Sciencemadness Discussion Board

Making sodium tellurite from tellurium dioxide?

woelen - 17-1-2010 at 08:02

I purchased some impure TeO2 from eBay. The material is green/brown and when it is added to concentrated hydrochloric acid, then a deep yellow solution is obtained, which contains a lot of copper(II) ions. When it is added to concentrated sodium hydroxide, then part of the material dissolves, giving a pale grey/blue liquid and a lot of dirty green/brown precipitate.

I want pure tellurium compounds for experimenting and the material as I have it now is not really useful. Before I do anything with the material I want to have confirmation or suggestions from others. I spent EUR 15 on 200 grams of the impure material and I do not want to loose it in a crappy/flawed process.

My idea is to add NaOH to the TeO2 such that there is a slight excess of (impure) TeO2. Theoretically I would have to add 100 grams to 200 grams of TeO2. I am thinking of adding 90 grams of NaOH and half a liter of water or so and then heating. This will result in a lot of green/brown stuff. Next step then would be filtering until a clear liquid is obtained (which most likely will be slightly grey/blue). This liquid I want to boil down somewhat and then let it evaporate on a dry warm place until crystals of Na2TeO3 separate from the liquid.

Do you think this is a good procedure, or do I overlook some potential problems?

unionised - 17-1-2010 at 11:20

It looks like it should work- at least you will lose the copper and someother metals (iron springs to mind).
IIRC you can electrowin Te from a solution of telurite and that would probably improve the purity still further (I'm not sure about the risk of making H2Te which you would want to avoid).

Telurium's not hugely toxic- it would take a gram or two to kill you; but it's known to have the unfortunate efect that even small amounts getting into the body make you smell of garlic.

12AX7 - 17-1-2010 at 12:40

I find H2TeO3 listed as pKa1 = 2.3, pKa2 = 8.0, so Na2TeO3 should be on the basic side but not quite as basic as CuO2(2-). Since you have a soluble form, you can recrystallize; I guess you'll have to see what the solubility vs. temperature is like. Finally, you can acidify to precipitate the free acid, something Cu won't do.

Tim

JohnWW - 17-1-2010 at 13:28

TeO2 and tellurites are quite readily oxidized to TeO3, telluric acid ( Te(OH)6 ), and tellurates(VI), especially in strongly alkaline solution as Woelen proposes. This may even happen on exposure to air. But it may be a means of separation from Cu++ and other soluble impurities, e.g. by subsequent precipitation as an insoluble tellurate e.g. of Ca or Ba.

I think we discussed on a recent thread elsewhere the strong garlicy smell that Te (along with Se) imparts to people who somehow ingest or absorb the stuff (in any oxidation state), due to the formation of H2Te and volatile organotellurium compounds. It was also noted there that the poisoning of Col. Litvenenko about 3 years ago, by Lugovoi's administering a massive lethal dose of a soluble polonium-210 compound into his teapot in a London hotel, may have made him similarly very smelly before he died a few days later of alpha-radiation poisoning, due to the formation of H2Po and volatile organopolonium compounds. Especially if the Po compound had been mixed with the similar Te compound as a macroscopic "carrier", as is done with microscopic amounts of some radioisotopes.

turd - 17-1-2010 at 14:34

Quote: Originally posted by woelen  
The material is green/brown and when it is added to concentrated hydrochloric acid, then a deep yellow solution is obtained, which contains a lot of copper(II) ions.

I've seen TeO2 p.a. from snow white to yellow (alpha- vs. beta-TeO2 or simply particle size?), but never brown. On dissolving in HCl it invariably gives an intensely yellow solution. I didn't realize that you can turn it into tellurite(IV) by simply refluxing with NaOH(aq), good to know!
Quote:
I think we discussed on a recent thread elsewhere the strong garlicy smell that Te (along with Se) imparts to people who somehow ingest or absorb the stuff (in any oxidation state), due to the formation of H2Te and volatile organotellurium compounds.

