Yet another nice experiment - selenium allotropes

[woelen]

Today, I did another nice experiment, now with selenium. I transformed the black allotrope into the red allotrope. Less risky than the chromyl chloride experiment, but fun also . This may also be interesting for people who collect elements.

 

Black selenium is affordable. It can be obtained for just over $10 per 30 grams, including shipping worldwide from http://www.emovendo.net .

This is sufficient for all the experiments with selenium you can ever dream of . For my experiment you only need approximately 10 mg, so you have some left for other experiments as well :D.

 

For the experiment, see my site again:

http://81.207.88.128/science/chem/exps/selenium/index.html

 

Have fun and stay green.

[jdurg]

Yeah, red selenium is really neat looking, but I'm now starting to wonder about it's 'stability'. I had a nice ampoule filled with red selenium, and over time it has darkend and is now a deep dark brown color as opposed to the bright red color seen in your photos or the charcoal gray/black color of standard selenium. I'll have to investigate that a bit further.

[jdurg]

Hey Woelen. Let's say somebody was going to make some red selenium and had the powdered black selenium, a bunch of glassware, a bunch of glass tubing, a working fume hood, nitric acid and time. The one thing he was missing were sulfites and/or bisulfites and he didn't have enough time to order some and have them arrive in time for the plans he has made. On the bright side, this individual does have many tanks of oxygen gas and plenty of pounds of elemental sulfur. Would it be possible to just burn the sulfur in an oxygen atmosphere and direct the SO2 gas into the reaction vessel with the HNO3 and Se in order to ppt out the red selenium?

[woelen]

Yes, of course that should be possible. The problem here is more of a practical and instrumental nature than of a chemical nature. Indeed, getting the SO2 from the burning sulphur into the HNO3 may be a pain.

 

A practical setup may be the following:

 

Take a large glass pot or beaker (1 liter or something like that). Take some burning sulphur and put this in the pot. Cover this with a glass plate or screw cap. The sulphur burns for some time and then stops burning, due to lack of oxygen. Then, let cool down and take out the sulphur, which is connected to a small wire. Do this quickly, so that you don't loose your SO2. Then pour in your selenium solution and swirl a few times. SO2 dissolves in water quite well. This setup can easily be done without all kinds of glassware and apparatus. Even better would be if the air in the beaker is enriched with some oxygen, before the burning sulphur is put in.

[jdurg]

Okay I'm back from the experiment with all appendages intact. I discovered that if you use a powdered form of selenium, absolutely no heating is needed at all. We took a large Erlenmeyer Flask and added all of the powdered selenium to it. We then added the nitric acid and a nice dark cloud of NO2 formed above the surface. My friend's fume hood works REALLY well as not once did we get an odor of the nitric oxides nor any selenium compound. The dark brown/red NO2 was neat to observe though.

 

When all of the NO2 finally dissipated, we were left with an orange solution. On your experiment page woelen, the third picture in your series of test-tube photos is exactly what we saw upon diluting the concentrated nitric acid solution with the selenium in it. We didn't add a single crystal of anything, but the solution was that exact same orange color. We then looked through the chemical supply and was disheartend to find that we had no reducing agents. No sulfites, no phosphites, no nothing. An attempt was made to reduce the selenium ions to selenium via iodides, but that just resulted in the formation of iodine and a thick, syrupy black tar which floated on the surface of the water. Makes me wonder what that was. (The only thing in the solution was the KI, HNO3, and the dissolved selenium).

 

In order to ppt out the red selenium, we had to have a source of SO2. There was a ton of sulfur available so we decided to try and burn that. It worked, but very poorly with a pathetic yield. The sulfur just wouldn't ignite no matter what we did. We heated it to the point where it was actually starting to vaporize, but we were never able to get it to react with much oxygen at all. A little bit did form and got into the selenium solution, and we wound up with maybe half a milligram of red Se. So the solution is now in storage until some sulfites can be found.

