Minimum amount of nitric needed per gram gold in AR

[Stibnut]

Hi, I'm just getting back into hobby gold refining after dabbling in it briefly in 2017. I've been reading the forum for a couple weeks now and working my way through Hoke.

I have a simple question that I can't seem to find an answer for in searching the forum, although it's no doubt been asked and answered numerous times. I know the preferred way to make AR is to add small amounts of HNO₃ to HCl in stages until the gold is fully dissolved so as to minimize excess nitric at the end.

But I'm not sure what the chemical equation or the theoretical minimum amount of nitric is to dissolve a given amount of gold. I know that this would just be a theoretical minimum because AR produces chlorine, nitrosyl chloride, and varying amounts of nitrogen oxides which evolve from the solution, but I'm thinking it would be a good place to start with the first nitric addition, with more as needed after that first addition until the gold dissolves completely.

I could write something like Au + HNO₃ + 4HCl --> HAuCl₄ + NO + 2H₂O. This would suggest that the minimum amount is just 1 mol HNO₃ per 1 mol Au. This comes out to 63.012/196.967 = 0.320 g HNO₃/g Au. Dividing by 68% and then by 1.41 g/mL, I get 0.334 mL 68% HNO3/g Au. But this seems far lower than is actually needed, far enough that it seems this reaction I wrote must be wrong by more than just a bit of loss from NOCl/NOx.

Does anybody know a more accurate amount for the minimum amount of nitric needed? And how much does it take in practice, in your experience?

 

[butcher]

Hoke's book states it takes about 4 fluid ounces (118.29 ml) 0f 32% HCl, and one fluid ounce of 70% HNO3 (29.57ml) will dissolve a troy ounce of gold.

This works out when you plug in the chemical reactions formula and figure the moles involved...
I have a post doing that math.

3.8 ml HCl and 0.95 ml HNO3 will dissolve a gram of gold.

 

[Geo]

In practice, it takes a little more. Due to NOx lost to the atmosphere and heating. Thermal decomposition of nitric acid occurs at around 182°F. Using a watch cover helps but only returns a small fraction of what's lost.

 

[Lino1406]

2 no3- = n2o4 + o2 + 2e-
This gives 1 electron per hno3 molecule. Gold needs 3 electrons. This gives 3 moles hno3 per 1 mole gold or 3x 63g per 197g, hence around 1:1 as Butcher said

 

[speed]

In practice it depends on many things. The surface area of the gold. The type of apparatus you use. What temperature (and pressure probably!) the solution is at and ofcourse if theres anything else in the mix like base metals.

You will need more than the theoretical in pretty much all cases amount but good practices can help you keep wastage to a minimum.

 

[Stibnut]

Oh right, I forgot about the electron transfer - that would push the amount of moles of HNO₃ per mol Au up to 3:1 and hence about 1 mL HNO₃ per g Au. That makes a lot more sense.

I saw that Hoke had a 4:1 ratio in her AR and said that it took about 1 fl oz AR per oz gold, but I wanted to see if the minimum might be a bit lower. Not that this minimum would ever be attained because of NOCl/NOx coming off the solution and impurities in the gold of course.

It seems like it would make sense to just add about 0.9 mL HNO₃ per g Au to excess HCl at the outset, give that plenty of time to react, and then add small increments of about 0.1 mL/g Au after that until it all dissolves.

A related question: is there any real disadvantage, other than wasting reagents, to overshooting the nitric addition and then denoxxing with sulfamic acid? Obviously the denoxxing is crucial if you do have excess nitric but I'm wondering if overshooting causes any problems by itself.

 

[kurtak]

In theory - 1ml nitric (70%) + 3 ml HCl (32%) to dissolve 1 gram gold

In truth - it is going to depend on conditions under which the gold is being dissolved

one of those conditions is the form the gold is in (foils - powder - shot/corn flake - solid coin/bar)

Theory - to dissolve 1 ozt gold = 31 ml nitric (70%) + (minimum) 93 ml HCl (32%)

Reality - 1 ozt gold foils MUCH less nitric (like +/- 20 ml) --- solid gold coin/bar - it will likely take MORE then 31 ml nitric

other condition that can effect amount of nitric needed/used --- heat - open or closed vessel - water no water - etc.

A related question: is there any real disadvantage, other than wasting reagents, to overshooting the nitric addition and then denoxxing with sulfamic acid? Obviously the denoxxing is crucial if you do have excess nitric but I'm wondering if overshooting causes any problems by itself.

other then what you listed - no - in fact - depending on the situation it can be to your advantage

Kurt

 

[Williamjf77]

I find I advantages to overshoot nitric when dealing with gold filled that may or may not have base metals still in there like eyeglass frames with stainless. I usually nuke it to make dissolving faster then clean it up. Sometimes those glasses take along time

 

[Stibnut]

I find I advantages to overshoot nitric when dealing with gold filled that may or may not have base metals still in there like eyeglass frames with stainless. I usually nuke it to make dissolving faster then clean it up. Sometimes those glasses take along time

Do you have trouble with silver chloride or other impurities when you do that? My impression is that going straight to AR tends to yield messes, but perhaps you mean that you digest with nitric or AP beforehand and you use AR on the results, which might still have base metals that survived the first treatment having been shielded by the gold.

