Concentration of Nitric Acid needed for refining Silver.

[rabbit60]

I understand the UK Gov are just starting consultations on possibly reducing the concentrations of certain acids available to buy on license by the public.
One possible outcome under consideration is that Nitric Acid concentration would be limited to a max 10% .
Just wondering if anyone has tried purifying Silver with Nitric of this strength and if it actually works without too many problems?

 

[Quiklearner]

I don't see why 10% wouldn't work, but the amount of waste (volume) you will likely generate will be incredible.

 

[butcher]

Nitric can be concentrated up to an azeotropic solution of 68% concentration, which we use to make aqua regia for gold and several of the platinum group metals,

The 68% solution would need to be diluted for use with silver to around 35% HNO3/H2O.

68% HNO3 that is as concentrated of nitric we need to recover or refine precious metals or dissolve metals reactive to the acid, you cannot concentrate an acid past its azeotropic point as the acid will vapor off with the water and 68% is as concentrated as you will get using evaporation or heat to drive off water.

Price could be a limiting factor, an alternative is to make poor man's nitric acid suitable for recovery or dissolution of gold,

Poor man's nitric acid would need to be distilled for use with silver, in order to remove the sulfates remaining that you cannot remove by simply chilling the solution.

Silver forms fairly insoluble sulfate's so we need a purer nitric acid, free of sulfates where possible, for use with silver.

Some sulfates in nitric or aqua regia are not a problem for gold, or most base metals and the sulfate could be somewhat helpful as it makes lead insoluble in the gold chloride solution.

 

[orvi]

10% is too much water i think. Working with very diluted stuff is tedious, and unusable on larger scale. If only 10% stuff could be purchased and i will have some chemical background, first thing i will try to do will be to concentrate it.

But distillation of nitric acid is quite bad, nasty operation. Only simplification with 10% acid would be, that you will firstly collect just water. Carefull fractional distillation with efficient column could be very straightforward if set up properly. From the top of the column, mostly water vapour will escape out with minor loss of acid. Conviniently scrubbed into bucket of cold water. No condenser needed (practically speaking).

Maybe another option would be to use H2O2 simultaneously with that diluted nitric, converting NO2 formed back to nitric acid. This could somehow eliminate good portion of "waste" volume.

 

[rabbit60]

10% is too much water i think. Working with very diluted stuff is tedious, and unusable on larger scale. If only 10% stuff could be purchased and i will have some chemical background, first thing i will try to do will be to concentrate it.

But distillation of nitric acid is quite bad, nasty operation. Only simplification with 10% acid would be, that you will firstly collect just water. Carefull fractional distillation with efficient column could be very straightforward if set up properly. From the top of the column, mostly water vapour will escape out with minor loss of acid. Conviniently scrubbed into bucket of cold water. No condenser needed (practically speaking).

Maybe another option would be to use H2O2 simultaneously with that diluted nitric, converting NO2 formed back to nitric acid. This could somehow eliminate good portion of "waste" volume.

Thanks for the advice Orvi, I will dilute some of my acid to 10% and see if i can distill the water off it to bring the conc back up.

 

[orvi]

Thanks for the advice Orvi, I will dilute some of my acid to 10% and see if i can distill the water off it to bring the conc back up.

Do it outside or in a fume hood, and in the all-glass apparatus. For efficient separation, rectifying column is needed. Without it, loss of the acid would be significant and concentration of the remaining acid won´t be that high.