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Barium and DTPA

Dear Reader,

Within my group a student will start soon on a project on the recycling of fireworks, now we have to be careful with fun sounding projects. While it is possible to enjoy doing chemistry which relates to fun topics it is important to make sure that the academic quality remains high. I would like to warn people working in fashionable areas of science and technology that for their own good that they need to avoid dumbing down their science and also avoid trying to hide poor quality work behind a fashionable label. But I will save this for another day.

Barium is commonly used in fireworks, a range of different barium compounds (barium nitrate, barium carbonate, barium oxalate and barium sulphate) are all used for different purposes in fireworks.

Today because my mind is fixed on barium we should consider some of the barium chemistry which occurs in oil fields. In an oil field the formation of barium sulphate is a major pain in the neck. Often the water in an oil field is very rich in barium but has little if any sulphate in it, while sea water is very sulphate rich. So if sea water is injected into an oil field to displace and push out the oil / gas then it is possible to form insoluble barium sulphate in the pores and cracks in the rock. This then glues up the oil field and slows down the extraction of oil or even gums up the well to the point that no more production is possible.

The great problem is that barium sulphate has a very low solubility in water, so it is not possible to rinse out the barium sulphate using reasonable amounts of water. For a poorly soluble 1:1 salt such as barium sulphate in water at a given temperature the following equation holds true.

Ksp = [Ba2+][SO42-]

When the water contains no other sources of barium or sulphate then we can write.

Ksp = [Ba2+]2

Now that was fun, now we should consider a bug bear of the oil and gas industry which is barium’s heavier sister which is radium. Now radium and barium sulfates are thought to form solid solutions (solids where radium and barium atoms are randomly replaced by each other) and their solubility constants (Ksp) are similar.

Now if we treat radium as a radioactive version of barium and assume that the properties of the two elements are identical then we can write

Ksp = ([Ba2+]+[Ra2+])[SO42-]

Now this is a great simplification of the system and I recall that the real world is a little more complex. The reason why the radium is a bug bear is that the formation of the barium sulphate scale on pipes and other equipment tends to deposit radium onto the same surfaces thus contaminating the oil field equipment. This then makes maintenance work more difficult to do.

One method of making the maintenance work more easy and also cleaning out the oil well is to use a barium sulphate scale dissolving mixture. One common method is to use DTPA, this is Diethylene Triamine Penta Acetic acid. This is a larger version of EDTA which is able to form water soluble barium complexes from the insoluble sulphate.

While I have searched the crystallographic literature for barium complexes of DTPA and I found none, but I did find a dinuclear EDTA complex of barium. It is important to note that barium is larger than calcium which is the classic metal for binding to EDTA. While calcium fits nicely inside an EDTA the cavity is too small for a barium ion.

The barium tends to spill out of the EDTA ligand’s cavity and then an oxygen from the second EDTA binds to it through an oxygen which bridges between the barium atoms. The complex contains two bariums and two EDTAs.

See G.G.Sadikov, A.S.Antsyshkina, V.S.Sergienko and A.L.Poznyak, Zh.Neorg.Khim., 2002, 47, 54 for details.

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Alpha decay part II

Dear Reader,

As I sit typing in a railway carriage on the way home sitting near a young lady who is sporting a ”Nuclear power no thanks” badge, I sit here thinking about nuclear processes hopeful that the young lady does not notice what I am typing.

It is interesting to note that one of the physical effects which regulate the reactions which go on inside the red sun of the “Karnkraft nej tak” badge are the electrostatic forces which oppose fusion. The same forces have an effect on the reverse reactions (alpha emission, fission and all the cluster emissions which come between those two extremes).

Now since I had a rather short hair cut recently I can not demonstrate electrostatic attraction using a comb dragged through my hair. I will let you try that at home, also I do not have a cat to rub on a bit of plastic so I can not use that either.

But back to nuclear processes and electrostatics, to a first approximation the atomic nucleus can be treated as a charged sphere. The size is given by the following equation.