 

[Geo]

Never intentionally use an excess of nitric acid. There are only a few instances where I would recommend and excess of nitric acid. For a beginner or even an acolyte, excess nitric acid can lead to losses. If you read about a gold solution that the gold will not drop, it is normally caused by excess nitric acid. It's better to add HCl to your material and heat. Once the material and HCl are hot, start adding the nitric acid in increments small enough that the solution does not boil over. Wait until the reaction stops completely and add some more. In this way, if more is added than is needed, it will only be the amount of the increments you are adding. Sulfamic acid removes nitric acid by converting the nNO2 to N2O. Do not even think about capturing the N2O for consumption. It still contains the other gaseous byproducts as well. Sulfamic acid conversion of NO2 should be done as hot and safely as possible. Add very, very small amounts at first because of the real possibility of a boil over as the reaction of the sulfamic acid on the hot solution is a very vigorous reaction and releases a lot of gas quickly.

 

[Geo]

BTW, the conversion of nitric acid using sulfamic acid is a 1:1 ratio by weight. Gram for gram.

 

[Stibnut]

Never intentionally use an excess of nitric acid. There are only a few instances where I would recommend and excess of nitric acid. For a beginner or even an acolyte, excess nitric acid can lead to losses. If you read about a gold solution that the gold will not drop, it is normally caused by excess nitric acid. It's better to add HCl to your material and heat. Once the material and HCl are hot, start adding the nitric acid in increments small enough that the solution does not boil over. Wait until the reaction stops completely and add some more. In this way, if more is added than is needed, it will only be the amount of the increments you are adding. Sulfamic acid removes nitric acid by converting the nNO2 to N2O. Do not even think about capturing the N2O for consumption. It still contains the other gaseous byproducts as well. Sulfamic acid conversion of NO2 should be done as hot and safely as possible. Add very, very small amounts at first because of the real possibility of a boil over as the reaction of the sulfamic acid on the hot solution is a very vigorous reaction and releases a lot of gas quickly.

I had a sulfamic boilover a week ago doing my first AR reaction in 4.5 years. Like most newbies I managed to add excess nitric despite knowing how to avoid doing that, but thought I'd get rid of it with sulfamic. I saw a thread where someone was having trouble getting sulfamic to react with known excess nitric, so I assumed it was a slow reaction and added it all at once. It boiled over and I ended up with ~2 g out of the ~7 g of gold I started with on my stirplate and concrete patio. After retreating inside briefly to let fumes die down, I used a solution of sodium carbonate to neutralize, wiped up the mess with paper towels and put them in a stainless steel pot containing more sodium carbonate solution. Now my latest project involves incinerating the paper towels and recovering as much gold as I can from my mistake.

Anyway, lesson learned - add sulfamic acid very slowly and always use an overflow bowl or beaker with any reaction that has a chance of overflowing. My stirplate still works and the patio has almost no sign that anything happened. I'm now doing my reactions with a 2 L beaker on the outside holding a smaller (250 - 500 mL) beaker containing the reaction.

Is there a reason you recommend adding the first dose of nitric only after the HCl is hot? I would think that while the reaction is faster hot vs cold, you'd lose more in NOx/NOCl fumes given the higher temperature, so it might be a good idea to add it while cold and then start heating to drive the reaction to completion and evaporate excess HNO3 then. But maybe AR that starts out cold causes some other issues, or your point is to avoid excess nitric, so losing some more of the first addition to NOx vapor is okay because it reduces the chance of adding too much in the first addition.

 

[Williamjf77]

Do you have trouble with silver chloride or other impurities when you do that? My impression is that going straight to AR tends to yield messes, but perhaps you mean that you digest with nitric or AP beforehand and you use AR on the results, which might still have base metals that survived the first treatment having been shielded by the gold.

I didn’t mean going straight to AR, I’ve had problems with some gold filled getting all base metals out with nitric, probably iron or something I guess, but yes there’s a little silver Chloride

 

[Williamjf77]

Never intentionally use an excess of nitric acid. There are only a few instances where I would recommend and excess of nitric acid. For a beginner or even an acolyte, excess nitric acid can lead to losses. If you read about a gold solution that the gold will not drop, it is normally caused by excess nitric acid. It's better to add HCl to your material and heat. Once the material and HCl are hot, start adding the nitric acid in increments small enough that the solution does not boil over. Wait until the reaction stops completely and add some more. In this way, if more is added than is needed, it will only be the amount of the increments you are adding. Sulfamic acid removes nitric acid by converting the nNO2 to N2O. Do not even think about capturing the N2O for consumption. It still contains the other gaseous byproducts as well. Sulfamic acid conversion of NO2 should be done as hot and safely as possible. Add very, very small amounts at first because of the real possibility of a boil over as the reaction of the sulfamic acid on the hot solution is a very vigorous reaction and releases a lot of gas quickly.

