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.
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.
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.
Length of unit cell (Å)
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.
Filed under: actinide, actinides, barium, barium sulfate, barium sulphate, calcium, Chemistry, crystals, cubic unit cells, fun science, home experiments, nuclear, nuclear chemistry, Nuclear fuel, nuclear physics, nuclear technology, radiation, radioactivity, radium, strontium, Uranium | 3 Comments »