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Rhodium
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<h2 id="history">History</h2> <p>Rhodium (from <a href="/facts/Greek_language/pzDd6eIz">Greek</a>: ῥόδον <i>rhodon</i>, meaning 'rose') was <a href="/facts/Discovery_of_the_chemical_elements/QY4X2RP3">discovered</a> in 1803 by <a href="/facts/William_Hyde_Wollaston/U0MXYKA3">William Hyde Wollaston</a>,<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> soon after he discovered <a href="/facts/Palladium/AXMwDFxs">palladium</a>.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a><a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> He used crude <a href="/facts/Platinum/qJC6lQf7">platinum</a> ore presumably obtained from <a href="/facts/South_America/PhrFEMk7">South America</a>.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> His procedure dissolved the ore in <a href="/facts/Aqua_regia/AHr1mobH">aqua regia</a> and neutralized the acid with <a href="/facts/Sodium_hydroxide/xjtdYtq7">sodium hydroxide</a> (NaOH). He then precipitated the platinum as <a href="/facts/Ammonium_chloroplatinate/cCvDEanc">ammonium chloroplatinate</a> by adding <a href="/facts/Ammonium_chloride/SfXdntlv">ammonium chloride</a> (NH4Cl). Most other metals like <a href="/facts/Copper/I8PEE8oX">copper</a>, <a href="/facts/Lead/EYljasJN">lead</a>, <a href="/facts/Palladium/AXMwDFxs">palladium</a>, and rhodium were precipitated with <a href="/facts/Zinc/80b1qgk3">zinc</a>. Diluted <a href="/facts/Nitric_acid/UDbwGp6s">nitric acid</a> dissolved all but palladium and rhodium. Of these, palladium dissolved in <a href="/facts/Aqua_regia/AHr1mobH">aqua regia</a> but rhodium did not,<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> and the rhodium was precipitated by the addition of <a href="/facts/Sodium_chloride/MWcXl3Mb">sodium chloride</a> as Na3[RhCl6]·<i>n</i>H2O. After being washed with ethanol, the rose-red precipitate was reacted with zinc, which <a href="/facts/Displacement_reaction/GpqWlKrz">displaced</a> the rhodium in the ionic compound and thereby released the rhodium as free metal.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p><p>For decades, the rare element had only minor applications; for example, by the turn of the century, rhodium-containing thermocouples were used to measure temperatures up to 1800 °C.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> They have exceptionally good stability in the temperature range of 1300 to 1800 °C.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p><p>The first major application was electroplating for decorative uses and as corrosion-resistant coating.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> The introduction of the three-way <a href="/facts/Catalytic_converter/KsP1ecPY">catalytic converter</a> by <a href="/facts/Volvo/JUFSUUYD">Volvo</a> in 1976 increased the demand for rhodium. The previous catalytic converters used platinum or palladium, while the three-way catalytic converter used rhodium to reduce the amount of <a href="/facts/NOx/aC4Squ7z">NOx</a> in the exhaust.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a><a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a><a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p> <h2 id="characteristics">Characteristics</h2> <table><tbody><tr><th><a href="/facts/Atomic_number/bgAr581S">Z</a></th><th><a href="/facts/Chemical_element/0jducgWD">Element</a></th><th><a href="/facts/Electron_shell/QxI30eh9">No. of electrons/shell</a></th></tr><tr><td>27</td><td>cobalt</td><td>2, 8, 15, 2</td></tr><tr><td>45</td><td>rhodium</td><td>2, 8, 18, 16, 1</td></tr><tr><td>77</td><td>iridium</td><td>2, 8, 18, 32, 15, 2</td></tr><tr><td>109</td><td>meitnerium</td><td>2, 8, 18, 32, 32, 15, 2 (predicted)</td></tr></tbody></table> <p>Rhodium is a hard, silvery, durable metal that has a high <a href="/facts/Reflectance/TT4sBwIw">reflectance</a>. Rhodium metal does not normally form an <a href="/facts/Oxide/DXtoVe1C">oxide</a>, even when heated.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> <a href="/facts/Oxygen/acyn6o8r">Oxygen</a> is absorbed from the <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a> only at the <a href="/facts/Melting_point/MY0w0h0k">melting point</a> of rhodium, but is released on solidification.