Dissolving Plaster CaSO4 with either NaHCO3 or NH4HCO3 ?

  • Thread starter as12
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In summary, CaSO4 is likely not going to be appreciably faster than either NaHCO3 or NH4HCO3 when dissolving set plaster. The difference may be due to the chemistry behind the dissolution, where ion exchange is taking place.
  • #1
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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  • #2
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.
 
  • #3
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?
 
  • #4
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).
 
  • #5
Why not use dilute acetic acid? Will dissolve in a jiffy.
 
  • #6
Kevin McHugh said:
Why not use dilute acetic acid? Will dissolve in a jiffy.

CaSO4? Care to elaborate on the chemistry behind?
 

1. What is the purpose of dissolving plaster CaSO4 with either NaHCO3 or NH4HCO3?

The purpose of dissolving plaster CaSO4 with either NaHCO3 or NH4HCO3 is to create a solution that can be used for various applications, such as in art conservation or as a cleaning agent.

2. What are the differences between using NaHCO3 and NH4HCO3 to dissolve plaster CaSO4?

NaHCO3 (sodium bicarbonate) and NH4HCO3 (ammonium bicarbonate) are both commonly used as solvents for plaster CaSO4. However, they have different chemical properties and may have varying effects on the dissolution process. NaHCO3 is a weaker base compared to NH4HCO3, meaning it may take longer to dissolve the plaster. NH4HCO3 may also produce ammonia gas during the dissolution process, which can be harmful if inhaled.

3. How does the dissolution process work?

The process of dissolving plaster CaSO4 with either NaHCO3 or NH4HCO3 involves the reaction between the bicarbonate ions and the calcium sulfate in the plaster. This results in the formation of calcium bicarbonate, which is more soluble in water and can be easily removed from the surface of the plaster.

4. What are the recommended ratios of plaster CaSO4 to NaHCO3 or NH4HCO3 for dissolution?

The recommended ratios may vary depending on the specific application. Generally, a ratio of 1:1 (plaster CaSO4 to bicarbonate) is recommended for a moderate dissolution rate. A higher ratio of 2:1 may be used for a faster dissolution rate, but this may also result in a more alkaline solution. It is important to carefully measure and adjust the ratios for the desired outcome.

5. Are there any safety precautions to consider when dissolving plaster CaSO4 with either NaHCO3 or NH4HCO3?

Both NaHCO3 and NH4HCO3 are considered safe to use, however, it is important to follow safety precautions when handling these chemicals. Always wear protective gear such as gloves and eye protection when handling them. Additionally, NH4HCO3 can produce ammonia gas during the dissolution process, so it is important to work in a well-ventilated area. In case of accidental ingestion or inhalation, seek medical attention immediately.

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