[YT2095]

a large glass vessel a cork with 2 holes in for 2 pipes, one goes to a fish tank air pump, the other is the outlet pipe.

in a crucible set fire to a mound of sulpher, and lower it into the vessel, put the cork into place and turn on the air pump, within a few seconds you`ll have a steady stream of SO2 gas coming out of the Outlet tube to bubble through whatever you like

 

that`s all I use, and it works great

[woelen]

When all of the NO2 finally dissipated, we were left with an orange solution. On your experiment page woelen, the third picture in your series of test-tube photos is exactly what we saw upon diluting the concentrated nitric acid solution with the selenium in it. We didn't add a single crystal of anything, but the solution was that exact same orange color.

Probably there were tiny amounts of reducing contaminants already present in the air. My experience with solutions of selenium (IV) compounds is that they are reduced a very little bit quite easily (e.g. dust, organic matter in the air).

 

 

We then looked through the chemical supply and was disheartend to find that we had no reducing agents. No sulfites, no phosphites, no nothing. An attempt was made to reduce the selenium ions to selenium via iodides, but that just resulted in the formation of iodine and a thick, syrupy black tar which floated on the surface of the water. Makes me wonder what that was. (The only thing in the solution was the KI, HNO3, and the dissolved selenium).

That "tar" is iodine, with dissolved selenium and SeO2. Due to its hydrofobic nature it tends to float on the water. Most likely there also are some bubbles of NO and/or N2O in it as contaminant. There also is another name for this stuff: "useless crap" .

 

In order to ppt out the red selenium, we had to have a source of SO2. There was a ton of sulfur available so we decided to try and burn that. It worked, but very poorly with a pathetic yield. The sulfur just wouldn't ignite no matter what we did. We heated it to the point where it was actually starting to vaporize, but we were never able to get it to react with much oxygen at all. A little bit did form and got into the selenium solution, and we wound up with maybe half a milligram of red Se. So the solution is now in storage until some sulfites can be found.

What sulfur do you use or how do you light it? My sulfur burns very neatly. I can light it fairly easily, when it is heated somewhat, with a match.

[woelen]

I did another interesting experiment with the selenium. I now have CS2 and this opens up new possibilities.

 

Black selenium does not dissolve in CS2 at all. I have some powdered black selenium, consisting of many minute crystals. It does not dissolve, nothing at all.

 

I also have those little corpuscles, 99.999% Se, from http://www.emovendo.net, an element-collectors supplier. These pieces are grey and they dissolve in CS2 a little bit. The smooth silky-looking surface if these corpuscles becomes a little pitted and irregular, so apparently they are a mix of different Se-allotropes, probably made by melting the element.

 

Again, I made red selenium with black Se, HNO3 and Na2SO3 and shook this solution with some CS2. The red stuff for a large part dissolves in the CS2, the solution in CS2 becomes yellow/green/brown.

 

Next, I let the CS2 evaporate. Nice little crystals remain. Unfortunately, these crystals are not red, but very dark red/brown/grey. I'm not sure what allotrope this is. it may also be a mix of allotropes.

 

But it is nice to see that there is a solvent for selenium.

 

Btw, sulphur dissolves in CS2 REALLY well. If you take 1 ml of CS2 and you add a large spatula of flowers of sulphur to this, then the sulphur dissolves almost at once and a nice yellow solution is obtained. It is really surprising to see how well sulphur dissolves in CS2. Selenium dissolves with more difficulty.

[jdurg]

Hey Woelen, here's another little 'experiment' to test out with your red selenium. You mentioned how you had made a bunch of red selenium and that it stayed red fro a long, long, long time. My question to you is, at what temperature was your selenium kept?

 

My reason for asking is that apparently the transition temperature at which red selenium begins to form gray/black selenium is about 45 degrees Celsius, and at higher temperatures the conversion takes place much more rapidly. (45 Celcius being equal to about 113 Fahrenheit). A friend experimented with some red Se and said that when he heated a vial up slowly to 45 degrees C it started to darken and turn gray. At 55 degrees C, it pretty much instantly turned black.

 

When I originally bought my ampoule of selenium, it was somewhat red but was quickly darkening. I guess what happened is that during shipping, the temperature of the ampoule was raised above that 45 degrees C mark and that started the change. In your experiments, you stated that the one you kept wet seemed to stay a brighter shade of red. I'm guessing that the high specific heat capacity of the water allowed the Red Se to stay red longer than the sample that was kept outside of the water.

