanoes. It is also used to fluoridate water in a lot of countries.
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Old 11-16-2008, 02:27 PM   #4
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Re: Chemistry question: hydrogen fluoride and silicon tetrafluoride

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Yes, hexafluorosilicic acid can be produced by hydrolysis of silicon tetrafluoride. I'm not sure why anyone would consider this a problem though. The same chemical is produced in huge quantities in volcanoes. It is also used to fluoridate water in a lot of countries.
http://forumserver.twoplustwo.com/34...arning-341741/
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Old 11-17-2008, 12:16 AM   #5
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Re: Chemistry question: hydrogen fluoride and silicon tetrafluoride

Thanks

Chris, like CR noted in that thread there is a lot of debate on whether these chemicals are safe for consumption or not. The evidence seems to me to point to the fact that there are very subtle negative side effects when consumed at the level water is fluoridated at.

I have had a hard time finding unbiased (ie, non fluoridealert.org) information on the chemicals that I can understand. I am clearly not a chemist.

Any information is appreciated.

So to be clear then, the two chemicals mentioned are not actually combining with each other but rather reacting to the water to turn into hydrofluorosilicic acid and hexafluorosilicic acid?
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Old 11-17-2008, 12:43 AM   #6
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Re: Chemistry question: hydrogen fluoride and silicon tetrafluoride

Hydrofluorosilicic acid and hexafluorosilicic acid are two names for the same compound, H2SiF6. The two compounds you mentioned do combine with each other, but it looks like the compound only exists in solution, because the reaction is reversible:

2 HF + SiF4 <-> H2SiF6

If the solvent evaporates, HF and SiF4 are released. I assume water serves to stabilise the anion.

About fluoridation of water, I can't really comment, but in the linked thread it makes me suspicious when people are raising the same old things that get blamed on whatever peoples' pet worries are (autism for instance). While there may be minor effects on health, if there were any dramatic impact then epidemiological studies ought to be able to pick it up pretty easily.
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Old 11-17-2008, 01:02 AM   #7
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Re: Chemistry question: hydrogen fluoride and silicon tetrafluoride

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Originally Posted by ChrisV View Post
Hydrofluorosilicic acid and hexafluorosilicic acid are two names for the same compound, H2SiF6. The two compounds you mentioned do combine with each other, but it looks like the compound only exists in solution, because the reaction is reversible:

2 HF + SiF4 <-> H2SiF6

If the solvent evaporates, HF and SiF4 are released. I assume water serves to stabilise the anion.

About fluoridation of water, I can't really comment, but in the linked thread it makes me suspicious when people are raising the same old things that get blamed on whatever peoples' pet worries are (autism for instance). While there may be minor effects on health, if there were any dramatic impact then epidemiological studies ought to be able to pick it up pretty easily.
Thanks.

Not to derail the thread, (though it's completed its function) the effects do seem to be rather minimal/hard-to-measure except in sub-groups (kidney patients especially). The question is more a moral one since fluoridating the water supply amounts to mass-medication without informed consent.

Studies are definitely showing some effects of fluoride, but the debate continues. The pro-fluoridation argument is generally flawed in that they are basing the burden of proof on the safety of fluoride, whereas if you are going to be adding something that everyone will be consuming you ought to be %100 certain it is safe and effective.

It is clearly not the case that sodium fluoride (nor hexafluorosilicic acid) is %100 safe and effective.
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Old 11-17-2008, 07:57 AM   #8
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Re: Chemistry question: hydrogen fluoride and silicon tetrafluoride

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So to be clear then, the two chemicals mentioned are not actually combining with each other but rather reacting to the water to turn into hydrofluorosilicic acid and hexafluorosilicic acid?
They do react with each other, but the reaction is ionic opposed to covalent. So they don't really "combine" - they just sort of bump in to each other to form Hydrofluorosilicic acid. But the real key to the reaction is the formation of the hexafluorosilicate anion. I'll elaborate a little.

Because the Silicon atom is large compared to the 4 Fluorine atoms that surround it, H2O and HF are capable of hybridizing with the inner electrons of the Si atom (something that isn't possible in most reactions). The reactions occur like this:

SiF4 + H2O ---> SiF3OH + HF
SiF4 + 2HF ---> SiF6[2-] + 2H[+]

**It looks as though only the 2nd equation is needed to describe the reaction between SiF4 and HF to create the acid. But I think (and I'm really not sure and need someone to back me up on this) H2O more readily reacts with SiF4 to create SiF3OH. This would allow SiF3OH to react at a lower energy to create SiF6[2-]**

The reaction that occurs when you add HF to an aqueous solution is:

HF + H2O <---> H[+] + OH[-] + F[-]

If you combine all of the above reactions you get something like this:

2SiF4 + 2HF + H2O <---> SiF3OH + SiF6[2-] + F[-] + 2H[+]

The above equation can be written in a bunch of different ways, but I think the way I wrote it best shows all of the different "parts" of a Hexafluorosilic acid equilibrium solution. Notice that the SiF6[2-] anion can ionically bond with the 2 hydrogen cations. But since this is an equilibrium solution there are other reactions taking place simultaneously. SiF3OH can take those 2 H[+] cations and create SiF3[-] and H2O; or F[-] can combine with H[+] to get back to HF.

So that was probably more than you wanted to know but I was bored an felt like testing my knowledge of chemistry. On that note I could be completely wrong, can anyone check this?
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