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Author: Subject: Dissolved in nitric acid
itchyfruit
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[*] posted on 15-9-2009 at 15:02
Dissolved in nitric acid


As far as i'm aware all metals will dissolve in nitric acid(except gold)
What i would like to know is would impure gold eg 9ct etc partially dissolve ?
Also a friend of mine asked me to check some scrap gold for him in this collection was a old gold watch the case was certainly gold but some of the cogs etc were clearly copper based(brown fumes and dark blue solution) the spring and a half moon shaped piece of silver coloured metal are all the remained,so i was just wondering what this metal might be?
The numbers from the face are little bits of gold flake in the solution.
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ammonium isocyanate
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[*] posted on 15-9-2009 at 15:15


Most platinum group metals aren't oxidized by nitric acid either.
The non gold part of gold alloys would dissolve in nitric acid (unless the other metal(s) is/are of the platinum group). But the gold would not to any appreachiable extent. The way to dissolve gold is Aqua Regia.

As for the other metals, are you saying the spring etc. didn't dissolve in concentrated nitric acid? In that case I'd guess platinum.




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itchyfruit
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[*] posted on 15-9-2009 at 15:32


I used 70% nitric acid, interesting about the platinum i didn't realise that it could be springy the half moon piece looks like it may have come from the face.
I've separated these and added some more nitric acid to them i'll check them again in the morning....
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merrlin
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[*] posted on 15-9-2009 at 21:54


Many ferrous alloys, such as stainless steels, will passivate in 70% nitric acid. Pure iron can passivate in concentrated nitric acid, but will be attacked by dilute nitric acid. Other metals that form a stable, impermeable surface oxide can also passivate in nitric acid.

[Edited on 16-9-2009 by merrlin]
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woelen
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[*] posted on 15-9-2009 at 22:33


Yes, passivation in fact is very common. The metals niobium and tantalum strongly passivate in any oxidizing solution and they do not dissolve, not even in aqua regia. The same is true for titanium and zirconium, but titanium does dissolve (albeit slowly) in conc. HCl and it dissolves very quickly when some HF is present.

So, in general, one can say that whether a metal dissolves or not depends on quite some factors, such as formation of a passivating layer which prohibits dissolving of the metal and formation of coordination complexes which promotes dissolving of the metal.

Many of the platinum metals and also gold are easier to get into solution than niobium and tantalum. For the latter two I did not yet find any aqueous solution, which is capable of dissolving them.




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[*] posted on 16-9-2009 at 07:30



Quote:

What i would like to know is would impure gold eg 9ct etc partially dissolve ?


Coin dealers use some type of nitric acid solution to determine the carat value of alloyed gold. Sorry I do not know more details.




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[*] posted on 16-9-2009 at 07:31


Can be aluminum, it passivates with HNO3 in almost any concentration.
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merrlin
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[*] posted on 16-9-2009 at 13:24


Quote: Originally posted by woelen  
Many of the platinum metals and also gold are easier to get into solution than niobium and tantalum. For the latter two I did not yet find any aqueous solution, which is capable of dissolving them.


Attached is a list of etchants used with tantalum and niobium. Since etchants are formulated to remove a small amount of material while enhancing microstructure contrast, they may not be that useful for dissolution, per se. The exception might be the combination of HF and sulfuric in an electrolytic cell. Perhaps one could sacrifice contrast for enhanced dissolution by using some of the other etchants in an electrolytic cell. I've used NaOH to electropolish tungsten.

Attachment: Tantalum_Niobium_etchants.pdf (19kB)
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[*] posted on 17-9-2009 at 01:31


Quote: Originally posted by Magpie  

Quote:

What i would like to know is would impure gold eg 9ct etc partially dissolve ?


Coin dealers use some type of nitric acid solution to determine the carat value of alloyed gold. Sorry I do not know more details.


General method is to make a small scratch in the object, then add a drop of 30-40 percent HNO3. Plated copper alloys turn the acid green to blue, 10 karat gives a brown colour - tiny bits of undissolved gold left when the more reactive metals dissolve, as the karat rating is increased the colour changes to a light brown at around 12 karat, to a yellow, to little to no effect at 14 karat. Gold over silver gives a pinkish colour, or if very thing a intensifying of the gold shade. Above that you might use a touchstone and alloy needle standards.

That a high enough percentage of gold protects more base metals from fully solution in HNO3 has to be taken in account in flame assay. In that procedure the sample is weighed, mixed with lead bits and wrapped in lead foil, then melting in stream of air in a bone-ash cupola which absorbs the oxides of lead and other base metals. After the lead has been fully oxidised the gold-silver alloy is cooled, hammered to break off any remaining lead oxide slag, weighed, and then treated with nitric acid and the remaining gold washed, dried, and weighed. The ratio of gold to sample weight is the percent gold, that of the mixed Ag-Au alloy minus the weight of gold the divided by the sample weight gives the percent of silver. To insure there was enough silver that it would be fully dissolved from the alloy, lead with precise known percentages of added silver would be used, weight the doped lead and calculate the amount of silver, subtracted that from the weight of the Ag-Au alloy button (the added silver should be at least 50% of the total weight)
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[*] posted on 18-9-2009 at 06:06


Quote: Originally posted by woelen  


Many of the platinum metals and also gold are easier to get into solution than niobium and tantalum.


So that goes to explaining why tantalum is often used for making small whip stirrers. i had always wondered about that. It is a nice springy metal however i always assumed platinum's inertness made it a better candidate.
Passivation is kind of fun, last week i did the greatest thing, i took seven old iron retort bases, that were corroded and had the epoxy paint peeling from them everywhere and had them sand blasted (six dollars each, seemed like too little money). I then 'parkerized' them, which is a passivation process i chanced upon when i ended up with 20L of 'Parkerizing 200' concentrate. It leaves the metal surface dark charcoal in appearance, but lighter if coarser sandblasting grit is used. They look great and are infinitely more practical, why lab suppliers do not do this already is beyond me.
If you google parkerizing, you can see endless web pages of americans dismantling their handguns, sandblasting them, parkerizing them and reassembling them.




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[*] posted on 30-9-2009 at 14:13
Gold dissolution


For dissolving gold, conc. HCl and H2O2 mixture also works and this mixture also dissolves Pt and some other metals. The aqua regia analogues thread also lists some more mixtures which will dissolve gold.
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