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Author: Subject: How To Divide a Radioisotope Source?
careysub
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[*] posted on 6-7-2016 at 17:22
How To Divide a Radioisotope Source?


I am half-owner of a Tc-99 source that I need to divide with the co-owner, so that we will both have a technetium sample for our element collections.

Upon receipt of the source, and examination of it, how best to divide it is not clear to me. It is not highly active, so safety is not a concern.

The source appears to be a minute fleck embedded in a small dab of epoxy deposited in a shallow recess in a machined plastic bar.

Due to the tiny size of the source my preliminary notions of how to go about it have to be discarded.

Removing the source from the bar is not difficult. Epoxy softens when heated, to a hot scraper will scrape up the dab easily (I am sure). But then what?

There are two approaches that occur to me.

The first is mechanical. Make a DIY microtome from a bolt, embed the dab in a bit of paraffin, then use a razor to slice off thin sections, checking each for activity (it is fairly easy to get 10 micron slices this way).

The second is to extract the Tc-99 from the source and then redeposit it. A minimum of manipulation would be desired.

Epoxy does not dissolve in anything, however it swells and softens in some solvents, DMF is best (and I have some). It appears the epoxy will swell and disintegrate in DMF after a while (museum conservationists use this on occasion).

But then what?

Technetium chemistry most closely follows rhenium, the rarest stable element. A rhenium carrier might be helpful.

The label on the source asserts that it is (C6H5OH)4TcAsO4 by the way, a rather odd compound. TcAsO4 is technetium arsenate, like manganese or rhenium arsenate, I don't know how 4 phenols would fit into the picture. But I would not assume this label is accurate.
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phlogiston
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[*] posted on 7-7-2016 at 00:52


Epoxy dissolves in hot fuming nitric acid. This is used to 'decap' IC's (dissolving the packaging's resin to expose the die).

No idea what that will do to your Tc compound though. If it does not dissolve, then you are left with a tiny speck of dust that will may be even more difficult to handle and split.

Is it important to you both to split the sample very precisely 50:50?
I imagine that the microtome method will still have the bulk of the activity in only a few slices.

How about powdering the epoxy-embedded sample (eg by cryogenic grinding)?




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careysub
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[*] posted on 7-7-2016 at 05:47


Certainly a sufficiently aggressive reagent can actually destroy epoxy chemically, which is what the hot fuming nitric acid does I imagine.

I would like to get the division as close to 50:50 as I can.

I have found the reason for the tetraphenylarsenate compound. From
Radiochemistry of Technetium:

"Because of the great differences in vapor pressures of the acids and heptoxides at low temperatures, Tc may be separated from Re by repeated alternating evaporation with HNO3 and HCl . The technetium In the distillate may be coprecipltated with tetraphenylarsonlum perchlorate."

So this is no doubt the precipitation end product after a separation process of technetium from manganese and rhenium present in the fission products.

The volatility of Tc is a concern. Distillation is how they separated it. ("Owing to the volatility of TC207, technetium the +7 oxidation state may be co-dlstllled with acids.") I do not want to volatilize the Tc, that sounds like a good way to lose it.

More useful information about the tetraphenylarsonium salt:

"The coprecipltatlon behavior of pertechnetate Ion has been Investigated In some detail. It forms sllghtly soluble salts with large cations, e.g., Tl+; Ag+, Cs+, nltron, and
(C6H5)4As+. At 0° C, precipitation with the last-mentioned reagent Is feasible at concentrations as low as 5 mg Tc/L. At lower concentrations, any of the XY4- anions may be used as carriers: e.g., Re04-, Cl04-, I04- and BF4- . In acid solutions, molybdenum also precipitates, but in basic solutlons, It does not Interfere. Decontamination from other flsslon products is excellent: a single-step decontamination factor of 100,000 has been reported. The principal remainlng contaminants are Zr, Nb, and Ru . Copreclpltatlon with (C6H5)4AsRe04 Is probably the fastest known separation method for Tc; when milklng 5 sec. Tc102
from 11.5 min. Mo102 , Flegenhelmer et al were able to precipitate and filter the sample In 5-6 seconds."

