Sample Preparation Guides

General Information

Ruthenium (Ru) and Osmium (Os) are the first of the platinum group metals (PGMs –Ru, Os, Rh, Ir, Pd and Pt) to be addressed in this series. Both elements are very rare and are typically found in their elemental states or as sulfides. They are found in ores containing one or more of the other PGMs but are mainly found in platinum ores. Sample types where preparations are typically focused upon include ores, alloys and catalysts. Ruthenium and osmium are grouped together because their preparation chemistry is essentially the same. Some of the similarities between Ru and Os are shown in Table 1.

Metallic radius0.134 nm0.135 nm

Metals

Ru° or Os° as the pure metals are not soluble in aqua regia. Ru° and Os° are soluble upon fusion in KOH/KNO₃. When Ru° is alloyed with platinum or gold it is soluble in aqua regia forming the chlorides of platinum, gold and ruthenium. Os alloyed with platinum or gold is not soluble in acids including aqua regia. Osmium occurs with platinum ores as a natural alloy with iridium (osmiridium) and remains undissolved in the form of hard, white metallic-looking grains when the ores are treated with aqua regia. There are several oxidative fusions that have been used to dissolve the metals such as K₂Cr₂O₇ in NaOH or Na₂O₂ melts but the KOH/KNO₃ fusion is the most popular. The KOH/KNO₃ fusion is used for samples containing all of the common forms of ruthenium and osmium including the metals, sulfides, alloys and oxides.

Ru oxides

Ru₂O₃ is formed when finely divided Ru° is heated in air forming a blue powder which is insoluble in acids. RuO₂ is formed when the metal is fused in an oxidizing atmosphere. RuO₄ is formed when the metal is heated at 1000°C. The tetroxide is also obtained when acid solutions containing ruthenium are heated with oxidizing agents such as MnO_4-, IO_4- or Cl₂. The tetroxide is volatile but can also be swept out of solution by a stream of gas at room temperature (M.P. °C: 25.4; B.P., °C: 40). The concern is loss of the volatile tetroxide. The aspiration of solutions containing the tetroxide using pneumatic nebulizers common to ICP instrumentation will cause false high signals due to the extra Ru entering the plasma due to its volatility. Fortunately the tetroxide is not formed in solutions of HNO₃.

Oxidative fusions such as the popular KOH/KNO₃ fusion convert all forms of ruthenium to the ruthenium tetroxide (RuO₄) mixed with some of the green ruthenate (VII). Due to the possible loss of the tetroxide the fusion temperatures are kept as low as possible. The KNO₃-KOH mixture forms KNO₃.2H₂O with a melting point of ~ 230 °C which defines the low temperature possible for this fusion. Nitrate fusions are known to become more oxidizing as the temperature is increased. KOH is added to stabilize a potentially explosive system. Since KNO₃ decomposes to give off O₂ at 400° C, care to keep the fusion temperature low (< 390 °C) is important. Loss of Ru may occur at fusion temperatures > 400° C. When the KOH-KNO₃ fuseate is dissolved in water the Ruthenate (VI) is formed. The conversion to the ruthenate (VI) occurs according to the following reactions:

1. 4RuO₄ + 4 OH^-  →  4 RuO_4- +2H₂O + O₂ (green ruthenate (VII))
2. 4RuO_4- +4 OH^-  →  4 RuO₄^-2 + 2H₂O +O₂ ( orange ruthenate (VI) )

For subsequent ICP-OES measurements the aqueous solution can be kept basic where use of a corrosion resistant introduction system is made or acidified with HCl where RuO₂Cl₄^-2 is formed. The presence of NO₃^-1 under acidic conditions will not produce the volatile RuO₄. Due to the complexity of the matrix, the calibration technique of standard additions is recommended.