R = Ro (A)0.3333

Where Ro is equal to 1.2 x 10-15 m, while A is the total number of nucleons (the sum of the number of protons and neutrons) in the nucleus.

So radius of a plutonium-238 nucleus is 7.44 fm, while its daughter (uranium-234) has a nuclear radius of 7.39 fm while an alpha particle has a radius of 1.90 fm. Using these radii we can calculate the energy required to push the alpha particles from plutonium-238 back inside the nucleus.

To do this we need a few more equations from A-level physics.

As the capacitance of a sphere is given by

C = 4 π ε r

Where ε is equal to the permittivity of free space which is 8.854187817620 × 10−12 F m−1 or just 8.85 × 10−12 F m−1

We can use this to estimate the electrostatic energy required to hold an alpha particle on the surface of the uranium-234 nucleus. As soon as the alpha particle is taken out of the nucleus it is no longer being strongly bonded by the very short ranged but very strong attraction between the protons and neutrons in the nucleus (the strong force). So suddenly only the electrostatic forces apply to the system (the weak force and gravity are far far weaker)

This energy (28.5 MeV) is far greater than the decay energy of the plutonium-238 (5.593 MeV), as a result the alpha particle, what has to happen is that the alpha particle must overcome this energy barrier before it can leave the nucleus of the atom. What happens is that by quantum tunneling the alpha particle leaves the nucleus of the atom and then goes on its merry way. Here is a graph of the electrostatic energy in MeV vs the distance from the centre of the nucleus for both the alpha particle and the carbon-12 nucleus.

Electrostatic energy as a function of distance from the centre of the daughter nucleus

When the calculation is repeated for the loss of a carbon-12 nucleus from plutonium-238 to form radium-226 then I have estimated that the energy barrier is 74.5 MeV, while the decay energy is now higher at 22.5 MeV you now have a bigger barrier and some other things also help slow down the release of C-12 nuclei.

I hope to get onto these things later.

The horrors of polonium

Dear Reader,

I read recently how it has been suspected that Yasser Arafat may have been murdered using polonium-210. This claim that polonium-210 has been used for another murder made me think for a moment about radium-226 and its daughters. I can tell you that polonium-210 is one of the radioisotopes which most radiation workers love to hate. Polonium-210 and radon-222 are both alpha emitters which are able to diffuse through rubber and plastics. This makes them more mobile than plutonium is. When you write plutonium you have to be careful to understand that not all plutonium is born equal.

Plutonium-239 is a long lived alpha emitter which has a moderate activity per gram (and emits few gamma photons and neutrons), pellets of plutonium dioxide which have been sintered are solids which are clean to handle inside a glove box. I used to work with a nuclear fuel chemist (John Pecket) who used to make plutonium dioxide fuel, MOX and some very werido fuels. He told me how Pu-239 was a nice radioisotope to work with, while plutonium-238 was a nightmare in comparison. Pellets of plutonium-238 dioxide emit so much heat that they tend to glow red hot, they also tend to emit plenty of radioactive dust. He told me that if you place a Pu-238 pellet in a glove box then within days every surface in the box will be crawling with radioactivity. But even plutonium-238 will not pass through a neoprene glove. Thus at least it will stay inside the glove box, the worst radioisotopes I know are alpha emitters which are very mobile.

You can think of low LET radiations (beta / gamma) as being a bit like a goblin with a big stick. This horrible little monster will chase you around the house before trying to hit you with the stick, it can do you some harm but in some ways there is something worse. For alpha imagine a big bath filled with boiling hot jam, while the bath might not be able to chase you, if you fall in then you are going to get a far worse injury. The alpha is very short ranged but if it does get you then it can go a lot more harm than the beta or gamma.

The super mobile alpha emitters are like a evil goblin armed with a steel bucket of boiling hot jam, this evil goblin is also equipped with running shoes or roller blades so it is able to chase you before throwing the boiling hot jam on you. In short this ones combine the some of worst features of alpha and the more long ranged nasties. The only way to stay safe from this wicked goblin is to lock all the windows and doors of the house and keep him sealed outside, with some luck he will die of old age (become weaker when he decays away) before you have to go outside to mow the grass.