Since the pertechnetate ion (+7) is stable in oxidizing conditions, frequently prepared using H2O2,

I am thinking perhaps of immersing the epoxy in DMF for some period of time, until is visibly disintegrates, then adding H2O2 solution, so that (one hopes) the pertechnetate ion goes into solution.

After that evaporation is no good, since DMF boils at 154 C, and I am concerned about volatilization. So reprecipitation or extraction into low BP solvent is called for, unless we decide to simply use the solution as the display sample.

Additional data about extraction:
"Tetraphenylarsonium chlorlde.
Pertechnetate also extracts into chloroform In the presence of tetraphenylarsonium
3 ion . The equilibrium constant for the reaction:
[(C6H5)4As+]aq + TC04- <-> [(C6H5)4AsTcO4]org
Is ~3000000. The corresponding constant for chloride is 200. At chloride concentrations of 1M or less, distribution coefficients as large as 100 can easily be realized at reagent concentrations of 0.007M. In neutral or basic sulfate solutlon the distribution coefficients are even more favorable. Perrhenate and perchlorate follow technetium in this extraction, but molybdenum does not Interfere in basic solution, and the decontamination from other fission products should be at least as good as in the precipitation reaction if care Is taken to remove iodine before extraction. Trivalent gold as chloro- or bromoaurate is also extracted and must be removed,by reduction with H2O2, or by distillation.

It is advisable to have small amounts of an oxidizing agent, such as hydrogen peroxide, present during the extraction, so that TcO4- will not be reduced by impurities In the reagent. Any wash solutions used should contain a sufficient concentration of tetraphenylarsonium ion (about 0.00005 M) to prevent premature back-extraction of TCO4- Into the aqueous layer.

The technetium may be recovered from the organic phase by back-extraction Into 0.2 N HClO4 or 12 N H2S04. In the
latter case, three back-extractions are required."

The extraction into chloroform could be performed, but I would need to get some tetraphenylarsonium ion I presume. Then reducing the pertechnetate might be a good idea before evaporation to reduce volatility.

I find in a paper about rhenium determination:
"The perrhenate ion unites with the tetraphenylarsonium ion to form a white, crystalline precipitate which is insoluble in cold water."

So perhaps the DMF-water extract could have ammonium perrhenate added, followed by tetraphenylarsonium chloride to precipitate it, carrying down (presumably) the Tc.

However, I don't have any tetraphenylarsonium chloride, it is expensive as a reagent (Sigma-Aldrich lists it at $75 for 2.5 g, about 1,000 times more than I would need) and making it is not an option (the Org-Syn process is quite hairy).

[Edited on 7-7-2016 by careysub]
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[*] posted on 7-7-2016 at 06:23


Here is something about the treatment of Tc/Re ion that has been removed from aqueous solution by ion exchange resin:

"After a second water rinse, both perrhenate and pertechnetate were displaced quantitatively with 2N NaClO4 or 2 HClO4. The eluent solution usually contained approximately 5-10 mg. of rhenium, and microgram or smaller quantities of
technetium, although sometimes as much as one gram of rhenium was present. After acidification (if necessary), the eluate was heated, treated with bromine water to oxidize any reduced technetium, and then treated with H2S gas to precipitate rhenium and technetium sulfldes."

So perhaps adding ammonium perrhenate to a DMF-water extract, then gassing with H2S would precipitate them both as sulfides. Much more convenient than messing with arsenic reagents.
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[*] posted on 7-7-2016 at 10:27


Here is a proposed procedure which I like since it is all done in one tube, and produces a material that can be divided by weight.

This would all be done at the microscale, which I am equipped for.

Immerse epoxy dab in DMF until it disintegrates. Ultrasonic agitation may be applied if necessary.

Add H2O2/water to create the soluble pertechnetate ion.

Add ammonium perrhenate (probably about 100 mg).

Bubble H2S through the mixture to precipitate the sulfides.

DMF is removed by repeated dilution and syringe decantation.

The residue is dried in a dessicator.

I can practice the whole procedure with an old dummy epoxy sample, before trying it with the source.

