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.

 

[didit]

I have an additional question to the original post as I have struggled to find a suitable answer in any of the readings, forum searches, or the internet.

For those of you who make your own fuming nitric acid, what are your recommendations on how to dilute fuming nitric acid (85-95%) to 68-70% concentrated nitric acid?

I have read procedures that in the simplest terms you must determine the acidity % of the fuming nitric acid you have and once that % is known a formula can be used to determine how much water that the acid can be added to to achieve 68-70% concentrated.

Does that sound like a recommended procedure?

Does diluting with water change the characteristics of the acid enough for use in standard precious metal recovery?

Some of the above responses already mention mixing with water so I assume that is acceptable but want to build on what I have learned thus far.

 

[Yggdrasil]

I have an additional question to the original post as I have struggled to find a suitable answer in any of the readings, forum searches, or the internet.

For those of you who make your own fuming nitric acid, what are your recommendations on how to dilute fuming nitric acid (85-95%) to 68-70% concentrated nitric acid?

I have read procedures that in the simplest terms you must determine the acidity % of the fuming nitric acid you have and once that % is known a formula can be used to determine how much water that the acid can be added to to achieve 68-70% concentrated.

Does that sound like a recommended procedure?

Does diluting with water change the characteristics of the acid enough for use in standard precious metal recovery?

Some of the above responses already mention mixing with water so I assume that is acceptable but want to build on what I have learned thus far.

First.
There is no use for fuming Nitric in refining.
Secondly.
There is no need for a specific strenght since it is "always" diluted anyway.
Anything above 30-50% will do.

 

[didit]

First.
There is no use for fuming Nitric in refining.
Secondly.
There is no need for a specific strenght since it is "always" diluted anyway.
Anything above 30-50% will do.

Yes that is what I understand as well. Fuming nitric acid is way too strong for refining. But, if you were to make you own nitric acid such as in the video in the original post, can you simply dilute it with water to 68-70% or even further to 30-40%?

Is addition of an appropriate amount of water sufficient to do that?

I ask because I see a number of other methods to reduce pure nitric to concentrated that are a quite more complicated than that.

 

[4metals]

For those of you who make your own fuming nitric acid, what are your recommendations on how to dilute fuming nitric acid (85-95%) to 68-70% concentrated nitric acid?

Fuming nitric acid is nothing to mess with but if you make your own nitric with nitrate salt (salt peter) and concentrated sulfuric acid and distill the acid you will have made fuming nitric acid. Open the cap and out comes the genie in the bottle wafting out into the room. (or preferably the lab hood) I diluted this acid to 68% to make the azeotrope strength which does not rise up to greet you when you open the cap.

First thing you need to do is determine the % of the concentrate you are starting with. You need a good balance which can weigh to at least 0.01 grams. You also need a 100 ml volumetric flask. (100 ml makes the math easy) Tare the dry flask and fill it with your concentrated acid. You need to know the temperature of the acid in degrees C and this is easiest to measure with an infrared thermometer. Then go on line and find a chart like this one that tells you the percent acid based on the weight and temperature of your acid.

Now for the math;
C1 x V1=C2 x V2

Where:
C1 is the concentration of the starting solution (which you determined by weighing your concentrate)
V1 is the volume of the starting solution you want to dilute
C2 is the concentration of the final solution (68%)
V2 is the final volume of the solution you want to prepare. (I used 2500 ml because that is the volume of a reagent acid acid bottle here in the US)

I solve for V1, added the required distilled water which is V2-V1, and slowly add the nitric to the water. It will warm up!

Be careful, nitric acid at any strength can hurt you.

I have always had the luxury of having 68% nitric available to use. I made my own only once to see how it was done to be better informed because some members were making their own. It is a viable method and if you do it routinely invest in a nice sized vessel and a heat mantle as well as the requisite glassware to hook up the condenser and receiver. And be careful!

 

[didit]

Fuming nitric acid is nothing to mess with but if you make your own nitric with nitrate salt (salt peter) and concentrated sulfuric acid and distill the acid you will have made fuming nitric acid. Open the cap and out comes the genie in the bottle wafting out into the room. (or preferably the lab hood) I diluted this acid to 68% to make the azeotrope strength which does not rise up to greet you when you open the cap.

First thing you need to do is determine the % of the concentrate you are starting with. You need a good balance which can weigh to at least 0.01 grams. You also need a 100 ml volumetric flask. (100 ml makes the math easy) Tare the dry flask and fill it with your concentrated acid. You need to know the temperature of the acid in degrees C and this is easiest to measure with an infrared thermometer. Then go on line and find a chart like this one that tells you the percent acid based on the weight and temperature of your acid.

Now for the math;
C1 x V1=C2 x V2

Where:
C1 is the concentration of the starting solution (which you determined by weighing your concentrate)
V1 is the volume of the starting solution you want to dilute
C2 is the concentration of the final solution (68%)
V2 is the final volume of the solution you want to prepare. (I used 2500 ml because that is the volume of a reagent acid acid bottle here in the US)

I solve for V1, added the required distilled water which is V2-V1, and slowly add the nitric to the water. It will warm up!

Be careful, nitric acid at any strength can hurt you.

I have always had the luxury of having 68% nitric available to use. I made my own only once to see how it was done to be better informed because some members were making their own. It is a viable method and if you do it routinely invest in a nice sized vessel and a heat mantle as well as the requisite glassware to hook up the condenser and receiver. And be careful!

Thank you for that detailed response! You have confirmed a great deal of what I have discovered, but I wanted to hear from the experts on this forum about the topic. Now at least I feel confident if one uses the proper equipment and safety precautions (definitely no nitrile gloves) I will be on the right course. I'm sure your explanation will be very beneficial to other less experienced members like myself.

 

[4metals]

I cannot say this enough. Be careful. While nitrile gloves are useless, I always preferred, while refining, to use thick neoprene rubber gloves which do offer protection from nitric acid. Not exactly the kind of gloves to pick up small things but lifting and decanting nitric solutions they are fine. And fortunately I never had to real time test them in a working environment.