Why diluting nitric acid?

[Blue Moonshine]

I totally understand that white fuming nitric acid (95%) is not necessary for gold refining, because at such a concentration passivation takes place (as illustrated in this video) and makes the reaction with copper extremely slow. Copper (and I'm assuming other metals) will react faster at the azeotropic concentration (68%) which is (I believe) what we commonly call concentrated nitric acid.

But what is the point of diluting the acid down to 25% or so?

 

[eaglekeeper]

I totally understand that white fuming nitric acid (95%) is not necessary for gold refining, because at such a concentration passivation takes place (as illustrated in this video) and makes the reaction with copper extremely slow.

That is true....but there is an additional reason FNA (fuming nitric acid) isn't recommended...and that is safety. Not only is it a fire hazard, but it can also nitrate certain materials making them energetic....think gun cotton.

But what is the point of diluting the acid down to 25% or so?

All the nitric acid does is convert the metal into a form that is water soluble. So you need the water to dissolve the metal.

azeotropic concentration (68%) which is (I believe) what we commonly call concentrated nitric acid.

Correct....The 68% does the work just fine, but I'm personally comfortable using 35-40%.....just a personal preference. If I switched to the 68% nitric I would have to recalibrate how much I use during a refine.

 

[kurtak]

But what is the point of diluting the acid down to 25% or so?

If you are using 68 - 70 % nitric you only need to dilute to 50/50 - a bit more to allow of water evaporation if you use heat to help dissolve the metals

because at such a concentration passivation takes place

This is why 68 - 70% needs to be diluted 50/50 --- in simple terms the water gives the metal atoms a place to go as nitric works at dissolving the metal

Kurt

 

[Thomas M Joseph]

Theoreticaly, hot concentrated nitric acid is about 1/2 as efficient as cold very dilute nitric acid, time being a factor.