Solubility Of Pt Research Articles

Phase relationship in the Fe-Pt-Pr ternary system at 500°C

The phase relationship in the Fe-Pt-Pr ternary system at 500°C was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) energy dispersion spectroscopy techniques. The 500°C isothermal section consists of 13 single-phase regions, 23 two-phase regions and 11 three-phase regions. At 500°C, the maximum solid solubility of Pt in α-Fe is about 10 at.% and Fe in Pt is about 18 at.%; the maximum solubilities of Pr in α-(Fe,Pt), Fe3Pt, FePt, FePt3 and (Pt,Fe) (the solid solution of Fe in Pt) are about 6 at.% 1.5 at.% 2 at.%, 2.5 at.% and 1.5 at.%, respectively. No Pr3Pt4 binary compound and new ternary compounds were found.

Solid state phase equilibria in the Fe–Pt–Dy ternary system at 900 °C

The solid state phase equilibria in the Fe–Pt–Dy ternary system at 900 °C (Dy ≤ 75 at.%) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques. It is found that the 900 °C isothermal section consists of 18 single-phase regions, 33 two-phase regions and 16 three-phase regions. At 900 °C, it is observed that the maximum solid solubility of Pt in Fe 2Dy and α-(Fe, Dy) is 5 at.% Pt and 3 at.% Pt, respectively. The binary phase Fe 5Dy was not observed. The existence of DyPt 5, DyPt 3, DyPt 2, Dy 3Pt 4, DyPt, Dy 5Pt 4, Dy 5Pt 3, Dy 2Pt and Dy 3Pt compounds is confirmed and the maximum solubility of Fe in them is below 1 at.% Fe; the maximum solubility of Dy in α-(Fe, Pt), γ-(Fe, Pt), FePt, FePt 3 and (Pt, Fe) (the solid solution of Fe in Pt) is 3 at.% Dy, 2 at.% Dy, 2 at.% Dy, 1.5 at.% Dy and 1.5 at.% Dy, respectively.

Solid‐State Phase Equilibria in the Fe—Pt—Pr Ternary System at 1173 K.

Abstract The solid-state phase equilibria in the Fe–Pt–Pr ternary system at 1173 K (Pr ≤ 70%) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques. The 1173 K isothermal section consists of 13 single-phase regions, 22 two-phase regions and 10 three-phase regions. At 1173 K, we have observed that the maximum solid solubility of Pt in α-Fe is below 1.5 at.% and the solid solution region of Pt in γ-Fe is from 2 to 35 at.%; the maximum solid solubility of Fe in Pt is 18 at.%. The maximum solubility of Fe in PrPt 5 , PrPt 3 , PrPt 2 , Pr 3 Pt 4 , PrPt, Pr 3 Pt 2 and Pr 7 Pt 3 is below 1 at.%. The maximum solubility of Pr in α-(Fe, Pt), γ-(Fe, Pt), FePt, FePt 3 and (Pt, Fe) (the solid solution of Fe in Pt) is 6, 2, 4, 4.5 and 1.5 at.%, respectively. In this work, it is found that the phase Pr 3 Pt 4 does not exist in the ternary system. The binary compounds Fe 7 Pr and Fe 2 Pr and any new ternary compounds were not observed.

Solid-state phase equilibria in the Fe–Pt–Pr ternary system at 1173 K

The solid-state phase equilibria in the Fe–Pt–Pr ternary system at 1173 K (Pr ≤ 70%) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques. The 1173 K isothermal section consists of 13 single-phase regions, 22 two-phase regions and 10 three-phase regions. At 1173 K, we have observed that the maximum solid solubility of Pt in α-Fe is below 1.5 at.% and the solid solution region of Pt in γ-Fe is from 2 to 35 at.%; the maximum solid solubility of Fe in Pt is 18 at.%. The maximum solubility of Fe in PrPt 5, PrPt 3, PrPt 2, Pr 3Pt 4, PrPt, Pr 3Pt 2 and Pr 7Pt 3 is below 1 at.%. The maximum solubility of Pr in α-(Fe, Pt), γ-(Fe, Pt), FePt, FePt 3 and (Pt, Fe) (the solid solution of Fe in Pt) is 6, 2, 4, 4.5 and 1.5 at.%, respectively. In this work, it is found that the phase Pr 3Pt 4 does not exist in the ternary system. The binary compounds Fe 7Pr and Fe 2Pr and any new ternary compounds were not observed.

