urea

[arthur kierski]

i read somewere a calculation for use of urea to reduce hno3 to hno2 ----i did at the time the calculation and came to the following numbers: for every 380cc of hno3(42%) one have to use 300grams of urea---since then i use this formula independently from the quantity of hno3 spent in dissolving itens in ar---example if i use an ar 3liters of hcl 1liter of hno3, i automatically put 784grams of urea---i know that i am using excess urea,but is safer to drop gold with smb.
i need help from someone that have read this information -where?
for me is working

 

[goldsilverpro]

Although I have played with it, I've never had any desire to use urea for this purpose and have, therefore, never been very interested in the chemistry of using it. However, I feel that your thinking is very wrong on this. When you combine HNO3 and HCl, you form NOCl and Cl2 gases. When the NOCl escapes, it breaks down to form NO and Cl2. As I understand it, the Cl2 formed in the solution is actually what is required to dissolve the gold. So, when you add urea, I would assume that it somehow suppresses the Cl2 production and, therefore, prevents the re-dissolution of the gold while you're trying to precipitate it. Maybe, the raising of the pH is what prevents Cl2 production?

To make this short, it only requires a small amount of urea to achieve this purpose. I think you are using way too much. I'm surprised you don't have an 3 inches of urea sludge in the bottom of your container. The standard way is to add urea until the pH is raised to 1.

Help, Lou! What in the hell is really going on?

 

[Lou]

You don't need much urea at all to do the job. Urea does not react with nitric acid or nitrate (in fact, nitric acid may be decolorized by putting urea in it). Instead, it reacts with nitrogen dioxide. Stopping the production of nitrogen dioxide in aqua regia means you've stopped the production/source of nitroysl chloride, which means you've stopped the dissolution of gold.

Cl2 is indeed what is dissolving the gold, but NOCl also does its share of work. While it's true that two equivalents of nitroysl chloride break down into NO2 and Cl2, NOCl is a potent oxidizing agent that attacks gold as well losing Cl as it does.

 

[arthur kierski]

thanks gsp and lou to make things a little more clear

 

[goldsilverpro]

Lou,

Thanks. The standard published ways of using urea is to add until there is no fizzing or add until the pH is raised to 1. Can you tell me why each of these methods works? Can you provide equations of both what happens when urea is added and of the gold dissolving mechanism (especially involving the NOCl)?

At one point, I played around with using urea. The fizzing method seemed very subjective and it was easy to add too much. If too much was added, it could form a solid in the solution. And, as Noxx noted earlier, the purity of the gold seemed to be affected when urea is used.

Can the urea be added, instead, by weight related to the amount of AR present? It would sure be nice to quantify this.

 

[Lou]

GSP, to be perfectly honest, this is a really, really difficult question. I really don't think I know enough of the chemistry of urea to give you complete satisfaction. I can take a crack at it, but it's on a best efforts/commonsense basis:

The fizzing is from the urea being oxidized to carbon dioxide and the nitrogen oxides going to nitrogen and water, with formation of two equivalents of ammonium cation. Also recall that urea is a base (see those two amines on the right and the left, both of them can be protonated). What steps really happen when you put the urea into a strong, strong oxidizing mixed acid solution would be guessing on my part, the obvious is that it breaks up in the presence of H+ and NO2/NOCl. As for what happens first, that would require some thought on my part. I believe that it is necessary that it be in very acid conditions (as urea is a relatively stable molecule all things considered) to be taken apart. Clearly, chlorine plays a role in this chemistry as it is what is responsible for ensuring that most of the nitrate from the nitric acid is in a form that is active and ready to be reduced.

I'm waiting on Noxx to put in a program called LaTeX (it's an in-board equation editor). After that, I will take a stab at the mechanism and the equations for it. I can say this for sure, it is acid-base first with protonation occurring before oxidation (this almost always occurs first). This is all driven by the main fact that gas is produced, and secondarily, ammonium cation is formed. Curiously enough, if there is enough residual NOCl present, ammonium is oxidized into nitrate (well nitric acid probably).

I've only used urea on a few occasions and I didn't like it. Technically speaking, it should be absolutely clean and never leave residues but like you said, it's easy to over add and that is what causes problems (especially when PGMs are present).