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> Rhodium has both a higher melting point and lower <a href="/facts/Density/92p6hh9I">density</a> than <a href="/facts/Platinum/qJC6lQf7">platinum</a>. It is not attacked by most <a href="/facts/Acid/FySU18n1">acids</a>: it is completely insoluble in <a href="/facts/Nitric_acid/UDbwGp6s">nitric acid</a> and dissolves slightly in <a href="/facts/Aqua_regia/AHr1mobH">aqua regia</a>. </p><p>Rhodium belongs to <a href="/facts/Group_9_element/YsFjV4Ey">group 9</a> of the periodic table, but exhibits an atypical <a href="/facts/Ground_state/ML7cfCug">ground state</a> <a href="/facts/Valence_electron/mXPA15VP">valence electron</a> configuration for that group. Like neighboring elements <a href="/facts/Niobium/3zNI8AKp">niobium</a> (41), <a href="/facts/Ruthenium/uzCm4ufn">ruthenium</a> (44), and <a href="/facts/Palladium/AXMwDFxs">palladium</a> (46), it only has one electron in its outermost <a href="/facts/S_orbital/tKhgl6mL"><i>s</i> orbital</a>. </p> <h3>Chemical properties</h3> <table><tbody><tr><th colspan="2">Oxidation statesof rhodium</th></tr><tr><td>+0</td><td>Rh4(CO)12</td></tr><tr><td>+1</td><td>RhCl(PH3)2</td></tr><tr><td>+2</td><td>Rh2(O2CCH3)4</td></tr><tr><td>+3</td><td>RhCl3, Rh2O3</td></tr><tr><td>+4</td><td>RhO2</td></tr><tr><td>+5</td><td>RhF5, Sr3LiRhO6</td></tr><tr><td>+6</td><td>RhF6</td></tr></tbody></table> <p>The common <a href="/facts/Oxidation_state/W53W6s6V">oxidation states</a> of rhodium are +3 and +1. Oxidation states 0, +2, and +4 are also well known.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> A few complexes at still higher oxidation states are known.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p><p>The rhodium oxides include <a href="/facts/Rhodium(III)_oxide/P90YBJ7y">Rh2O3</a>, <a href="/facts/Rhodium(IV)_oxide/xN0qwEDt">RhO2</a>, RhO2·<i>x</i>H2O, Na2RhO3, Sr3LiRhO6 and Sr3NaRhO6.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> None are of technological significance. </p><p>All the Rh(III) halides are known but the hydrated trichloride is most frequently encountered. It is also available in an anhydrous form, which is somewhat refractory. Other rhodium(III) chlorides include sodium hexachlororhodate, Na3RhCl6, and <a href="/facts/Pentaamminechlororhodium_dichloride/E9NkH6sF">pentaamminechlororhodium dichloride</a>, [Rh(NH3)5Cl]Cl2. They are used in the recycling and purification of this very expensive metal. Heating a methanolic solution of hydrated rhodium trichloride with <a href="/facts/Sodium_acetate/CU1Yc8Jp">sodium acetate</a> give the blue-green <a href="/facts/Rhodium(II)_acetate/mKM9nIsq">rhodium(II) acetate</a>, Rh2(O2CCH3)4, which features a Rh-Rh bond. This complex and related <a href="/facts/Rhodium(II)_trifluoroacetate/fGu9NUvB">rhodium(II) trifluoroacetate</a> have attracted attention as catalysts for <a href="/facts/Cyclopropanation/8EmwKzrK">cyclopropanation</a> reactions. Hydrated rhodium trichloride is reduced by <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a>, <a href="/facts/Ethylene/QMNBYLYn">ethylene</a>, and <a href="/facts/Trifluorophosphine/feeXrTsB">trifluorophosphine</a> to give rhodium(I) complexes Rh2Cl2L4 (L = CO, C2H4, PF3). When treated with <a href="/facts/Triphenylphosphine/nYFNCVcJ">triphenylphosphine</a>, hydrated rhodium trichloride converts to the maroon-colored RhCl(P(C6H5)3)3, which is known as <a href="/facts/Wilkinson%2527s_catalyst/c7lrFXfo">Wilkinson's catalyst</a>. Reduction of <a href="/facts/Rhodium_carbonyl_chloride/VazkzXPS">rhodium carbonyl chloride</a> gives <a href="/facts/Hexarhodium_hexadecacarbonyl/EngW4jeo">hexarhodium hexadecacarbonyl</a>, Rh6(CO)16, and <a href="/facts/Tetrarhodium_dodecacarbonyl/37KH2KUC">tetrarhodium dodecacarbonyl</a>, Rh4(CO)12, the two most common Rh(0) complexes. </p><p>As for other metals, rhodium forms high oxidation state <a href="/facts/Binary_phase/pQ7zq0Cv">binary fluorides</a>. These include <a href="/facts/Rhodium_pentafluoride/PdbXWm8z">rhodium pentafluoride</a>, a tetrameric complex with the true formula Rh4F20) and <a href="/facts/Rhodium_hexafluoride/HGrRWsFQ">rhodium hexafluoride</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> </p> <h3>Isotopes</h3> <p class="note">Main article: <a href="/facts/Isotopes_of_rhodium/fYAb2dz8">Isotopes of rhodium</a></p> <p>Naturally occurring rhodium is composed of only one <a href="/facts/Isotope/KD9B1y6o">isotope</a>, 103Rh. The most stable <a href="/facts/Radioisotope/5WpCfbdq">radioisotopes</a> are 101Rh with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 