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Polverone
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[*] posted on 7-1-2003 at 12:33
a convenient radiation source


Have you ever wanted to experiment with radiation? Many mad scientists have. But even if regulations don't prevent you from ordering low-level radioactive materials like thorium and uranium compounds, the special shipping required is really a pain.

However, some electrodes used for TIG and plasma welding are "thoriated" - they contain 1% or 2% thorium oxide. The oxide is supposed to stabilize the arc or somesuch. There is some trend to move away from thorium for health reasons, but a recent visit to a welding supply outlet discovered a large variety of thoriated electrodes.

I obtained a couple of the smallest-diameter electrodes they had since I wished to dissolve them. But... problem! These electrodes are made out of tungsten, and extremely resistant to common acids. Lange's Handbook of Chemistry suggested that I dissolve tungsten in a mixture of HNO3 and HF - no thank you! Fortunately, a search found the following:

Quote:

The dissolution of tungsten by hydrogen peroxide is little known... Consequently, when it was recently observed that commonly encountered commercial tungsten products such as powder, wire, and sheet are completely dissolved in 30% hydrogen peroxide within minutes to hours, it was thought that these findings should be reported because of their interesting implications for both analytical and inorganic chemistry, and for metallurgy.

...

The rate of dissolution was observed to increase with temperature; however, at temperatures above 60 degrees Celsium, an increasing rate of spontaneous decomposition tended to slow the reaction. Approximately 60 degrees C was found to give the most rapid dissolution.

...

Tungsten may be converted into various of its compounds via dissolution in hydrogen peroxide. Evaporation of the solution and drying at not over 100 degrees C produces yellow crystalline pertungstic acid which is freely soluble in hot water. Higher drying temperatures (ca. 180 degrees C and higher) convert the pertungstic acid to tungstic acid, or anhydrous tungsten trioxide, wich may be reacted and dissolved in accordance with their known properties. Alternatively, the alkali tungstates may be produced by addition of alkali hydroxide to the pertungstic acid solution, followed by boiling to expel both the free and combined hydrogen peroxide.



One could presumably fail to boil the solution mentioned above to recover sodium pertungstate instead of sodium tungstate. The authors found that 5 mil tungsten wire dissolved in 30% H2O2 at 60 degrees C in 1 1/2 hours, 10 mil wire took 2 1/3 hours, 15 mil wire 2 3/4 hours, 25 mil wire 3 hours.

Dissolution of Tungsten by Hydrogen Peroxide
P. C. Murau;
Anal. Chem. ; 1961; 33(8); 1125-1126.

So now you know convenient methods for preparing pertungstic acid, tungstic acid, and tungsten trioxide, as well as releasing the radioactive component of thoriated electrodes.

This TIG electrode has been sitting in warm H2O2 all night, and it is slowly but surely dissolving. There is a thin layer of white powder at the bottom of the container that I'm guessing is the thorium oxide. I will have to be careful when it comes to handling and separation lest I end up an unwitting player on Who Wants to Inhale an Alpha Emitter?
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blazter
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[*] posted on 7-1-2003 at 15:10
safety anyone?


If I were going to be working with any kind of radioactive materials, I would certainly become the proud owner of a geiger counter and some hefty lead shielding first. Not only would you be able to make sure that your clothes weren't carrying radioactive dust afterwards, but it should make identifying that mystery precipitate much easier... just my $0.02
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[*] posted on 7-1-2003 at 15:51


The high density of Thoriumdioxide would probably make the inhaling hazard insignificant. Also, alpha radiation doesn't even penetrate paper, let alone clothing.



One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Polverone
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[*] posted on 7-1-2003 at 17:00


Thorium oxide is quite dense. But the powder may be very fine, so I will be careful anyhow. Thorium isn't terribly radioactive; otherwise it wouldn't be in over the counter items like (older) lantern mantles and TIG electrodes in the first place. Alpha particles are easily blocked by the skin but they can cause considerable damage if their emitters get inside the body. If I had a geiger counter I would like to compare the activity of what I get from the electrodes with a sample of uranyl nitrate that I have - although I'm not sure an ordinary geiger counter can even register alpha particles, given their low penetrating ability. If anything, I'll be making a small cloud chamber to watch the particle tracks.
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[*] posted on 9-1-2003 at 07:06


That's an interesting bit of info you've found, I'll have to try it. Maybe the pertungstates would be interesting too.
Do you think the powder is thorium oxide or thorium pertungstate?
If I were you I'd add something like gelatine or a bit of PVA glue to the wet powder after decanting off the solution and washing it by decanting it from distilled water. That way it shouldn't create as much of a dust problem.
The thorium you get should have a very low activity compared to things like smoke detectors (37kBq from a few ug of Am-241!), it has an activity in the order of kBq's/g (more active than U-238 though).
A normal geiger counter will detect alphas of reasonable energies, because normal ones are not cheap. But some of the cheaper ones will only detect gamma! Yeah, definitely make a cloud chamber, they're neat and the diffusion type can be put together in a few minutes from a glass jar.
And you could do cool experiments with the radium gas, seal some activated charcoal in a chamber with the thorium for a few weeks, and the daughters of radon will "plate out" on the charcoal, providing a short half-life source which you can make measurements with if you have a good counter.