I'm not convinced that this isn't a myth. I know the garlic breath of TeO2 well enough - but none of my colleagues ever noticed and they are not shy to point out these kind of things. :P I think for others to notice you'd need to ingest quite a lot of material.
Quote:
TeO2 and tellurites are quite readily oxidized to TeO3, telluric acid ( Te(OH)6 ), and tellurates(VI), especially in strongly alkaline solution as Woelen proposes. This may even happen on exposure to air.

Really? People heat tellurites (IV) to 600°C in air without much oxidation, but of course elements in this region of the periodic table have a crazy redox chemistry.

unionised - 18-1-2010 at 13:21

Cu tellutite is, so I understand, insoluble. If you dissolve the bulk material in base you will leave the Cu behind, but it will take some Te with it.
I guess you could reduce the CuTeO3 to CuTe then acidify to get the last of the Te as H2Te but it's not nice stuff.
NaTeO3 can be reduced to Te by SO2. That should remove the Te from solution fairly selectively (for example it leaves Se behind) to give relatively pure Te

Jor - 18-1-2010 at 16:29

I think the most effective way would be generating H2Te from this (Zn/HCl?) and decomposing this. Ofcourse it is REALLY TOXIC, but if you take a long tube, wich is heated strongly, I can't imagine anything coming out.
I think this is also the best way to make very pure arsenic, and possibly antimony, it's just shame that these hydrides are so toxic.

Precitipating as an insoluble tellurate, as JohnWW, wouldn't be a very good idea I think. To convert this back to a soluble Te compound would be hard, requiring reduction with Al or something wich creates lots of toxic dust. But, I'm not sure, maybe the tellurite is a good oxidiser and can oxidise a reducing agent in aqeous solution.
So first we really need to know how strong oxidisers tellurates are, and Te(IV) and both acidic as alkaline solution. But I do not have accees to this data. AFAIK selenates are very strongly oxidising, but tellurates less so.

Is it all inorganic impurities? Maybe boil some solid with some DCM (or other good solvent like THF), decant, and see if any residue remains on evaporating.

turd - 18-1-2010 at 23:03

Quote: Originally posted by Jor  
Precitipating as an insoluble tellurate, as JohnWW, wouldn't be a very good idea I think. To convert this back to a soluble Te compound would be hard [...]

Add hot HCl and stir for a few minutes - Te is amphoteric. Of course that wouldn't get rid of unwanted cations, but just saying..

JohnWW - 19-1-2010 at 01:21

About Po and Te compounds, and the garlicy odor caused by their ingestion: see my latest addition to the thread about the poisoning of Col. Litvinenko in November 2006, at:
http://www.sciencemadness.org/talk/viewthread.php?tid=7081&a...
. I am fairly sure that the microscopic amount of the Po-210 compound administered by Lugovoi was a water-soluble one mixed with a macroscopic amount of the same compound of Te to "bulk it up", probably alkali metal polonite(IV) and tellurite(IV).

woelen - 19-1-2010 at 02:53

I certainly will not go along the H2Te route. I really fear being smelly for weeks with a bad garlic/rotten leek smell around me. So making H2Te is out of the question.

Based on your comments I decided to go the NaOH+TeO2 way, as I proposed in my first post. I add 90% NaOH based on stoichiometry and accept the 10% loss (probably less, because the material is not pure and in that 10% there will be impurities as well).

I already tried the making of elemental Te from this and that works great. Simply dissolve some of the (impure) TeO2 in conc. HCl and then add a slight excess of a solution of sodium sulfite. Fine black Te precipitates from the solution after adding the Na2SO3. I however, don't want Te-powder (I already have several tens of grams of very pure Te), I really want Na2TeO3 (or pure TeO2). I think that it really is easiest to go through the NaOH route and let the solution (after filtering) evaporate until a large crop of crystals is obtained. The final waste solution I will convert to Te, together with the remaining precipitate after adding only 90% of stoichiometric amount of NaOH.

This method has the big advantage that no gases are evolved, so no bubbling of solutions in which aerosols with tellurium can be created. The only critical step is transferring the dry powder to a flask. Once it is wet or on solution, there is no need anymore to fear having Te-based dust in the air.

