# Denox silver nitrate solution



## Luca.83r (Jul 27, 2022)

Hi everybody,
a question: is it possible to use sulfamic acid to Denox silver nitrate solution or does it work well and I need to add silver to the solution?
Thank you


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## FrugalRefiner (Jul 27, 2022)

Why waste the acid? Add more silver till no more dissolves.

Dave


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## Luca.83r (Jul 27, 2022)

FrugalRefiner said:


> Why waste the acid? Add more silver till no more dissolves.
> 
> Dave


yes, I do this, I wanted to know if you could avoid adding silver and eliminate nitric acid with sulfamic acid


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## 4metals (Jul 27, 2022)

Luca.83r said:


> and I need to add silver to the solution?


What are you doing with the solution once the nitric is consumed? You already said you need to add Silver to the solution. Dave gave you the right answer, no sense killing the nitric off if you need more Silver in solution.


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## Luca.83r (Jul 27, 2022)

4metals said:


> What are you doing with the solution once the nitric is consumed? You already said you need to add Silver to the solution. Dave gave you the right answer, no sense killing the nitric off if you need more Silver in solution


because I wanted to know if you can avoid adding silver. I don't always have silver available


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## Ultrax (Jul 27, 2022)

1) Ag + 2*HNO3 → AgNO3 + NO2 + H2O
2) HNO3 + H3NSO3 → H2SO4 + N2O + H2O
3) 2*AgNO3 + H2SO4 → Ag2SO4 + 2*HNO3

Sulfamic acid isn't good idea to neutralize free HNO3 in nitrate solution.
You will receive additional nitric acid + insoluble silver sulfate precipitate in your solution.


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## Martijn (Jul 28, 2022)

Are your questions theoretical or have you overshot your nitric?


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## Luca.83r (Jul 28, 2022)

Martijn said:


> Are your questions theoretical or have you overshot your nitric?


no no, theoretical question. sulfamic acid is used to Denox in AR and I wanted to know if it could also be done with a silver nitrate solution so as not to add additional silver


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## Martijn (Jul 28, 2022)

As ultrax said, you will create silver sulphate. One way to get that out is converting to silver chloride. 
So skipping the sulfamic step and just adding HCl would work. 

Or leaving some silver undigested in the nitric treatment and then cement it out on copper.

Edited for spelling.


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## 4metals (Jul 28, 2022)

Just as adding too much nitric in an aqua regia dissolve should be avoided, the same is true for a Silver dissolve. With aqua regia the sulfamic and the sulfuric by-product is actually beneficial if lead is present which is why it has become a preferential method. I see no benefit in adding it to a Silver dissolve in nitric. Best to underestimate your nitric requirements leaving a small amount of Silver undissolved than to do anything to remove excess nitric with more chemistry. 

Again you never stated what you intend to do with the silver nitrate you are preparing. The desired end result is helpful in determining the path to success and members trying to help you won’t have to guess what your desired goals are.


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## Luca.83r (Jul 28, 2022)

4metals said:


> Just as adding too much nitric in an aqua regia dissolve should be avoided, the same is true for a Silver dissolve. With aqua regia the sulfamic and the sulfuric by-product is actually beneficial if lead is present which is why it has become a preferential method. I see no benefit in adding it to a Silver dissolve in nitric. Best to underestimate your nitric requirements leaving a small amount of Silver undissolved than to do anything to remove excess nitric with more chemistry.
> 
> Again you never stated what you intend to do with the silver nitrate you are preparing. The desired end result is helpful in determining the path to success and members trying to help you won’t have to guess what your desired goals are.


Sorry 4metals.
from the solution, which is the result of the treatment of quartered gold, I have to cement out silver with copper. 
however, I asked the question only because I had thought that there might be the possibility of eliminating excess nitric acid in this way.


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## 4metals (Jul 28, 2022)

If that is the case the best bet is to use copper to burn up the excess nitric. Seeing as copper consumes more nitric it would be more effective than Silver and copper is needed to cement the Silver out anyway. The copper will drop the metal fairly clean and any silver sulfates won’t gum up the cemented Silver.


