theobromacacao - 12-6-2008 at 07:57
I have recently acquired some maple-leaf shaped thin metal "stampings", which according to the seller, are composed of solid rhodium! There is approx.
3.5g in weight and I am very sceptical about this for obvious reasons! Does anyone know of a way that I could test for the Rh content with reagents
available outside of well-equipped labs, or have the set up themselves, or access, to be able to confirm this for me? I'd be happy to offer some fair
remuneration for this of course 
My aqua regia had no success, although some
sources state it does not dissolve in it, whilst others do. Any information/help would be greatly appreciated
12AX7 - 12-6-2008 at 09:03
Density is definitely a good start. 3.5g should take up near zero volume...
Probably NaOH + KNO3 fusion would be the next step if regia isn't doing anything. Should be able to prepare some pink compound, as the name suggests.
Tim
panziandi - 12-6-2008 at 09:38
Rh doesn't dissolve in aqua regia. Do you have a micrometer that can measure the thickness? If so perhaps cut a square out, work out the volume and
weigh it and calculate density & compare to text book data of the Pt group elements, Even a rough idea should give you a clue? Another idea is to
heat a small piece in a test tube and pass dry chlorine over it, RhCl3 (red).
Fleaker - 12-6-2008 at 11:12
Bulk Rh doesn't dissolve in aqua regia, even boiling. I am almost positive that rhodium black will dissolve (any one else here experience that?).
Your best bet is to try a fusion with sodium bisulfate. It is cheap and very easy to find. Do this in a porcelain or nickel crucible. It will dissolve
most all of the rhodium and give you a brown solution of rhodium (III) sulfate.
Another option is simple heating. Rhodium will stay shiny until red heat, then the piece will gray over until yellow heat when it will become shiny
again from the oxide, Rh2O3, decomposing. EDIT: palladium does this too and its density is close to Rh, but you said your aqua regia had no success so
that rules Pd out. Pd dissolves readily in aqua regia at room temperature. Pt is much harder to dissolve!
Hope this helps you out.
[Edited on 12-6-2008 by Fleaker]
not_important - 12-6-2008 at 14:30
The density measurement is a good start, eliminated many things quickly.
Download Standard Methods of Chemical Analysis, by Wilfred Welday Scott
http://www.archive.org/details/standardmethodso00scotrich
(google books also has it, sometimes you want both version because of fouled up scanning)
Rhodium will dissolve in aqua regia, but it is slow; heat on a waterbath and add fresh A.R. to keep the volume up.
Fusion with KHSO4 is done at low red heat, for some time. Cooling and then extracted with hot dilute HCl (1 N); the solution will be rose coloured.
Of interest is that rhodium is stated to dissolve in a solution of NaCl through which chlorine is bubbled, forming Na3RhCl6
Quick tests given include boiling the solution to eliminate free Cl, even evaporate it a bit; if made with A.R. then add NaOH solution to bring the pH
around 2-4 (DON'T overshoot to the basic side)
Take a drop, add it to a couple of drops of distilled water in a small test tube, add a solution of NaOH or KOH to excess. Set aside, a yellowish ppt
will form; adding alcohol turns it black. The yellow ppt is slow forming from the NaRhCl6 complex.
Take another drop or two, add to equal volume distilled water, heat in a waterbath. Add hot KNO2 (nitrite) solution, yellow ppt of the double nitrite
(similar to cobalt).
A bit added to some aqueous NH3 gives a yellow ppt that does not dissolve in 5 N HCl.
Quantitative analysis is more complex, check the book I listed.
A reference
Fleaker - 12-6-2008 at 17:51
This may also help you out.
The sodium bisulfate is probably the easiest way to do it in my opinion. If it dissolves in bisulfate and gives a brown coloured solution, it is proof
positive that it is rhodium.
Attachment: Reactions involving colored complexes of PGMs.pdf (751kB)
This file has been downloaded 940 times
497 - 12-6-2008 at 21:24
Well the simplest way to tell would be price. Unless they were totally ignorant, they should have charged at least $1000 for that much rhodium. If it
was much less then chances are you got ripped off. Also, why would someone make a maple leaf out of rhodium sheet? Chemical tests will have the final
say though.