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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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