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## Luca.83r (Jul 28, 2022)

4metals said:


> If that is the case the best bet is to use copper to burn up the excess nitric. Seeing as copper consumes more nitric it would be more effective than Silver and copper is needed to cement the Silver out anyway. The copper will drop the metal fairly clean and any silver sulfates won’t gum up the cemented Silver.


Thanks, this is great advice. another question if I may. to check if there is still silver in the solution to be cemented I do a test with a little solution and hydrochloric acid or is there another better method?


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## 4metals (Jul 28, 2022)

The way to test if there is Silver remaining in your solution is, as you said, with Hydrochloric Acid. But it’s really the chlorine doing the indicating by forming Silver Chloride. The most sensitive way to do this is to take a clear clean beaker full of chlorinated tap water and with a glass stir rod, dip the rod into the solution you want to test and allow a drop of this solution to drip into the chlorinated water. Just 1 drop is all that is needed. If there is Silver in solution you will immediately see a white haze in the beaker. If there is no Silver there will be no cloud. There may be blue color from any Copper but it will be clear without a haze. 

If you feel your tap water does not have enough chlorine, or you are not sure if it is even chlorinated, use tap water with just a few drops of Hydrochloric Acid added. 

Very sensitive test that will detect a few ppm of Silver.


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## Geo (Jul 28, 2022)

Ultrax said:


> 1) Ag + 2*HNO3 → AgNO3 + NO2 + H2O
> 2) HNO3 + H3NSO3 → H2SO4 + N2O + H2O
> 3) 2*AgNO3 + H2SO4 → Ag2SO4 + 2*HNO3
> 
> ...


Not true. The nitric acid in complex with silver will remain the same unless the solution is concentrated and then further reduced by evaporation.


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## Geo (Jul 28, 2022)

I have used sulfamic to remove excess nitric acid from nitric acid leaches on per batch lots. I've never had silver sulfate form. Not once.


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## Luca.83r (Jul 28, 2022)

4metals said:


> The way to test if there is Silver remaining in your solution is, as you said, with Hydrochloric Acid. But it’s really the chlorine doing the indicating by forming Silver Chloride. The most sensitive way to do this is to take a clear clean beaker full of chlorinated tap water and with a glass stir rod, dip the rod into the solution you want to test and allow a drop of this solution to drip into the chlorinated water. Just 1 drop is all that is needed. If there is Silver in solution you will immediately see a white haze in the beaker. If there is no Silver there will be no cloud. There may be blue color from any Copper but it will be clear without a haze.
> 
> If you feel your tap water does not have enough chlorine, or you are not sure if it is even chlorinated, use tap water with just a few drops of Hydrochloric Acid added.
> 
> Very sensitive test that will detect a few ppm of Silver.


Many thanks!


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## Ultrax (Jul 29, 2022)

Geo said:


> Not true. The nitric acid in complex with silver will remain the same unless the solution is concentrated and then further reduced by evaporation.



HI -> HClO4 -> HBr -> HCl -> H2SO4 -> HNO3 -> HMnO4 -> H2SO3 -> H3PO4 -> HF -> HNO2 -> H2CO3 -> H2S -> H4SiO4

The strength of acids decreases in this series. Therefore, each acid can displace the next from the salt. And sulfuric acid will displace nitric from its salt. Other results you obtained are not a rule, but the result of your specific conditions in solution for impurities, temperature, concentration, etc.

A good example, of how to make nitric acid at home: KNO3 + H2SO4 -> K2SO4 + HNO3

Of course, concentration limits and the level of dissociation will affect reactivity. It is obvious.
In addition, even in your case, you could well not notice the colloidal silver sulfate in the solution and drain it without waiting for precipitation.


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## Geo (Jul 30, 2022)

Ultrax said:


> HI -> HClO4 -> HBr -> HCl -> H2SO4 -> HNO3 -> HMnO4 -> H2SO3 -> H3PO4 -> HF -> HNO2 -> H2CO3 -> H2S -> H4SiO4
> 
> The strength of acids decreases in this series. Therefore, each acid can displace the next from the salt. And sulfuric acid will displace nitric from its salt. Other results you obtained are not a rule, but the result of your specific conditions in solution for impurities, temperature, concentration, etc.
> 
> ...


