Bedlasky
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Tungsten chemistry
Hi.
I am researching tungsten chemistry in last weeks. And I am really confused in redox chemistry. My sources are mostly: Remy's inorganic chemistry,
Mellor volume 11 (from SM library), A Text-Book of Inorganic Chemistry Volume VII Part III (also from SM library) and this article.
According to what I read in this sources it should be easy to recognize particular tungsten oxidation states. But there are few confusing things.
I dissolved some sodium tungstate in concentrated HCl in hot water bath. Then I added piece of aluminium foil and cover top of the test tube with
paper tissue. Very vigorous reaction occurred, lots of HCl gas and water vapor escaped from the test tube and tungstate is reduced in to greenish-blue
W(V), even when I used relatively lots of aluminium. Reduction didn't work at room temperature.
W(V) can be easily recognized. If you let greenish-blue solution stay for a while, dark blue chloride complexes are formed. Also after addition of
oxalic acid there is a formation of dark blue complex.
When I added more aluminium, W(V) is reduced further in to some lower oxidation state. Solution turned reddish-brown. If I let it stood in a hot water
bath, it slowly turned in to greenish-brown solution. When I added some aluminium, it turned back to reddish-brown. When I quickly cooled
reddish-brown solution to room temperature, it didn't change colour. When I diluted greenish-brown solution, it looked yellow.
When W(V) is reduced, W(IV) and W(III) are produced. W(III) forms two chloride complexes - red [WCl5]2- and yellow-green [W2Cl9]3-. [W2Cl9]3- is in
dilute solutions yellow. [WCl5]2- after heating is transformed in to [W2Cl9]3-. It looks simple - redish-brown solution when heated is changed in to
greenish-brown. So it looks like ligand exchange. But why when I added aluminium in to greenish-brown turned back in to redish-brown? This looks like
reduction. There is also green complex of W(IV) [W(OH)Cl5]2-.
I also tried comproportionation - just mixed equal volumes of dark blue W(V) and reddish-brown solution, which have the same concentration of
tungsten. If reddish-brown is W(III), it should form green W(IV). After mixing solution remained dark blue. So I tried to add W(V) in to reddish-brown
drop by drop - it formed green solution. The same result with adding W(V) in to greenish-brown.
I did another test by adding NaOH solution. Reddish-brown formed brown precipitate, greenish-brown also formed brown precipitate, but few times it
looks more light green than brown, green solution formed light green precipitate, dark blue W(V) didn't form precipitate at all and it was quickly
oxidized in to colourless W(VI). This doesn't explain anything. Hydrated WO2 is brown, W2O5 is blue. When I added solid NaOH in to these solutions,
they heated up and they were immediately oxidized in to colourless W(VI).
Greenish-brown for some time reacted with KSCN to form a orange complex, exactly as reddish-brown, but after some time it didn't react with KSCN at
all.
Green solution should be W(IV), reddish-brown W(III), but what is greenish-brown? The most logical explanation is that it is some kind of mixed
W(IV)-W(III) complex, but I never found a mention about it. What do you think?
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j_sum1
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I have not read in detai: I will come back to this.
But you may find this link helpful -- or at least something that opens up a couple more ideas.
http://www.explorechem.com/tungsten_top.html
J.
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MidLifeChemist
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Wow, what a great website. Looks like the owner has a SM id but hasn't been active here in 6 years
[Edited on 11/24/2020 by MidLifeChemist]
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woelen
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My experience with the second row and third row transition metals is that their chemistry is remarkably complex and also descriptions in literature
are quite vague and not completely according to my observations. This makes me believe that the chemistry of these elements only is understood
partially (usually the highest oxidation state is described quite well, e.g. +6 for tungsten, +5 for niobium, and so on, while lower oxidation states
are described much more vaguely).
I have done experiments with rhenium , iridium, tungsten and the results of all these experiments loosely are in accordance with literature, but not
precisely so. I think that many textbooks and papers simply do not want to dive really deep and only touch the surface of truly complex chemistry.
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Bedlasky
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J_sum1: Thanks. But in redox chemistry there is just a mention about tungsten blue.
Woelen: This is sadly true. Find decent informations is sometimes pain. You must put together info from multiple sources and even with it you are not
certain in all problematic.
Maybe manganometric titration answer on some of my questions.
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DraconicAcid
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Interesting. I saved some filaments from some old light bulbs a number of years ago, intending to do some W chemistry, but never did get around to
it.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Bedlasky
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If you wait some time, I'll post article about tungsten redox/complex chemistry. I did more than just this redox reaction.
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MidLifeChemist
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Quote: Originally posted by Bedlasky  | | If you wait some time, I'll post article about tungsten redox/complex chemistry. I did more than just this redox reaction. |
That would be great, my bottle of Sodium Tungstate is ready!
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Bedlasky
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According to manganometric titration, oxidation state of W in reddish brown solution is 4,23. I don't know if I can believe this, because when I tried
to determine oxidation state of Mo in solution from dissolving Mo powder in dilute HNO3 + H3PO4, result was oxidation state 1,58, which is nonsense.
Lets say that oxidation state is 2. Mo(II) is very strong reducing agent, which cannot be prepared even by reduction of molybdate with zinc powder.
Why this oxidation state should be formed with oxidizing nitric acid? I read that electrolysis HNO3 with Mo cathode leads firstly to formation of
Mo2O5 on surface of cathode. Formation of Mo(V) complex in nitric acid sounds far more reasonable.
If reddish brown tungsten complex really contain W(IV), I am more confused than in beginning.
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