Dissolving Plaster CaSO4 with either NaHCO3 or NH4HCO3 ?

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Discussion Overview

The discussion revolves around the dissolution of plaster (CaSO4) using either sodium bicarbonate (NaHCO3) or ammonium bicarbonate (NH4HCO3). Participants explore the effectiveness and mechanisms of these reactions, particularly at room temperature, while expressing a preference to avoid strong acids or bases.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant inquires about the relative speed of dissolution of CaSO4 using NaHCO3 versus NH4HCO3, noting that NaHCO3 has been effective at elevated temperatures.
  • Another participant expresses skepticism about the speed difference between the two bicarbonates, highlighting the odor of NH4HCO3 due to ammonia release and suggesting NaHCO3 for indoor use.
  • Several participants question the mechanism by which bicarbonates might enhance the solubility of gypsum, noting that gypsum dissolves to produce Ca++ and SO4-- ions and asking how HCO3- ions would affect this process.
  • One participant proposes that the observed dissolution might not be due to bicarbonate but rather an ion exchange process involving calcium hydroxide, especially at higher temperatures.
  • Another participant suggests using dilute acetic acid as an alternative method for dissolving CaSO4, prompting a request for further elaboration on the chemistry involved.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness and mechanisms of using bicarbonates versus acetic acid for dissolving plaster, indicating that multiple competing perspectives remain without consensus.

Contextual Notes

Participants highlight the potential for ion exchange and the role of temperature in the dissolution process, but the discussion does not resolve the underlying chemical mechanisms or assumptions regarding the reactions.

as12
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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?

Thanks!
 
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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.
 
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?
 
Merlin3189 said:
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?
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).
 
Why not use dilute acetic acid? Will dissolve in a jiffy.
 
Kevin McHugh said:
Why not use dilute acetic acid? Will dissolve in a jiffy.

CaSO4? Care to elaborate on the chemistry behind?
 

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