A lot of text book chemistry just doesn't work in every day practical applications. We have run into things that should be or should not be happening occasionally. When using it myself, I take pains to keep conditions where I'm comfortable with the reaction happening. I don't typically stray far from the process parameters.


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## Ultrax (Jul 30, 2022)

If the book laws of chemistry did not work, it would be alchemy  
Fortunately, when something goes "wrong" in chemical reactions, there are always gaps in our insufficiently deep knowledge of chemistry. At least that's how it's always been for me


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## Yggdrasil (Jul 30, 2022)

Ultrax said:


> If the book laws of chemistry did not work, it would be alchemy
> Fortunately, when something goes "wrong" in chemical reactions, there are always gaps in our insufficiently deep knowledge of chemistry. At least that's how it's always been for me


I think he means that in the real world it is not “clean” reactions.
There will always be some kind of extra side reactions, which means that the anticipated reaction may not go as planned.
So the “book” and the world often seem to be two different entities.

And every now and then one may be surprised by things that should not happen within the envelope you envisioned.
That just means that things got more complicated than the book/you planned to do.


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## orvi (Jul 30, 2022)

If I can add to this, yeah, in nitrate enviroment, little bit of sulfate isn´t going to precipitate Ag2SO4. I regularly do batches of AgTa capacitors, which are basically pure silver casings, oxidized tantalum chunk inside and sulfuric acid electrolyte... And bit of plastic spacers, paper and resin seal.
We just dump them straight into nitric acid and dissolve. No cloudiness, no precipitate. And ammount of sulfates in that electrolyte isn´t negligeable.

De-noxing the nitrate solution in silver processing is wasteful in my opinion.
4metals and FrugalRefiner gave you best hints what you can do with it. Easiest thing is to add more silver and boil for couple of hours to spend the acid. Comparably easy, but much quicker is to eat the nitric with copper - copper reacts faster with nitric than silver. Silver will precipitate during this, but cement silver reacts much faster than bulk silver. So it will go back and forth, till all acid is spent.

You can also use HCL to "revive" nitric acid and precipitate quite pure silver chloride. Simply add just enough HCL to still form a slight white cloud when dropped to the solution. Best is to titrate small sample and then add like 95-97% of the theoretical ammount of HCL. If you overshoot just a little bit, you can add more silver nitrate solution to scavenge free chlorides. And I must admit that it is easy to overshoot without previous titration 

But by doing this, you can recover all the nitric acid that was in the form of AgNO3 (the portion that escaped as NOx is obviously lost, if you do not regenerate NOx-es). Fairly dirty (based on what you dissolved) and dilute, but essentially OK for many purposes. I use it as diluent for new batch of impure technical silver scrap - I add this dirty HNO3 and new clean one in ratio based on concentration of "dirty" acid to get 30-35% nitric for dissolving.
But I do not advise to use it twice in a row - too much junk inside. Then, it is much more practical to cement the next batch on copper, and let the copper nitrate solution evaporate down to slush/crystals.
These can be used as nitrate source for AR, or can be dissolved in water, slaked lime is added, and you obtain metal hydroxides cake and calcium nitrate solution. If you add H2SO4 to this solution, you precipitate calcium sulfate mud and supernatant will be fairly clean dilute nitric acid with bit of sulfuric acid contamination.
It is quite tedious and messy process, but it can be done if one is interested in recovery of nitrates due to various reasons. Not my prefered way. There are more elaborate ways how to save nitric, or even use it as catalytic oxidant. But these require pressure and corrosion-resistant vessels and much more expensive stuff than just beaker on the hotplate


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## orvi (Jul 30, 2022)

Ultrax said:


> If the book laws of chemistry did not work, it would be alchemy
> Fortunately, when something goes "wrong" in chemical reactions, there are always gaps in our insufficiently deep knowledge of chemistry. At least that's how it's always been for me


