Metals in medicine
Metals in medicine

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Metals in medicine

Barium sulfate

Barium has a high atomic number and absorbs X-rays extremely well. A ‘barium meal’ consisting of an insoluble barium salt such as barium sulfate, BaSO4, is given to patients to swallow in the form of a milky-looking drink, and its progress through the digestive system is followed with X-rays. This is typically used to visualise the structures of the upper gastro-intestinal tract. For the lower parts of the intestines, including the bowel, a barium enema is given instead.

  • Why is it desirable for this contrast agent to be insoluble? (The solubility product of BaSO4 is 1.1 multiplication 10 super negative 10 times m times o times l squared postfix times d times m super negative six)

  • The very low solubility product of BaSO4 means that this is not absorbed in the body but is simply excreted with no danger. (In fact, soluble salts of barium are highly poisonous.)

  • You may recall that the units of solubility product will differ depending on the expression for the sparingly soluble salt concerned. Account for the units shown above for barium sulfate.

  • The equilibrium for barium sulfate is:

    BaSO sub four times open s close right harpoon over left harpoon Ba super two plus times open aq close postfix plus SO sub four times super two minus times open aq close

    So the solubility product is given by

    equation left hand side cap k sub s times p equals right hand side open cap b times a super two postfix plus times open a times q close close times open SO sub four times super two minus times open a times q close close

    and in this case the units will be:

    left parenthesis m times o times l times d times m super negative three right parenthesis equation left hand side open m times o times l times d times m super negative three close equals right hand side m times o times l squared postfix times d times m super negative six

Abnormalities such as ulcers in the stomach wall and abnormal growths can be picked up using a barium meal.

Figure 4 shows an X-ray of the large intestine of a patient using this method. In this example, the contrast of the image has been reversed to see the intestines better and hence allow the medical practitioner to make a diagnosis.

Figure 4  X-ray image of a barium meal passing through the bowel.

Contrast agents are also used to enhance the image in organs such as the kidneys, liver and bladder, as well as in bronchography (imaging of the lower respiratory tract).

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