3.3 years, 102Rh with a half-life of 207 days, 102mRh with a half-life of 2.9 years, and 99Rh with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with <a href="/facts/Atomic_weight/A3vbuz2n">atomic weights</a> ranging from 92.926 <a href="/facts/Atomic_mass_unit/IMATU3vf">u</a> (93Rh) to 116.925 u (117Rh). Most of these have half-lives shorter than an hour, except 100Rh (20.8 hours) and 105Rh (35.36 hours). Rhodium has numerous <a href="/facts/Meta_state/L8DHcqnk">meta states</a>, the most stable being 102mRh (0.141 MeV) with a half-life of about 2.9 years and 101mRh (0.157 MeV) with a half-life of 4.34 days (see <a href="/facts/Isotopes_of_rhodium/fYAb2dz8">isotopes of rhodium</a>).<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> </p><p>In isotopes weighing less than 103 (the stable isotope), the primary <a href="/facts/Decay_mode/Ya6FVjt4">decay mode</a> is <a href="/facts/Electron_capture/EIccoV1u">electron capture</a> and the primary <a href="/facts/Decay_product/hT0obfLv">decay product</a> is <a href="/facts/Ruthenium/uzCm4ufn">ruthenium</a>. In isotopes greater than 103, the primary decay mode is <a href="/facts/Beta_emission/vxTBlckp">beta emission</a> and the primary product is <a href="/facts/Palladium/AXMwDFxs">palladium</a>.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> </p> <h2 id="occurrence">Occurrence</h2> <p>Rhodium is one of the <a href="/facts/Abundance_of_elements_in_Earth%2527s_crust/UDSVHH56">rarest elements in the Earth's crust</a>, comprising an estimated 0.0002 <a href="/facts/Parts-per_notation/PozRuoLM">parts per million</a> (2 × 10−10).<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> Its rarity affects its price and its use in commercial applications. The concentration of rhodium in nickel <a href="/facts/Meteorite/NsmHlBlX">meteorites</a> is typically 1 <a href="/facts/Parts-per_notation/PozRuoLM">part per billion</a>.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> Rhodium has been measured in some <a href="/facts/Potato/F8jRVExs">potatoes</a> with concentrations between 0.8 and 30 ppt.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> </p> <h3>Mining and price</h3> <p>Rhodium ores are a mixture with other metals such as <a href="/facts/Palladium/AXMwDFxs">palladium</a>, <a href="/facts/Silver/xXe41BGg">silver</a>, <a href="/facts/Platinum/qJC6lQf7">platinum</a>, and <a href="/facts/Gold/1znknojp">gold</a>. Few rhodium <a href="/facts/Mineral/LkUFbNmv">minerals</a> are known. The separation of rhodium from the other metals poses significant challenges. Principal sources are located in South Africa, river sands of the <a href="/facts/Ural_Mountains/gzNoPkt6">Ural Mountains</a> in Russia, and in North America, especially the <a href="/facts/Copper/I8PEE8oX">copper</a>-<a href="/facts/Nickel_sulfide/2UWCgMew">nickel sulfide</a> mining area of the <a href="/facts/Greater_Sudbury/IojLaSQn">Sudbury</a>, Ontario, region. Although the rhodium abundance at Sudbury is very small, the large amount of processed nickel ore makes rhodium recovery cost-effective. </p><p>The main exporter of rhodium is South Africa (approximately 80% in 2010) followed by Russia.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> The annual world production is 30 <a href="/facts/Tonne/gMymLmWv">tonnes</a>. The price of rhodium is highly variable. </p> <h3>Used nuclear fuels</h3> <p class="note">Main article: <a href="/facts/Synthesis_of_precious_metals/dDPVCRIn">Synthesis of precious metals</a></p> <p>Rhodium is a fission product of <a href="/facts/Uranium-235/pe0RznDM">uranium-235</a>: each kilogram of fission product contains a significant amount of the lighter platinum group metals. <a href="/facts/Used_nuclear_fuel/jWDWvJ6d">Used nuclear fuel</a> is therefore a potential source of rhodium, but the extraction is complex and expensive, and the presence of rhodium radioisotopes requires a period of cooling storage for multiple half-lives of the longest-lived isotope (101Rh with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 3.3 years, and 102mRh with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 2.9 years), or about 10 years. These factors make the source unattractive and no large-scale extraction has been attempted.