Talk:Noble metal

message: More on history.
:Hopkins J 1918, "Earliest Alchemy", The Scientific Monthly, vol. 6, no. 6, pp. 530–537 (534):
::In the elements of Empedocles, the sequence was from the lowest to the highest: earth, water, air, ﬁre. Following this order, the alchemists had the base metals, earthy; the fusible metals (like tin and mercury) having the property of liquidity; the bright or "noble" metals (gold and silver) remaining clear like air; and the bronzes of a higher spiritual nature, playing like ﬁre on the surfaces of the metals. Between the base metals, such as lead and copper, and the noble metals came tin, and later mercury, considered stages of transmutation.
So Cu was a base metal, and Hg was a fusible (not noble) metal.
While Pt wasn't discovered until 1735 it came to be recognised as a noble metal, alongside Ag and Au.
The rest of the PGM appeared much later: Pd 1802; Ir, Os 1803; Rh 1804; Ru 1844; Re 1928. I presume a textbook dated 1810–1850 or so would only recognise six noble metals.
Three or four noble metals?
:Cronstedt AF 1788, An Essay Towards a System of Mineralogy: In Two Volumes, vol. 2, p. 507:
::…the noble metals, viz. platina, gold, and silver…Quicksilver holds a kind of middle place; for, like the base metals, it may be calcined, though not readily; and like the noble ones, it may be reduced by heat alone….we may therefore reckon four noble or perfect metals; viz gold, platina, silver, and mercury; because, when calcined, they recover their phlogiston, without the addition of any phlogistonic substance.
In the following extract from 1838, there are only seven noble metals including Ag, since Ru had not been discovered yet.
:Griffin JJ 1838, Chemical Recreations: A Compendium of Experimental Chemistry, Part 1, pg. 143
::Most of the metals fuse before the blowpipe, and, excepting the noble metals, become oxidised on exposure to the outer flame. Of the noble metals, Gold and Silver both melt, without suffering any further alteration. Platinum, iridium, palladium, rhodium, and osmium are all infusible; but the last by exposure to the outer flame, becomes oxidised, and volatises in the state of osmic acid.
We know that silver leaf was employed in illuminated manuscripts but was subject to corrosion.
:Shaw H  1866, Handbook of the Art of Illumination, as Practised During the Middle Ages p. 59:
::The metal next in importance to gold is silver, which was employed in illuminations of all periods, but more largely in drawings of an early date than in those of a later time…The original beauty and brilliancy of these…have been sadly marred by the blackened state of the silver from oxidation.
::In more recent examples we have met with many instances in which this metal has retained in a remarkable manner its original freshness. We would particularly allude to the prayer-book of Henry VI…The almost unchanged appearance of he drawings in these books could only arise from their being rarely exposed to the action of light and atmospheric influences, or to the silver having being fixed to the vellum in leaves on a preparation of size, and secured from all sinister action on its surface by the protection of a varnish.
More on copper as a seminoble metal.
:Ostwald W 1906, Conversations on chemistry, vol. 2, p. 315, 337:
::Copper…may be looked upon as representing the transition stage between the base and noble metals.
::You already know mercury so well that all I have to do to complete the description of its properties...is only to give you a few ﬁgures…At the ordinary temperature neither damp nor dry have any effect on it, but it oxidizes at or about 300°. However, the mercuric oxide, when subjected to a greater heat, decomposes, as you know, into the metal and oxygen. Therefore, the adjective noble may be applied to mercury for all practical purposes."
Perhaps sometime in the 1930s, the physics-based definition of a noble metal as being Cu, Ag, Au arises. Here's a sample extract:
:Seitz F & Johnson RP 1937, Modern theory of solids. II,. Journal of Applied Physics, vol. 8, no. 3, 186–199 (194) 
::…using the zone theory, that in the alkali metals and the noble metals (Cu, Au and Ag)…
And then there is the confusion with precious metals.
:Anusavice DMD, ‎Chiayi Shen ‎H & Dawls R 2012, Phillips' Science of Dental Materials, p. 369:
::These [seven] noble metals and silver are sometimes called precious metals, referring to their high economic value, but the term precious is not synonymous with noble. Silver is reactive in the oral cavity and is not considered a noble metal.
Here's a good defintion of the term.
:Louis C & Pluchery O 2017, Gold Nanoparticles For Physics, Chemistry And Biology, p. 393:
::…gold has long been considered as an inert material since bulk gold does not react with many molecules typically present in the ambient atmosphere, the property that renders gold the most noble metal.
Sandbh (talk) 07:26, 3 April 2022 (UTC)
thread_title: Comments cont.

