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DO NOT heat HCl on your stove. When it's cold it releases fumes that corrode stainless steel. When it's hot it eats your stove.

Just get Ammonium or Sodium Persulphate.

Mike.
P.S.What is HPO?

I second that.

MG Chemicals Sodium Persulphate cat No. 4101 - 1kg
Mouser 590-4101-1KG

 

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I only heat the water on the stove. Then place the sealed container of etchant in the water. Before that the solution is at room temp.