Platinum–silica aerogels via supercritical drying and impregnation

The preparation of a platinum (Pt)–silica aerogel nanocomposite has been investigated using supercritical impregnation of platinum (II) acetylacetonate (Pt(II)(acac)2) into silica substrate. The Pt(II)(acac)2 was dissolved in supercritical carbon dioxide (scCO2) and was impregnated into the silica gel during the supercritical drying stage. Due to low viscosity, high diffusivity, and weak interactions with the dissolved Pt complex, scCO2 can yield a good dispersion of Pt particles at short treatment times. A number of silica aerogels were prepared with differing surface roughness and degree of hydrophobicity in order to investigate how the surface of the substrate would affect the loading of Pt per sample. The use of tetrahydrofuran during the aerogel preparation stage to affect the solubility of Pt(II)(acac)2 was also investigated. Platinum–silica aerogels were successfully prepared with Pt contents up to 1.59wt% and an average particle size <10nm. We discuss the effects of the impregnating conditions and of the modification of the substrates as described above.

The solubility of Pt in liquid Fe-sulfides

The partitioning of Pt between liquid sulfides with approximately stoichiometric FeS and Pt–Fe alloys was experimentally studied. Experiments were conducted in corundum crucibles enclosed in evacuated quartz vials heated to 1100, 1200 and 1300 °C. Starting materials were Pt metal and sulfides of Fe 70S 30 composition. Run products were coarse Pt–Fe alloys and quenched sulfide melts crystallized to FeS and dendrites of Pt–Fe alloys. All phases were analyzed with the electron microprobe (EMP) and the composition of average sulfide melt was calculated. The Pt contents of sulfide melts increase from 0.01 to nearly 10 at.% with S contents increasing from 45 to 50 at.%. Metal/liquid sulfide atom ratios of Pt range from 3 to 1400. After recalculating the data to unit Pt activity in metal, rather constant Pt contents in sulfides were obtained for S-rich systems with f S 2 above 10 −4 atm. On average, 13.1±6 at.% Pt (or 53±22 wt.% Pt) would be dissolved in liquid sulfides in equilibrium with Pt at unit Pt activity. By using the Pt solubilities in sulfides obtained here and those in silicates measured by Borisov and Palme [Geochim. Cosmochim. Acta 61 (1997) 4349] and by assuming the f O 2 to be defined by the QFM buffer, Pt liquid sulfide/silicate partition coefficients, D Pt Sul/Sil, were calculated. These partition coefficients are several orders of magnitude higher than those experimentally determined. The most likely reason is the formation of Pt-rich nuggets in partition experiments.

