Question about dilute nitric

[goldscraphobby]

I use dilute nitric (~50% water) on stainless and aluminum material with a nickel plate (some may have Cu, but mostly Ni) and gold. After the gold is loose, I remove the remaining base pieces.
Usually, I will add more material and nitric to the let over solution and let it sit. Check it once and while, add nitric as needed
My question(s),
Which is better or more efficient use of nitric
Adding more nitric to the remaining solution and then add more material or discard the old solution and start over at 50% dilution?
Or is a different percent better?
Is there a point where the nitric is too diluted to be of use or will it still attack the Ni or Cu at lower dilutions, albeit just slower?

I would hate to waste any remaining nitric in the old solution so how could I verify if it's used up, if it's not I could put in small pieces and let sit?
Thanks

 

[orvi]

I use dilute nitric (~50% water) on stainless and aluminum material with a nickel plate (some may have Cu, but mostly Ni) and gold. After the gold is loose, I remove the remaining base pieces.
Usually, I will add more material and nitric to the let over solution and let it sit. Check it once and while, add nitric as needed
My question(s),
Which is better or more efficient use of nitric
Adding more nitric to the remaining solution and then add more material or discard the old solution and start over at 50% dilution?
Or is a different percent better?
Is there a point where the nitric is too diluted to be of use or will it still attack the Ni or Cu at lower dilutions, albeit just slower?

I would hate to waste any remaining nitric in the old solution so how could I verify if it's used up, if it's not I could put in small pieces and let sit?
Thanks

Nitric start to loose it´s oxidizing power with dilution. Then, it start to behave like an ordinary acid. Nitric isn´t particulary very strong acid, so attack on the stainless is minimal, even at very low concentrations. If you do not heat it, it´s usually fine.

In terms of adding/discarding issue - anytime you add more fresh 50% nitric to the leach, you will create less concentrated and junk-polluted acid. To dissolve just the Ni underplating, the acid will serve you for a long time, before it needs to be replaced. But if needed, I will definitely replace the spent one with new one, and not carrying all of the dissolved salts from batch to batch.

 

[nickvc]

I always used HCl on stainless it’s easier to find and buy than nitric for many but you do need to heat it .

 

[goldscraphobby]

Nitric start to loose it´s oxidizing power with dilution. Then, it start to behave like an ordinary acid. Nitric isn´t particulary very strong acid, so attack on the stainless is minimal, even at very low concentrations. If you do not heat it, it´s usually fine.

In terms of adding/discarding issue - anytime you add more fresh 50% nitric to the leach, you will create less concentrated and junk-polluted acid. To dissolve just the Ni underplating, the acid will serve you for a long time, before it needs to be replaced. But if needed, I will definitely replace the spent one with new one, and not carrying all of the dissolved salts from batch to batch.

Thank you

 

[goldscraphobby]

I always used HCl on stainless it’s easier to find and buy than nitric for many but you do need to heat it .

Thanks, forgot HCl does not bother ss. Will do that on may next batch of connectors

 

[Yggdrasil]

Well I have used HCl to dissolve Stainless so I would say it depends on the alloy.
I'd check it thoroughly first.

 

[Geo]

Highly diluted acid is still acid, only diluted. To force the reaction, apply heat. If you add nitric acid to a base metal and let it react passively, the reaction will build heat on it's own to a certain point. When enough of the acid has been depleted, the reaction will slow and the solution will cool. When the solution, which has a lower percentage of available free nitric acid cools enough, the reaction will stop completed or slow until the reaction is undetectable with the naked eye. That does not mean that the nitric acid is completely used up. If there is base metal left, heat the solution and the reaction will begin again. Only on heat will you actually use all the available nitric acid. If you are not heating the reaction, you are wasting acid. Harold_V would preach about ir all the time. "I don't understand how people claim to refine without using heat." It's like people are afraid to heat a reaction as though the precious metal will just evaporate away if it's heated. It's the only way to get the most out of your acid.

 

[Geo]

HCl, as CuCl2, will attack stainless steel with little problem. If you are using HCl to leach and there is copper in the mix, heat and oxygen from the atmosphere will dissolve some copper. This copper in solution acts as a catalyst for the dissolution of stainless steel. I would not trust that HCl will not attack SS if there is copper available and it is heated.