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Samples Containing Platinum

General Information

Occurrence – Platinum comes from the Spanish word platina (little silver).  Platinum (Pt) has an abundance of 0.003 ppm in the lithosphere and is the most abundant Pt group metal being only slightly more abundant than Ru.  It is found with the other platinum group metals in alluvial deposits or in nickel, copper or iron sulfide ores. 

Uses - Pt is a gray, malleable metal.  It has a variety of uses and consequently a variety of sample types for analysis, namely:

  1. Geological samples such as sulfide ores of Ni, Cu, Fe, rocks, minerals and soils.
  2. Catalysts (auto exhaust, nitric acid production, petroleum refining)
  3. Electronic components (wires, alloys, electrodes)
  4. Jewelry
  5. Chemotherapy ( biological materials)
  6. Pharmaceutical preparations

Chemical Properties – Pt can have oxidation states of 0, +2, + 4, +5 and +6.  Pt does not form the +3 oxidation state.  The +4 and +2 states are common with the +4 being the most common for Pt.  The most important compounds for Pt are the chlorides (Pt(Cl)42- , Pt(Cl)62- ).  Inorganic Venture’s produces single element CRMs of Pt in the +4 oxidation state primarily as Pt(Cl)62-  in dilute HNO3 and/or HCl that can be mixed with most other mineral acids.   Avoid basic media due to precipitation of black/brown Pt(OH)4 from Pt(Cl)62-

Pt is reduced to the metal by formic acid, stannous chloride, and metallic Zn, Mg, Fe, Al and Cu. Treatment of Pt(Cl)62- solutions with H2S will form PtS2 which is soluble in Aqua Regia. Pt is stable with most metals and inorganic anions in acidic media.   Solutions of   Pt(Cl)62- can be diluted in HNO3  where 2 - 10 ppb Pt  is stable for 2 months (1% HNO3 / LDPE container) , 100 ppb is stable for 5 months  in 1% HNO3 / LDPE container. 1-10,000 ppm solutions are chemically stable for years in 10 % HCl / LDPE container.

Fusion of H2PtCl6 with NaNO3 at 500 ºC yields Pt(OH)4  i.e. oxidation to a higher oxidation state does not occur.

At Inorganic Ventures - Inorganic Ventures uses PtCl4 as the starting material with a documented purity that is ~ 4-9’s +.  The pure starting material is dissolved in electronic grade HNO3 for nitric acid matrices or HCl for HCl matrices.  Impurities for each lot are measured using ICP-MS and ICP-OES and are reported on the Certificate of Analysis.   Following is some information about the handling and use of IV’s Pt CRMs:

Location:  Group 10, Period 6

Atomic Weight:  195.08

Coordination Number:  6

Chemical Form in Solution:  Pt(Cl)62-

Storage & Handling:  Keep tightly sealed when not in use. Store and use at 20 ± 4°C. Do not pipet from container. Do not return portions removed for pipetting to container.

Chemical Compatibility:  Stable in HCl and HNO3, as the chloride complex. Avoid basic media. Stable with most metals and inorganic anions in acidic media.

Stability of ICP-MS/OES calibration mixtures (following stability is for 70 element mixtures made using IV CRMs where the other elements are at equivalent concentrations.  Trace amounts of fluoride and chloride are present):  2-10 ppb Pt is stable for 2 months in 1% HNO3 / LDPE container. 100 ppb is stable for 5 months in 1% HNO3 / LDPE container. 1-10,000 ppm solutions chemically stable for years in 10 % HCl / LDPE container.

Pt Containing Samples (Preparation & Solution):  Metal (aqua regia); Oxides (soluble in HCl); Ores (dissolve in HCl / HNO3).

Atomic Spectroscopic Information:  (red text indicates severe at ~ concs.)

Technique / Line

Estimated D.L.*

Order

Type

Interferences

ICP-OES 214.423 nm

0.03/.003 µg/mL

1

ion

W, As, Ir, Cd

ICP-OES 203.646 nm

0.06/.006 µg/mL

1

ion

Co, Hf

ICP-MS 195 amu

5 ppt

n/a

M+

179Hf16O

*ICP-OES D.L.'s are given as radial / axial view

Sampling and Handling:   Pt contamination risk is low.  There are no special precautions above to be considered unless your laboratory handles a wide variety of sample types.  In developing a sampling approach it is helpful to have a ballpark idea of the concentration of Pt in the sample.  Naturally occurring Pt requires relatively large samples collected using detailed sampling schemes with sample reduction according to statistically and experimentally methodology.  The many and diverse uses of Pt result in a wide variety of sample types and concentrations ranging from metallic alloys to organic carbon bonded (σ and π C to Pt bonds) Pt compounds.  Pt like the other Pt Group metals is rare and therefore environmental contamination is not of particular concern but great care should be exercised in keeping sample handling areas for sample types low in Pt free of sample handling/preparation activities for samples containing high Pt levels.  Catalysts, ores, geological materials, alloys and jeweller’s sweeps are examples of samples high in Pt and require sophisticated sampling methodology where large samples are collected and reduced to a laboratory sized sample use specialized grinding, and crushing equipment.  The aforementioned should not in any way occupy laboratory space or use the same glassware as used for biological samples where sub ng quantities of Pt are the rule.