Temperature dependence of Pt and Rh solubilities in a haplobasaltic melt

The temperature dependence of the solubilities of Pt and Rh in a haplobasaltic (anorthite-diopside 1-bar eutectic composition) melt has been investigated at 1 bar and 1300 to 1550°C using the mechanically assisted equilibration technique (Dingwell et al., 1994). The experiments were performed at almost constant oxygen fugacity (log fO 2 = −2.5 ± 0.3) over the entire temperature range. Major element concentrations in the quenched glass samples were determined using an electron microprobe. Pt and Rh concentrations were obtained by laser ablation inductive coupled plasma mass spectrometry. From our data, we obtain the following expressions for the solubilities of pure Pt and pure Rh in anorthite-diopside eutectic melt at 1 bar and log fO 2 = −2.5: log[Pt](ppm)=− 3320(340) T( K) +2.0(0.2) r 2=0.96, log[Rh](ppm)=− 5440(450) T( K) +3.9(0.3) r 2=0.97. Metal-silicate partition coefficients estimated from these results at likely conditions of core formation (3000 K and an oxygen fugacity 2 orders of magnitude below the iron-wüstite oxygen buffer; e.g., Righter and Drake, 1997) are 6.1 × 10 8 and 3.9 × 10 6 for Pt and Rh, respectively. On the basis of these results, high temperature is not sufficient to explain the abundances of these highly siderophile elements in the Earth’s mantle as a consequence of metal-silicate equilibrium during core formation.

COMPOSITION OF THE PLATINUM-GROUP MINERALS IN THE SALMON RIVER PLACER DEPOSIT, GOODNEWS BAY, ALASKA

The composition and the associations of platinum-group minerals from the Salmon River placers, Goodnews Bay, Alaska, are compared with those of assemblages in other Alaskan-type complexes. Pt–Fe alloy, with an average value 25 at.% Fe, corresponds to isoferroplatinum (Pt 3Fe). Ir concentrations in isoferroplatinum reach 15.4 wt.%, and Rh concentrations, as much as 2.27 wt.%. Tulameenite, which replaces the isoferroplatinum grains, appears as a rim with distinct boundaries. It forms a Pt 2CuFe–PtFe series and also contains up to 2.64 wt.% Ni. Iridium and osmium occur as inclusions in the isoferroplatinum matrix and are depleted in Ru. The Ir content of osmium reaches 26 at.%, and the proportion of Os in iridium varies from 12 to 20 at.%. The concentration of Pt in iridium reaches 21.79 wt.%, which exceeds the limit of solubility of Pt in Ir at the maximum temperature of equilibration (~850°C). The composition of PGE-sulfarsenides corresponds to irarsite, with accessory platarsite and osarsite components: (Ir,Pt,Os)AsS. As predominates over S where substantial Pt exists, and excess S is present in irarsite with a high Os content. Solid solution along the erlichmanite (OsS 2) – sperrylite (PtAs 2) series is restricted. A wide range of compositions and levels of minor elements (Ir, Pd, Rh) in Pt–Fe alloys from different Alaska-type occurrences reflects geochemical features of t he ore-forming system at the source. Fractionations of the PGE in the ore-forming system, from Ir-rich nuggets to Rh-rich and to P drich alloys, reflect the decrease in temperature of the ore-forming system. Compositional variation and temperature range of al loy genesis decrease in the order from Goodnews Bay and Inagli to Galmoenan, Fifield, Nishnii Tagil, then to Tulameen and Yubdo, and down to Pustaya and Itchayvayam occurrences.

Solubility of platinum in aqueous solutions at 25°C and pHs 4 to 10 under oxidizing conditions