Continuing with the radioactive sources theme, uranium glass is easy to find on eBay in the form of marbles or chunks and is slightly active, IIRC it contains about 1% U depending on the exact glass. You can also get uranium minerals, I got some autunite and this and the glass looks really good under UV, I was surprised how bright it was.
If you're the kind of person that likes dismantling things to see what's inside, like me, then an old magnetron will have a thick filament inside which is usually thorium doped, probably similar in % Th to welding rods. Flourescent lights often have thoriated filaments too.
Radium dials are another obvious source.
If you want a source of a more exactly known activity and emission type, then smoke detectors are good. They're practically a point source too.
Also I got a cool keyring, with a tritium vial inside. It glows quite bright, you can read with it in the dark if your eyes are adjusted. It probably contains MBq's, but all that escapes are weak X-rays from betas hitting the glass.
Potassium salts are active, IIRC a 25kg bag of KNO3 will have an activity of around 141KBq, but that's from memory and may not be accurate. There's B, Y and some B+ from K-40. Anyone know how to do an isotope concentration on potassium? I've always wanted to try that (electrolysis maybe).
Rubidium is radioactive, c. 800Bq/g IIRC. A third of it is an isotope with a very long half life.
I was looking at a site a while ago, and LOADS of elements are naturally radioactive, but at VERY low levels due to relatively, but not quite, stable isotopes. Zn and W were I think, and a load of lanthanides, and some in the platinum group, and more.
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Polverone
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[*] posted on 9-1-2003 at 12:16


I don't think that the thorium oxide has reacted with the tungsten compounds. I do think that I may need to add some more liquid and raise the heat, though, because I now have a layer of white sludge that is too plentiful to be the thorium oxide alone. The pertungstate and tungstate (unlike most tungsten compounds) are supposed to be soluble, at least in hot water, so we'll see. One thing I've worried about is that my H2O2 may be stabilized with phosphoric acid, so that I'm precipitating tungsten phosphate. I will have to try testing a sample to see if it has phosphoric or other mineral acids in it.

There is a considerable amount of naturally radioactive potassium about, but like you I don't know how I would go about concentrating the one isotope.

Glowing tritium items can have a surprising amount of activity. I was at a church a few weeks ago that has tritium-powered exit signs around all of the doors. Each one contained 7 curies of tritium, and there were six signs total.

I do have a smoke detector around here that I intend to dismantle for the radiation source. Hopefully I'll be able to identify the source by process of elimination with the other components.
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[*] posted on 10-1-2003 at 09:43


The source in smoke detectors is very easy to identify, it's in the chamber with the radiation symbol on, the cover just snaps off and you'll see basically some sort of metal plate with a small circle of metal in the middle - that's the source. 0.3ug of Am-241 I worked out - not much!

Hmmm... I was under the impression that only a tiny amount of acid was used to stabilise H2O2 - ie, not enough to create much ppte? But maybe I was wrong. I don't know what it could be apart from what you suggested though. Let us know if you find out for sure.
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[*] posted on 10-1-2003 at 11:58
it's okay


When I added more water and heated it, most of the white sludge dissolved to form a yellow solution. There's now only a thin layer of very fine white silt on the bottom of the container. I assume that this is the thorium oxide.

Something strange, though:

I wanted to check to see if my H2O2 contained phosphoric acid. I thought I could use barium nitrate solution to test since barium phosphate is insoluble. But first I wanted to confirm that the test would work. So I dissolved a gram of barium nitrate in a small amount of water, added a few drops of a cleaning product that I know is about 30% phosphoric acid, and... Nothing. Nothing happened even when I added some 85% phosphoric acid. What have I missed? Why can barium nitrate be used to detect sulfuric acid but not phosphoric?
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[*] posted on 21-1-2003 at 10:21
More on dissolving tungsten...


I've found a few more methods to dissolve tungsten:

It will dissolve in molten NaOH, Na2CO3, and/or NaNO3, or when used as the anode to electrolyse a solution of NaOH and/or NaNO2.
That last method is the one I'm trying now. I originally tried it with just an NaOH solution (didn't weigh the NaOH, it's probably a few M in concentration), but it only produced a very slow reaction, gradually turning the surface a dark matt grey. I then added a teaspoon of NaNO2, and now it's dissolving much faster.
I'm using a car battery charger, 12V and probably a few hundred mA at most, and after 45 minutes the diameter of the tungsten electrode has been reduced from 1mm to just under 0.5mm, with 5cm immersed in the solution.