As far as the standard ways go, adding urea until the pH is raised to 1 seems rather more scientific than until it just stops gas production since they probably have the same underlying chemical reason. For me, that implies several possibilities: 1. at pH 1, the solution is no longer acidic enough to properly ensure urea's decomposition to gas, 2. at pH 1, the solution is no longer acidic enough to sustain significant concentrations of NOCl which further implies that there would be small/negligible amounts of active chlorine.

When you put nitric and hydrochloric acids together (in any ratio), the end product is a reduction-oxidation reaction with nitrate oxidizing the chloride from HCl to chlorine, again this only occurring in highly acidic milieu. Lots of things come out of an aqua regia solution: chlorine gas, nitrogen oxides, and even oxygen (which usually oxidizes any NO).
2 HCl + 2 HNO3 --> 2 NOCl + 2 H2O + O2 <--> 2 NO + Cl2 + O2 (+ H2O) -->2 NO2 etc
It is not a simple situation because even above, things are in equilibrium, and there are competing reactions.

 

[goldsilverpro]

Thanks, Lou,

That was very helpful. I guess the obvious question is if, instead, NaOH could be used to adjust the pH to 1? Would it have the same effect?

 

[Platdigger]

Good question Chris, I like the way you think......!

 

[Lou]

It would raise the pH, but not as efficiently as urea would since it can accept two protons and NaOH only one.

Would it remove the nitrogen oxides and make it all dandy for precipitating gold? No, almost certainly not.

 

[peter i]

I'm glad there is no simple answer to the urea-question, I've been wondering about that myself!


I have used urea with good results, but you should be careful when adding it, as the gas has a tendency to evolve rather suddenly, making the beaker overflow. A magnetic stirrer and a little patience does it nicely.
(My beakers always stand in a plastic try when doing something expensive for the same reason. I once spilled quite a bit of gold chloride, and I never want that to happen again )


Adding a strong base in controlled amounts is a little hard to do when you don't know what amount you want to add, and adding too much might lead to the formation of hydroxides, that are in general a pain in the b... to work with. They will normally dissolve in acid, but that brings you back where you started.

 

[Noxx]

If one wants to produce pure gold, urea is not the solution in my opinion. Unless you get ACS grade or better, you will add too much contaminants.
One case where I can see it useful is when you already have a very dirty and large AR solution that you want to neutralize and get the gold quickly.

The resulting gold will need a second refining.

Lou, I can't seem to be able to install LaTex. I am supposed to remplace some code line in the forum but I don't find them in the files.

 

[eagle2]

The purpose of adding urea is to sidestep the tedious procedure of evaporating the Gold AR solution just to dryness, then adding slightly more HCl and heating again to drive out the remaining unused Nitric acid, plus all those complex NO(subset x) compounds.

Its a necessary step, because those complex compounds tend to suppress the complete precipitation of Gold. and I would also think alter the actual precipitant, what ever one used.
So adding Urea to Ph 1 seems like a good workable solution.

Adding NaOH or Carbonates to raise the Ph somewhat before adding Urea to bring the Ph finally to 1, in order to save on Urea, may or may not work. Someone could try some experiments on this. It would be essential to have enough Urea present to neutralize all the active compounds anyway.

An AR ratio of 4 to 1 or, even 5 to 1 would dissolve Gold just as well. The finer the particles of Gold you are dissolving the less Nitric acid you will need to do the work and the less Urea needed to eliminate the unused excess.

Al

 

[Harold_V]

The finer the particles of Gold you are dissolving the less Nitric acid you will need to do the work

Are you sure about that?

Discounting any losses to gassing, or evaporation, it appears to me that a specific amount of acid is required to dissolve a specified amount of gold, regardless of how it is divided. Finely divided gold will dissolve faster, but it all requires the same amount of nitric.

Harold

 

[eagle2]

Hi Harold
Yes. I have dissolved, like for instance, 1 gram of Gold precipitate in 20 cc of HCl and a few drops of Nitric acid. When the action seemed to stop I would add a few more drops. The total amount of Nitric needed was much less than of a 3 to 1 or 5 to 1 ratio.
Gold compounds with the Chlorine anyway and HCl is always present in excess. What happens in AR is complex but Nitric works to free up the -Cl ions. Its not strickly true, but I think of Nitric acid as similar to a catalyst.

I was happy to find out later that I could dissolve my precips in HCl and H2O2, which is the cleanest way to go.

Al