Platinum has been found to be mobile under supergene conditions, including placers and weathering profiles. To elucidate the nature of Pt mobility in supergene environments, the dissolution of platinum metal was investigated under physicochemical conditions similar to those found in such environments. The solubility of platinum metal was measured at 25°C in several systems: Pt-K-HC8H4O4-H2O (pH 4.02), Pt-Na-HCO3-Cl-H2O (pH 6.40), Pt-Na-K-H2PO4-HPO4-H2O (pH 6.90), Pt-Na-HCO3-H2O (pH 8.30), Pt-Na-OH-H2O (pH 8.54), and Pt-Na-HCO3-H2O (pH 9.91). The redox conditions of these experiments were relatively oxidizing, with measured Eh values ranging from +280 to +590 mV. The ionic strength of the aqueous solutions did not exceed 0.30 (molal scale).The interpretation of the solubility results, in terms of the following reaction and its equilibrium constant, Pt(s)+nH2O ⇔ Pt(OH)2−nn+nH++2e− served to identify the importance of the hydroxylated complex PtOH+ in the pH range (4 to 10) and to determine its stability constant at 25°C. Linear regression of the solubility data using the function log [Pt]−2pe=npH+logKn yielded a value of 1.01 ± 0.07 for n, the average ligand number, and −31.76 ± 0.55 for the thermodynamic equilibrium constant of reaction. The resulting stability constant (β1) of PtOH+ (Pt2+ + OH− ⇔ PtOH+) is 24.91 ± 0.50, assuming the same value of the free energy of formation of Pt2+, ΔGfo(Pt2+) as that given by Glushko et al. (Thermodynamic Constants of Matter, Academy of Science, USSR, 1972). The range of values of ΔGfo(Pt2+) proposed to date is +185.63 to +258.74 kJ/mol. The value of Glushko et al. (1972) (+244.11 kJ/mol) appears to fit better with our measurements at pH 4 to 10 and with those of Wood (Wood S. A., “Experimental determination of the hydrolysis constants of Pt2+ and Pd2+ at 25°C from the solubility of Pt and Pd in aqueous hydroxide solutions,” Geochim. Cosmochim. Acta55, 1759–1767, 1991) at pH 9 to 15.5. Finally, according to these new measurements of the solubility of platinum, the recommended values for Gibbs free energy (ΔGfo, in kiloJoule per mole) of the different aqueous species of Pt are +244.11 (Pt2+), −55.96 (PtOH+), and −234.48 (Pt(OH)2(aq)).The integration of data from the literature for chloride and sulfate complexes was used to calculate the speciation of platinum in seawater and in solutions with variable chlorinity (0.1, 1, and 3 mol/L NaCl) at 25°C. These calculations showed that in the absence of strong ligands (i.e., S2O32−, CN−), the transport of platinum in supergene environments primarily occurs in the form of PtOH+ (90%) and Pt(OH)2(aq) (9.7%). Chloride complexes (PtCl42− and PtCl3−) account for less than 1% of the dissolved platinum.This study clearly shows that the hydroxylated complexes (PtOH+ and Pt(OH)2(aq)) can play a very important role in controlling platinum transfer mechanisms in surface fluids (soils, placers, weathering profiles, etc.). Because the charged species PtOH+ is largely predominant, the mobility and transfer of platinum can also be affected by adsorption-desorption mechanisms onto oxides and hydroxides.

Solubilities of noble metals in Fe-containing silicate melts as derived from experiments in Fe-free systems

The solubilities of noble metals (NM: Ir, Pd, Au, Pt, and Ru) in FeO-free silicate melts are known from recent experimental work. In this paper, calculations are presented that relate solubilities of NM in FeO-free melts to those in FeO-containing melts. The main difference between these two systems is the formation of Fe-NM alloys in FeO-containing melts. At fO₂ conditions of the QFM buffer and 1200 °C, binary alloys of Fe with Au, Ru, Ir, Pd, or Pt containing 1, 2, 10, 17, and 25 at% Fe, respectively, are in thermodynamic equilibrium with a silicate melt with 10 mol% FeO. Thus alloy formation leads to a significant reduction in the solubility of Pt and to a lesser reduction in the solubility of Pd. The effects for Ir and Ru are small, and for Au almost negligible. The reduction in solubilities depends on temperature, oxygen fugacity, and FeO content of the silicate melt. Formation of FePt-alloys would lead to a preferred depletion of Pt in partial melts from the Earth’s mantle, which, however, is not observed. One explanation is that mantle melting occurs under very oxidizing conditions (QFM+2). The calculations presented here should be considered a first step toward gaining a better understanding of the behavior of NM during igneous processes

Platinum group element behaviour and thermochemical constraints in the ultrabasic–basic complex of the Vizcaino Peninsula, Baja California Sur, Mexico

