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Encyclopedia of Physical Science and Technology EN010b-481 July 14, 2001 18:45

478 Noble Metals (Chemistry)

1. Gold 3. Platinum
In most cases, the gold sample involves gold metal or High concentrations of platinum can be determined
salts that can be converted easily to the chloride complex with X-ray emission spectroscopy (e.g., as platinum
for further analysis. This method of putting the gold into metal or in alloys). Low to trace levels of metal can
solution can be used for samples that are predominantly be analyzed with optical emission spectroscopy or by
gold (e.g., ingot) or predominantly extraneous material spectrophotometric analysis using stannous chloride and
(e.g., alloy or ore). Gold is usually converted to [AuCl 4 ] − p-nitrosodimethylaniline. Platinum analysis can be done
by gravimetric analysis using thermal decomposition of a
by dissolving it in aqua regia, purifying to remove NO x
and then concentrating the sample by solvent-extraction compound, but since no specific reagent is known, the
if necessary. The most common analytical methods are metal must be isolated from the interfering elements.
gravimetric, titration, and spectrometric. Fire assaying in- The platinum must be in the (IV) state and free of nitrate.
volves fusing the sample with fluxes, litharge, and a reduc- The platinum is isolated by treatment with NH 4 Cl, fol-
ing agent to obtain a lead button containing the precious lowed by reduction with an agent such as formate. Atomic
metals. The base metals and lead can be removed to yield absorption is an excellent method as long as the sample
the gold, silver, and PGMs, which can then be analyzed can be put into solution. Methods for polarographic and
−
further. The precipitation of gold from a [AuCl 4 ] solu- X-ray fluorescence techniques are also known. Volumet-
2−
tion can be done by using C 2 O ,SO 2 , or hydroquinone. ric methods, in general, are not satisfactory for platinum
4
Hydroquinone is best because it does not bring down the or the other PGMs.
PGMs. A third gravimetric method involves the precipi-
tation of a gold complex. The common chelating agents
4. Palladium
used in this method are dimethylglyoxime, tetraamisole,
thiophenol,and8-quinolinol.Determinationofgoldbythe For the other PGMs, aside from atomic absorption, the
titration of gold(III) to gold(0) can be followed potentio- most common analytical methods are gravimetric and
metrically or coulometrically using hydroquinone as the spectrophotometric. For the palladium gravimetric pro-
reducing agent. Interference by other metals can be de- cedure, there are over 50 reagents; the most common are
creased by extraction of the gold complex into an organic the oximes. Gold is the only interfering metal, and it can
solvent (e.g., diethyl ether, ethyl acetate, methylisobutyl be removed by reduction with oxalate. Since PdO will
ketone, or dichloromethane–tetrahydrofuran). Very small form on burning, the combustion product must be treated
quantities of gold in other metals or trace impurities in with methanol vapor to reduce the surface oxide to the
gold can be determined by emission spectroscopy, atomic metal. The common colorimetric reagents are thiocyanate,
absorption spectroscopy, and neutron activation. p-nitroaniline derivatives, and 8-mercaptoquinoline. The
colorimetric method does not have a significant advantage
over the gravimetric method.
2. Silver
The methods used for silver analysis are similar to those 5. Iridium
used for gold. Spectrophotometrically, silver can be de- Iridium is best determined gravimetrically as IrO 2 , which
termined to trace levels by a rhodanine derivative, with can be reduced to the metal. There are no specific gravi-
mercury and iron interfering, or a pyrogallol red complex, metric reagents for iridium. Spectrophotometrically, the
where common anions can interfere. Cation interference [IrCl 6 ] 2− complex can be used to determine iridium, or
can be minimized by using EDTA as a masking agent. the reagent SnBr 2 can be used if rhodium, palladium, and
Silver can be determined volumetrically, either indirectly, platinum are absent.
by reacting copper metal with silver(I) and then titrat-
ing the resulting copper(II) with EDTA, or directly, by
6. Rhodium
titrating silver (I) with a halide using dithizone in car-
bon tetrachloride as an indicator. Gravimetric determina-
Rhodium is quantified spectrophotometrically with SnBr 2
tions can be made using 1,2,3-benzotriazole and its deriva- or gravimetrically by precipitating the rhodium with H 2 S,
tives in acid, 1-amidino-2-thiourea in base, or a halide. burning, and then reducing with hydrogen.
Silver can be determined electrochemically by ampero-
metric titration using sodium diethyldithiocarbamate or
7. Osmium
by polarography. As with gold, silver can be determined
by atomic absorption, emission spectroscopy, and neutron Gravimetric analysis of osmium is accomplished by the re-
activation. action of osmium(VIII), as OsO 4 with 1,2,3-benzotriazol