Electrochemical series of metals are nice for educational purposes, but in real world, it is often bit different.
That is only one thing from the list of things, that do not fit the theory for various reasons.
If we took the electrochemical series, and for example reduction potential of Au+ or Au3+, we couldn´t get it into the aqueous solution, because it will react with water and oxidize it to oxygen. And in real life, we all dissolve the gold in AR without problems. In complexed form, AuCl4- anion has redox potential "low" enough for it´s existence in solution. Same story with PGM chlorocomplexes. If it was just cations in solution, electrolytical separation wouldn´t be that difficult. But they have too much coordination places to stick chlorine, water, nitrito or whatever anion in their reach, change it and mess it all up 
Also, when we came back to the Ag2SO4 issue, Ag2SO4 is relatively insoluble in pure water. That is the table number given, if I remember correctly aroung 8g/L at room temperature. But if you go to the concentrated sulfate solutions and even sulfuric acid, it´s solubility enhances greatly. I do not know if the same is true for heavy nitrate solutions, but somehow, silver even in presence of sulfates refuse to form a precipitate. If it is due to strong binding to nitrate, complex stuctures in solution, complexation with another nitrate anion or different solubility of silver sulfate in nitrate solution, I am not sure.
The same is when people and textbooks said platinum and rhodium are insoluble in nitric acid. With bulk metal, it is practically true. With cemented Rh or Pt, not true at all. Alloys of few % Pt with Ag and Pd goes into the solution without any residue, despite the platinum should stay insoluble.

But I get your point and agree that many times, the existing theory is missing some important starting points and facts which makes it useless until they are uncovered


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## Lino1406 (Jul 30, 2022)

Silver sulphate is soluble to some extent, hence you may not see it (unlike silver chloride)


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## orvi (Jul 30, 2022)

Lino1406 said:


> Silver sulphate is soluble to some extent, hence you may not see it (unlike silver chloride)


Yeah, I know that and understand everything behind. Thing is that I also done some FeAgTa capacitors with firstly with plain dilute nitric, than second batch with HNO3/H2SO4 mixture. I don´t remember who from the knowledged members here advised me to use sulfuric acid additions to fight iron sludge and passivation of the casings... But, it worked wonders. In the end, stoichiometric sulfuric for the iron was added and practically zero precipitate formed in solution. And there was three times more iron than silver in the feed. So plentiful of sulfates to precipitate silver sulfate. But it did not happened. Volume of the liquid was insufficient to hold even 10% of Ag2SO4 compared to it´s solubility in regular water.


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## 4metals (Jul 30, 2022)

Often a minor trade off between efficiency of processing and a quasi-quantifiable loss is just a fact of life in refining. And also underlines the benefits of all of your acid wastes spending some time in a cementation process.


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## Geo (Jul 30, 2022)

I am in no way saying the laws of chemistry are wrong. I'm saying that in practice, text book reactions are not always observed.


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## Geo (Jul 30, 2022)

Just as 4metals alluded to, there will have to be acceptable losses. I don't think that there is ever 100% recovery.


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## Shark (Jul 30, 2022)

Geo said:


> Just as 4metals alluded to, there will have to be acceptable losses. I don't think that there is ever 100% recovery.


So many do not realize the need for a stockpot. Those not quite “text book” reactions is a part of knowing how to correctly use it. Many smaller people such as myself may need to cut a small corner here and there in order to do some reactions using what we have on hand. We then have to make a call as to acceptable loses or not being able to do a job at all. I can understand a commercial refiner not going this route, but for the small home processor it is an individual call. Sometimes I do and sometimes I don’t. It also has to do with the learning process. As I learn more about the chemistry I may take an odd step to test some of the things I have learned recently. Having this forum to fall back on for help is a great help.


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## 4metals (Jul 30, 2022)

Geo said:


> I am in no way saying the laws of chemistry are wrong. I'm saying that in practice, text book reactions are not always observed.


You go to college to study the chemistry of what theoretically happens. Then you go to work in the real world and see just how it actually happens. 

There are so many possible combinations of metals, impurities, temperatures, chemicals, and concentrations that a chemistry textbook couldn’t possibly cover it all.


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## Palladium (Jul 30, 2022)

Ain't that the truth!

Hold on boys.... were going in for a hard landing!


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