Sulfuric acid turns black

[Bobby Walker]

I bought some silver plated bowls and cups. I tested them then cleaned and heated with a torch. I put 600 ml of sulfuric acid in a beaker then added the plated material. I increased the heat slowly at first it turned blue or green then black. I removed some of the material hoping it would clear up. I then added 200 ml of sulfuric acid to the beaker and continued to heat but it remained black. Any help would be greatly appreciated.

 

[butcher]

I cannot see that as a very good way to try and recover a tiny amount of silver plate, by dissolving loads of base metals in loads of acids, creating very large waste steam to deal with for almost no return or gain.

The solution is saturated with metal sulfate salts some of which will give color to the solution as ions as long as those metal ions are in the solution they will color the solution.

Silver sulfate does not have a high solubility, so much of your silver may not be in solution, what is can be tested with a little salt or HCl.

Silver corrodes as a sulfide giving a blackish oxide color to silver, that is sulfides but it may be you are seeing something similar with the silver sulfates, I am not sure if this could have something to do with what you are seeing, also silver salts will react with light (normally after the acid or oxidizers are washed from the salts).

Metals like iron can color sulfates black or sulfuric acid black. Copper will give blue.

With plated scrap metal you can be dealing with almost any metal or its alloys.
I would be surprised if your solution was not colored at this point.

 

[Bobby Walker]

Thank you for your reply. I am only playing around trying to find the best way to remove plating. I have a lot of copper busbar with silver on it. The copper busbar is worth a good bit more if I remove the silver so for the difference in price would pay for the chemicals and disposal with the silver free. I have a waste disposal at work that I am allowed to bring in the contaminated aqueous for $5 a gallon as long as I get the PH to between 6 and 8.

 

[butcher]

There are several methods, I prefer copper refining using electrolysis, electrowinning copper, and collecting the silver.

with the above you do not produce too much waste as much can be reused, if you do have waste treat it as described in our safety section, it will remove more toxic metals than just neutralizing the solution, it will also give you a better understanding of the chemistry we use.

 

[upcyclist]

Depending on how much you have and the form factor allows (i.e., literal bus bars without connections), you could simply file (or otherwise abrade) the silver off with a bastard file and process the shavings in nitric. That's a lot of manual work, though, and your time is also worth money. I'd only do it if you couldn't or didn't want to mess with electrolysis.