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Should cadmium sulfate affect the pH of my buffer?

  1. Mar 3, 2009 #1
    I have a 0.2M solution of Sodium Acetate, (which surprisingly seems to be at pH7.8, (we calculated it should be around pH9)). We then add Cadmium Sulfate, and adjust the pH using acetic acid to pH5. However, when I add the Cadmium Sulfate the pH drops.

    Surely this can't happen?
     
  2. jcsd
  3. Mar 3, 2009 #2

    Borek

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    Are you adding solid sulfate, or solution? If solution - what is its pH?

    Cations of many metals are able to lower pH of the solution reacting with water.
     
  4. Mar 11, 2009 #3
    Solid Cadmium Sulfate
     
  5. Mar 11, 2009 #4
    How come a metal can affect pH if pH is a measure ph hydrogen ions?
     
  6. Mar 11, 2009 #5

    Borek

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    See my previous answer. Hydrolyzis.
     
  7. Mar 12, 2009 #6
    [tex]Cd^{++}\ +\ H_2O\ \rightarrow\ CdOH^+\ +\ H^+\ \ \ pKa\ =\ 7.6[/tex]

    So with Cadmium the reduction of pH is slight. Try to do the same with Hg++ or Fe+++.
     
  8. Mar 12, 2009 #7
    I love you
     
  9. Mar 12, 2009 #8

    Borek

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    You would love yourself, you just ignored what I have posted earlier. This is metal cation reacting with water, lowering pH. This reaction is called hydrolysis. It is all in this thread.
     
  10. Mar 12, 2009 #9
    Yes, I read your post. I also read about hydrolysis. I also understand it. I also have a sense of humour. Get a grip.
     
  11. Mar 12, 2009 #10
    HA! I just saw you have an award for humour!
     
  12. Mar 12, 2009 #11

    symbolipoint

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    The target is becoming lost here. Cadmium would be reacting with something, not necessarily water, affecting the pH.
     
  13. Mar 12, 2009 #12

    Borek

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    Cadminum will also get complexed by acetate, up to four ligands per cation. Looks like in solution that is 0.1M in both Cd and acetate about 40% of cadmium will be complexed. However, concentration of undissociated acetic acid goes up (even if concentration of acetate anion goes down; one may expect that will shift dissociation right), so it must be cadmium hydrolysis that is responsible for the pH change.

    0.1M CdCl2 and 0.1M NaAcetate.
     

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    Last edited: Mar 12, 2009
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