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Techniques for CollectorsHow to test for sulphur using a silver coin?

30th Oct 2014 09:19 UTCHenri Koskinen Expert

Here is a Mindat photo of löllingite by E.L.Clopton


http://www.mindat.org/photo-643412.html


Text says that "Crystals of löllingite to 3 cm embedded in albite var. cleavelandite and quartz matrix. Löllingite crystals are rimmed by an unidentified massive black mineral, larger masses of which contain blocky crystals of an unidentified dark brown mineral; see second photo. Original label (Gene Bearss collection) states "misnamed as arsenopyrite but no sulfur found with silver coin test". "


Does anyone know the details of how to perform this silver coin test?


Henri

30th Oct 2014 13:04 UTCMatt Neuzil Expert

09018620016016029425427.jpg
books.google.com/books?id=p2g4AAAAMAAJ

30th Oct 2014 13:08 UTCAlfredo Petrov Manager

As most so-called "silver" coins don't contain silver anymore, people might get negative results for S because they used the wrong coin.

30th Oct 2014 13:24 UTCMatt Neuzil Expert

I think it's funny kids these days don't know that. Whenever I find one I get excited and get the response is usual "aren't they all silver" ?

30th Oct 2014 15:11 UTCHenri Koskinen Expert

Thanks Matt

30th Oct 2014 16:48 UTCEd Clopton 🌟 Expert

I assumed the "silver coin test" worked something like that, with the sulfur from the sample somehow tarnishing the coin, but I hadn't looked up the details. Thanks for asking and pursuing it!

30th Oct 2014 18:15 UTCHenri Koskinen Expert

Hello E.L.


Imagine prospecting in the wilderness, long ago, with no advanced testing or tools available. You find some promising metallic mineral. You hit it with a spike and smell arsene. You pulverize a small piece, add some soda, grind a small hole in a piece of charcoal. You place the pulver in the charcoal and heat it with a blowpipe until pulver melts and fuses. While doing this your bacon and beans are warming up in the campfire. You take the fused bead from the charcoal, grind it between two spoons and spit in the pulver. You fish a silver dollar from your pocket and place the wetted pulver on top of the coin, sip your coffee and the coin appears tarnished yellow from sulphur... arsenopyrite!


That's how I imagine the scene anyhow.


I checked Donald Peck's book titled Mineral Identification. The book includes a couple of paragraphs about this method of fusing carbonates and a lot of tips about analyzing fused beads. Charcoal is not necessary as Bunsen burner can be used and Pecks book advises also on the relative amounts of sodium of carbonate and the tested metal to be used.


Henri

30th Oct 2014 18:31 UTCAlfredo Petrov Manager

The test works on sulphates too, not just sulphides.

30th Oct 2014 20:29 UTCFred E. Davis

Some time ago out of curiosity, I looked at the composition of world-wide coins using EDXRF.

Silver coins with a high-percentage silver (>93%):

1924 Venezuela ? bolivar

1936 USA Liberty 50 cents

1915 Cuba 20 centavos


Many were Cu-Ni, Cu-Zn-Mn-Ni, Ni, two were Ni-Fe (1999 Jamaica 10$, and center of 1993 Mexico 2 N$), and two were Fe-Cr (1967 Italy 50 lire, and edge of 1993 Mexico 2 N$)

30th Oct 2014 20:49 UTCPeter Haas

The procedure described above is basically an alkaline melt extraction. Upon heating the soda, it does what all carbonates do upon sufficient heating: it will decompose, giving off carbon dioxide and leave NaOH behind (note that "soda" contains ten moles of water per formula unit and not all of this will evaporate). An alkaline melt - or, generally speaking, an alkaline solution - is a good extraction solvent for all anions, not only sulfide. In classical wet chemical anion analysis, one starts with extracting the sample with a soda solution and all subsequent testing is done on that extract.


Problems with both procedures are (1) low soluble sulphides (e.g. cinnabar, acanthite, covellite, etc.), and (2) selectivity, of course, since there may be other species present that get extracted as well and produce false positives. To care about low soluble sulfides, the charcoal comes into play, as it will reduce the metal in some fraction of the low soluble sulphides and set sulphide ions free. However, elementary sulphur, if present, is reduced as well in this process, as are sulphite, rhodanide, thiosulphate and many organic sulphur compounds. They all end up as sulphide in the reaction product and will produce false positives when testing with the silver coin (Sulphate, however, will not interfere). While most of these potential interferences can be safely ignored as they are very unlikely to occur in natural mineral samples, the sulphur is certainly an interference to worry about.