message: , , I'd wondered why Ag was ever regarded as a noble metal given that when sulfur is present, usually as hydrogen sulfide, it reacts with silver to form the well-known tarnish of Ag2S. It transpires that from about the middle of the 19th century onwards (at least in the US) sulfur levels increased by up to 25 times, due to the industrial revolution/s, peaking in about 1920, and staying at that level until falling away after about 1980, when stricter sulfur emission standards were adopted in parts of the world. That's why it came to be that silver plate or silver wasn't used as extensively as it had been in the past. The rest of the noble metals i.e. the PGM + Au are not so affected at room temperature. That's why, I guess, while Ag was traditionally regarded as a noble metal back in alchemical times, it later came be dropped off of some lists of noble metals.
I've also found a source (Russell & Lee 2005, Structure-property relations in non-ferrous metals, p. 293) saying that the PGM, and their "exceptional resistance to oxidation and chemical attack in corrosive aqueous solutions", originates in their high EN, and does not rely on a protective oxide layer. The lack of an oxide film is also the case for Ag and Au neither of which react with O; Ag only reacts at a pressure of 1.5 MPa and 300 °C, caveating that Ag does not have an exceptional resistance to being oxidised by sulfides. The EN values are interesting: 1.9 Re; 1.93 Ag; Hg. 2.0; 2.2 Ir, Os, Pd; 2.28 Pt, Rh; 2.54 Au. One needs an EN of 2.2 or more, to be on the starting blocks. Unanswered question: why is W at 2.36 not a noble metal? Pourbaix says it is a base metal.
It seems that nearly all the other metals and metalloids form an oxide film on their surfaces when exposed to oxygen at RT. The only exception appears to be Ge which, in any event, is not noble in the electrochemical sense (GeO2 + 4e– → Ge = –0.25), and its EN at 2.01 falls short. Sandbh (talk) 08:05, 5 April 2022 (UTC)
replies: text: I'm not convinced that the Pauling electronegativity for W is really correct. On the Allen scale it is much lower, in keeping with its neighbours (Ta 1.34, W 1.47, Re 1.60). If the Allen scale has a better handle on the real situation for Mo and W, then it would pretty neatly explain why tungsten is left off. Double sharp (talk) 08:13, 5 April 2022 (UTC)

text: Regarding why W is not a noble metal.  I'm going to posit that there are four traditional tests which might make one think an element is a noble metal.  1.  I find it as a pure metal--  W is not found in pure form.  2.  I can purify the metal/reduce its oxide by heating it in air-- cannot be done with W.  3.  I have some aqueous reactivity criteria such as I put it into sulfuric and see if it bubbles, dissolves in nitric, etc.-- W resists being dissolved in acids very well, but can be dissolved in bases.  4.  I can put it into an aqueous solution and reduce it electrochemically to bulk, pure metal-- cannot be done with W.  Ag, on the other hand, definitely passes criteria #1 and #4.  A15730 (talk) 14:04, 5 April 2022 (UTC)
thread_title: The picture becomes clearer

