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• Overview

  Osmium tetroxide is an oxide of osmium (OsO4). Osmium tetroxide is the most important and most easily prepared compound of osmium. It has a number of specific applications in organic chemistry and in biochemistry. It is also a useful source of osmium compounds. It is remarkable in that it is one of the few volatile oxides of a heavy metal and that although the osmium is octavalent (of all elements only osmium and ruthenium reach as high an oxidation state) it is a reasonably controllable oxidising agent. It is from this latter property that most of its applications derive. Being an expensive chemical, available commercially only in small amounts, it is still considered a serious toxic compound for small-scale terrorist devices. On 6 April 2004, news agencies reported that the British police foiled a plot by members of El-Qaeda to prepare and detonate a bomb containing OsO4 in London^[1]. ;

• History and preparation

  The compound was discovered in 1803 by Smithson Tennant (1761-1815), and in the same year he isolated metallic osmium from it^[2]. Fusion with alkali of the black residue remaining after treatment of native platinum with aqua regia followed by extraction and acidification of the melt gave. Industrially, OsO4, is made from crude platinum concentrates by oxidative acid distillation and is then separated from ruthenium tetroxide. In the laboratory it is best made by direct oxidation of osmium metal^[3] or by the acid distillation with chlorate of almost any osmium compound ;

• Properties

  Osmium tetroxide is a non-combustible, colorless to pale yellow solid that has a disagreeable chlorine-like odor^[4]. It slowly develops when powdered osmium metal is exposed to air. OsO4 is fairly soluble in water and in several organic solvents, but reacts as an oxidant with many of them. The substance is used in organic syntheses, mainly to oxidize unsaturated carbon-carbon bonds to dihydroxy compounds (glycols). Its most common usage is as a staining agent and a “fixant” in transmission electron microscopy. This chemical sublimes at room temperature, having a remarkable vapor pressure of about 7 mmHg at 20ºC (more typical for a liquid compound than for a solid), as compared to 17 mmHg for water, 2 mmHg for the nerve agent sarin, or 0.07 mmHg for the blistering agent sulfur mustard under the same conditions. It is highly poisonous, even at very low concentrations, and must be handled according to the appropriate precautions. Hours may pass between exposure and the appearance of noticeable symptoms. In almost all of its chemical reactions, a number of which are summarized in the diagram, OsO4, is reduced to compounds containing lower oxidation states. With ammonia, however, the tetrahedral osmiamate ion[OsO3, N]is formed, which is isoelectronic with OsO4. ;

• Applications

  It is used mainly for the cis-hydroxylation of olefinic double bonds to give glycols, for which purpose it is the smoothest and most efficient general reagent known. It tends to react faster with strained olefins, particularly in the presence of pyridine^[6]. Aromatic hydrocarbons are hydroxylated only at the most reactive aromatic site (e.g. phenanthrene at the 9,10 position^[7]). The reagent may be used either alone in an inert solvent, the resulting osmate (VI) ester being decomposed by bisulphite or hydrogen sulphide^{[5, 6]}, or, more commonly and economically, in the presence of an additional oxidant such as chlorate, hydrogen peroxide or periodate. These regenerate the OsO4, which is therefore functioning as a catalyst. Examples of simple hydroxylations with OsO4, are the production of glycerol from ally1 alcohol and of ethylene glycol from ethylene. The compound has been used in the synthesis of such species as cortisone, progesterone, and of reserpine-type alkaloids, and also in the degradative investigation of natural products such as columbin^[5]. The glycol cleavage properties of periodate may be used together with the oxidising properties of OsO4, to convert olefins to aldehydes (e.g. trans-stilbene to benzaldehyde, cyclohexene to adipaldehyde), to ketones or to epoxides^{[8, 9]}.
  Osmium tetroxide is also a reagent that is used in the dihydroxylation process in the synthesis of 3-[1-(2,4-Dichlorophenyl)-2-(1H-imidazol-1-yl) ethoxy]-1,2-propanediol (D436535). 3-[1-(2,4-Dichlorophenyl)-2-(1H-imidazol-1-yl) ethoxy]-1,2-propanediol is involved in biological studies as microsomal cytochrome P 450 isoenzymes from Penicillium italicum interaction with sterol demethylation inhibitor fungicides.
  In biochemistry field, the compound is extensively used (normally in 2 percent aqueous solution, often called “osmic acid”) for cell and tissue studies, its unique fixation and staining properties having been recognised and used since 1861. It is used for both visible and electron microscopy of biological materials, but now the latter application is probably the more important. The purpose of fixation is to “freeze” cells without destruction or disruption of their organization or structure; staining is necessary for the resolution of cellular structure by increasing the apparent density of some parts of the tissue only. OsO4, is unique in that it both fixes and stains biological material. For the electron microscopist its most important functions are the preservation of sub-cellular ultrastructure and its ability to fix and stain membranes. For staining purposes it is often used with polar species such as uranyl or lead ions. The normal method used is to pre-treat the tissue with aldehydes, then to treat it by immersion in a dilute aqueous solution of OsO4, (or the tissue is exposed to OsO4, vapor) followed by washing, additional staining if required, dehydration with alcohol, embedding in resin and cutting into thin sections suitable for microscopy.
  Like some other heavy metal tetra-oxo species, osmium tetroxide in electrolytes has the property of passifying iron electrodes^[10]. ;