OK time to return from analogyland back to reality

With these mobile alpha emitters special extra precautions are needed to keep them contained, for example with polonium-210 some people have been known to put a glove box inside another glove box to increase the thickness of plastic through which the polonium needs to diffuse. Also for radon-222 some people trap the radon on an absorbent material rather than allow it to wander freely around their glove box.

I feel that many members of the general public have a great misunderstanding of what it is like to work with radioactivity.

I have heard of radiochemists being asked “do you glow in the dark”, the short answer is “no”. While the long answer is that is close to impossible to get sufficient contamination on you to make you glow, the only creditable cases I have heard are of some of the radium dial painters who painted their bodies with the radium based glow in the dark paint.

A paper on the legal battle for compensation can be seen here.

I think that the worst aspect of the radium dial industry was the fact that many workers would lick their brush to get a better shape tip. If you look at this document you will see that radium-228 may have been the real villain rather than radium-226. I hold that the pre 1926 two radium dial painting industry could well be the worst part of the radioactivity sector. It is interesting that it appears that only 20 % of the radium which is taken orally is retained in the human body.

Also according to Norris et. al. as cited in this report the human body is quite good at eliminating radium from its self. Some years ago in America as part of a crazy attempt at curing mental patients some people at Elgin state hospital were injected with radium. Using the data from these medical treatments it was possible in the 1950s to work out a mathematical model for the retention of radium in a human. I have rearranged this equation and used a standard bit of maths which allowed me to calculate a biological half life for radium of only 1.33 days.

This value is rather shocking to me, as a chemist I have always been taught and held the view that radium, strontium and lead are calcium mimics which have very long biological half lives because they become part of the bones. I suspect that if radium is injected or swallowed that only part of the radium which enters the blood stream will end up in the bones. While the biological half life of the radium in the bones may be very long, the biological half life of the radium in other parts of the body will be much shorter.

As a result the half life will appear to change if you consider the whole body after a single intake of radium, I suspected that the biological half life will appear to become longer with increasing time after the intake of the radium. Reading more of the report I found that Norris in 1955 published a mathematical equation which predicts how radium is slowly lost from a human. An article in Nature (March 1969, 221, 1059) suggests that the biological half life for radium in humans (long after the intake) is between 10 and 36 years.

The review of radium in humans points out that Dudley in the 1960s suggested an experiment using short lived radiotracers, the experiment was done using humans and it was found that only 20 % of the radium in a mock dial paint was absorbed when it was taken by mouth while only 0.02 % of the thorium in the dial paint was absorbed.

Another interesting point from the review is the fact that if radium-226 sulfate is deposited in the lungs then 25 % of the radon formed can be exhaled, while if radium-226 is deposited in the bones of a person then 60 to 70 % of the radon can be exhaled. This is an interesting difference, which I suspect is due to the difference in the mobility of radon in bone tissue and radium/barium sulfate. I may well get back to this point later.

Cesium in fish

Dear Reader,

I imagine since the horrible reactor accident in Japan that a lot of questions have arisen. One of these has been will fish be safe to eat ?

I suspect that some “experts” will make statements suggesting that the Fukuashima fish will either be safe/unsafe to eat (delete as appropriate to suit the political stance of the “expert”). But rather than just making or accepting a blanket statement as to the safety (or otherwise) of fish lets look at some of the evidence.

In one study Japanese cat fish (Silurus asotus Linnaeus) were placed in a big fish tank where the water had been spiked with 137Cs. This paper was written by M.A. Malek, M. Nakahara and R. Nakamura, Journal of Environmental Radioactivity, 2004, volume 77, pages 191 to 204.

To cut a long story short what happened was that the fish did absorb radioactivity. It is important to understand that just because a fish becomes radioactive it does not mean that a human would absorb any radioactivity from eating the fish. For examine 90Sr absorbs into the bones of the fish, as long as you do remove the bones before cooking the fish (and do not boil the bones into soup) then the 90Sr in the bones of a fish that you buy in the supermarket are unlikely to be ingested when you eat a fish dinner. 137Cs on the other hand is an isotope which goes into the edible part of the fish (the muscle tissue), so the cesium in the fish is a possible threat to the consumer (bad news).