message: , I’ve checked 201 books so far (1925 to 1993) and found 45 hits for noble metals = 22.4%.
Zumdahl, in Chemistry, (1993, 3rd, ed., p. 834) had something interesting to say about the topic:
:"Some metals, such as copper, gold, silver, and platinum, are relatively difficult to oxidise. These are often called noble metals.
:Corrosion can be viewed as the process of returning metals to their natural state—the ores from which they were originally obtained. Corrosion involves oxidation of the metal. Since corroded metal often loses its structural integrity and attractiveness, this spontaneous process has great economic impact. Approximately one-ﬁfth of the iron and steel produced annually is used to replace rusted metal.
:Metals corrode because they oxidize easily. Table 17.1 shows that, with the exception of gold, those metals commonly used for structural and decorative purposes all have standard reduction potentials less positive than that of oxygen gas. When any of these half-reactions is reversed (to show oxidation of the metal) and combined with the reduction half-reaction for oxygen, the result is a positive E° value. Thus the oxidation of most metals by oxygen is spontaneous (although we cannot tell from the potential how fast it will occur).
:In view of the large difference in reduction potentials between oxygen and most metals, it is surprising that the problem of corrosion does not completely prevent the use of metals in air. However, most metals develop a thin oxide coating, which tends to protect their internal atoms against further oxidation. The metal that best demonstrates this phenomenon is aluminium. With a reduction potential of –1.7 V, aluminium should be easily oxidized by O2. According to the apparent thermodynamics of the reaction, an aluminium airplane could dissolve in a rainstorm. The fact that this very active metal can be used as a structural material is due to the formation of a thin, adherent layer of aluminium oxide (Al2O3), more properly represented as Al2(OH)6, which greatly inhibits further corrosion. The potential of the “passive,” oxide-coated aluminium is –0.6 V, a value that causes it to behave much like a noble metal.
:Iron also can form a protective oxide coating. This is not an infallible shield against corrosion, however; when steel is exposed to oxygen in moist air, the oxide that forms tends to scale off and expose new metal surfaces to corrosion.
:The corrosion products of noble metals such as copper and silver are complex and affect the use of these metals as decorative materials. Under normal atmospheric conditions, copper forms an external layer of greenish copper carbonate called patina. Silver tarnish is silver sulfide (Ag2S), which in thin layers gives the silver surface a richer appearance. Gold, with a positive standard reduction potential of 1.50 V, significantly larger than that for oxygen (1.23 V), shows no appreciable corrosion in air."
The following caught my eye:
#Thinking of corrosion as returning the elements involved to their natural state.
#The table of SRP values includes Hg (0.80) but not As, Sb, or Bi.
#Mention of a passivated Al potential, although it's not clear why a potential which is still negative would cause it to behave much like a noble metal.
#Saying noble metals are "relatively" difficult to corrode and OTOH saying the corrosion products of Cu and Ag affect their use as decorative materials.
#The reference to the SRP of oxygen makes Huang (extract #31) clearer for me. Sandbh (talk) 02:02, 12 April 2022 (UTC)
replies: text: Hmmm, Mercury has almost no atmosphere, and a crust largely made up of Mg–Fe–Ca-rich sulfides. If it did have an atmosphere it'd contain practically no oxygen. If there was some H2S in the atmosphere I don't know that this would matter since the reaction of silver with H2S requires the presence of oxygen. I see the average temperature on Mercury is 452 K, and that sulfur melts at 318 K, which may suggest there's no free sulfur on Mercury. I just don't know how e.g. silver would behave in the presence of H2S at 452 K, and in the absence of oxygen. Sandbh (talk) 06:48, 14 April 2022 (UTC)
replies: text:  For base metals that react strongly with O but not (so much) with S, Ac has super-high oxophilicity but below the bottom of the scale thiophilicity, see Kepp, K. P. (2016). A quantitative scale of oxophilicity and thiophilicity. Inorganic Chemistry, 55(18), 9461–9470, table 1, p. 9643,  Sandbh (talk) 04:29, 19 April 2022 (UTC)
thread_title: Zumdahl (1993)

message: , : As a culmination of this thread I've updated the article lede and image, temporarily trimmed the EN v MP image which was taking too much room, and trimmed rhenium from the image gallery. I hope you'll be OK with this. The rest of the article will need more work, including as outlined by A15730, which I expect'll occur slowly. I temporarily trimmed mention of semi-noble metals at least until the basics of the article get better. Sandbh (talk) 07:30, 24 April 2022 (UTC)
thread_title: Updated lede and image

message: I am not sure why, but presently the article uses "sulfur" despite the article being tagged "use British English". Collins and Oxford both cite "sulphur" as correct in BrE and "sulfur" as being US English. 81.96.145.214 (talk) 10:16, 25 August 2023 (UTC)
replies: text: This is the standard international spelling used worldwide in chemistry and in Wikipedia chemistry articles, regardless of the variety of English in the article. See WP:SULF. --Ben (talk) 15:37, 25 August 2023 (UTC)
replies: text: Apologies, I did not know about that policy. 81.96.145.214 (talk) 21:30, 25 August 2023 (UTC)
thread_title:  "Sulfur" used in article instead of "sulphur"