Stannous chloride for detecting excess nitric in AR

[alvaschein]

Hi all

I recently was digesting a sample of gold plated pins in AP. All was processing good, the gold foil was swimming. A few days later I checked again the process and found everything has been dissolved - even the gold foils!
Oh! And now? Knowing that in the solution I had only chloride ions, I tried to precipitate the dissolved gold with SMB. But nothing happened! Well ...
How can I get the gold out of the solution? I tried the next in simply pouring stannous chloride in this solution - just to see what happens. Surprisingly the solution turned turbid and gold particles started to precipitate! I thought "Cool, I found Columbus' egg! (just kidding...). Let's try it with an other gold solution out of AR.". Thought and made. I took a sample of the AR solution and poured stannous chloride in it. It happened like above, gold started dropping out.
I let it stand for a few minutes or so and looked again. Gosh! What happened? Everything is like before I poured in stannous chloride ... and my brain cells started to work. There must have been excess nitric in the AR solution that reversed the precipitation effect.
I took an other sample of AR and treated it to get rid of excess nitric. And the the same test as before, with the difference that the gold colloid remained in solution and didn't disappear. What a discovery! (sorry, kidding again)
No, what I'm pointing to is, that stannous chloride can be used for qualitative determinate if there is any excess nitric present or not. I simply don't know how sensitive this test could be.
There is an other test propagated to detect nitric, called brown ring-Test, but seems to be more complicated and stannous chloride is available in every gold refiner's kitchen.

I appreciate any comments to my observation, even if you find it is a truism...


Roy

 

[patnor1011]

Your gold foils did not dissolved. What you think that were foils it was actually probably gold plated nickel foil and as that dissolved, your plating came down as fine powder. Most of pins are flash plated only.

 

[Palladium]

This don't make sense. You started off talking about the gold was dissolved with ap, then it goes on to say you had an excess of nitric?

 

[butcher]

Now a problem with tin.

Not the best way to tell if excess oxidizer, but can be used as an indicator, is using the stannous chloride, as we normally test for gold, with oxidizer in solution the stannous test will not form the familiar violet color or will begin to form and quickly disappear, I would not add stannous chloride to a gold solution except in a test sample of a few drops of solution, in a spot plate or Q-tip or filter paper...

Tin salts can reduce and precipitate a wide variety of metals, besides making a solution a bear to filter.

 

[alvaschein]

This don't make sense. You started off talking about the gold was dissolved with ap, then it goes on to say you had an excess of nitric?

I wrote about testing it on different solutions: First on AP and then on AR.
The story behind is this: It isn't the first time that dissolving pins in AP, all get dissolved. The gold foils first started floating on the AP and after a few days all disappeared. Even with a lamp I couldn't find the slightest turbidity that could come from very fine gold particles. The stannous chloride test always was positive. Even in the last case mentioned above. I don't know why this happens. Impurities in HCl or H2O2? Or simply dirty glassware from tests before?
Anyway, after "precipitating" with stannous chloride and washing the supposed gold, I redissolved it with AR and made with it the stannous chloride test, which resulted positive.
But this isn't what I was pointing to. I just wanted to say that stannous chloride might be a very simple method to detect nitric in an AR solution due to following reason: Stannous chloride produces colloidal gold in the solution. Is there still any nitric in the solution, the colloidal gold will redissolve again, more or less rapid, depending on how much nitric is still left in the AR solution.

Hope this helps against the confusion.

Roy

 

[alvaschein]

Now a problem with tin.

Not the best way to tell if excess oxidizer, but can be used as an indicator, is using the stannous chloride, as we normally test for gold, with oxidizer in solution the stannous test will not form the familiar violet color or will begin to form and quickly disappear, I would not add stannous chloride to a gold solution except in a test sample of a sew drops of solution, in a spot plate or Qtip or filter paper...

Tin salts can reduce and precipitate a wide variety of metals, besides making a solution a bear to filter.

Of course I wouldn't use it on the whole batch or even as precipitating agent. I use only drops.

 

[Palladium]

I see now.

 

[g_axelsson]

It isn't the first time that dissolving pins in AP, all get dissolved. The gold foils first started floating on the AP and after a few days all disappeared. Even with a lamp I couldn't find the slightest turbidity that could come from very fine gold particles. The stannous chloride test always was positive. Even in the last case mentioned above. I don't know why this happens. Impurities in HCl or H2O2? Or simply dirty glassware from tests before?

