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Peroxy-Sulfuric Etching Module
Chemical Analysis/Precipitating Excess Copper
Precipitating excess copper
If the test indicates that too much copper is present, it will be necessary to change the acid content of the bath to force the copper to precipitate out as copper sulfate pentahydrate. The procedure presented here is straightforward, but requires care when handling the extremely corrosive etchant.
If you have not done so, review the safety procedures for handling corrosive materials and follow them explicitly!
- Use the sulfuric acid analysis procedure to determine the sulfuric acid content.
- If necessary, add enough acid to bring the total concentration up to a minimum of 10% (by volume). The level of acid will bring the pH of the bath down to a point where the excess copper will spontaneously precipitate out as the bath cools.
BE VERY CAREFUL WHEN ADDING THE ACID TO THE BATH.
READ THE ACID ANALYSIS PROCEDURE AND FOLLOW IT EXPLICITLY.
- the etchant should now be drained into carboys (mark each container ETCHANT / DANGER CORROSIVE) and allowed to sit in a cool place until it's temperature drops below 20°C (68°F). At that point the copper will begin to precipitate out of solution as copper sulfate pentahydrate, a bright blue crystal that will settle to the bottom of the carboy. Precipitation will commence as soon as the etchant has cooled, but the carboys should be allowed to sit for at least a couple of days to ensure that adequate separation occurs. Note: using a lower temperature will accelerate precipitation, as long as the electrolyte does not freeze.
- Instead of precipitating the entire bath at one time, many users have found it preferable to drain about 1/2 of the bath every Friday to let this smaller volume precipitate over the weekend. If your production level is fairly low, you may be able to maintain the copper concentration by precipitating a gallon or two per week.
- Once precipitation is complete, the etchant should be pumped back into the etcher with care being exercised to avoid picking up any of the sediment. After the etcher is refilled, it should be re-analyzed for both hydrogen peroxide and acid content.
- Decant the copper sulfate pentahydrate crystals into a holding container (plastic) and allow it to air dry. These crystals can be used to make a concentrated solution of copper sulfate, which, after carbon treating, can be added to the acid-copper plating bath to reduce erosion of the anode material.
To insure adequate precipitation of excess copper, the acid content of the bath MUST be adjusted to 10%
(digital titrator count = 187).
HANDLE WITH CARE
Always use the proper equipment as outlined above in the SAFETY section when handling concentrated sulfuric acid. If contact should occur, flush affected area with cool tap water for 15 minutes.
CONTACT A PHYSICIAN IMMEDIATELY.
Always wear protective clothing, acid resistant gloves, and a full coverage face shield when handling this or any other corrosive material.
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