Any comments about the proposed procedure?
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[*] posted on 7-7-2016 at 12:28


One of you has the source Jan Mar May... etc and the other has it the rest of the time.
That way you avoid the frankly dangerous trick of playing with unsealed sources.
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[*] posted on 7-7-2016 at 13:44


Quote: Originally posted by unionised  
One of you has the source Jan Mar May... etc and the other has it the rest of the time.
That way you avoid the frankly dangerous trick of playing with unsealed sources.


Frankly dangerous?

I discarded the temptation to use irony or sarcasm to respond on a site where all manner of dangerous chemicals and procedures are discussed many times a day without such vague scare-mongering. I don't see you over on the Energetic Materials thread that today that was discussing making high N nitrocellulose, warning that this is dangerous and no one should ever do it.

I did mention that the source was "not highly active" as a notice that we are not dealing with anything that could ever be a health concern, but clearly this did not register.

To be more specific, the amount of activity we are dealing with is about the same total activity as the potassium in your body. Further it is a mild beta emitter, with no gammas.

A bag of this in my local grocery store:
https://www.instacart.com/stater-bros/products/2753074-morto...
has 80 times as much radioactivity.

Radioactivity is not magic. Its not witchcraft. Honest its not. Risk can be assessed and controlled just like any toxic chemical (which means, nearly all of them).

In a number of ways the risk with radioactivity is easier to manage than with other chemicals. You can detect contamination easily with a single reliable instrument - try that with a carcinogen or toxic metal that might contaminate any surface in the lab. It is impossible to create more radioactivity, or different radioactivity, in the lab. Toxic side reactions do not exist. If you start with a safe quantity, like what we are working with it, it will always be a safe quantity.

[Edited on 7-7-2016 by careysub]
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[*] posted on 7-7-2016 at 15:08


KCl WILL make a Geiger counter go wild. Got several bags of the stuff sitting around in storage and to feed the softener with.
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[*] posted on 8-7-2016 at 00:41


Couldn't it be that this sample looks like it is not very active because the beta-radiation is blocked by the resin? IIRC beta radiation doesn't penetrate a sheet of paper right?
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[*] posted on 8-7-2016 at 01:22


alpha doesn't go through paper. Beta will be stopped with about an eighth of an inch of sheilding.
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[*] posted on 8-7-2016 at 01:26


alpha radiation is easily blocked by a sheet of paper (or your skin epidermis). Beta radiation can typically pass through a few millimeters of metal, but the penetrating power depends strongly on the energy of the particles. Very energetic beta particles are best blocked with low-atomic weight materials (acrylic is often used because it is conveniently transparent), to reduce brehmstrahlung formation.
The decay energy of Tc-99 is approx 294 keV, which is pretty low. The epoxy will probably shield some of the radiation, but I suspect careysub knows exactly what the activity of the sample is because it will have been specified when he purchased the source.

edit: BTW, Considering the widespread complaint on this forum about chemophobia, the mad scientists here are surprisingly radiophobic. Apparantly, fear of the unknown/poorly understood is not alien to forum members either.



[Edited on 8-7-2016 by phlogiston]




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[*] posted on 8-7-2016 at 08:40


Quote: Originally posted by phlogiston  
Apparantly, fear of the unknown/poorly understood is not alien to forum members either.
[Edited on 8-7-2016 by phlogiston]


This is only human nature!

But I agree, everyone take a deep breath.
Now exhale.

PS. The profundity of home technetium chemistry seems to have passed over everyone's heads. Or maybe it's not that extraordinary?


[Edited on 8-7-2016 by DoctorOfPhilosophy]
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[*] posted on 8-7-2016 at 12:55


Yes, It even seems to be a first for the forum, search for 'technetium' only yields this thread. Boy, that must be the only element on the periodic table that hasn't been mentioned here before!

Edit: never mind, I misspelled it. To compensate for my mistake, I searched every element name on the periodic chart and found that all of them have been mentioned at least once. Einsteinium and Mendelevium only twice though (now three times :) )

[Edited on 8-7-2016 by phlogiston]




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