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a><a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a><a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> </p> <h2 id="applications">Applications</h2> <p>The primary use of this element is in automobiles as a <a href="/facts/Catalytic_converter/KsP1ecPY">catalytic converter</a>, changing harmful unburned hydrocarbons, carbon monoxide, and nitrogen oxide exhaust emissions into less noxious gases. Of 30,000 kg of rhodium consumed worldwide in 2012, 81% (24,300 kg) went into this application, and 8,060 kg was recovered from old converters. About 964 kg of rhodium was used in the glass industry, mostly for production of fiberglass and flat-panel glass, and 2,520 kg was used in the chemical industry.<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a><a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> </p><p>In 2008, net demand (with the recycling accounted for) of rhodium for automotive converters made up 84% of the world usage,<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> with the number fluctuating around 80% in 2015−2021.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> </p> <h3>Carbonylation</h3> <p>Rhodium <a href="/facts/Catalyst/LPBS7TQC">catalysts</a> are used in some industrial processes, notably those involving <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a>. In the <a href="/facts/Monsanto_process/HdDDK3WC">Monsanto process</a>, rhodium iodides catalyze the <a href="/facts/Carbonylation/Jq8jghNR">carbonylation</a> of <a href="/facts/Methanol/EIfpoNCg">methanol</a> to produce <a href="/facts/Acetic_acid/uyBKA2Pd">acetic acid</a>.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> This technology has been significantly displaced by the <a href="/facts/Iridium/TFQutoj1">iridium</a>-based <a href="/facts/Cativa_process/h829bEle">Cativa process</a>, which effects the same conversion but more efficiently. Rhodium-based complexes are the dominant catalysts for <a href="/facts/Hydroformylation/KCnbNIu7">hydroformylation</a>, which converts alkenes to <a href="/facts/Aldehyde/xSIRkW03">aldehydes</a> according to the following equation:<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a><a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> </p> RCH=CH2 + H2 + CO → RCH2−CH2CHO <p>Rh-based hydroformylation underpins the industrial production of products as diverse as detergents, fragrances, and some drugs. Originally hydroformylation relied on much cheaper cobalt carbonyl-based catalysts, but that technology has largely been eclipsed by rhodium-based catalysts despite the cost differential. </p><p>Rhodium is also known to catalyze many reactions involving hydrogen gas and <a href="/facts/Hydrosilane/bIkgTqf1">hydrosilanes</a>. These include hydrogenations and hydrosilylations of alkenes.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> Rhodium metal, but not rhodium complexes, catalyzes the hydrogenation of <a href="/facts/Benzene/Fguue7nr">benzene</a> to <a href="/facts/Cyclohexane/FDAW5j8v">cyclohexane</a>.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> </p> <h3>Ornamental uses</h3> <p>Rhodium finds use in <a href="/facts/Jewelry/DYt7cj0R">jewelry</a> and for decorations. It is <a href="/facts/Electroplated/HbKf0SBb">electroplated</a> on <a href="/facts/White_gold/B1gj1kqS">white gold</a> and platinum to give it a reflective white surface at time of sale, after which the thin layer wears away with use. This is known as rhodium flashing in the jewelry business. It may also be used in coating <a href="/facts/Sterling_silver/fa6mHoon">sterling silver</a> to protect against tarnish (<a href="/facts/Silver_sulfide/muzsPR6v">silver sulfide</a>, Ag2S, produced from atmospheric <a href="/facts/Hydrogen_sulfide/UbCqjMMk">hydrogen sulfide</a>, H2S). Solid (pure) rhodium jewelry is very rare, more because of the difficulty of fabrication (high melting point and poor malleability) than because of the high price.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> The high cost ensures that rhodium is applied only as an <a href="/facts/Electroplating/HbKf0SBb">electroplate</a>. Rhodium has also been used for honors or to signify elite status, when more commonly used metals such as silver, gold or platinum were deemed insufficient. In 1979 the <i><a href="/facts/Guinness_Book_of_World_Records/Az88wVWk">Guinness Book of World Records</a></i> gave <a href="/facts/Paul_McCartney/TfhC0oaF">Paul McCartney</a> a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist.