Sciencemadness Discussion Board

Silver thiosulfate or Silver sulfide

joseph1990 - 4-6-2017 at 22:09

I'm in a serious blunder. I used two pure silver plates to do electrolysis with sodium thiosulfate solution. The H2S produced was strong so I used a fume hood. When the solution turned black I had no idea what this was so I added hydrochloric acid in order to achieve silver chloride. The whole thing turned black and then I added sodium hydroxide. My goal was to get silver oxide so that I can heat it up to get pure silver but once I took the precipitate and burned it it just kept getting darker and flames got much more blue. Someone please help with this experiment. I'm a computer science major but I really need to achieve a B in this class. The professor wants us to form pure silver and he explains little.

Melgar - 6-6-2017 at 16:25

You need cyanide to plate silver. You may see a reference here and there about ways that don't need cyanide, but believe me, if they'd have found a better way they would have switched to it long ago. If you're heating up silver sulfide with a propane torch or similar, you'll be adding sulfur faster than you're removing it, just from the mercaptan odorizer in the gas.

What you need is nitric acid. It'll oxidize the sulfur away, and form nice, water-soluble silver nitrate, which is the starting silver compound for any silver chemistry that you'd be able to think of.

edit: Do not get silver nitrate on porcelain or your skin. It will leave dark, black-purple stains on either. They'll last about a week on your skin, forever on porcelain.

[Edited on 6/7/17 by Melgar]

clearly_not_atara - 7-6-2017 at 10:54

I assure you the professor of a college chemistry class does NOT expect the students to figure out for themselves that they need to use cyanide. That's insane, and it could easily cost him his job or land him in prison.

Silver can easily be plated from diamminesilver solutions by the addition of a reactive aldehyde like glucose or formalin. No idea what you're doing though OP, it sounds like you're trying to make silver from silver?

[Edited on 7-6-2017 by clearly_not_atara]

Melgar - 7-6-2017 at 12:29

First you're chastising me about how deadly H2S is, now he can't possibly be expected to use KCN just after having been making clouds of H2S by following instructions. So which is it? I never said he SHOULD use cyanide, in fact I gave him a way to make silver nitrate instead. My point was that trying to plate silver using a non-cyanide bath is probably going to fail, because every attempt to do so thus far has been an abject failure.

The method you're suggesting using will produce a surface layer that can be scratched off by a fingernail. OP, if you just need to get pure silver, there's a very easy way to do it. No need to use cyanide unless your goal is actually electroplating silver. All you do is take your silver sulfides and oxides, dissolve in dilute nitric acid overnight, then neutralize the acid with bicarbonate and add a piece of pure copper to the solution. Over time, large silver crystals will grow on it. Copper electrical wire is pure enough for your purposes. To get large crystals takes time though, and I'm not sure how much you have. You should ideally dilute the solution a lot so that the crystals form more slowly and thus are nicer looking.

[Edited on 6/7/17 by Melgar]

k

joseph1990 - 7-6-2017 at 23:24

Quote: Originally posted by Melgar  
You need cyanide to plate silver. You may see a reference here and there about ways that don't need cyanide, but believe me, if they'd have found a better way they would have switched to it long ago. If you're heating up silver sulfide with a propane torch or similar, you'll be adding sulfur faster than you're removing it, just from the mercaptan odorizer in the gas.

What you need is nitric acid. It'll oxidize the sulfur away, and form nice, water-soluble silver nitrate, which is the starting silver compound for any silver chemistry that you'd be able to think of.

edit: Do not get silver nitrate on porcelain or your skin. It will leave dark, black-purple stains on either. They'll last about a week on your skin, forever on porcelain.

[Edited on 6/7/17 by Melgar]



Thanks for the post. Wouldn't silver sulfide and nitric acid produce solid sulfur, nitric oxide, and then silver nitrate? The question now is how do we get rid of the sulfur, adding excess nitric acid will only create sulfiric acid. Are you saying to add excess nitric acid and then add aluminum oxide so it will produce flocculant aluminum sulfate. Sounds like a good idea but this would require lots of calculations to be exact. Unfortunately the use of Cyanide has been outdated for a couple of years now even in the mining industry due to health concerns, pretty sure they wouldn't want a couple of students to get sick if they're avoiding this route even for poor old miners.

well

joseph1990 - 7-6-2017 at 23:25

i think the best route would be to create silver sulfate then do displacement with aluminum

Melgar - 8-6-2017 at 02:48

Quote: Originally posted by joseph1990  
i think the best route would be to create silver sulfate then do displacement with aluminum

Silver sulfate is only slightly soluble in water, but a lot more soluble than silver chloride, for example.