Os oxides

An oxyhydrogen flame oxidizes the metal but does not melt it. When strongly heated in contact with air, finely divided Os° burns and is converted into OsO₄, commonly called osmic acid. OsO is not a likely byproduct of common sample preparations. The formation of Os₂O₃ and OsO₂ are also unlikely to be encountered. All of these oxides are formed when the corresponding salts are ignited in a current of CO₂ i.e. a condition that is not possible in a straight carbonate fusion. Of greatest concern is the formation of OsO₄. The tetroxide is formed when finely divided Os° is heated at about 400°C where it may catch fire with the formation of OsO₄. The tetroxide is also formed (slowly in very dilute solutions and more rapidly in 4M solutions) in aqueous HNO₃ solutions of osmium. The concern is loss of the volatile tetroxide. The aspiration of solutions containing the tetroxide using nebulizers common to ICP instrumentation will cause false high signals due to the extra Os entering the plasma due to its volatility. Please note that OsO₄ is very poisonous. The odor is easily detected because it is highly disagreeable like chlorine (Osmium is derived from the Greek work osme-a smell).

The KOH/KNO₃ fusion converts all forms of osmium to the potassium osmate (K₂OsO₄) which forms the soluble OsO₄^-2 upon dissolution of the melt in water. The KNO₃-KOH mixture forms KNO₃.2H₂O with a melting point of ~ 230 °C which defines the low temperature possible for this fusion. Nitrate fusions are known to become more oxidizing as the temperature is increased. KOH is added to stabilize a potentially explosive system. Since KNO₃ decomposes to give off O₂ at 400° C, care to keep the fusion temperature low (< 390 °C) is important. Loss of Os may occur at fusion temperatures > 400° C. When the KOH-KNO₃ fuseate is dissolved in water the Osmate (VI) is formed. For subsequent ICP-OES measurements the aqueous solution should be kept basic where use of a corrosion resistant introduction system is made.

If the aqueous solution of the fuseate is acidified with HCl the presence of NO₃^-1 under acidic conditions will produce the volatile OsO₄. If acidification is necessary, this author has found that the addition of ammonium 1-pyrrolidinedithiocarbamate (APDC) will prevent the high readings by either complexation or by reduction to one of the lower valence osmates thereby eliminating the OsO₄volatility. Either mechanism seems reasonable. OsO₄ is a strong oxidizing agent in acidic media as show here:

OsO₄ + 8H⁺ +6 Cl^- +4e^- =OsCl₆^-2 + 4H₂O         +1.0 v vs N.H.E.

It is reasonable to expect that it will oxidize the APDC. However, APDC is also a bidentate ligand. The OsO₄(OH^-)₂ is known to exist and it is very reasonable to expect that OsO₄(APDC)^-1 could also exist. Regardless of which mechanism is correct the addition of APDC to aqueous acidic solutions containing OsO₄ does eliminate the vaporization interference. With all of this said, the analysis of the Os in a basic solution formed from the aqueous dissolution of the KOH/KNO₃ fuseate is encouraged thereby eliminating formation of the volatile tetroxide.

Due to the complexity of the matrix the calibration technique of standard additions is strongly recommended.

A summary of some solution chemistry of osmium and ruthenium as it pertains to the preparation of calibration standards

CRMs manufactured by Inorganic Ventures contain both ruthenium and osmium in the +4 oxidation state as a chloride complex. The ruthenium and osmium are prepared in an HCl matrix and are believed to exist as the RuCl₆^-2 and the OsCl₆^-2 complexes. In HCl matrices these CRMs have been observed to be both stable and homogeneous. These CRMs have also been shown to be stable for years as well as stable to both extremes in temperature during shipping (short term stability).

The Handling and use of Ru CRMs – A Summary

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

Chemical Compatibility: Stable in HCl. Stable with most metals and inorganic anions as the [RuCl₆]^2- in dilute acidic media.

Stability: 2-100 ppb levels stable for months in 1% HNO3 / LDPE container. 1-10,000 ppm solutions chemically stable for years in 10% HCl / LDPE container.

Ru Containing Samples (Preparation & Solution):  Metal (fuse with KOH/KNO₃ in a Ag⁰ crucible); Oxides (fuse with KOH / KNO₃ in a Ag⁰ crucible); Ores (see Oxides); Alloys (see Oxides). Organics (the RuO₄ is volatile and acidic oxidizing preparations should be used with caution. The preferred approach is the KOH / KNO₃ fusion and dissolution of the fuseate in HCl).

