Sciencemadness Discussion Board - My silver powder won't dissolve in nitric acid

My silver powder won't dissolve in nitric acid

marethyu - 9-6-2018 at 13:35

I have recently refined a batch of silver powder by dissolving 300 grams of sterling silver chains in nitric acid
adding sodium chloride to precipitate silver chloride
adding sodium hydroxide to convert the silver chloride to silver oxide
adding sugar to reduce the silver oxide back to silver metal
washing with hydrochloric acid to remove copper and nickel hydroxides
vacuum filtering and washing with copious amounts of water
and then oven drying the precipitate
After oven drying the silver powder for 4 hours at 500 degrees I was left with lumps of what seemed to be dry silver but the weight came out to 325 grams which I found very odd considering there was 300 grams of metal initially and there should only have been 277 grams of silver metal. I just assumed there must have been a bunch of water trapped in the silver that was struggling to boil off due to the extremely small particle size of the powder that made up the huge lumps of silver after it was oven dried.
Today I attempted to redissolve the silver lumps with nitric acid to make silver nitrate. I used an excess of azeotropic nitric acid that I believed to be contaminated with a small but still significant amount of ammonium nitrate. Very little silver would go into solution so I decided to filter off the acid and rinse the remaining silver powder and start again. I now put the silver powder into a beaker with some very pure nitric acid with about an 85% concentration but even after hours of refluxing the powder in the acid it a large amount of silver remains.
Help!?

fusso - 9-6-2018 at 14:14

Maybe the sugar didnt reduce Ag2O to Ag so AgCl formed on surface when HCl added to the "Ag" powder, protecting it from reacting with HNO3.
Also why use HNO3 contaminated with NH4+ when u have the pure acid?
Why buy .925 instead of .999(9) silver when u have the $?

Edit: what sugar did u use?

[Edited on 09/06/18 by fusso]

marethyu - 9-6-2018 at 14:34

I'm fairly sure the sugar reduced the Ag2O to Ag because the particle size was around the 300-500 mesh range and I added a large excess of sugar. Also, there was a complete color change from black to the tan color of silver metal. Also, I've never had any issues dissolving silver contaminated with small amounts of silver chloride in the past. I used the contaminated HNO3 because I had a liter laying around from a large batch I made. Also, I used .925 silver because I can buy it at $11 an ounce. Also, after further testing, the contaminated acid I used doesn't dissolve .99 silver anyways for reasons I'm still trying to figure out (I'm thinking I may have stilled over small amounts of sulfuric acid along with it as well). I used Sucrose sugar. I need to redistill that contaminated acid =(
Can you think of any way to salvage this because I really do not want to smelt the whole batch down and start over.

Quote: Originally posted by fusso

Maybe the sugar didnt reduce Ag2O to Ag so AgCl formed on surface when HCl added to the "Ag" powder, protecting it from reacting with HNO3.
Also why use HNO3 contaminated with NH4+ when u have the pure acid?
Why buy .925 instead of .999(9) silver when u have the $?

Edit: what sugar did u use?

[Edited on 09/06/18 by fusso]

aga - 9-6-2018 at 15:01

Attempting to mess with Gold, Silver or PGMs (with an eye to Profit) by any method other than a well established procedure just results in a lot of toxic waste.

I speak from experience- still processing toxic waste at least a year later.

Very little unsaleable Gold (it has no proveable provenance)

http://goldrefiningforum.com is where you'll get the best info.

Texium - 9-6-2018 at 15:15

Sucrose isn't a reducing sugar. It breaks down in water to produce glucose and fructose, which are reducing sugars, but that reaction only proceeds quickly in hot water. This may be part of the problem. Trying to use sucrose to give a silver mirror at room temperature is not effective, so it's likely that it didn't do much here.

marethyu - 9-6-2018 at 16:05

I tried the gold refining forum but they didn't know what the problem was

Quote: Originally posted by aga

Attempting to mess with Gold, Silver or PGMs (with an eye to Profit) by any method other than a well established procedure just results in a lot of toxic waste.

I speak from experience- still processing toxic waste at least a year later.

Very little unsaleable Gold (it has no proveable provenance)

http://goldrefiningforum.com is where you'll get the best info.

marethyu - 9-6-2018 at 16:11

I believe that the sucrose was able to reduce the Ag2O because I have seen others perform the same reaction and when I added the sugar the water quickly came to a boil and I left it on heating over night. Also, I just loaded up a small amount of the material into a crucible and melted it and was left with some insoluble black slag. It appears that the compound decomposed into silver once hot enough because the sides that were in direct contact with a flame are coated in silver. I added borax as a flux. The black compound also melted around 700C i think. Any idea what this compound is?

Quote: Originally posted by Texium (zts16)

Sucrose isn't a reducing sugar. It breaks down in water to produce glucose and fructose, which are reducing sugars, but that reaction only proceeds quickly in hot water. This may be part of the problem. Trying to use sucrose to give a silver mirror at room temperature is not effective, so it's likely that it didn't do much here.

[Edited on 6-10-2018 by marethyu]

unionised - 10-6-2018 at 00:29

It's entirely possible that only the surface of the AgCl particles reacted at all.
So you would get a mixture of Ag and AgCl - only the Ag will dissolve in HNO3

zed - 10-6-2018 at 13:24

Umm. Seems to me, spent etching solution, may be purged of Silver, by dissolving Copper in it. Copper rapidly goes into solution, and Fine Silver precipitates out of the solution.

I don't know, how far this reaction will go. Or if it is useful, to your refining Silver.

But, personally, I would be reluctant to form Silver Chloride.

Yields are more or less, quantitative..... But, to my way of thinking, Silver Chloride is hard to deal with.

markx - 13-6-2018 at 00:47

You should try to dissolve the metallic silver in a more dilute nitric acid....say below 50% or around it. Silver is actually quite reluctant to dissolve in highly concentrated nitric acid, so perhaps therein lie the solution to this problem. From my personal experience even the common 65% nitric acid will take forever to dissolve any decent amount of metallic silver, but once the concentration is diluted down the reaction becomes really fast and violent.