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Ion Association of Dissolved Bases

  1. Feb 4, 2012 #1
    I was told that when a base is dissolved in water, the ions form ion pairs so it is difficult to separate them completely; is this true? And if so, why wouldn't this apply to acids?
     
  2. jcsd
  3. Feb 5, 2012 #2

    Borek

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    Staff: Mentor

    Without context (solvent?) it is hard to comment. What does it mean "to separate them"?

    And yes, ion pairs are not limited to bases.
     
  4. Feb 5, 2012 #3
    Well, my science fair project was to build a magnetohydrodynamic generator and to test different electrolytic fluids running through the generator. When I presented my project to a research scientist, he told me that my results (NaOH and KOH were by far the least-voltage producing, with the salts NaCl and KCl as well as acetic acid producing a medium amount and HCl producing a high amount) were due to the fact that the hydroxides form ion pairs with the positive charges, so that it is difficult to separate them. In case you are unfamiliar, the purpose of MHD generators is to separate the ions and send opposite charges to opposite electrode plates using a magnetic field.
     
  5. Feb 6, 2012 #4

    Borek

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    There is something wrong with this explanation (it doesn't have to be entirely wrong, could be it is just some minor confusion). K+OH- ion pair doesn't have a positive charge - it is neutral.

    Was it a water solution? Of what concentration? In water solutions - because of the high dielectric constant of water - ion pair creation is usually not observed, unless the solution is highly concentrated.
     
  6. Feb 6, 2012 #5
    They were all 1 M, so low concentration.
     
  7. Feb 7, 2012 #6

    Borek

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    I did some digging in my books. Association constant (which means an equilibrium constant for reaction of ion pair creation) for Na+/OH- pair is 0.2, that means in 1M solution about 15% of both ions will be paired. For KOH, NaCl and KCl solutions association constant was not determined (which probably means it was too small to be detected). 1M acetic acid is dissociated in less than 1%, so number of ions in the solution is 100 times smaller than in the NaCl or KCl case (and still orders of magnitude lower than in the case of NaOH).
     
  8. Feb 7, 2012 #7
    Oh, thanks! That's really helpful. But my results indicated that acetic acid worked the third best our of all 6-what could have caused that?
     
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