The Vizcaino complex is characterized by tectonised harzburgites overlain by serpentinized dunite bodies and wherlites layers containing several cumulate chromitite deposits and by fractional crystallization sequences: cyclic repetititons of dunites, pyroxenites with pegmatoidal textures, and gabbros overlain by a succession of gabbronorites to diorites in which olivine is followed by pyroxene and by plagioclase. Enriched sulfide horizons generally associated with pegmatoidal rocks are lacking. The fractionation of platinum-group element (PGE) displays evidence of erratic dispersion throughout the intrusion. PGE patterns of chromitites show high Ir and Ru contents, relatively high Rh content and an unusual Pt–Pd depletion lower than in ophiolitic chromitites. These chromitites do not contain laurite-type sulfides, which are commonly observed in association with Os–Ir–Ru alloys. Dunites show relatively flat patterns and gabbros present typical trends with depletions in the Iridium-group (IPGE) relative to PPGE. Ru, Rh, and especially Ir are very low in these silicate rocks, indicating the precipitation of these elements during the earliest stages of fractional crystallization and their concentration in chromitites. Spilitized dolerite dykes from the metamorphic basement show significant PGE enrichments similar to those of gabbros. Based on our previous thermochemical data, an extremely low f S 2 (<10 −2 atm) was in equilibrium with the silicate melt as indicated by the absence of laurite included in chromitites. In the absence of complexation of Pt (+Pd) by sulfur in the early stage of magmatic differentiation, a complexation of these elements by Cl as chloride complexes in a Cl-rich fluid phase may be expected. This hypothesis is reinforced by the notable absence of any sulfide horizon throughout the intrusion. Part of precious metals (Pt+Pd+Au) may have been remobilized at low temperature and transported as chloride or hydroxide complexes by fluid phases introduced during low-grade metamorphic processes or later during deformation events. In this interpretation, the erratic behaviour of PGE at Vizcaino was partially controlled by solubilities of Pt and Pd as chloride complexes at magmatic temperatures as early as chromite crystallization and the control of local saturation of the magma by variation of f S 2 was overshadowed.

Structural investigation of platinum solubility in silicate glasses

The coordination environment of 20-200 ppm Pt in yellowish glasses from the CaO-Al 2O3-SiO2 (CAS) ternary was studied using X-ray absorption fine structure spectroscopy at the Pt-L III edge. Analysis of the Pt-L III edge region suggests that Pt in these glasses is mainly tetravalent and sixfoldcoordinated by O (with a mean Pt-O distance of 2.08 ± 0.02 A). No evidence for Pt 2+ or Pt 6+ was found in any of the glasses studied, suggesting that one can not derive valence information easily from solubility data. No second-neighbor contribution was observed around Pt 4+ O6 polyhedra. However, bond-valence modeling suggest that these polyhedra are likely to bond mostly to [VI] Ca 2+ , which should promote high positional disorder of second -neighbor cations around Pt. This particular bonding arrangement may explain the relatively high solubility of Pt in these relatively depolymerized melts, as CaPtO3-type units.

