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Author: Subject: removing trace copper impurities from AgNO3
Jor
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[*] posted on 6-2-2010 at 16:41
removing trace copper impurities from AgNO3


Well recently I bought a 1 oz silver bar, to make a good amount of silver nitrate, but this was stolen.
So now, I still had about 8 grams of silver wich i purified before, by dissolving alloy, precitipating AgCl, wash with boiling water, and cold water, reduce with sugar in alkaline solution, and melt the powder. Well I dissolved these silver nuggets in HNO3 today, and evaporated. Now in the evaporating dish, there is mostly just white crystals, but on the sides there is some slight blue coloration, so i think these is some Cu present, but this was not observable when the silver was dissolved, so it is trace.
Still I'd like to remove this. How is this achievable, without having to the AgCl+reduction route again?
I was thinking of adding a excess of strong NaOH to the AgNO3 (in solution), as copper dissolves in strongly alkaline solution. But I'm not sure if silver does, e.g. as Ag(OH)2(-).

Do you guys have any ideas?
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Myfanwy
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[*] posted on 6-2-2010 at 17:02


adding sodium hydroxide will form insoluble Ag2O and Cu(OH)2.

adding copper to the AgNO3 solution will form elemental silver, what can be washed with distilled water and react with HNO3 to form AgNO3 again.

i have an idea.
try treating the solution with sodium acetate.
Copper(II)-acetate will stay in solution and silver(I)-acetate will fall out.

You will lost maximal 2g Ag this way, but you will get a pure product.




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Jor
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[*] posted on 6-2-2010 at 18:08


Quote: Originally posted by Myfanwy  
adding sodium hydroxide will form insoluble Ag2O and Cu(OH)2.

adding copper to the AgNO3 solution will form elemental silver, what can be washed with distilled water and react with HNO3 to form AgNO3 again.

i have an idea.
try treating the solution with sodium acetate.
Copper(II)-acetate will stay in solution and silver(I)-acetate will fall out.

You will lost maximal 2g Ag this way, but you will get a pure product.

I wanted to add NaOH, because copper is amfoteric and will dissolve giving cuprate(II)-ion. I'm sure silver also does, as the Ag(OH)2(-) ion is mentioned in literature, but i can't currently find the K-value for this reaction. If it is much lower than for copper(so 10x or more), I think this would be the right way to go.
I don't really like the acetate idea, as silver(I) acetate is moderately soluble. Their should be better options.

The copper idea is an option, but then I will have to dissolve elemental silver again, wasting nitric acid and generating a lot of toxic gas again.

[Edited on 7-2-2010 by Jor]
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bahamuth
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[*] posted on 6-2-2010 at 19:07


Dissolve in HCl and do the chloride way once more, wash the silver chloride with several liters of RO water or better, either by decantation of filtration. Use good (pure) chemicals and your silver will be 99%+





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[*] posted on 7-2-2010 at 00:50


You really have to go the AgCl-route again. Separating with hydroxide is not a real option and will be much more hassle than going through the AgCl route again.

Are you really sure that the blue coloration is due to the presence of copper? Test your AgNO3. Dissolve some of the crystals which have the blue coloration on them in distilled water. Prepare a solution of yellow prussiate of potash and then mix the solutions. If you have copper in the mix, then you'll obtain a reddish precipitate, otherwise you'll get a pale yellow precipitate.

Another option is to make a solution of KSCN or NH4SCN and add this to the solution of silver nitrate. If there is no copper in this, then you get a purely snowwhite precipitate, if there is copper in the silver nitrate, then the precipitate will be dirty, not snow white.




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a_bab
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[*] posted on 7-2-2010 at 01:43


Or you may consider cupelation. It will render your silver pure enough to get pure AgNO3.

I also remember there was something about washing the AgNO3 with alcohol, Cu(NO3)2 being soluble in it.

Attachment: Pure silver from coin - cupelation.txt (4kB)
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unionised
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[*] posted on 7-2-2010 at 06:33


The easy way is just to recrystallise the AgNO3.
Unfortunately, it's very soluble, so you will get significant losses.
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Myfanwy
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[*] posted on 7-2-2010 at 06:50


i think the best is to wash it with ethanol,like a_bab said.
AgNO3 is just slightly soluble and Cu(NO3)2 is good.




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[*] posted on 7-2-2010 at 07:28


If amount of Cu is small (<1%), then dissolve AgNO3 in small amount of water and take 3 % of prepared solution and precipitate Ag2O (+Cu(OH)2) with NaOH.
Wash it and add to remaining solution.
Ag2O will react with Cu(II) quantitatively to give Cu(OH2).
Filter the solution and evaporate it. You should get AgNO3 Cu free.
(this is from my book about inorganic stuff preparation)
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[*] posted on 7-2-2010 at 08:59


Use a flame test on the 'blue colouration': copper is unmistakable in a Bunsen or alcohol flame. Unless you know what the contamination really is purifying will always be difficult...

If copper, could the slightly different solubilities of CuS (solubility product Ks1 = 6 x E-37) and Ag2S (Ks2 = 6 x E-51) be used for separation?

Thus : [Cu2+] = Ks1/Ks2 x [Ag+]^2 = E+14 x [Ag+]^2

At the right [S2-] concentration [Cu2+] would thus appear to remain in solution, especially if the Cu contamination level was low. Just a thought...



[Edited on 7-2-2010 by blogfast25]
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Jor
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[*] posted on 7-2-2010 at 10:15


Well it seems there was only copper nitrate on the surface of the crystal layer, it was a really small amount.
Rising the AgNO3 with a few mL of EtOH took care of it. Now I have a nice amount of pure white crystals.

Thanks for all the help!
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[*] posted on 8-2-2010 at 02:06


Even though the problem has been solved, I feel compelled to mention one more method of purification. Fusing the AgNO3 results in any copper salts decomposing into copper oxides; after cooling. dissolving the AgNO3 in DW and filtering through a very fine filter removes the copper oxides.

Another way is related to what you did do. Dissolve the impure AgNO3 in DW (as a cool/cold solution), add a couple of drops of HNO3, then precipitate the AgNO3 with ethanol. Some AgNO3 will remain in solution, recover by allowing the solution to evaporate and add the mixed nirates to the next batch of crude AgNO3.
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