This is interesting:

http://www.blackcatsystems.com/GM/experiments/

Dust from a TV screen having an activity of 1300 CPM!
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[*] posted on 21-1-2003 at 14:27


It sounds like the electrolytic method is considerably faster than using H2O2 - although H2O2 does let you just stick a bunch of rods in a tall, narrow vessel, cover them with liquid, and heat for 24 hours or so to dissolve. Are you getting soluble tungsten compounds? That was an important point for me, since I wanted to isolate the thorium oxide by filtration or decantation. The tungsten had to go into solution.
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[*] posted on 22-1-2003 at 03:45


So far it seems that most of the tungsten has gone into solution. If it was totally insoluble there'd definitely be more solid on the bottom, although I think there might be too much for it to be pure ThO2.
I'm about to do a larger test, with eight TIG electrodes and a power supply rated at 15V, 5A DC. This should make it more obvious whether or not any of the W compound is ppting. I haven't worked out what the reaction is, I'm not sure how the nitrite helps. Any ideas?
I'll also try the fused NaOH method if I can find a crucible that isn't broken... Haha, last time I tried anything with molten NaOH, I forgot about its hunger for glass and it ended up eating through the bottom of a long neck 250mL RB flask :(. It was very sad.
BTW, you know thorium doped mantles? Well, I always thought the "doped" term commonly applied to them suggested that the thorium was a small % of the total mantle, like uranium doped glass, but apparently the ash of a used mantle can contain 95% ThO2! I'm sure it varies, but it seems that the concentration is generally quite high, the rest being BeO/MgO/CaO and rare earth oxides.
I was lucky enough to find a cheap geiger counter, a CDV-700 6B, one of the better Civil Defence models from the cold war. It can detect beta and gamma with a good range of sensitivity, makes a clicking noise and has an analogue meter, so I thought I'd buy it. It was only about twenty quid, which I thought was a bargain. It should arrive soon, so I'll be able to check what the white solid is after I've finished my electrolysis and other experiments. No alpha detection, but it'll detect the gamma part and betas from decay products. And if I ever find a cheap window probe I could modify it...
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[*] posted on 23-1-2003 at 05:33


Well my power supply has been abused too much, and fails to work.
So... back to the car battery charger. I made up a solution of 20g NaOH and 5g NaNO2 (random amounts) in 200mL dH2O, and used eight rods as the anode and a length of copper wire for the cathode.
A suitable resistor was found to limit the current to the high end of the "medium charge rate" zone on the charger's meter, this happened to be the primary coil from a MOT.
After six hours, most of the tungsten in the solution had dissolved, although some of the rods had broken off half way. So I fished out all the remaining tungsten, and it weighed 8g, which means that 8g had dissolved. So, theoretically, there should be around 0.16g of sludge on the bottom, and it looks about right. I've sucked it all up with a pipette and washed it with clean water, it's a very slightly grey powder.
My plan now is to add a mineral acid, H2SO4 probably, to dissolve the ThO2 but leave all tungsten compounds insoluble. Then I can filter it and basify it to ppte Th(OH)4...
I'll do that now, and post what happens...
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[*] posted on 30-1-2003 at 10:46


My counter's arrived, and I was able to confirm that the residue on the bottom after electrolysis is indeed the ThO2.
After reacting the equivalent of four 1mm x 150mm 2% rods, the solid (suspended in around 10mL of water) produced a count rate of roughly 200 CPM, beta + gamma, through 2mm thick glass (and an average of about 5mm water). Not bad for what is predominantly an alpha emitter. Although I should point out that the needle jumps all over the scale at these low levels because it doesn't take an average over time.
I have 36 more rods to process, so I should end up with a very measurable source!
It did not dissolve when I added H2SO4, which is strange since I know Th(SO4)2 to be soluble. It turned from slightly grey to slightly yellow. I later read that "thorium oxide is soluble in hot concentrated sulphuric acid", so perhaps it needed heating...
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Polverone
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[*] posted on 31-1-2003 at 02:32
nice


Have you tried the H2O2 method yet?

[temptation] You wouldn't have to mix the residue with hot, concentrated H2SO4 to separate the oxide. Water alone will dissolve the tungsten compounds produced and leave the thorium oxide behind.[/temptation]
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[*] posted on 31-1-2003 at 10:05


No, haven't tried H2O2 yet. I don't have any H2O2 atm., and it's not something that I actually use much so I might not be buying any more for a while. Not until my bank account is happier, anyway.
The ThO2 is easy to seperate without H2SO4, since all the W goes into solution, but I mainly wanted to convert it from such a fine powder into coarser crystals to make it easier to handle when dry.

A smoke detector Am-241 source produces well over 300 CPM @ 0.5cm from the low-energy gamma and maybe a couple of betas from decay products, and a sample of autunite that I have registers at over 10,000 CPM! I was quite surprised by this, since it's a very small amount and it isn't even registering any alpha particles, but of course the U and daughters are in equilibrium, with something like 8 daughter products, meaning that it'd be 8 times as radioactive as the equivalent amount of uranium! I hadn't thought about that before, but it seems obvious now - at eqbn, each daughter decays once per parent iostope decay, leading to high total rates.
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