Stability of Ru spiked into matrices resulting from the KOH/KNO₃ aqueous solution of the fuseate: Due to the complex nature of matrices resulting from this preparation method, it is recommended that the technique of standard additions be used. These CRMs can be spiked into these matrices and will produce a stable solution. A good practice is to perform an analysis of the sample and sample + spike solutions on the same analytical day. Stable solutions also result from the addition of the Ru CRM to aqueous fuseate solutions acidified with hydrochloric acid.

The Handling and use of Os CRMs – A Summary

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. Stable with most metals and inorganic anions as the [OsCl₆]^2- in dilute HCl media. DO NOT EXPOSE TO NITRIC ACID - FORMATION OF THE VERY VOLATILE AND TOXIC OsO₄ WILL RESULT. Any oxidizing condition must be avoided.

Stability: 2-100 ppb levels are NOT stable in 1% HNO₃ / LDPE container. The stability of HCl solutions at ppb levels has not been determined by our laboratory. 1-10,000 ppm solutions are presumed chemically stable for years in 10% HCl / LDPE container; stability studies have not been completed. Please call 1-800-669-6799 for current stability information .

Os Containing Samples (Preparation & Solution): Oxides (fuse with KOH / KNO₃ in a Ag⁰ crucible and dissolve in water being sure to avoid addition of any acid); Ores (see Oxides); Organics (the OsO₄ is volatile and acidic oxidizing preparations should be used with caution. The preferred approach is the KOH / KNO₃ fusion and dissolution of the fuseate in water. Our laboratory has used APDC to help stabilize Os solutions, but more work is required to define the chemistry.

Stability of Os spiked into matrices resulting from the KOH/KNO₃ aqueous solution of the fuseate: Due to the complex nature of matrices resulting from this preparation method it is recommended that the technique of standard additions be used. These CRMs can be spiked into these matrices and will produce a stable solution. A good practice is to perform an analysis of the sample and sample + spike solutions on the same analytical day. Stable solutions WILL NOT RESULT from the addition of the Os CRM to aqueous fuseate solutions acidified with hydrochloric acid.

NOTE: The presence of the OsO₄ will give false high results due to its enhanced nebulization efficiency (volatility). Only dilutions in HCl should be made where no nitrate or nitric acid is present. The use of nitric acid should be strictly avoided. Preparations from caustic nitrate fusions should be diluted in water not acid.

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Preparation of samples containing Ru and Os using KOH- KNO₃ fusion

This Chapter 8 has been written around the following fusion sample preparation method where many explanations can be found:

1. Convert the sample to a fine powder. Contamination issues, which are a problem with many determinations, are not an issue with either Ru or Os. Therefore, any grinding or crushing equipment that is available with suitable hardness will suffice.
2. Weigh up to 1 gram of sample into a Ag crucible followed by 7 grams of KOH and 1 gram of KNO₃. Keep the ratio of KOH to KNO₃ between 5:1 and 9:1. The presence of the KOH stabilizes the KNO₃ consequently more vigorous reactions will be observed with a smaller ratio. Larger relative amounts of the nitrate (5:1 ratio) is suggested for metals, alloys and harder to oxidize samples. Organic samples will be oxidized by air during heating with only KOH mixed with the sample prior to the addition of the KNO₃ . When only KOH is present the temperature can go to 450 °C, otherwise the temperature should be kept at ≤ 350 °C.
3. Fuse the sample at an absolute maximum temperature of 390 °C in a silver crucible for 30 to 60 minutes using a muffle furnace. Temperature calibration is suggested.
4. After cooling, dissolve the fuseate in DI water. The addition of any acid is not recommended for reasons explained above.
5. Quantitatively transfer the solution to a plastic container. The final weight of the solution will depend upon the grams of potassium salts used for the fusion. If ICP-OES is the measurement tool, then a final solution weight of 100 grams is possible. ICP-MS will require a larger final weight due to ion shielding.  Corrosion resistant introduction systems are required.

For general information on sampling and sub-sampling see: https://www.inorganicventures.com/sample-preparation-guide/samples-containing-nickel