I am a little confused by a catalytic reaction

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Discussion Overview

The discussion revolves around the use of sodium hydroxide (NaOH) in removing aluminum from a steel sputtering mask. Participants explore the nature of the reaction, questioning whether NaOH acts as a catalyst or if it is consumed in the process, and consider the implications of aluminum saturation in the solution.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant, Gary, suggests that NaOH is a catalyst in the reaction and questions why its effectiveness diminishes over time, proposing the possibility of "poisoning" by steel or saturation with aluminum.
  • Another participant asserts that NaOH is not a catalyst but is instead chemically dissolving aluminum.
  • A later reply acknowledges the misunderstanding about the catalytic nature of the reaction and discusses the complexity of the reaction, suggesting that under certain conditions, hydroxide ions (OH-) may act similarly to a catalyst but ultimately are consumed in the process.
  • The same participant elaborates on the reaction mechanisms involving aluminum hydroxide and hydroxide ions, indicating that the base can be consumed depending on the concentrations present.

Areas of Agreement / Disagreement

Participants express differing views on whether NaOH functions as a catalyst or is consumed in the reaction, indicating a lack of consensus on the nature of the reaction.

Contextual Notes

There are unresolved assumptions regarding the concentrations of aluminum and hydroxide ions in the solution, as well as the definitions of catalytic versus non-catalytic reactions in this context.

fly_boy_bc
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When Sodium hyroxide is used to remove aluminum from a steel sputtering mask the reaction is energetic at first but after a while the solution gets dirty and the reaction slows down until we need to replace it. As I understand it the caustic soda is a catalyst in this reaction and is not consumed or changed by it. So why does it lose effectiveness? Is it "poisoned" by the steel? Is there another material I could use for the mask that would not poison the Lye? (I am amused by the fact that it has so many names).

Or is the water too saturated with aluminum?

Gary
 
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NaOH is not a catalyst, it is chemically dissolving Al.
 
Well! That would explain THAT then wouldn't it? Hmmmm I guess I figured it was a catalytic reaction just because using it as an electrolyte to separate water is catalytic. Dumb guy thank chemistry ppl for help! ugh!
 
fly_boy_bc said:
Well! That would explain THAT then wouldn't it? Hmmmm I guess I figured it was a catalytic reaction just because using it as an electrolyte to separate water is catalytic. Dumb guy thank chemistry ppl for help! ugh!
The reaction is not so simple as it seems. With little amounts of Al hydroxide dispersed in the solution and little concentrations of OH^-, I'd say that OH^- acts a bit as a catalyst:

2Al\ +\ 6H_2O\ -(basic\ solution)\rightarrow 2Al(OH)_3\ +\ 3H_2\uparrow

But if the concentration of OH^- is enough and there is enough hydroxide dispersed, OH^- reacts with the aluminum hydroxide:

Al(OH)_3\ +\ OH^-\ \rightarrow Al(OH)_4^-

and so the base is consumed.
 

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