Sodium tungstate

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Beatsy
Posts: 1648
Joined: Fri Jul 05, 2013 3:10 am
Location: Malvern, UK

Sodium tungstate

Post by Beatsy »

Hope it's OK to post this here - a chemistry question really - but I don't fancy joining and asking a random chemistry forum. Main reason is suspicion I'll have to wade through pages of "safety advice" suggesting a noob shouldn't be messing with stuff they don't understand, and stuff like that. I know they mean well - but. There's always someone here who can give succinct pointers to relevant info and will trust me not to go and melt my head off (and therefore spare me unnecessary lecturing)... :D

Since discovering electrolysis of tungsten needles, my interest has also been piqued by the discovery that electrolyte and tungsten produce hydrogen and oxygen while leaving sodium tungstate in solution (as the NaOH electrolyte is used up).

Sodium tungstate is very interesting to me because it can be used to make a "heavy solution" that is tuned by dilution to float silica (diatoms) but sink minerals (sand and silt). Perfect for separating tiny, sub 10-micron forms from minerals and other heavy detritus. This is difficult and time-consuming using the techniques that work well for larger forms.

My question (finally) is how to extract pure tungstate from the depleted electrolyte. I guessed I could just sizzle a load of tungsten until the NaOH was used up and sizzling stopped. But there's no clear "stop" and I've run out of wire I'm willing to burn. The solution is still strongly alkali and would damage or destroy diatoms. I can neutralise that with HCl, but the remaining solution would probably not be dense enough. So how can I get pure tungstate out?

Any pointers, even just clues or ideas appreciated as I'm a bit stumped at the moment (with my limited chemistry knowledge). Thanks.

Edit: just found out that HCl would convert the tungsten to WO3 (trioxide), so that's not an option anyway. Told you my knowledge was limited :D Also, I may need Sodium polytungstate, but not sure about that. Is there a route from tungstate to polytungstate? And finally - I found a recipe using sodium nitrite, sodium hydroxide and tungsten - and I have those things in hand. Highly exothermic and a bit scary though, so I'll leave exploring that until all the other options (of extracting from depleted electrolyte) are proven impractical. Hopefully they won't be.

Ichthyophthirius
Posts: 1075
Joined: Thu Mar 07, 2013 5:24 am

Post by Ichthyophthirius »

Hi,

Just from the literature; no personal experience.

https://books.google.co.uk/books?id=2eK ... g=PA63&lpg

This section describes the purification of sodium tungstate (Natriumwolframat) from commercial stock. It gets to a step where sodium tungstate is present in an alkaline solution with NaCl and NaOH at approx. pH 9-10.

The instruction is to add 2/3 volume ethanol which precipitates the tungstate overnight. Then wash the precipitate with 50 % ethanol and dry.

However, the tungstate is present at a high concentration in the protocol (50 % in the first precipitation) and it's not clear if this will work with your very dilute sample.

BTW you can easily buy the stuff online once Corona is over.

Stay safe and wear PPE!

Regards, Ichty

Beatsy
Posts: 1648
Joined: Fri Jul 05, 2013 3:10 am
Location: Malvern, UK

Post by Beatsy »

Ichty, you are a star!

I wasn't sure what "add 2/3 volume" meant for ethanol - 2 parts ethanol to one part tungstate mix? Anyway, since I'm dealing with max 10% w/v but unknown quantities, I just put a few ml of used electrolyte in a vial and added twice as much pure ethanol. The mix immediately showed slight opalescence and a layer of stronger milkiness was visible a minute later. I gave it all a good shake and it went back to being near clear and opalescent. Just checked it again, two hours later and it is clearly "settling out" with the top third clear and the rest slightly milky looking.

Even better, although I had to use a 15x loupe to see it, there are loads of tiny little white crystals sliding around on the bottom. I fancy some might be slightly needle-like, but they're very small and that could just be distortion through the glass.

So thanks very much - this is a fantastic start. Fingers crossed it all pans out as I hoped. Feeling optimistic. Oh - and sodium tungstate solution is £250 for 100ml (with tax) here - £2500/litre!!! I probably couldn't buy it anyway due to our draconian restrictions on chems for private individuals. Even if it's permitted, fewer and fewer suppliers are willing to deal with anyone but pros.

Ichthyophthirius
Posts: 1075
Joined: Thu Mar 07, 2013 5:24 am

Post by Ichthyophthirius »

Hi,

I agree, it's not a very good way of writing the protocol. It's always good to give the final concentration as well. I think it means adding a 2/3 volume of ethanol to the starting volume (final concentration approx. 40 vol%) rather than 2/3 of the final volume (final conc. 67 vol %) but it is not clear from the text.

Regardless, NaOH dissolves well even in absolute ethanol. So if you get a precipitate, it will hopefully be the tungstate. Na2CO3 might be an issue as it doesn't dissolve in ethanol as well as NaOH.

Regards, Ichty

Beatsy
Posts: 1648
Joined: Fri Jul 05, 2013 3:10 am
Location: Malvern, UK

Post by Beatsy »

Quick morning update. The suspension has settled and I have mostly clear liquid (alcohol and near saturated NaOH solution?) with an oily looking drop at the bottom. Looks "heavy" and just rolls around without mixing but I'm not sure what that is. However, there is white powder or micro-crystals sliding around on the bottom within that drop. I suspect (hope) that's the stuff I'm after.

Your "steer" was really good. Helped me find the right key words and phrases to dig out more info. The "washing" in alcohol now makes sense - tungstate is soluble in water, but not ethanol (which NaOH does dissolve in).

One thing has me a bit confused. Washing in 50% ethanol. I guess that means 50/50 ethanol and water!? So surely the water would dissolve some of the tungstate too? Perhaps the NaOH is preferentially dissolved and "saturates" the water before tungstate can get in there. Dunno? More research needed. Appreciated if anyone can shed light on that part.

No matter though - I have enough info to bumble forwards over time now. I will have to get more "industrial" on converting the tungsten. The tiny little bit consumed in needle making won't make enough (quickly).

Thanks again...

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