When etching a board with Cupric Chloride it really doesn't matter what strength we use as long as we have the correct proportions of H2O2 (hydrogen peroxide) and HCL (hydrochloric acid). We only need enough volume to cover our board. A stronger solution would be easier to store as regenerating with weak H2O2 and HCL would increase the volume greatly. Starting off with weak H2O2 and HCL should be sufficent for most small runs.
This is how our reaction works:
First we are making Cupric Chloride (CuCl2):
Cu + H2O2 + 2HCl -> 2H2O + CuCl2
The Copper reacts with the CuCl2 from the previous reaction to form Cuprous Chloride (CuCl):
Cu + CuCl2 -> 2CuCl
The Cuprous Chloride continues to react with the H2O2 and HCL to continue the reaction.
2CuCl + H2O2 + 2HCL -> 2H2O + 2CuCl2
It's a chain reaction and these reactions are all happening simultaneously but each mole of HCl used dissolves 1 mole of copper.
H2O2 has a molecular mass of 34.0147 grams/mole and HCL is 36.46 grams/mole. With 100 grams of 3% H2O2 we would have 3 grams or 0.088 moles of H2O2 (3 grams divided by 34 grams/mole). We would need 0.176 moles of HCl for the reaction. That would be 0.176 moles x 36.5 grams/mole = 6.42 grams. With 27% HCl that would be 6.42 divided by 27% equals 23.78 grams of 27% HCl Solution. 23.78 grams of HCl per 100 grams of H2O2 would be a 1:4.2 ratio. This would of course change with different concentrations.
When regenerating our solution by adding more H2O2 and HCl we are adding it to a solution of CuCl and water. The reaction above starts with 114.374 grams of water and generates another 3.178 grams of water so we have roughly 117.5mL of water when the reaction is complete. This is why to regenerate the etchant it would be best to use more concentrated chemicals so it doesn't get too diluted. If the reaction doesn't use up all of the H2O2 and HCL the H2O2 will break down in the light to H2O and O2 so regenerating with only H2O2 may work. Add H2O2 to the CuCL and water solution first. If the reaction starts there was HCL left in the solution. When adding H2O2 causes no results it's time to add proportionate amounts of both H2O2 and HCl to continue the reaction.
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