Hi Roy, do you have any vessel close by with nitric acid in it? Gases from one beaker easily travels between beakers and it don't take much to start dissolving.
Quite often I get a purple film on top of my silver nitrate solution, even on a table outside, just by placing it close to a beaker with HCl in it. Another contamination source could be chloride in tap water.

I have had gold foils in AP for months without any gold dissolving. I'm not using any peroxide since the first time, new batches are started with a mix of old copper chloride and fresh acid.

By the way, you are using peroxide, what strength do you use? Too concentrated can easily dissolve gold with HCl.

Göran

 

[alvaschein]

It isn't the first time that dissolving pins in AP, all get dissolved. The gold foils first started floating on the AP and after a few days all disappeared. Even with a lamp I couldn't find the slightest turbidity that could come from very fine gold particles. The stannous chloride test always was positive. Even in the last case mentioned above. I don't know why this happens. Impurities in HCl or H2O2? Or simply dirty glassware from tests before?

Hi Roy, do you have any vessel close by with nitric acid in it? Gases from one beaker easily travels between beakers and it don't take much to start dissolving.
Quite often I get a purple film on top of my silver nitrate solution, even on a table outside, just by placing it close to a beaker with HCl in it. Another contamination source could be chloride in tap water.

I have had gold foils in AP for months without any gold dissolving. I'm not using any peroxide since the first time, new batches are started with a mix of old copper chloride and fresh acid.

By the way, you are using peroxide, what strength do you use? Too concentrated can easily dissolve gold with HCl.

Göran

Hi Göran
Now that you mention it: I have acids and beaker (with cover) on the same shelf. Maybe this could be one reason of dissolving gold too.
My peroxide has 30%, but always dilute it by 1:3. Still too high?

Thanks for your hints.

Roy

 

[g_axelsson]

I think that the recommended strength of peroxide is 3%, I think we have the answer....

HCl and strong peroxide is a good mix to dissolve fine grained gold. I use it when I have dropped gold from a dirty solution and have problem with the fine grained precipitation that never settles. A splash of 30% H2O2 in the wash (already HCl in it), swirl it around a little and the gold goes into solution, then drop it again and the gold clumps together and is so much easier to wash. It takes me less time to dissolve and drop a second time than waiting for the fine grained gold to settle.

As said before, I don't use peroxide, only air in my copper chloride process. But then I'm usually in no hurry with the material that takes that way. My first experiment with pins in AP took 6 years before I cleaned up that mess. :mrgreen:

Göran

 

[Geo]

ive been using AP solution since before i started on the forum. i havent used peroxide in my "copper chloride" solution since the first time i made a batch for this reason. try to keep some copper chloride in a sturdy plastic jug such as an empty bleach bottle or a washed and dried drain cleaner bottle. when you want to process using copper chloride, use clean water and fresh hcl and add some of the pregnant copper chloride solution from the jug. no peroxide needed beyond the first time. as a matter of fact, you can take AP solution that is so saturated that it looks black and store a bottle for about a month and the resulting liquid will be a clear emerald green with green CuCl crystals formed in the bottom of the container.

be sure to properly label that bottle. just to be safe.

 

[goldsilverpro]

No, what I'm pointing to is, that stannous chloride can be used for qualitative determinate if there is any excess nitric present or not. I simply don't know how sensitive this test could be.

In one of my gold dissolving processes, I use a form of AR containing 50% nitric acid, by volume. While dropping out the gold with hydrazine, I tested for gold on a spot plate using freshly made stannous chloride solution. I used 1 drop of the AR solution and 1 drop of stannous chloride. The black gold appeared immediately but quickly redissolved and disappeared. I then tried 1 drop of AR solution plus about 5 drops of water and added a drop of stannous chloride. The black gold that formed did not disappear. If I remember right, it was still there several hours later.

Please note that all I did was dilute the nitric to a point where it wouldn't contribute to the redissolving of the black/purple colloidal gold. All the original nitric was still there. In other words, stannous chloride can't be used to detect the presence of nitric, as far as I can see. Maybe, somewhat, the strength, but not the presence.

Actually, the brown ring test is extremely simple (I have used it many times, many years ago), but, in my view, a little too sensitive for this.

SMB can tolerate a little free nitric. 1.5g of SMB will eliminate about 1ml of excess 70% nitric, producing about 1.34g of sodium nitrate, which remains in the solution, and 1g of SO2.