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Nitric Acid Saturated

Hello Everyone,

Question regarding the use of nitric acid (55% dilution). We are a small production shop that turns over around 50 pieces/day. We use a sulphuric acid pickle (10% dilution) followed by a nitric acid dip to remove the cuprous oxide that appears after soldering.

Over time this nitric acid bath becomes a deep blue color due to it becoming saturated with said cuprous oxides. At this point the time it takes for the acid to do it’s job gets lengthier and lengthier, but if I go to neutralize this acid I can tell it still has a lot of strength in it (due to the immense about of bubbling and vapors that occur when I pour it into a giant bucket of lime). I’m thinking the longer time needed is due to it being over saturated with cuprous oxides and it just can’t take much more.

My question is this, is it possible to remove the cuprous oxides from the nitric acid and then keep on using it? If so, would this return the nitric acid to its original strength and short time needed to remove the oxides? I remember reading something somewhere on here about using steel wool to soak up the copper that gets put into a pickle pot, would something like this work?

Appreciate any advice offered!

What are you pickling?
Copper pieces?
The blue color is most likely Copper Nitrate or Copper sulphate, likely a mix. They are both blue.

If it get oversaturated it will start forming crystals on the inner surface.
How do you clean the pieces between the baths?

If I’m not mistaken, the only viable way of keeping/cleaning the acid is an electrolytic cell.
Platinum anode and copper cathode.

Have you tested the Ph and Eh?
The acid may just be depleated, it will still be quite potent, but everything will take more time.

Regards Per-Ove

We are pickling brass pieces only, well silver posts attached to the brass earrings as well.

Normally we will go straight from the sulphuric acid pickle into the nitric acid if the piece doesn’t require further soldering. If it does require more work, we neutralize the piece in baking soda/water mixture, rinse in fresh water, and then go into the nitric acid.

I have not tested the Ph or Eh, that’s a good idea as long as I can source some test strips!

You happen to have any information regarding the use of platinum anodes and copper cathodes to keep it clean? Will this prolong the useful life of the nitric acid?

Thank you!

Thanks for the information.

You have basically two options.
Neutralize the acid and the copper will precipitate out as a hydroxide, this will of course destroy the acid.

Electrowinning will force the copper into metal form again with electrons from the anode.
The acid will not be affected by this, but one need to remove the cathode from the solution immediately after shutting off the current.

Some pollutants may still be in solution, but should not affect its function, after this evaporate down a bit and refresh with new acid.

A Platinum electrode is unaffected by Nitric, but has a high cost, so it may be better to test with graphite first, it may prove good enough.

How much solution do you need to treat?
Regards Per-Ove

You will not be able to test Eh (RedOx) with strips. You will need a meter for this.
Ph gives information about the “strenght” of the acid species in the solution within reasonable concentration, Eh gives information on the Reducing Oxidizing capacity of the solution.

In simple, inaccurate terms, concentration of acid.

For your application not very important, just add some more Nitric after evaporating some of the solution.

If I wanted to try the option with the graphite and copper, what sort of voltage am I looking at needing to make it work? Would a simple wall wort from a phone charger provide enough voltage/current to do this or what do you think?

The best would be a regulated dc power supply.
You do not want to add too much voltage.

At around 5V you risk creating Hydrogen and Oxygen.
This will be explosive and counteractive as you will destroy the acid you are trying to preserve.

Best keep it below 3-4V, but as many Amps as you can get. The current is what does the job.

For testing a usb/phone charger can be used, but add a resistor to the line. Higly ineffective but as proof of concept it will do.

You did not answer how much solution you need to process, but if it is less than 10L you can find a variable powersupply in thrift shops, ebay, alibaba and so on and let it run in spare times.

Regards Per-Ove

Good to know! haha

Okay I’ll try to see what I can find regarding the DC power supply.

As for quantity, it will be anywhere from 1 to 3L, definitely not upwards of that, so glad to know a small unit is all that will be needed.

Thanks for all the information!

You are welcome :slight_smile:

Gentle heating will make the HNO3 somewhat speedier. Do you have an old hotplate, or a ceramic hot-pot? I also occasionally drop some “new” HNO3 into the blue solution, which also speeds up its removal of cuprous oxide. But, yes, over time the acid must be neutralized.
Judy Bjorkman

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As a retired chemist I have some advice.
In this case and with even dilute HNO3 pH paper is useless. PH is only useful for weak acids. If your solution is greater than pH 0.5 then your acid is depleted.
While you may still have a lot of protons from the acid available to react with your brass two things may be effecting your efforts.

  1. obvious reduction of acid concentration (but not even close to zero) recognizing that concentration and reaction rate is not a linear relationship.
  2. Presence of other ions (all the metal dissolved by your processes that is now metal nitrate salts) that will physically and significantly interfere with the acid getting next to the metal you want to ‘clean’.

Three things you might do to extend your pot life.

  1. Easiest, heat the solution up (rule of thumb 10 degrees doubles the reaction rate)
  2. Mix the solution with a magnetic mixer, don’t use air as the bubbling will release noxious gases into your studio.
  3. You might try freezing the solution to remove reaction salts…HNO3 should not be affected and the nitrate crystals might pop out (unfortunately Nitrate salts are usually the most soluble) haven’t done this myself??