When the aqueous extraction procedure is chosen, the insolubles are dried and extracted with chloroform or carbon disulfide (in which sulphur is readily soluble) to remove any elementary sulphur present. The residue is then treated with acid and zinc granules added; reduction of protons on a metallic surface produces "hydrogen in statu nascendi", i.e. hydrogen atoms in the first step which then recombine to form molecular hydrogen (i.e. H2). Hydrogen atoms are a more powerful reducing agent than molecular hydrogen and do the job of reducing some part of the metal in low soluble sulfides. The resulting solution is then neutralized and combined with the soda extract for subsequent testing.


It's worth mentioning that a strip of filter paper that has been soaked in a lead(II) acetate solution may be used for final testing instead of the silver coin. In contact with sulphide, the paper will turn black immediately (this also works well for testing for trace H2S in the gas phase where a silver coin will not be of a great help).



Though, there's an even better test procedure: the so-called iodine-azide reaction:


(1) Prepare a reagent solution by dissolving 2.5 g of iodine (I2) and 3 g of sodium azide (NaN3) in 100 ml of distilled water. This solution is perfectly stable and may be stored for an unlimited period of time at ambient temperature.


(2) Fill a few ml of the reagent solution in a test tube. Grind a small amount of your sample (a few milligrams are sufficient) to a fine powder and add it to the solution. If there's any sulfide present in the sample, a rather strong production of nitrogen gas will start almost immediately (this will not stop until the iodine is consumed whence the solution will have turned colourless).


The principle of this test is a catalytic decomposition of the iodine-azide solution. Sulfide ions act as the catalyst, according to the following equations:


S2- + I2 -> S + 2 I-

S + 2 N3- -> S2- + 3N2


Since sulfide is not consumed in the overall reaction and also retains its oxidation state, even a minute amount of sulphide gives rise to a strong reaction. In other words, this test is extremely sensitive. Even low soluble sulphides will give a positive test result without pre-treatment (this is because there aren't any really insoluble compounds: their solubility products may be abyssmally low, but there's always a minute fraction that actually dissolves).


The only interferences are thiosulphate, rhodanide and several organic sulphur compounds. Elementary sulphur will not interfere at any concentration.

1st Nov 2014 11:32 UTCHenri Koskinen Expert

Thanks Peter.


Henri.

12th Nov 2014 11:04 UTCHoward Heitner

I think that the iodine-azide test is a little two sensitive because only a catalytic amount of sulfide is needed. The reaction may be caused by a very minor component of the sample, when the objective is determine if the major component contains sulfur. Sodium azide is an explosive. It is what explodes to rapidly fill automobile "air bags".. At least in the US, it is highly unlikely that you can buy it. The sodium carbonate fusion combined with lead acetate or silver is more useful. Yes there are no more silver coins. However if you rummage around in the attic or in a thrift store,there is a good chance of finding an old knife or serving spoon with a sterling silver handle

12th Nov 2014 16:59 UTCD. Peck

Any old silver spoon will do, and the dark silver sulfide can be polished off it quite easily to renew the surface. I kept one in my "lab" kit.

27th Nov 2014 20:07 UTCDana Morong

Here is another one: "Hepar Reaction and other tests for Sulfur" on RockNet, 4-22-2002, at www.rockhounds.com/rocknet/archive/messages/17816.shtml

(note the shtml on the end)

(no use to try to contact him via that e-mail it is long since disused)

1st Sep 2020 02:19 UTCDana Morong

The previous link is outdated, and to find it by title by internet search is difficult.  The site for "Hepar Reaction and other tests for Sulfur" (by "Martin" in 2002) is now at:
If this doesn't work, try archive.is/www.rockhounds.com 

2nd Sep 2020 01:34 UTCDana Morong

I found that if i click on it as a link, it does not work, but if I type it in myself, then it works.  Sometimes takes a while.  Sometimes it is useful to copy it, or copy and paste, into the webaddress place on one's machine, to find an internet site.  Anyhow, it is still available.

2nd Sep 2020 02:35 UTCKevin Conroy Manager

This should work: https://archive.is/KmK2J

(An extra space after the address was in the original.)
 
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