[Edited on 19-1-10 by woelen]

woelen - 28-6-2010 at 11:54

The method with NaOH is a total failure. This does not work.

I dissolved 10 grams of technical grade TeO2 in a solution of NaOH. I had to add more NaOH than I would expect, based on the stoichiometry. When this is done, an impure yellow/brown solution is obtained.
Next, I filtered the solution. I needed three filter steps through the same filter, before the liquid was totally clear. It remained brown/yellow. This is a true solution, a test with a laser pointer shining through the liquid did not show any turbidity.
I allowed the liquid to stand on a warm dry place in order to obtain crystals from the solution. After one day or so, a white crust appeared at the bottom of the evaporating dish. This white crust probably is Na2TeO3. I decanted the brown liquid, rinsed the white material with distilled water and allowed this to dry. The remaining brown liquid I transferred to the evaporating dish again.
The white solid had a weight of only 1.3 grams after drying. When it is added to concentrated hydrochloric acid, then it dissolves, giving a deep yellow solution, but also a lot of gas is produced. So, the white solid is very impure Na2TeO3, it contains considerable amounts of Na2CO3, which indeed is easy to imagine, because I had to use excess NaOH in order to get all of the technical grade TeO2 dissolved.

From the remaining liquid no crystals can be obtained. After some time it simply forms a yellow/brown crust, but no separation occurs. I did many attempts, sometimes very slowly, but no success. So, using NaOH for purifying TeO2 is not a good idea.

I now was left with a brown solution. I put it aside for further processing and now, a few months later, while cleaning up my lab, I decided to give it a last chance. I dumped all of it in a lot of water (appr. 200 ml) and added 20 ml of concentrated HCl to this. When this is done, the liquid starts bubbling vigorously (due to all NaOH being converted to Na2CO3, not good at all, the aerosol contains Te-compounds!) and hence I quickly left the room setting ventilation at full power. Tens minutes later I came back and found that a thick white precipitate had formed at the bottom with a very pale yellow/brown clear liquid above it. I stirred with a glass bar and more precipitate was formed (and also some bubbles were formed again).

I filtered the liquid on a buchner filter and rinsed three times with distilled water. I now have a wet paste in a petri dish, which now is drying. The paste looks very good, it only has a very faint yellow/brown color, but that hardly is noticeable (only when the paste is compared with the pure white of e.g. table salt one can see the difference). The paste is not yet dry, so I cannot yet measure the yield.

So, summarizing, when purifying 10 grams of TeO2 from technical grade the following can be done:

- Dissolve the TeO2 in water with NaOH. Use so much NaOH that all of it dissolves, but try to keep the amount of used NaOH as low as possible. This will result in 50 ml of liquid.
- Filter the resulting turbid liquid multiple times on the same filter (use vacuum).
- Discard the crap in the filter and keep the clear yellow/brown liquid.
- Dilute this liquid with a lot of water (e.g. 5 times its own volume).
- Add appr. 25 ml of concentrated hydrochloric acid (30% HCl) and stir.
- Allow the precipitate to settle and decant the clear liquid above it (this must not be flushed down the drain, treat as chemical waste).
- Add distilled water and then filter the whole mess. Rinse the white paste on the filter with distilled water a few times and filter on a vacuum.
- Scrape the paste from the filter and allow to dry in a warm and dust-free place.

Pseudomonia - 28-6-2010 at 17:39

I wouldn't suggest getting on any busses for a while. Not unless you want the bus to clear :P

woelen - 29-6-2010 at 00:01

Things fortunately are not that bad. I have done experimenting with Te-compounds before and never had any troubles with this. Apparently the stories are exagerated. I also am very careful and always do a thorough cleanup after such experiments and I work in a separate room with strong suction ventilation.

I do not perform experiments with gaseous Te-compounds though, such as H2Te.

turd - 29-6-2010 at 07:32

Quote:
Apparently the stories are exagerated.

