blogfast25
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Precious Metal Mini Mystery?
I bought 2 silver 925 table spoons weighing about 36 g in total off eBay and dissolved them in a 20 % excess of hot 50 % HNO3. A bluish solution was
obtained but some insoluble residue (about 1 g at a guess) remained, even after cranking up the heat for some time.
I filtered off the solution and set it aside for copper/silver separation.
The residue, still on the filter, was then submitted to fresh, hot 70 % HNO3 and showed some reactivity towards it but not all of it dissolved. The
filtrate reacted clearly positively for silver with HCl and the AgCl (a small amount) obtained was set aside too.
The remaining residue appeared mauve and was subjected, still on the filter, to Aqua Regis (5 ml HNO3 70 % + 20 ml HCl 36 %) in which it dissolved
(and part of the filter too, of course) enthusiastically, leaving no further residue. After filtering I was left with a urine-coloured solution, a
small sample of which was treated with sat. NH4Cl(aq). No precipitate formed which appears to exclude Pt and point to HAuCl4 instead.
The filtrate was part neutralised with 10 g NaOH (predissolved in water) and then a good dollop of strong sodium metabisulphite was added and the
solution heated. Quite slowly (over a period of minutes) the solution clouded over, going from clear yellow to very turbid black. Whatever’s causing
this is now slowly precipitating to the bottom of the beaker. This doesn’t look anything like the small amount of gold powder (< 0.1 g at a
guess) I recovered from a mobile phone a few weeks back.
Any ideas? I think the spoons were no more than about 100 years old, probably much less.
[Edited on 1-7-2012 by blogfast25]
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Wizzard
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How about Rhodium? I believe it's chlorides can be yellow and red, could have been tainted to other colors by copper.
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blogfast25
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Quote: Originally posted by Wizzard  | | How about Rhodium? I believe it's chlorides can be yellow and red, could have been tainted to other colors by copper. |
Wiki:
"White gold is often plated with a thin rhodium layer to improve its optical impression while sterling silver is often rhodium plated for tarnish
resistance."
Not impossible. The spoons were weren't tarnished at all.
Also, it's very unlikely that the second residue still contained any copper...
[Edited on 1-7-2012 by blogfast25]
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cyanureeves
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more often than not i have gotten a grey and sometimes whitish precipitate that wouldnt dissolve further when dissolving sterling. but i never
persisted in dissolving it with aqua regia but have wasted nitric unecessarily trying to dissolve it further to no avail.in my cases it would always
react with sodium chloride to form silver chloride and in some cases it would produce more silver chloride than what was all in solution.no doubt my
silver is not 99.9 but this mud when turned to the chloride also turns to metal "silver?" with zinc/hcl displacement so i just thought it was silver.
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blogfast25
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| Quote: Originally posted by cyanureeves | | in my cases it would always react with sodium chloride to form silver chloride and in some cases it would produce more silver chloride than what was
all in solution. |
You mean the residue reacted with NaCl solution to yield AgCl??? That sounds near impossible to me but I'm not an expert on silver, sterling or
other.
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cyanureeves
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that has been the case for me many times blogfast25. i would venture to say that i never achieved total dissolution with my nitric which was not very
stout nitric back when. but really even dilute nitric can dissolve silver fairly quick. i have a total of about 4 oz. of silver gotten from sterling
and i assumed that the only contaminant was zinc from the displacement procedure.yes the mud would would turn very white with salt solution and as a
matter of fact i would have to crushed the very dense white silver chloride. if i did not crush and dispers the white almost bluish clump it would
take longer to turn black with the zinc/ hydrochloric solution but eventually it would turn black.
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blogfast25
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It should be possible to remove zinc from the silver sponge with acetic acid solution.
I'm now in the process of isolating the black residue to weight it and do some more tests on it.
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blogfast25
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After a few decantations, each time replacing the water with fresh water, and drying at about 150 C I obtained 0.041 g of residue, dark gray/black.
The quantity could be consistent with a thin coating.
To it was added 2 ml HNO3 (70 %) (no reaction ensued) and then 8 ml HCl (36 %) upon which reaction started immediately and on a bit (< 5 min) of
heating a clear urine coloured solution was obtained, as per previous. Based on rhodium, the solution would have been about 0.1 M.
