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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.

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3 Responses

  1. […] 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. […]

  2. I’ve found your blog to be keenly insightful and well put together. As a fellow chemist though, I feel in the interest of factual disclosure that I should add my corrections. Your explanation of the solubility product concept is excellent and correct. For radium sulphate however, the Ksp concept has a notable caveat. It’s perfectly acceptable and governs the solubility behavior of dilute solutions of sulphuric acid up through very concentrated solutions, but fails when the concentration reaches approx. 96% (w/w%) or 18 molar. At this concentration up to, but not including anhydrous sulphuric acid (oleum), a hitherto undiscussed process takes place–complexation. The complex ion Ra(SO4)2 ^-2 is formed.

    Additionally, I find the citing of “security” concerns to be either an elitist position of “I can do it, but you can’t.” or if truly believed, a very weak rationale. Considering Curie herself detailed a method of isolating the radium from the barium sulphate carrier as part of her Nobel prize winning research, and given you are located in Sweden, the home of the Nobel prize, it makes it even more laughable. Science is not about keeping secrets, especially when they aren’t secrets. It’s about sharing. Certainly telling your audience of at least one easy way couldn’t hurt.

    After co-precipitating barium and radium sulphate and rinsing, Censored for public safety reasons. As RaCl2 is less soluble than BaCl2, it will precipitate out first. A learned student of chemistry can easily figure out the point when to stop and separate rinsing the filtrate, and running another fractional crystallization until ~85% of the radium has been recovered as the soluble radium chloride. From here, while it would be inadvisable, the radium could be electrolyzed from molten radium chloride. This assumes an enormous recovery, so it’s better to work with a solution of the the salt. There you have it. Fractional crystallization.

    Marie spilled the beans first…

    • While the solubility of radium in concentrated sulphuric acid might be higher due to the formation of the anionic [Ra(SO4)]^-2, when the concentration of an acid is increased greatly the change in activity coefficents and the failure of sulphuric acid to fully dissociate into hydrogen ions and sulphate anions will make the system more complex. Even if the radium dissolves in the concentrated sulphuric acid when the system is diluted then the radium will reform the insoluble radium sulphate.

      Also while some things might be somewhere in the public domain, it is not a good idea for public safety for me to provide an easy to follow trail of breadcrumbs leading to well tested ways of doing some things. For example I have seen in the public domain (open journal) a method of making sarin together with the experimental details, I think that the authors should never have published it. While a well trained phosphorus chemist (I have a PhD in phosphorus chemistry) could work out how to make sarin, I do not think it is right to save a criminal or terrorist the effort of having to optimise the synthesis. I am sure that even a PhD educated P chemist might need to spend a lot of time working out how to get a good yield and purity in each step.

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