I really hope that nobody actually thinks about capturing the N2O for inhaling, that would be insane

 

[Geo]

I had a sulfamic boilover a week ago doing my first AR reaction in 4.5 years. Like most newbies I managed to add excess nitric despite knowing how to avoid doing that, but thought I'd get rid of it with sulfamic. I saw a thread where someone was having trouble getting sulfamic to react with known excess nitric, so I assumed it was a slow reaction and added it all at once. It boiled over and I ended up with ~2 g out of the ~7 g of gold I started with on my stirplate and concrete patio. After retreating inside briefly to let fumes die down, I used a solution of sodium carbonate to neutralize, wiped up the mess with paper towels and put them in a stainless steel pot containing more sodium carbonate solution. Now my latest project involves incinerating the paper towels and recovering as much gold as I can from my mistake.

Anyway, lesson learned - add sulfamic acid very slowly and always use an overflow bowl or beaker with any reaction that has a chance of overflowing. My stirplate still works and the patio has almost no sign that anything happened. I'm now doing my reactions with a 2 L beaker on the outside holding a smaller (250 - 500 mL) beaker containing the reaction.

Is there a reason you recommend adding the first dose of nitric only after the HCl is hot? I would think that while the reaction is faster hot vs cold, you'd lose more in NOx/NOCl fumes given the higher temperature, so it might be a good idea to add it while cold and then start heating to drive the reaction to completion and evaporate excess HNO3 then. But maybe AR that starts out cold causes some other issues, or your point is to avoid excess nitric, so losing some more of the first addition to NOx vapor is okay because it reduces the chance of adding too much in the first addition.

You will not get a complete usage of all the nitric acid you add unless the solution is hot. Using nitric acid at ambient temps leads to over usage and excess nitric acid in the solution.

 

[Geo]

I didn’t mean going straight to AR, I’ve had problems with some gold filled getting all base metals out with nitric, probably iron or something I guess, but yes there’s a little silver Chloride

Nitric acid will not effect stainless steel that is at least 304 or better even hot. Add a small amount of sulfuric acid as a catalyst to force the nitric acid to attack the stainless steel. You will get better results.

 

[kurtak]

Never intentionally use an excess of nitric acid. There are only a few instances where I would recommend and excess of nitric acid.

Geo

Per the bold print

I have to disagree - there are actually many reasons why you would want to use an excess of nitric to dissolve your gold --- some of those reasons are where you actually NEED to use excess nitric - other times - though you don't need to it can be to your advantage

It's better to add HCl to your material and heat. Once the material and HCl are hot, start adding the nitric acid in increments small enough that the solution does not boil over.

I will agree that when teaching beginners this is good advice &/or when talking about dissolving gold foils or gold powders

I don't have time today but outside of dissolving foils &/or powders - or teaching a beginner - in most other situations I would use an excess of nitric ether because it is "needed" - or - it is to my advantage

Will post more about this when I have time this weekend

Kurt

 

[Geo]

Geo

Per the bold print

I have to disagree - there are actually many reasons why you would want to use an excess of nitric to dissolve your gold --- some of those reasons are where you actually NEED to use excess nitric - other times - though you don't need to it can be to your advantage



I will agree that when teaching beginners this is good advice &/or when talking about dissolving gold foils or gold powders

I don't have time today but outside of dissolving foils &/or powders - or teaching a beginner - in most other situations I would use an excess of nitric ether because it is "needed" - or - it is to my advantage

Will post more about this when I have time this weekend

Kurt

Thank you Kurt. I did try to convey that in the next sentence that there a few instances where an excess is needed. After leaching base metals from gold filled material. An excess of nitric acid is recommended due to the unknown amount of base metal mixed with the karat solids left behind for example. I may not be able to articulate my thoughts as well as I would like. Sorry I didn't make myself understood more clearly.

 

[FrugalRefiner]

Never intentionally use an excess of nitric acid. There are only a few instances where I would recommend and excess of nitric acid.

I agree with Geo, that in general it is best to avoid excess nitric.

One of those exceptions Geo mentioned, which I learned from Harold, is when toll refining professionally.

Because he offered quick turnaround on his customers' gold, he had to process their material as fast as possible. So he would add excess nitric, instead of dosing it in increments as the reaction died down, to get the material into solution as fast as possible.

Because he was fair, he wanted to ensure he had dissolved all of his customers' values, and having excess nitric assured him he had done that.

So he developed his technique of getting all the customers' material in solution fast, then adding a weighed button of refined gold to the AR. By weighing the button after the nitric was exhausted, he could determine how much of it had gone into the solution, deduct that weight from the final weight of all the dropped gold, and credit his customer with the rest.

Again, I would recommend that most members add their nitric in increments and avoid adding too much nitric. For most of us, nitric is expensive and can be difficult to find, so we try not to waste any.

Dave