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Dissolving Plaster CaSO4 with either NaHCO3 or NH4HCO3 ?

  1. Jan 22, 2017 #1
    Which of the reactions between CaSO4 with either NaHCO3 or NH4HCO3 would be faster?

    I'm trying to dissolve some set plaster at room temp. I've experimented with NaHCO3 but I have yet to get my hands on any NH4HCO3 though.... The simple baking soda worked really well at elevated temps, quite impressive actually.

    I've read here btw and would really like to stay away from the strong acids/bases...

    Any insight?

  2. jcsd
  3. Jan 23, 2017 #2


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    I doubt that one would be appreciably faster than the other. Ammonium bicarb will be significantly smellier than baking soda, though, since it releases ammonia upon dissolution. So if you're working indoors or in an enclosed space with inadequate ventilation, I'd stick with baking soda.
  4. Jan 23, 2017 #3


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    I'm puzzled as to why the bicarbonate (either) should increase the solubility of the gypsum? If gypsum dissolves in water, it produces Ca++ and SO4-- ions. How does adding HCO3- ions remove any of these ions?
    When we try to dissolve chalk by adding CO2, it helps dissolution by converting CO3-- ions to HCO3- ions, so that the CaCO3 ⇔ Ca++ + CO3-- moves to the right.
    But I can't see any mechanism for CaSO4. Would it not be just as good to use plain water?
  5. Jan 23, 2017 #4


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    It's a good question. I can't speak for OP, but it might be the case that they're not actually seeing dissolution, but rather the ion exchange between calcium sulfate and calcium hydroxide is breaking up the solid plaster. Especially at higher temperatures I could see some sort of ion exchange happening (where you're pushing the equilibrium more toward the hydroxide by eliminating CO2).
  6. Feb 1, 2017 #5
    Why not use dilute acetic acid? Will dissolve in a jiffy.
  7. Feb 1, 2017 #6


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    CaSO4? Care to elaborate on the chemistry behind?
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