# urea



## arthur kierski (Jul 29, 2008)

is there a calculation on how much urea one adds for every 100cc of nitric acid in an aqua regia solution?

i for every 100cc of nitric use 79grams----
i did this calculation a few years ago and i am not sure if it is correct
adding urea until the solution stops fizzing or bubling is not very cientific and is a bit of guessing


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## goldsilverpro (Jul 29, 2008)

Seems way too much. I don't know the number. Adjust to a pH of 1 with urea and don't worry about it.


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## lazersteve (Jul 29, 2008)

Arthur,


I seldom, if ever use urea anymore, but here's an easy trick I came up with to determine when I've added enough urea:

Try adding a sprinkle of SMB to the pregnant solution, if the brown powdered gold that forms immeadiately fizzes and starts redissolving, add a teaspoon of urea and stir until the urea dissolves. Test with SMB again and repeat the process until the gold that forms no longer fizzes and redissolves.

A few *better* tricks you should adopt:

You can avoid excess nitric all together by placing a spare gold button (weigh first and after) into the hot solution and waiting for it to stop fizzing before filtering and adding SMB. Be sure the button is large enough that it won't all dissolve if you are heavy handed with the nitirc. It also makes for really neat crystallized gold buttons when you are done. 8) 

The second way to avoid excess nitric levels is to add your nitirc from a small squeeze bottle. As the added nitric quits producing fizzing and bubbles on the dissolving scrap add anothe rsqueeze to get it going again. When all the scrap is dissolved you won't need any more nitirc and the leftover nitric is very little. This also conserves on waste nitirc and the serve fuming that is accompanied by large nitric additions. 

Be sure to thank Harold and GSP for the two excellent tips above as they taught them to me. Thanks guys!  

One last way to avoid nitric in the solution is to boil the solution down and add HCl until no more brown fumes are produced when the HCl is added. This method is how Hoke did it, but is very slow and not preferred (by me) for that reason. GSP's trick (second one above- small nitric additions) works with PGM solutions too.

Steve


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## Lou (Jul 29, 2008)

Yes, but that trick will not work for Pt Steve. A large piece of Pt foil even will take so long to dissolve, it will do little to remove any NOx. I strongly recommend keeping urea OUT of PGM solutions.


Lou


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## lazersteve (Jul 29, 2008)

Lou,

I didn't even realize this post was in the PGMs section! :lol: 

I thought he was asking about gold solutions... although he really didn't specify what he had dissolved.

Boiling down to remove nitirc really sucks, it takes so long. Lately I've been trying out other ways of getting the PGMs out of the bulk of the solution.

In the past week I've researched and tested a few more ways to precipitate from acid and neutral soultions. It's amazing how many ways you can skin the PGM cat. There's a lot of good literature on the subject from the late 1800's. I'm currently working on translating another very old German text that should prove very useful.

My collection of PGM metallic powders is really starting to grow now. 

Steve


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## Jill (Jul 11, 2013)

I bought some rose fertilizer that has urea in it. Is that good for the urea,....and can I put it in as is, or do I need to add water to it first. I tried not diluting it, the tried the smb but it doesn't seem to be working.


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## butcher (Jul 11, 2013)

Jill, 
Urea will destroy NOx gas in solution but it will not destroy nitric acid or nitrates dissolved in solution, you will read many places on the internet and in very early posts on the forum suggesting its use to denox or remove excess nitric acid from a solution containing gold, most of the forum has decided urea is not good for this purpose, with PGM's it can form complexes making recovery difficult if not impossible, urea can form ammonium compounds in solution thus has a possibility of forming a possible explosive mix in your treatment or in your waste, it has been reported causing spontaneous combustion in the lab, urea nitrates are not something that should be taken lightly especially with metals and possible ammonium compounds being formed, urea in my opinion is only good to use a prill or to to test the solution, but is useless to just plain dangerous to use otherwise.

Besides it what you have is a rose fertilizer with urea in it, and several other chemical compounds, you do not have urea, so by adding this rose fertilizer you may just lose your gold, or just make it hard to get any gold back from the mix, or mess.

Use only the amount of nitric needed for the reaction and the nitric is consumed by the reaction (GSP's trick).

Slow evaporation (3 times) to a concentrated solution to vapor of free HNO3, HCl to re-wet before each evaporation, Study Hokes book.

Adding a gold button to consume HNO3 while heating solution (Harolds trick).

Or leaving a little gold undissolved while making only minimum HNO3 additions to HCl and gold while heating to dissolve gold.

Sulfamic acid to denox, creating sulfuric acid as a byproduct and added benefit (for lead removal), sulfamic can be found as grout cleaner in the flooring section of Lowe's hardware store.


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## solar_plasma (Jul 12, 2013)

> Sulfamic acid to denox, creating sulfuric acid as a byproduct and added benefit (for lead removal), sulfamic can be found as grout cleaner in the flooring section of Lowe's hardware store.



That almost sounds to good to be the whole truth, - are there any disadvantages in using sulfamic acid?


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## Lou (Jul 12, 2013)

Yes, if you add the solid to a beaker or flask which is heated not by steam but by a hot plate or mantle, you risk the decompositon of the sulfamic acid into an ammonium cation, which causes precipitation of ammonium salts of the PGMs. All of those salts re-dissolve (some faster than others) in aggressive AR, but with significant NOx production vis-a-vis oxidation of ammonium ion.

Solution, dissolve the most sulfamic you can in boiling hot water, and slowly add that hot solution in until N2O ebullition ceases. Obviously, the vessel you do this in must be setup to collect acid mists or you will have values lost in entrainment from the bubbling/off-gassing.


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## solar_plasma (Jul 13, 2013)

I am impressed again, thanks for the answer!

If this is the only disadvantage, I understand that especially in the case no PGMs are involved, but only gold, sulfamic acid (combined with a chain of washbottles to collect vapors) is just great. And if PGMs are involved, it could be the better choice to denox the classical way.


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## lazersteve (Jul 17, 2013)

The absolute optimal route to controlling the excess nitric acid is not to add excess nitric acid in the first place. 

When using sulfamic acid I add it in crystal form to a hot solution. Add the sulfamic a little at a time until new additions do not cause the solution to foam. Excess sulfamic crystals can be removed with a little hot water, and this liquid can be used to kill trace excess nitric acid in another solution. Your container will need to be large enough to contain the large amount of foaming that occurs when the sulfamic is added. Always work with a catch basin around your pregnant solution containers in case of foam overs. The end point of the sulfamic additions with is clear when new sulfamic additions with stirring produce no foaming.

Steve


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