• Toxicity and health effect

  OsO4 has an odor threshold of 0.0019 parts/million. Its lethal concentration time (LCt50) is considered to be 1316 mg/min/m3, similar to that for sulfur mustard^[11]. Humans may tolerate a maximal concentration of 0.1 ppm in air for 1.5 hours or 0.0001 ppm for 6 hours without harmful effects^[12]. McLaughlin and co-authors^[13] reported that workers exposed to 0.01–0.53 ppm (0.1–0.6 mg/m3) suffered from lacrimation, conjunctivitis, vision disturbances, headaches and cough.
  OsO4 can be compared to chemical warfare agents in terms of toxicity. Exposure to even low doses can be lethal. In addition, both OsO4 and chemical warfare agents share similar physiological effects. The first appearance of a physiological effect, also known as the threshold effect, is observed at a lower concentration for osmium tetroxide vapor than for chemical warfare agents such as phosgene, sulfur mustard or even the nerve agent sarin. The LCt50 of OsO4 is comparable to that of sulfur mustard, but since sulfur mustard has a much lower vapor pressure OsO4 can pose a greater inhalational threat. Under the same environmental conditions there will be much more OsO4 vapor in a closed space than sulfur mustard vapor^[1].
  Osmium tetroxide is a rapid, indiscriminative oxidizer that does not distinguish between organic tissue and inorganic materials. An inhalational toxicity study with rabbits proved futile, because of the rapid reduction of OsO4 by the skin, hair, mucous membranes, etc., as well as by the chamber walls^[14]. Inhalation, ingestion, contact with skin and with mucous membranes may all result in severe consequences. Due to its high vapor pressure, most exposures are to vapor. These can cause severe chemical burns to the eyes, skin and respiratory tract^[15]. Very short-term contact with the vapor may cause lacrimation, accompanied by cough, headaches and dizziness. OsO4 may cause irreversible blindness by turning the cornea black. Symptoms may not be noticeable until several hours after the exposure, which may be an appealing feature for terrorists. Affected people may not realize immediately the extent of its toxic effects. Another severe delayed effect following inhalational exposure is acute lung injury, which may be followed by non-cardiogenic pulmonary edema^[1]. Direct contact with osmium tetroxide solution will turn the skin black (severe chemical burns due to strong oxidizing properties). Painful burns or contact dermatitis may result, depending on the concentration. OsO4 is not considered a carcinogen^[1]. Death is mainly the result of complications due to the exposure. ;

• Medical care

  Neutralization of the chemical on surfaces can be conveniently achieved by covering it with unsaturated oil (vegetable oil)^[1]. Osmium tetroxide does not have a medical antidote; therefore the treatment is supportive and symptomatic, depending on the route of exposure. Initial treatment should focus on preventing further exposure. Victims should be removed from the contaminated area, undressed, and decontaminated by running water as soon as possible. ;