Solubilities of Pt and Rh in a haplobasaltic silicate melt at 1300°C

The solubilities of Platinum (Pt) and Rhodium (Rh) in a haplobasaltic melt (anorthite-diopside eutectic composition) have been determined experimentally by using the mechanically assisted equilibration technique at 1300°C, as a function of oxygen fugacity (10 −12 < fO 2 ≤ 1 bar), imposed by CO-CO 2, N 2-H 2-H 2O, Ar-O 2, and air gas mixtures. Samples were analyzed by sample nebulization (SN) inductively coupled plasma-mass spectrometry and, using some of these samples as standards, also by laser ablation (LA) inductively coupled plasma-mass spectrometry. The latter is a true microanalytical technique that allows small-scale sample heterogeneity to be detected. At each oxygen fugacity step, a time-series of samples was taken, to demonstrate that the solubilities converge on a constant value. In addition, solubilities were measured after both increasing and decreasing the imposed fO 2. The results fall into three groups, according to oxygen fugacity. At high fO 2s, (fO 2 ≥ 10 −2 bars), samples are homogenous at all sampling scales. Both Pt and Rh predominantly dissolve in the silicate melt as 2+ species, with some evidence for Pt 4+ and Rh 3+ at the highest fO 2s studied (air and pure O 2). From these data, we obtained the following expressions for the solubilities of Pt and Rh: Pt/ppb = 2100(fO 2) + 10980(fO 2) 1/2 Rh/ppb = 68630(fO 2) 3/4 + 31460(fO 2) 1/2 At fO 2 < 10 −5 bars, the true solubilities of Pt and Rh appear to be obscured by Pt-Rh micronuggets, which remain suspended in the melt despite stirring on time scales of 10 3 h, resulting in samples that are heterogenous on the laser sampling scale. Samples at intermediate fO 2 (10 −2 to 10 −5 bars) are affected by the micronugget problem on the sampling scale of the conventional SN-inductively coupled plasma mass spectrometry, but these can be filtered out by analyzing on the laser sampling scale.

Experimentally Determined Solubilities of Pt in a Haplobasaltic Melt Under Well Constraint Conditions of fo2 P, T, and H2O Activity

Experimentally Determined Solubilities of Pt in a Haplobasaltic Melt Under Well Constraint Conditions of fo2 P, T, and H2O Activity

Experimental determination of the solubility of platinum in silicate melts

We report here new experimental results on the solubility of Pt in silicate melts at one atmosphere total pressure. Experiments were made with two different melt compositions: an anorthitediopside eutectic and a CaO-free system (3SiO2·MgO·Al2O3, by weight). Oxygen fugacities ranged from pure oxygen to 10−8 atm and temperatures from 1300 to 1560°C. Glass samples were analysed after the experiments by instrumental neutron activation analysis (INAA). In experiments with An-Di melts, isotopically enriched 40Ca (and correspondingly depleted 46Ca) was used to reduce the background from 47Sc produced by neutron activation of 46Ca. In this way, the interference was reduced by a factor of 500, allowing the determination of Pt in glass samples by INAA down to a concentration of about 50 ppb.Over five orders of magnitude in oxygen fugacity, from pure oxygen to an fO2 of 10−5 atm, a systematic decrease of the Pt solubility with decreasing fO2 was observed. The slope of the log (Pt solubility) vs. log fO2 correlation suggests a formal valence of +2 for Pt in the silicate melt, independent of temperature. The solubility in the CaO-free melt is about one-fourth of an order of magnitude lower than for the An-Di melt. At oxygen fugacities below 10−5 atm, results show a large scatter. We suspect that formation of Pt nuggests leads to very inhomogeneous distribution of Pt in the melt. A literature survey shows large variations in Pt solubilities determined by various authors. These discrepancies primarily result from the formation of Pt nuggets. A new mechanism of PGE nugget formation is suggested.The formation of PtFe alloys predicted to occur in terrestrial magmatic melts may be important for fractionating Pt from other noble metals.

Experimental investigations of the hydrothermal geochemistry of platinum and palladium: V. Equilibria between platinum metal, Pt(II), and Pt(IV) chloride complexes at 25 to 300°C