Nitric acid is a strong oxidizer, and would HOPEFULLY only oxidize your sulfide to SO2, allowing it to leave the solution. It could also potentially oxidize it to SO3 which would immediately react with water to form H2SO4, sulfuric acid, and then precipitate out silver sulfate. This isn't ideal, but since silver sulfate is slightly soluble in water, it'd eventually plate onto your piece of copper as well.

Don't do displacement with aluminum. Aluminum is too reactive, and will react with water in many scenarios. Also, its passivating oxide layer makes it nonreactive with nitric acid, and so either you're reacting too fast and generating black silver powder, or not reacting at all, and generating nothing. Copper works better because it's in the same group as silver, it's easy to tell apart from silver, and copper's surface oxidizes only slowly. It also will gradually turn the water blue as it's displaced by silver, which is a nice visual indication that it's working.

Nitric acid WOULD release NOx fumes, but in a dilute solution, they'll often be oxidized back to nitric acid by atmospheric oxygen. You can reduce their formation by adding H2O2 to the solution, but I don't think you're supposed to do that for silver. It works spectacularly for copper and nickel though.

edit: One more thing: if you neutralize the nitric acid with bicarbonate, STOP if you start to see precipitate. That'd be the very-insoluble silver carbonate. If you accidentally do this anyway, a bit of nitric acid added drop by drop should dissolve it. Oh, and chloride ions in the solution will make the silver nearly impossible to recover, so definitely avoid that.

edit 2: Oxidation of sulfides nearly always skips elemental sulfur. In order to separate a sulfur atom from a metal sulfide, both the metal and sulfur typically have to be oxidized, and the sulfur won't usually leave until it's at least at SO2.

[Edited on 6/8/17 by Melgar]

well

joseph1990 - 10-6-2017 at 08:08

Quote: Originally posted by Melgar  
Quote: Originally posted by joseph1990  
i think the best route would be to create silver sulfate then do displacement with aluminum

Silver sulfate is only slightly soluble in water, but a lot more soluble than silver chloride, for example.

Nitric acid is a strong oxidizer, and would HOPEFULLY only oxidize your sulfide to SO2, allowing it to leave the solution. It could also potentially oxidize it to SO3 which would immediately react with water to form H2SO4, sulfuric acid, and then precipitate out silver sulfate. This isn't ideal, but since silver sulfate is slightly soluble in water, it'd eventually plate onto your piece of copper as well.

Don't do displacement with aluminum. Aluminum is too reactive, and will react with water in many scenarios. Also, its passivating oxide layer makes it nonreactive with nitric acid, and so either you're reacting too fast and generating black silver powder, or not reacting at all, and generating nothing. Copper works better because it's in the same group as silver, it's easy to tell apart from silver, and copper's surface oxidizes only slowly. It also will gradually turn the water blue as it's displaced by silver, which is a nice visual indication that it's working.

Nitric acid WOULD release NOx fumes, but in a dilute solution, they'll often be oxidized back to nitric acid by atmospheric oxygen. You can reduce their formation by adding H2O2 to the solution, but I don't think you're supposed to do that for silver. It works spectacularly for copper and nickel though.

edit: One more thing: if you neutralize the nitric acid with bicarbonate, STOP if you start to see precipitate. That'd be the very-insoluble silver carbonate. If you accidentally do this anyway, a bit of nitric acid added drop by drop should dissolve it. Oh, and chloride ions in the solution will make the silver nearly impossible to recover, so definitely avoid that.

edit 2: Oxidation of sulfides nearly always skips elemental sulfur. In order to separate a sulfur atom from a metal sulfide, both the metal and sulfur typically have to be oxidized, and the sulfur won't usually leave until it's at least at SO2.

[Edited on 6/8/17 by Melgar]



Hey thanks for the help. What about oxygen? Silver sulfide with o2 can produce silver elemental and sulfur dioxide but with the solution being in water what problems can I encounter? Just trying to avoid smelling fart as my last experience before death...yes I've already smelled a little and it's quiet unpleasant. Would the sulfur dioxide evaporate or still be in solution?