The cat fish are fresh water fish, I have to confess I do not know much about fish but I reasoned that if the fish are concentrating potassium out of the water into their bodys then in the sea the higher concentrations of potassium may tend to block the absorption of cesium into the fish. But when I checked I. Antovic and N.M. Antovic, Journal of Environmental Radioactivity, 2011, volume 102, pages 713 to 717 it stated that for mullet that does cesium does concentrate in sea fish. I have not worked out quite how the cesium gets into the fish.

So attention needs to be paid to the cesium levels in the fish near Japan.

Crystals and the home made nuclear reactor

Dear Reader,

I feel that nature has not taken its course yet with the Swedish home made nuclear reactor but it is high time I wrote again on the subject of crystals. So lets do both at once !

I commented on how I thought that it was a bad idea to try to use sulphuric acid to dissolve up the radium which is in solid form. I suspect that the radium in a radioactive source or on the surface of ye olde glow in the dark clock will be as the insoluble radium sulphate.

Much of radium chemistry is shrouded in darkness when compared with other metals, for example only four crystal structures have ever been published which contain radium. One of the key gaps in our knowledge is radium sulphate; we will assume for a moment that radium sulphate is isostructural to barium sulphate. The word isostructural is a big technical sounding word which means that the basic structure is the same, but the exact distances between the atoms in the unit cell might differ.

For example calcium fluoride and uranium dioxide are isostructural, the fractional coordinates of the uranium / oxygen atoms match those for calcium and fluorine atoms. But the size of the cubic unit cells are different. But lets get back to our barium and radium chemistry.

I think that the radium will have a lower solubility in sulphuric acid than it will in tap water. Tap water is normally low in sulphates; this lack of sulphate will become clear in a moment.

For many poorly soluble metal salts a thing called a solubility product is known.

This is often written as Ksp.

For barium sulphate Ksp = [Ba2+][SO42-]

[Ba2+] and [SO42-] are the concentrations of barium and sulphate in the solution.

Now those of you who paid attention in your GCSE maths lessons should understand that when barium sulphate is placed in pure water and stirred (until it reaches equilibrium) that

[Ba2+] = (Ksp)½

But when the barium sulphate is placed in 0.01 sulphuric acid, then the concentration of the barium will be given by a new equation.

[Ba2+] = Ksp / [SO42-]

It should be clear to you that by increasing the sulphate concentration that the equilibrium concentration of the barium will go down. It is very likely that the radium will behave the same way as the barium; Marie Curie isolated the radium from uranium ore together with the barium fraction. As I said yesterday for public safety reasons I will not tell you how she converted the barium / radium fraction into a water soluble form. If you are keen to know, please do not ask me about that chemical step as refusal often offends! If you want to know about other bits of chemistry then feel free to ask.

But now we have thought about solubilities lets look at the solid.

The unit cell of barium sulphate is 8.884 by 5.458 by 7.153 Å and it has atoms with the following fractional coordinates.

Ba 0.1846 0.2500 0.1581
Ba 0.6846 0.2500 0.3419
Ba 0.3154 0.7500 0.6581
Ba 0.8154 0.7500 0.8419
S 0.0630 0.2500 0.6914
S 0.5630 0.2500 0.8086
S 0.4370 0.7500 0.1914
S 0.9370 0.7500 0.3086
O 0.0814 0.0298 0.8190
O 0.1808 0.2500 0.5515
O 0.0814 0.4702 0.8190
O 0.9122 0.2500 0.6062
O 0.4122 0.2500 0.8938
O 0.5814 0.4702 0.6810
O 0.5814 0.0298 0.6810
O 0.6808 0.2500 0.9485
O 0.9186 0.9702 0.1810
O 0.8192 0.7500 0.4485
O 0.9186 0.5298 0.1810
O 0.0878 0.7500 0.3938
O 0.5878 0.75 0.1062
O 0.4186 0.5298 0.319
O 0.4186 0.9702 0.319
O 0.3192 0.75 0.0515

If you build a unit cell with these atoms then I think you need a prize from your teacher! I am not sure how it will apply to those of us who either left school twenty years ago or used a copy of ORTEP or some other computational aid.