Absolutely. I've made a Pr-Te-Te-Pr/Pr-Te-Pr mix with ~1:2 molar ratio and it's not really worse than the S and Se analogues. I tried making H2Te from Al2Te3 with HCl, but it hydrolyses/decomposes very quickly, so it poses not much of a problem. From working with Te(IV) and Te(VI) inorganics you may get a strange taste in the mouth, but nobody else will notice.

In your case I think I would have tried sublimation in evacuated "quartz" tubes, but the needed gear is usually not available to the home experimenter. Maybe you should try going via Te(0). TeO2 is easily made from Te. Though the best option is probably buying pure TeO2. It's not terribly expensive.

benzylchloride1 - 29-6-2010 at 20:48

I recently prepared selenium dioxide from crude selenium purchased off of Ebay. The selenium dioxide was prepared by dissolving the selenium in concentrated nitric acid, evaporating to dryness and subliming the selenium dioxide. A method of purifying selenium and presumably tellerium is to dissolve the crude material in nitric acid and to reduce the selenious acid to metallic selenium using sulfur dioxide or sodium sulfite. Here is a way to purify crude selenium dioxide from Vogels Textbook of practical organic chemistry. This may work for your tellerium dioxide. (i) The crude selenium dioxide is treated with sufficient water to
dissolve it, and, after filtering from any insoluble matter, 10 ml. of concentrated
hydrochloric acid are added. A slow stream of sulphur dioxide
is passed into the solution until heat is no longer evolved (2-5 hours);
the reaction is complete when the red precipitate settles to the bottom
in a coherent mass. Upon standing for a few hours or, better, on boiling
for a short time, the red selenium passes into a pasty grey form, which
eventually becomes brittle. The precipitated selenium is filtered off,
ground in a mortar, washed free from acid, dried, and finally heated over
a Bunsen burner. The cold mass is then dissolved in nitric acid and
evaporated as before. To make certain that all the nitric acid has been
completely removed, the residue is dissolved in 70-80 ml. of water and
evaporated again. White selenium dioxide is obtained in about 90 per
cent, yield.

woelen - 29-6-2010 at 23:03

Unfortunately that does not work for TeO2. TeO2 is more like SiO2, it does not dissolve in water. It does dissolve in concentrated alkalies, giving tellurites and in concentrated hydrochloric acid (not in aqueous sulphuric and nitric acids). In concentrated hydrochloric acid it forms a deep yellow chloro-complex.

woelen - 3-7-2010 at 13:33

Quote: Originally posted by woelen  
[...]
So, summarizing, when purifying 10 grams of TeO2 from technical grade the following can be done:

- Dissolve the TeO2 in water with NaOH. Use so much NaOH that all of it dissolves, but try to keep the amount of used NaOH as low as possible. This will result in 50 ml of liquid.
- Filter the resulting turbid liquid multiple times on the same filter (use vacuum).
- Discard the crap in the filter and keep the clear yellow/brown liquid.
- Dilute this liquid with a lot of water (e.g. 5 times its own volume).
- Add appr. 25 ml of concentrated hydrochloric acid (30% HCl) and stir.
- Allow the precipitate to settle and decant the clear liquid above it (this must not be flushed down the drain, treat as chemical waste).
- Add distilled water and then filter the whole mess. Rinse the white paste on the filter with distilled water a few times and filter on a vacuum.
- Scrape the paste from the filter and allow to dry in a warm and dust-free place.


The material now is totally dry. The funny thing is that on drying the material becomes brown. The wet material is white and the dry material has a quite strong brown color.

I tested the brown material by adding a spatula full of this to a solution of NaOH in water. All of the brown material dissolves and a clear and practically colorless solution is obtained. The quality of my end product is MUCH better than the quality of the product I purchased from eBay and it certainly is good enough for performing interesting experiments with TeO2.

Yield of the procedure above is quite sensitive on the amount of HCl used. If too much HCl is used, then quite some TeO2 remains in solution (as some chloro-complex), if too little HCl is used, then also quite some TeO2 remains in solution (as tellurite). I had too much HCl, the solution was strongly acidic (pH < 1). If I repeat this experiment, then I'll add so much HCl that the solution only becomes moderately acidic (pH > 2).