# aqua regia solution



## arthur kierski (Sep 9, 2008)

i had an ar solution containing pgm---i elevated the ar to ph=6 and added zinc powder until the solution became white and precipitated the zinc with the pgms---to the precipitate i added hcl (expecting to dissolve the excess zn and stay with the pgm) and all dissolved with the hcl becoming a chloride solution of zinc+pgm----i had to add fe powder to this solution to precipitate the pgm.
question?could i precipitate the pgm with fe at ph=6 directly with fe powder?
thanks for replies


----------



## lazersteve (Sep 9, 2008)

Arthur,

I have never tried to use powdered iron to precipitate the PGMs, but I have used a solid cold rolled steel bar to do so from strongly acidic solutions. The reaction was slow compared to zinc and the resulting precipitate appeared much lighter in color than the precipitate from zinc. A portion of the precipitate also strongly adhered to the bar and I was therefore unable to completely recover that portion. The precipitate would also redissolve if not filtered out quickly. 

Your descripiton of the zinc precipitation process strikes me as strange. Firstly, the pH sounds higher than I'd expect. Typically I go for a more acidic solution (pH 3-5) as opposed to a solution that is close to neutral as you did. This could account for the reactivity of the precipitate to HCl. You likely formed hydroxides and/or carbonates that readily reacted with HCl. When I use zinc I add it in very small increments after I have added soda ash with lots of stirring unitl a slightly white precipitate forms. This white precipitate readily dissolves in a little added water leaving behind the precipitated metals after the zinc addition is completed. 

As the zinc is added in small amounts (1 teaspoon per addition) precipitates form. Be sure to stir it in and let the zinc fully react before adding more. When the zinc levels reach the proper concentration the entire solution will change from an orange-yellow color to a very dark black/gray color and the bulk of the PGMs will precipitate then. After everything settles I filter the solution. The liquid that is left should test barren and appear as a pale green to clear color.

Only after filtering and rinsing the black/gray powder do I perform a few washes of HCl (wash until the powder no longer fizzes). I test the washes for PGMs and discard it if barren.

I think iron should work as you have stated as long as it is finely divided and you throughly wash the resulting precipitants until they are free of iron. Since iron is not as reactive as zinc, it will be difficult to know when the iron is gone from the preciptate as iron will not fizz as zinc does. If your iron is in slivers or shavings, removal of the excess pieces will require boiling in HCl to be sure the iron shavings are completely removed.

Steve


----------



## arthur kierski (Sep 9, 2008)

thanks steve for your imediate reply--i will tomorow do the precipitation from a (new) ar solution the way you explained and then give you a return ---thanks again


----------

