Dissolving Plaster CaSO4 with either NaHCO3 or NH4HCO3 ?

[as12]

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!

 

[TeethWhitener]

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.

 

[Merlin3189]

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?

 

[TeethWhitener]

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).

 

[Kevin McHugh]

Why not use dilute acetic acid? Will dissolve in a jiffy.

 

[Borek]

Why not use dilute acetic acid? Will dissolve in a jiffy.

CaSO₄? Care to elaborate on the chemistry behind?