The solubility of metallic Pt in HCl solutions was determined at 200 to 300°C at oxidation states buffered near the aqueous Pt(II)/Pt(IV) boundary. Equilibrium constants were obtained for the following disproportionation reactions: log K, 200° 250° 300°C 2PtCl 4 2− = PtCl 4 2− + Pt(s) + 2Cl − 1.47 1.70 1.54 (a) 2PtCl 3 − = PtCl 5 − + Pt(s) + Cl − 1.77 1.74 1.37 (b) with experimental uncertainties of approximately ±0.20 log units. These results are found to be in good agreement with previously published estimates for reaction at 60 to 152.5°C. The data indicate that the relative stability of the Pt(II) and Pt(IV) chloride complexes does not change appreciably with temperature. This is in contrast to previous work in the Au(0)/Au(I)/Au(III) system which demonstrates that the Au(I) chloride complexes are unstable with respect to Au (III) at low temperature, but become the dominant aqueous species at 300°C. Pt(IV) chloride complexes are unlikely to be important in high temperature hydrothermal fluids, as unrealistically high aqueous platinum concentrations are required to stabilize these species relative to Pt(II). In contrast, thermodynamic calculations suggest that Pt(IV) chloride or hydroxychloride complexes may be the dominant form of dissolved platinum in low temperature brines that are strongly oxidized (e.g., seawater). In oxygenated, Cl-rich solutions, the solubility of Pt is extremely high at pH < 6, such that the mobility of this metal will most likely be limited by surface adsorption reactions and/ or its abundance and rate of dissolution in the enclosing rock or soil. At neutral to alkaline pH, calculated solubilities are much lower, and saturation with Pt oxide phases may occur, as has recently been described in nature.

Solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions

An experimental study of the solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions was conducted at temperatures from 200° to 350 °C and at saturated vapor pressure. A 500-mL Bridgemantype pressure vessel constructed of titanium, and equipped with a motor-driven magnetic stirrer was employed. The pH and the oxidation state were buffered by the coexistence of H2S/HS−/SO inf4 sup2− . The pH at temperature was calculated to be in the range 5.91–9.43, and ∑S was 0.3–2.2 m. Under the experimental conditions, the measured solubility of gold is about two to three orders of magnitude greater than that of either platinum and palladium, and the measured solubility of platinum is, in general, approximately equal to that of palladium, in molal units. The solubilities are found to be in the range: platinum 4–800 ppb, palladium 1–400 ppb, and gold 2–300 ppm. The solubility data can be modeled adequately using the following reactions: Au+H2S+HH−=Au(HS) 2 − +1/2H2 (K14); PtS+HS−+H+=Pt (HS) 2 0 (K15); PdS+HS−+H+=Pd (HS) 2 0 (K16); PtS2+H2=Pt (HS) 2 0 (K21).

The solubility of Pt and Pd sulfides and Au in bisulfide solutions

Two sets of measurements of the solubility of Pt and Pd in bisulfide solutions have been carried out at low temperatures. The first involved the equilibration of Pt metal with bisulfide solutions at 25 °C and pH = 6–12 for periods up to five years. These experiments yielded Pt concentrations on the order of tens of μg/L at sulfide concentrations as low as 0.001 molal under conditions too reducing to permit significant contributions from hydroxide complexes. The second set of experiments consisted of reacting PtS, PdS and Au with H2S-saturated solutions having pH values of 3–4 at 25°, 50° and 90 °C. These experiments showed that the solubilities of all three metals increased with temperature. The observed order of solubility was Au > Pt ≈ Pd. Solubilities ranged from 10 to 75 μg/L Au, 4 to 20 μg/L Pt and 0.5 to 10 μg/L Pd. The data do not permit definitive identification of the Pt and Pd species present in either set of experiments, but do strongly suggest that the species present under acidic and basic conditions are different. The measured solubility of gold at 25° and 50 °C is consistent with that measured in previous studies.