For those of you who are not motivated to draw or build a unit cell here is a unit cell for BaSO4.

A unit cell of barium sulphate, barium is in green, sulphur in orange and oxygen in red

Now the unit cell for strontium sulphate is a 8.377 by 5.350 by 6.873 Å box, all the atoms have the same fractional coordinates except the bariums are now strontiums. I suspect that radium sulphate has the same structure as barium sulphate and that the cell will be slightly bigger than that of barium sulphate. The fluorides of calcium, strontium, barium and radium all have the same fluorite structure, but the unit cells differ in size. Here is a table of the lengths of the sides of the unit cells of the fluorides.

Element

Length of unit cell (Å)

Ca

5.450

Sr

5.800

Ba

6.196

Ra

6.381

Sadly magnesium and beryllium has a different structure so we can not compare it to these other alkaline earth fluorides. Well I suspect that I have given you something to think about for a while.

Home made nuclear reactor part II

Dear Reader,

I was going to tell you some more about crystals and crystallography but that will have to wait for a while. Instead I am going to tell you some of what I think about garden shed nuclear experiments. In case you have come here hoping to read how to make your own reactor, then I have to apologise and tell you that for a series of reasons (including security reasons and a lack of time/space here) it is not possible for me to give you a how to guide to build a reactor in the space we have today.

The way that David Hahn and the Swede were discovered were very different. David gave himself a fright and then in his panic he attracted the attention of a random policeman, while the Swedish man wrote a letter to SSM asking if it is legal to build nuclear gadgets at home. Before we go any further I would like to stress that almost all nuclear and radioactive activities and equipment are regulated by criminal law in all the countries I have heard of. The IAEA have said publicly that they want the penalty for the illegal possession of some nuclear materials to be as high as murder, so we are dealing with “serious stuff” here!

David Hahn wanted to breed uranium-233 which is fissile from natural thorium-232 which is not fissile but just fertile. Fertile means that an isotope can be converted by a simple nuclear reaction into an isotope which either is (or decays into) a fissile isotope.

What David did was to burn up a large number of gas mantles to obtain thorium oxide; he then cut up a large number of lithium batteries to get the lithium. Then using the lithium he converted the thorium into the metal. I think that this was never needed, many reactors using metal oxides as fuels or as targets for radioisotope production. While neutrons can react with oxygen to form radioisotopes such as nitrogen-16 (by a np reaction) this is normally not a major problem.

David then tried to bombard his thorium with neutrons which were from a homemade neutron source. He used a combination of radium-226 and beryllium. This is a horrible mixture; the radium is properly one of the most radiotoxic isotopes in the world while the beryllium is the most toxic non-radioactive element. It causes a series of horrible diseases including a horrible lung rotting disease!

Radium-226; Properly the nastiest radioisotope in the world!

When David was doing his experiment he noticed that the radioactivity level of the thorium target was increasing, this increase in radioactivity prompted him to get into a bit of a panic. What was likely to be happening was during the pyrochemical processing of the thorium dioxide he would have separated the radium-228 and radium-224 from the thorium. These two radioisotopes would have then started to reappear because of the alpha decay of the thorium-232 and thorium-228.

When I get around to it I will plot some graphs of the radioactivity levels as a function of time in a sample of thorium which has been purified. But that will have to wait for another day.

The Swede was spotted in a different way, he wrote a letter to SSM asking for legal advice regarding his home made reactor. I have read the reactor builder’s blog and he does seem to be having some trouble with some concepts.