To it, 0.4 g of NaCl was added and a few ml of DIW to dissolve it. No red Na3RhCl6 (sodium hexachlororhodate) was observed, not even on complete
evaporation of the solvent: a yellow/white residue was left behind.
Could it be good old gold? But then why is the powder so black, not brown-reddish as with Au?
I’m out of ideas on this one… 
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Fleaker
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Gold powder is black when parted from silver. It is only brown when precipitated from chloroauric acid. All of the anodes slimes we see are black.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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blogfast25
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@Fleaker:
Except that the material has already been precipitated once from a solution containing no silver whatsoever, from HAuCl4 (presumed)...
But I think you're right: the chemistry seems to strongly point to gold. Presumably the grain is so fine the powder is black, not brown. I will try
and re-precipitate it once again.
[Edited on 7-7-2012 by blogfast25]
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Fleaker
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Gold powder won't always be brown even when
For instance, if we dissolve 50 t oz up in aqua regia or with HCl/Cl2 (11 t oz per liter) and precipitate 80% of the gold from a cold solution with
SO2, 90% of it will be a beautiful light brown powder that rinses like a dream. It will be above 9999 fine after rinsing/pulping. Filter that off and
precipitate the last bit and it will be a fine black-brown sand. This we will pulp with sulfuric to turn it into a nice brown powder.
The blacker it is, the higher the impurity levels and the smaller the grain size (typically). We've still had fine black gold powder melt to 3N5 pure.
Remember well that AgCl is considerably soluble in HCl solutions! Always keep your gold as concentrated possible until you dilute by chilling with
ice. The AgCl will crash out and can be filtered and then the gold precipitated.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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blogfast25
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Fleaker:
Very interesting.
What do you mean by 'pulping [Au] with sulphuric [acid]' ?
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Fleaker
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Put precipitated gold into 60% hot sulfuric and sonicate. It considerably upgrades the purity.
Visually you can see the gold go "blonder". Heated for a long time, even AgCl converts to Ag2SO4 which is more soluble, especially in excess sulfuric
acid.
The liquid can be decanted or the gold rinsed on polypropylene felt (see Kavon Filter products--good stuff!)
[Edited on 26-6-2016 by Fleaker]
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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Fleaker
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925 is sterling silver. 925 parts silver balance usually copper. Flatware is usually close or spot on with this, but jewelry itself, at least when
melted 4-5K ounces at a time, is invariably less and almost always carries about 3 ounces of gold per 10,000 ounces of sterling.
EPNS can still be recovered. One can melt and plated out a silver enriched deposit in dilute nitric acid. Melt that up again and repeat to get pure
silver. Or it can be digested in hot sulfuric acid and cemented with copper metal to give 99% silver cement.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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Boffis
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The main elements present in "silver" used in cutlery are silver copper and antimony. Other impurities are mostly accidental although some
manufacturers used bismuth to harden the silver alloys too.
Assuming that you are dealing with a silver alloy and not EPNS, which is a silver plated copper nickel alloy, then the insoluble residue from the
nitric acid leach is likely to be mainly antimony oxides which are modestly soluble in aqua regia and slightly soluble in nitric acid alone.
To check for this element try washing the insoluble residue with water and then dissolving it in 5M HCl. If you have a microscope try placing a couple
of large drop on a slide and adding caesium chloride to one and KI to the other. In the former drop white snowflakes and hexagonal crystals indicate
antimony while iridescent plates that are almost invisible indicate bismuth, both may form. In the iodide drop a deep yellow solution indicates Bi the
antimony solution remains almost colourless. If a tiny grain of solid caesium chloride is now added to the almost coloureless iodide bearing drop and
red-orange hexagonal crystals are thrown down this indicates antimony. If the KI bearing drop is deep yellow the red crystalline ppt indicates bismuth
but antimony is masked by just a small amount of bismuth hence the need for the caesium chloride test.
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Fleaker
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Antimony isn't really significant in flatware in my experience.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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Eddygp
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Ruthenium? Really obscure addition to that object, but still... wait, does Ru dissolve in aqua regia?
there may be bugs in gfind
[ˌɛdidʒiˈpiː] IPA pronunciation for my Username |
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diddi
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for Ru: no. use hypochlorite 
Beginning construction of periodic table display
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Fleaker
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Yes, aqua regia doesn't really touch Ru unless mixed with other PGMs in which case things all start getting weird...
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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