[woelen]

I did your experiment with heating the red selenium. I made some red selenium and left this suspended in water for a day. I decanted the liquid with the SO2 and HNO3 and left the red precipitate. I added new water and swirled. This results in water, with a lot of fine red precipitate. This precipitate remains red for a long time.

 

Next, I heated the water with the red suspension, until the water was steaming (not boiling yet). Indeed, the color of the selenium shifts towards dark grey. The fine particles also stick together and finally I had a larger somewhat lumpy piece of precipitate, which was black.

 

The shift from red to black took a few minutes when heating. So, indeed, you are right with your explanation about transition temperature. I could not precisely determine the transition temperature (I do not have such a nice lab thermometer), but I certainly could confirm the existence of a transition temperature.

Once the product is black, it does not revert to red. On cooling down, the selenium remains black.

[jdurg]

Awesome! Thanks for the confirmation Woelen. I'll have to make sure that my red selenium stays nice and cool and doesn't heat up. What's strange is that on the shattered black-glass selenium slug that I have, on some of the broken edges the black selenium has slowly turned red. It's truly remarkable the differences in the allotropic forms. I should take a photgraph of my red selenium next to my red phosphorus. The two look nearly identical, with the phosphorus being slightly brighter.

[jdurg]

Another little update on the selenium. It arrived in the mail the other day sent in a cold pack so it arrived nice and bright red. Similar in color to my red phosphorus, but not nearly as bright. The only problem was that the vial had a 'fog of red Se' attached to the glass. I decided to transfer the red Se from the vial to another vial I have that is more inline with how the rest of my sample containers look. (Hey, I'm anal retentive. That's how my collection got where it is. ) Anyway, I discovered quite a few things.

 

Even after being filtered and dried, the selenium holds onto remnant vapors of nitric acid VERY well. I opened up the vial and was nearly taken aback by the strong nitric oxide fumes that rose out. I also noticed that selenium containing gases smell REALLY bad. WOW! I'm not sure if it was hydrogen selenide that I smelled, or some selenium oxide, but it smelled like rotting flesh and nitric oxide becuase of the two smells. Uggh. I now have it in a new container where the glass seems to be fogging up a bit. I wonder if I should filter the selenium even more. The problem is that I don't want to lose the red Se that I do have, and it's pricey getting some nitric acid. I may have access to some stuff that a lab is getting rid of at the end of the year, but concentrated HNO3 is definitely not something I want hanging around me.

[woelen]

I left my selenium in the open air in a warm place for a few days, taking care of that no dust can come in(remember, it was summer when I did my experiments) and it is nicely dry and has no nitrous vapors clinging on it. Nor does it have a bad smell. All nasty stuff has outgassed quite well.

But, there is another bad thing with my sample. It has become darker . Not much, but definitely darker than when I made the picture for my web site. I'm afraid that in the long run, red selenium simply is not stable when it is dry and that if you really want it in your collection you have to replace it every year or so . If my sample indeed becomes dark, then I'll stop with this red selenium stuff and then I just store the grey corpuscles and keep my nice picture as an in memoriam for the red allotrope . I do not want to keep a wet sample under a dilute acidic sulfite solution.... and besides that, how long will that last ?

 

But why don't you want to have some nitric acid around? If you store it in a well capped bottle and put the bottle in a plastic bag, what can happen with it? In my home lab I have a few liters of concentrated acids and a few liters of solvents around. Of course, you should not store these chems at places where you sleep, prepare your food or are living for several hours a day.

[woelen]

Time to bump this old thread again. I found another way to make red selenium from black selenium. Not for my element collection anymore, but I found it by coincidence.

 

Take some concentrated H2SO4 (96+ %)

Add some black selenium. If it is in the form of small corpuscles, then crush it to a powder before adding to the acid.

Heat the acid, until it is a mobile liquid like water and faint white fumes are formed.

The liquid becomes dark green, due to formation of cationic species of selenium, which are dissolved in the acid.

