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Techniques for CollectorsHow to test for sulphur using a silver coin?
30th Oct 2014 09:19 UTCHenri Koskinen Expert
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 13:08 UTCAlfredo Petrov Manager
30th Oct 2014 13:24 UTCMatt Neuzil Expert
30th Oct 2014 15:11 UTCHenri Koskinen Expert
30th Oct 2014 16:48 UTCEd Clopton 🌟 Expert
30th Oct 2014 18:15 UTCHenri Koskinen Expert
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
30th Oct 2014 20:29 UTCFred E. Davis
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
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
Henri.
12th Nov 2014 11:04 UTCHoward Heitner
12th Nov 2014 16:59 UTCD. Peck
27th Nov 2014 20:07 UTCDana Morong
(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
If this doesn't work, try archive.is/www.rockhounds.com
2nd Sep 2020 01:34 UTCDana Morong
2nd Sep 2020 02:35 UTCKevin Conroy Manager
(An extra space after the address was in the original.)
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Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: March 28, 2024 20:04:48