For more on sample contamination risks see chapters 8, 9 and 10 of the Inorganic Ventures ‘Trace analysis Guide’:

https://www.inorganicventures.com/trace-analysis-guide/environmental-contamination

For general information on sampling and sub-sampling see:

https://www.inorganicventures.com/trace-analysis-guide/sampling-and-subsampling

Sample Preparations of the Metal and Alloys

Metal – Platinum metal is not affected by air or H2O at any temperature.  At red heat Pt forms PtCl4 with Cl2 which is widely used with the Pt group metals for synthesis of the chloride salts.  Pt metal is not attacked by HNO3 or dilute H2SO4 but hot and concentrated H2SO4 very slowly forms Pt(SO4)2.  Aqua Regia, HCl plus HClO3, and HCl + H2O2 all dissolve Pt metal forming H2PtCl6.   Pt is not as reactive as Pd but it is the second easiest/ most readily dissolved Pt Group metal. 

Pt can be fused with alkali nitrates, hydroxides and peroxides forming Pt(OH)4 which is soluble in dilute HCl.  In an attempt to remove contamination of Ni and or Fe and or Ag as contaminants during the preparation, Inorganic Ventures is currently working on a NaOH/NaNO3 fusion method using a glassy carbon crucible.  The method will be published on our web-site when finished (probably during the year 2019

Alloys- Platinum is often found with the other Pt Group metals in addition to a variety of other metals.  Fusion of the sample with Na2O2 followed by dissolution of the melt with dilute HCl is often used or by performing a fire assay with nickel sulfide collection followed by dissolution of the bead in Aqua Regia. Pt/Ag, Cu, Pb alloys can be dissolved in HNO3 where the Ag, Cu, Pb and other impurities are dissolved along with much of the Pt.  The remaining residue, after washing, is Pt and Au which are dissolved in Aqua Regia.

Oxides, Minerals and Ores

Oxides –The oxides and hydroxides are soluble in dilute HCl.

Ores and Minerals  The are many techniques that have been reported.  The following link to “Sample Preparation Methods for the Platinum Group Metals and Gold Compatible with AAS, ICP-OES and ICP-MS Measurement” is recommendedThis is an excellent review paper citing 153 references discussing the platinum group metals (PGMs) + gold (Au) sample preparation methods and one that is easily obtained using this  link: 

(https://www.researchgate.net/publication/11245280_Sample_Digestion_Methods_for_the_Determination_of_Traces_of_Precious_Metals_by_Spectrometric_Techniques) is:

Balcerzak, M. , ANALYTICAL SCIENCES, Vol. 18, July 2002, pp 737 -750

This review compares the following sample preparation methods that are commonly used for the PGMs +Au:

  1. Fire Assay
  2. Wet Chemical Acid Digestion
  3. Oxidizing Fusion
  4. Chlorination

The reader is encouraged to consult this paper for detailed information on these sample preparation methods.

Organic Matrices: Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step can be expedient for samples containing Pt.  If ashing is used it is suggested to keep the temperature low (400 to 450 deg C max) and to use an ashing aid such as high purity sodium carbonate.  Acid digestions using nitric, perchloric and sulfuric acids are also suggested.  Wet Ashing is suggested for oil and petroleum products using sulfuric acid in combination with magnesium nitrate as an ashing aid.  Do not use ashing temperatures above 450 deg C.  If sulfuric acid is added to the petroleum sample than heat on a hot plate slowly until foaming stops and a char is produced i.e. wet ashing is very time consuming but it is a common practice in the petroleum industry.  For more on ashing please see the following paper: https://www.inorganicventures.com/trace-analysis-guide/ashing-procedures

Although Pt is not listed in the scope for EPA Methods 3050A and 3050B (Open Vessel Acid Digestion..) and 3051 and 3052 (Microwave Assisted Acid Digestion) it is suggested that these methods should be explored for environmental samples (sediments, sludges, soils and oils). 

Samples containing mid to low ppm levels of Pt can be digested with nitric/perchloric. For more detailed information about acid digestions of organics please see the following article:  https://www.inorganicventures.com/trace-analysis-guide/acid-digestions-of-organic-samples