I am doubtful that even if he had been left to get on with his experiments that he would ever have managed to get his reactor to work. From the little information I have obtained from his blog it looks like he was trying to build a similar gadget to David’s one. It looks like he wanted to build a subcritical reactor which would be driven by an external neutron source.

He describes how he wanted to dissolve radium in 96% sulphuric acid, I think that this was a bad idea for several reasons. Firstly radium is horrible and radiotoxic, while secondly radium sulphate is very insoluble. One of the classic ways to extract uranium is to boil up uranium ore in sulphuric acid, the uranium dissolves while the radium together with the barium will form an insoluble sulphate. For those of you reading in America sulphate = sulfate, and sulphuric acid = sulfuric acid.

I have written some more about barium and radium sulphate, if you want to read about how it applies to this case then go here.

Then the mixed barium/radium sulphate is dissolved, I think in the interests of public safety I will not tell you how to do it here!

So the choice of reagents for the dissolution of the radium was a poor choice, I have also noticed that an ash tray was close to the cooker. I know that you all know that smoking is a dirty habit but smoking anywhere near radium is a very dirty habit. The problem is that in a radium contaminated environment such as an old uranium mine that the air contains radon-222. This ‘evil creeping death gas’ might be able to diffuse through rocks into caves and houses. But the real villain is the polonium-218 and the other radon daughters. These tend to stick to dust and smoke particles.

If you were to breathe radon contaminated but totally dust free air then it would not be good for you but compared with smoke mixed radon daughters the dust free radon air is positively health giving! What happens with tobacco smoke is that the particles get coated with the radon daughters and then they stick in the lungs. The smoke thus acts like glue to stick these nasty alpha emitters into the lungs. As a result a combination of smoke and radon has a far greater baneful effect on the lungs than the sum total of the two if they are done separately in time and space.

Before you are inclined to feel sorry for the Swede bear in mind that while he did stop to ask if home made reactors were legal, he did not think to ask until he had already started his experiments. I suspect that if a random person in Sweden was to write a letter to SSM asking if it was legal to build a nuclear reactor in the basement, then they would get investigated. But if the person had neither acquired nuclear/radioactive materials nor had started to try to build a reactor then they might at worst get their home searched, but I think that after a stern warning that SSM would send him on their merry way.

I do not quite know what the text of the warning would be but maybe the following might be a good one

The unauthorised construction and use of nuclear reactors in the home is dangerous and may result in a large fine or lengthy imprisonment. I strongly suggest you take up some alternative recreational activity such as …………..

I once had my lab inspected by a team of UN inspectors who wanted to check that I was not doing some undisclosed nuclear activity in my lab. I found the UN inspectors to be a courteous and professional group of men. What they did was to look in my lab at what sort of equipment I had, they wanted to ask me what sort of chemistry I was doing and to take a gamma spectrum to check what sort of radioisotopes were present in my lab (They found nothing as I have next to no radioactivity in the lab).

While I have no experience of being investigated for illegal back street nuclear reactor operation, I suspect that SSM might use similar tools such as the portable gamma spectrometer to check what radioisotopes were present in the man’s flat. I would also expect them to take some samples in the form of wipe samples to allow for different tests to be done later. One option would be to use an alpha/beta scintillation type contamination meter to search the surfaces in the man’s flat for hot spots of activity. When a hot spot is found it would be logical to sample it and then use a more sensitive and selective counting method to evaluate the sample.

Home made nuclear reactor

Dear Reader,

I would like to bring you a public information message.

Please do not try to build nuclear reactors or other radiological gadgets at home, I have just discovered that someone in Sweden has just tried to build a reactor in his flat.

The flat has been visited by SSM together with the police. (SSM is the Swedish radiological protection organisation who regulate the use of radioactive sources, X-ray equipment and non-ionizing radiation sources such as lasers, mobile phones and sunbeds)

For those of you who do not care about your own health and safety, I would like to remind you that in every country I have ever lived in or had dealings with the “ownership, use, production, disposal and construction of nuclear and other radiological gadgets is strongly regulated by law

I will comment in greater depth in the near future on this person’s and David Hahn’s radiological misdeeds.

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