metallic precips from ammonium chloride

[johnon]

Hi Folks. Been washing me Au precips. (from AR) w/ ammonia, then HCl. Problem is, when i combine solns (wanna make ammonium chloride) i get metallic precip. Both solns. test non-oxidising to potassium iodide/starch test paper. That's also how i rest to make sure nitric is gone from AR. Have SnCL and DMG, but waiting to make std Solns. for testing against (per Hoke-great book) I do a lot of work w/ores so maybe interference? Thanks.

 

[Harold_V]

Hi Folks. Been washing me Au precips. (from AR) w/ ammonia, then HCl. Problem is, when i combine solns (wanna make ammonium chloride) i get metallic precip.

Doesn't surprise me a bit. Washing is intended to remove trace elements that hitch a ride when you precipitate. By changing pH, you precipitate anything that hitches a ride, so what you're doing isn't in your best interest, assuming you want to make use of the ammonium chloride.

When you begin the wash cycle, your first wash is best done with HCl. I typically boiled the material for a prolonged period of time, then I'd give a hard boil with a tap water rinse. Only then did I go to ammonia. I'm not convinced that starting the first wash with ammonia is in your best interest.

Harold

 

[samuel-a]

harold, by ammonia wash, you mean ammonium chloride + HCL solution?
or ammonium hydroxide solution?

 

[Harold_V]

harold, by ammonia wash, you mean ammonium chloride + HCL solution?
or ammonium hydroxide solution?

Ammonium hydroxide, for washing. The only place I ever used ammonium chloride was in the precipitation of platinum and palladium.

Harold