Let acid cool down. It remains dark green and clear. It becomes viscous again, like H2SO4 always is.

 

Pour the acid in a large amount of water. The red allotrope of selenium is separated as a fine brick-red powder.

 

A word of warning to anyone wanting to repeat this experiment: Be VERY careful with heating concentrated sulphuric acid to the temperatures involved in this experiment. Temperatures are well over 200 C. You absolutely must trust your glasswork. Cracking glasswork can be really destructive with this hot concentrated sulphuric acid.

[Tartaglia]

The toxicity of selenium would put me off doing the last prep more than heating conc H2SO4

[woelen]

No, you do not need to worry about any selenium escaping from this hot liquid. According to my book (Chemistry of the Elements, from Greenwood and Earnshaw), a cationic species is formed, which dissolves in the sulphuric acid. Because an ionic species is formed, it cannot escape (just like that a solution of NaCl on heating never will give off any NaCl). The exceedingly corrosive ultrahot sulphuric acid is more scary to me.

 

But I agree with you, selenium compounds are quite toxic at higher doses (although you need small doses). Remarkably, the pure element hardly is toxic, it leaves your body unchanged. Compounds like H2Se and SeO2, however, are VERY toxic, much much more than the related compounds H2S and SO2.

[Tartaglia]

Woelen - No offence but you strike me as an amateur. I used to have an Office next to Norman Greenwood and I know Alan Earnshaw too. In the eight years in which I made a living as a synthetic chemist I saw people leave labs when others were using Selenium. A horrendously toxic element. I personally would not recommend that anybody does an unnecessary syntheses using selenium

[woelen]

If selenium is so horrendously toxic, then why are there still selenium toners in photography, used by many photographers? These are not old-fashioned, removed-from-market curiousities, but you can still purchase them at many ordinary photography shops. Another thing, why can I buy pure elemental selenium for just $9 per 30 grams without any question asked? The stuff even is shipped without any restriction overseas, in just a normal letter envelope for $1.60 US to Europe postage. See http://www.emovendo.net/store/customer/home.php?cat=256 for more info. At this place I purchased a few of these 30 gram packages.

 

On the other hand, if I want to buy 30 grams of I2 from the same seller from the USA (shipping to Europe), then I have to pay hazmat fees like crazy (starting at $20, going up to $150 if quantities are raised somewhat). In practice this seller does not ship I2 at all internationally, because it is not worth the cost and the hassle.

[Tartaglia]

Unfortunately price is not correlated with toxicity. Please be my guest and carry on using it. I won't be joining you.

The basic problem with selenium is the low ratio of required amount to lethal amount. From my days as a chemist I believe the ratio is the lowest of all required trace elements

[woelen]

Of course price is not correlated with toxicity, but my point is that the selenium is shipped in an ordinary envelope at very low cost, while for instance I2, but also Ag2O, K2Cr2O7 and many other chems require outrageous hazmat fees, when they have to be shipped overseas. If you can give an explanation for this, then you're welcome. I also found another supplier for selenium (JD Photochem) and this one also sends the material to me without any hazmat hassle.

Apparently it is not THAT toxic. But of course, I agree with you that it is toxic and one should not play with it without knowing its risks. But this is true for most chemicals.

[Tartaglia]

The inconsistencies of poison regulations has always been a surprise to me and is nothing new.

[jdurg]

I've inhaled numerous amounts of H2Se and other selenium containing gases. I'm still alive and in quite good health. The supposed toxicity of selenium is a bit overdone, I'm afraid.

 

And here's some data to back that up http://www.oehha.org/air/chronic_rels/pdf/selenium.pdf: In 1991 a clinical study was undertaken by 142 patients in western South Dakota who lived in areas where a great deal of selenium exists in the environment and large amounts of selenium were ingested. NONE of the subjects showed any clinical toxicity to selenium even though many of them had ingested amounts greater than 200 ug/day. The study goes on to show that intakes of up to 0.853 mg/day of selenium results in no toxicological findings.