• Analytic methods

  Spectrum method
  In general, the 1960-90s saw a big growth in spectral and optical methods for determining osmium. One of the first methods was X-ray fluorescence, with a lot of benefits such as accuracy, non-destructiveness, and the ability to detect osmium without chemical extraction from biological samples. Also, this method is independent of the chemical state of osmium atoms.
  It was observed that the reaction of OsO4 with thiourea in acid medium gave a red combination of[Os(thio)6]Cl3[16]. The results were not affected by systematic errors. The convenient, sensitive, reproducible, and accurate method for the spectrophotometric determination of osmium has been developed.
  Electrochemical methods
  A rapid potentiometric method was proposed by Zaky et al.[17]. The method is based on the addition of arsenite to Os(VIII) to reduce it to the metallic state. The excess of arsenite was oxidized by iodine dissolved in acetic acid. The liberated iodide was then potentiometrically titrated against mercury (II) using silver amalgam as the indicator electrode. Some binary and ternary mixtures were completely analyzed. Amin and Saleh described a simple, rapid, and accurate potentiometric method for the determination of Os (VIII) in the concentration range 0.4-4.0 mg•ml-1[18]. Hydrazine hydrochloride was added to Os (VIII) to reduce it to Os (IV). The excess of hydrazine hydrochloride was oxidized by iodine dissolved in acetic acid. ;

• References

  1. Baker M, Kosal ME. Osmium Tetroxide – a New Chemical Terrorism Weapon? http://CNS miis edu/pubs/week/040413 htm 2004
  2. D. McDonald, Platinum Metals Rev., 1961, 5, 146; Smithson Tennant, Phil. Trans., 1804, 94, 411
  3. G. Brauer, Handbook of Preparative Chemistry, Academic Press, New York, 1965, p. 1603
  4. Material Safety Data Sheet (MSDS). Osmium tetroxide. http://www proscitech com au/catalogue/msds/c010 pdf 2007.
  5. L. F. Fieser and M. Fieser, Reagents for Organic Synthesis, Wiley, New York, 1967, PP 475,759
  6. R. Criegee, B. Marchand and H. Wannowius,Ann., 1942, 550, 99
  7. G. M. Badger, Qzturt. Rev., 1951,5,160
  8. R. J. Collin, W. P. Griffith, F. Phillips and A. C. Skapski, Biochim. Biophys. Acra, 1973, 320, 745; J. Chem. Soc., Dalton Trans., 1974, I094
  9. R. Pappo, D. S. Allen, R. U. Lemieux and W. S. Johnson, J. Org. Chem., 1956,z1,478
  10. G. H. Cartledge, Corrosion, 1967,18,316t
  11. CBWInfo.com. Improvised Chemical Agent: Osmium Tetroxide. http://cbwinfo com/Chemical/HistandMisc/oso4 shtml 2004.
  12. Grant WM. Toxicology of the Eye. 2nd edn. Springfield, IL: Charles C. Thomas, 1974:769.
  13. McLaughlin AIG, Milton R, Perry KMA. Toxic manifestations of osmium tetroxide. Br J Ind Med 1946;3:183–6.
  14. Osmium Tetroxide. MSDS Division of Occupational Health and Safety. http://dohs ors od nih gov/pdf/Osmium%20Tetroxide%20REVISED pdf 2007.
  15. National Academy of Sciences. Prudent Practices in the Laboratory: Handling and Disposal of Chemicals.Washington, DC: National Academies Press, 1995:364.
  16. BRATULESCU G., GANESCU I., GANESCU A. Thiocyanatochrome complexes in analytical chemistry. Determination of osmium(III). J. Serb. Chem. Soc. 70, (8-9), 1113, 2005.
  17. ZAKY M., KILLA H.M., ISSA Y.M. Some observations on the application of arsenite reduction to the potentiometric determination of osmium(VIII) and to the analysis of mixtures. Microchem. J. 44, (1), 54, 1991.
  18. AMIN A.S., SALEH H.M. Utilization of hydrazine hydrochloride in the potentiometric determination of osmium(VIII): Analysis of binary and ternary mixtures. Sci. Pharm. 69, (2), 367, 2001.
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