Experimental investigation of the hydrothermal geochemistry of platinum and palladium: I. Solubility of platinum and palladium sulfide minerals in NaCl/H 2SO 4 solutions at 300°C

Approximately 150 silica tube experiments were used to determine the solubility of Pt and Pd sulfides in NaCl/H 2SO 4 solutions at 300°C. Oxidation state was controlled by equilibrium between aqueous sulfate and coexisting solids. Buffering assemblages included Pd + Pd 4 S, Pd 4 S + Pd 16 S 7, Pd 16 S 7 + PdS, Pt + PtS, and PtS + PtS 2. Solution pH was controlled by the dissociation of H 2SO 4 (0.1 or 1.0 m), adjusted for ion pairing. Salinities ranged from 0.01 to 3.0 m NaCl. Under these conditions, measured Pt and Pd solubilities varied over a wide range (<0.1 ppb to >1000 ppm), indicating dissolution as chloride complexes of high ligand number. Detailed interpretation of the results gives equilibrium constants for the following reactions: Pd(s) + 2H + + 4Cl − + 1 2O 2(g) = PdCl 4 2− + H 2O (a1) Pt(s) + 2H + + 3Cl − + 1 2O 2(g) = PtCl 3 − + H 2O , (a2) log K(300° C, P sat ) = +12.65 ± 0.5 (reaction al) and +14.84 ± 0.3 (reaction a2). Our experimentally derived constants are in fair (Pt) to excellent (Pd) agreement with previously published estimates based on extrapolation of low-temperature data. The temperature dependence of reaction (a1) is described by the following polynomial (valid between 25 and 300°C): log K al = 22.53 − 7.398 e − 02 T + 1.433 e-04 T 2 − 1.034 e − 08 T 3 ( T = ° C). Our study indicates that chloride complexes may contribute significant solubilities (> 1 ppb) of Pt and Pd at low to moderate temperature, but only under conditions which are highly oxidized (hematite stable), highly saline (>3 m NaCl eq), and/or unusually acidic ( pH < 4). In the presence of aqueous sulfide, Pt-group element (PGE) solubilities as chloride complexes are extremely low (<1 ppt) and are probably less than the total contribution from other aqueous species (e.g., bisulfide, hydroxy, or ammonia complexes). However, the very strong temperature dependence of reaction (a1) suggests that much higher PGE mobility may be expected at supercritical temperatures. PGEs dissolved as chloride complexes may be deposited in response to reduction, increase in pH, dilution, and/or temperature decrease. Of these, reduction is the most effective mechanism and explains the common association of anomalous PGEs with strongly reduced horizons within sedimentary basins.

Experimental determination of the hydrolysis constants of Pt 2+ and Pd 2+ at 25°C from the solubility of Pt and Pd in aqueous hydroxide solutions

The solubilities of Pt and Pd metal were measured at 25°C in 10 −4 to 10.0 molal NaOH solutions under a reduced oxygen atmosphere in order to determine the stoichiometry and stability constants for Pt and Pd hydroxide complexes. Equilibration times of over one year were employed. The Pd data are consistent with the existence of Pd(OH) 2 0(aq) from pH 9 to 12 and Pd(OH) 3 − from pH 12 to 15.5. No conclusive evidence for a Pd(OH) 4 2− complex was obtained, but the data do not preclude its existence at high pH. For Pt, the data are consistent with a single complex for pH = 9 to 15.5, I.E., Pt(OH) 2 0(aq). A graphical treatment of the data yields the following cumulative stability constants: log β 2 = 18.9 ± 1.0 and log β 3 = 20.9 ± 1.0 for Pd and log β 2 = 29.9 ± 1.0 for Pt. The stepwise stability constant for Pd(OH) 3 − log K 3 = 2.0 is in relatively good agreement with that derived from data in the literature ( log K 3 = 1.8 ). However, the cumulative stability constants for Pd measured in this work are considerably smaller than those reported in the literature. The log β 2 = 29.9 ± 1.0 value measured for Pt compares relatively well with a theoretically estimated value of 28.3. The data suggest that the predominant inorganic form of Pt and Pd in freshwaters may be the neutral hydroxide species. In seawater, the hydroxide complex of Pt is also predicted to predominate over the chloride complex, but, in the case of Pd, the hydroxide complex appears to be less stable and it is presently not clear whether the chloride or the hydroxide complex will predominate. In fluids responsible for serpentinization, Pt and Pd may also be mobilized as hydroxide complexes.