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As Harvey said, it is to put the Oxygen back into the etchant without diluting it too much. It is much easier to dilute 30% H2O2 than it is to concentrate the 3% solution. Adding sufficient O2 with the 3% stuff adds a lot of water that is not necessary for the chemistry. Using the 30% H2O2, you can add as much or as little water as necessary to keep the chemistry where it belongs.

The goal is controlled etching, and too much water defeats that.

(I'm not a fan of bubbling to put the O2 back in - too much drag-out into the atmosphere.)

Donald.
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On 2019-03-23 3:24 p.m., Jim Higgins wrote:

Received from Harvey White at 3/23/2019 07:08 PM UTC:

On Sat, 23 Mar 2019 14:33:48 -0400, Mark wrote:

High concentrations of H2O2 result in VERY exothermic reactions. Etching takes place in seconds. Lots of heat. Lots of acrid fumes. Do outside or with a good hood. Have water available to put your board in to stop the etching. I did a lot with those high mixtures but decided it wasn't worth the problems and went back to 3% peroxide. I usually use a sponge with a small quantity of etchant. Takes a minute or two but is very controllable and uses only a fraction of the volume, making disposal much easier. I don't make a lot of boards, so it's not worth keeping vats of chemicals around.

The "top up the solution" is meant to add enough H2O2 to restore the solution to good operation, but NOT to replace a lot of water. You want to maintain the overall concentration. The goal (I think) is to maintain the same concentration you get with 3%, but not end up diluting the overall mixture. You do lose some to evaporation, etc.
Harvey

I absolutely don't understand the desire some have to complete etching in "seconds." As Mark says above, "it wasn't worth the problems."

And Harvey is right... if you regenerate with H2O2 (and you'll also need some HCl eventually) you may find the volume of your solution growing and growing. OTOH, if you regenerate with bubbling, and some HCl eventually same as with H2O2 regeneration, you may actually find you have to maintain the etchant volume by adding water due to water loss from evaporation while bubbling. That's if you turn on the bubbler and forget about it.

Jim H

Received from Rob via Groups.Io at 3/24/2019 02:55 AM UTC:

I'm not a chemist.

Some of you guys a talking chemistry I haven't seen since high school 42
years ago.

I etch with Muriatic acid and peroxide ...... 30% HCl 3% H2O2



which is diluted with a LOT of water.

What is the advantage of using tanks and bubblers and all these precision
mixtures calculations?

I add 50ml of HCL to 100ml H202 and it always "just works".

Yep, it works, but then what do you do with the solution that we can only
assume you mix up fresh each time? Few if any of us have a place to legally
dispose of multiple one-time batches of solution containing copper chloride,
so we're discussing solutions that can be regenerated and reused many, many
times.

Jim H

Received from Rob via Groups.Io at 3/24/2019 02:55 AM UTC:

I'm not a chemist.

Some of you guys a talking chemistry I haven't seen since high school 42 years ago.

I etch with Muriatic acid and peroxide ...... 30% HCl? 3% H2O2



which is diluted with a LOT of water.

What is the advantage of using tanks and bubblers and all these precision mixtures calculations?

I add 50ml of HCL to 100ml H202 and it always "just works".

Yep, it works, but then what do you do with the solution that we can only assume you mix up fresh each time? Few if any of us have a place to legally dispose of multiple one-time batches of solution containing copper chloride, so we're discussing solutions that can be regenerated and reused many, many times.

Jim H