 

So the whole reason why there is no major hubbub over Selenium toxicity is because it isn't really toxic. Is it something you want to be ingesting like it's candy? No, but then again I don't know of anything that is. I've worked in labs where people constantly left the lab any time an aluminum compound was used, but does that mean that aluminum is the most horribly toxic thing on earth? No. They left the room because it was a crowded lab and the sodium aluminum hydride they were using required special care. As a scientist you should know that it's not a good idea to make assumptions without having a full set of data.

 

The human body requires about 65 ug/day. Based on the VERY thorough study linked to above, up to 853 ug/day of selenium can be intaken without ANY toxicological symptoms being shown. That's a ratio of about 13.12 to 1. For Chromium, if my memory serves me right the suggested daily intake is about 100 ug/day, but toxicity begins to show at around 1,200 ug/day. That's a ratio of 12:1 which makes chromium the most toxic trace element that I have found. (Though I think that the fluoride ion takes the cake there).

[woelen]

I agree with you, Jdurg. Of course, I'm very careful with selenium and on my website I warn for Se-compounds, but we should not exaggerate on things.

 

 

For Chromium, if my memory serves me right the suggested daily intake is about 100 ug/day, but toxicity begins to show at around 1,200 ug/day. That's a ratio of 12:1 which makes chromium the most toxic trace element that I have found. (Though I think that the fluoride ion takes the cake there).

Just for fun : This is a very funny definition of toxicity. According to this definition, I think water is the most toxic compound.

Suggested daily intake is around 1.5 liters for a grown up healthy person. A dangerous dose, however is 10 liters per day (for many people even much less). That's a ratio of at most 7 : 1, probably even lower .

[Tartaglia]

I find it incredible that I appear to be on te backfoot here when I consider safety to be the most important aspect of synthetic chemistry. I will continue to put safety first. This thread was put up without drawing any attention to selenium's toxicity. The thought of someone, who doesn't know what they are doing, reading this thread and going off a boiling up selenium in concentrated sulphuric acid in their garden shed doesn't bear thinking about.

 

Jdurg - I have a friend with substantial burn scarring from an ether explosion caused by LiAlH4 coming in contact with water. There are many reasons to be careful in a lab. Toxicity is not the only one

[jdurg]

I find it incredible that I appear to be on te backfoot here when I consider safety to be the most important aspect of synthetic chemistry. I will continue to put safety first. This thread was put up without drawing any attention to selenium's toxicity. The thought of someone' date=' who doesn't know what they are doing, reading this thread and going off a boiling up selenium in concentrated sulphuric acid in their garden shed doesn't bear thinking about.

 

Jdurg - I have a friend with substantial burn scarring from an ether explosion caused by LiAlH4 coming in contact with water. There are many reasons to be careful in a lab. Toxicity is not the only one[/quote']

 

Undoubtedly. And safety should NEVER be ignored when working with chemicals. In the case of the experiment spelled out by woelen here, anybody who has the equipment and materials to carry out the experiment would have to inherently be aware of the safety requirements of the experiment. Those who do not would simply not understand what is going on and would have no need to understand.

 

With selenium one has to be concerned with the ingestion of the compound. I don't know about you, but I would NEVER drink ANY beaker of ANYTHING in a lab. In addition, I would never have any food, cigarettes, or drink in a lab that is currently functional. Therefore, the main route of entry would be inhalation of selenium. The only gas I know of that has selenium in it which may be encountered in a home lab,and may cause some problems, is H2Se. SeO2 is a crystalline solid (Unlike SO2) so therefore it will not be volatile. In the reaction all Selenium is in the form of an ionic species which is 100% not volatile. As a result, it cannot escape the solution. An analogy to this is working with chlorides and chlorine containing compounds. Chlorine is a horribly toxic substance, but when working with the chlorides one doesn't have to worry because the chlorine atoms are ionic in nature and won't escape any solution. Safety wise there is nothing wrong with the experiments laid out by woelen. I can appreciate your concern in regards to the toxicity of selenium, but it is crucial to remember that some things can be perfectly safe in one form but horribly toxic in another. (Nitrogen in the form of an ammonium ion is perfectly safe, but nitrogen in the form of an azide or cyanide ion will kill you pretty quickly).