<a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> </p> <h3>Other uses</h3> <p>Rhodium is used as an alloying agent for hardening and improving the corrosion resistance<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a> of <a href="/facts/Platinum/qJC6lQf7">platinum</a> and <a href="/facts/Palladium/AXMwDFxs">palladium</a>. These alloys are used in furnace windings, bushings for glass fiber production, <a href="/facts/Thermocouple/FW9cYU5D">thermocouple</a> elements, <a href="/facts/Electrode/251PaKX8">electrodes</a> for aircraft <a href="/facts/Spark_plug/Y9lG10hg">spark plugs</a>, and laboratory crucibles.<a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a> Other uses include: </p> <ul><li><a href="/facts/Switch/vqbNDgYm">Electrical contacts</a>, where it is valued for small <a href="/facts/Electrical_resistance/hxSt0UAj">electrical resistance</a>, small and stable <a href="/facts/Contact_resistance/m0D7MTpj">contact resistance</a>, and great <a href="/facts/Corrosion/lIC7H4we">corrosion</a> resistance.<a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a></li> <li>Rhodium plated by either <a href="/facts/Electroplating/HbKf0SBb">electroplating</a> or evaporation is extremely hard and useful for optical instruments.<a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a></li> <li>Filters in <a href="/facts/Mammography/mN9jkyf9">mammography</a> systems for the characteristic X-rays it produces.<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a></li> <li>Rhodium neutron detectors are used in nuclear reactors to measure neutron flux levels—this method requires a digital filter to determine the current neutron flux level, generating three separate signals: immediate, a few seconds delay, and a minute delay, each with its own signal level; all three are combined in the rhodium detector signal. The three <a href="/facts/Palo_Verde_Nuclear_Generating_Station/jLLeSk1b">Palo Verde</a> nuclear reactors each have 305 rhodium neutron detectors, 61 detectors on each of five vertical levels, providing an accurate 3D "picture" of reactivity and allowing fine tuning to consume the nuclear fuel most economically.<a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a></li></ul> <p>In automobile manufacturing, rhodium is also used in the construction of headlight reflectors.<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a> </p> <h2 id="precautions">Precautions</h2> <p>Being a <a href="/facts/Noble_metal/Pe9EKIvU">noble metal</a>, pure rhodium is inert and harmless in elemental form.<a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a> However, chemical complexes of rhodium can be reactive. For rhodium chloride, the <a href="/facts/Median_lethal_dose/PSTer8Wv">median lethal dose</a> (LD50) for rats is 198 mg (RhCl3) per kilogram of body weight.<a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a> Like the other noble metals, rhodium has not been found to serve any biological function. </p><p>People can be exposed to rhodium in the workplace by inhalation. The <a href="/facts/Occupational_Safety_and_Health_Administration/dCGbW5Ax">Occupational Safety and Health Administration</a> (OSHA) has specified the legal limit (<a href="/facts/Permissible_exposure_limit/IrQeETWy">Permissible exposure limit</a>) for rhodium exposure in the workplace at 0.1 mg/m3 over an 8-hour workday, and the <a href="/facts/National_Institute_for_Occupational_Safety_and_Health/5WjdDNMN">National Institute for Occupational Safety and Health</a> (NIOSH) has set the <a href="/facts/Recommended_exposure_limit/nexWYeNv">recommended exposure limit</a> (REL), at the same level. At levels of 100 mg/m3, rhodium is <a href="/facts/Immediately_dangerous_to_life_or_health/ektUbSdQ">immediately dangerous to life or health</a>.<a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a> For soluble compounds, the <a href="/facts/Permissible_exposure_limit/IrQeETWy">PEL</a> and REL are both 0.001 mg/m3.<a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/2000s_commodities_boom/vIIenKsU">2000s commodities boom</a></li> <li><a href="/facts/2020s_commodities_boom/tbat1Smb">2020s commodities boom</a></li> <li><a href="/facts/Bullion/qzeJXQB1">Bullion</a></li> <li><a href="/facts/Bullion_coin/9z2saSfc">Bullion coin</a></li> <li>Rhodium compounds</li></ul> <h2 id="external-links">External links</h2> Look up <i>rhodium</i> in Wiktionary, the free dictionary. Wikimedia Commons has media related to Rhodium. <ul><li><a href="http://www.periodicvideos.com/videos/045.htm">Rhodium</a> at <i><a href="/facts/The_Periodic_Table_of_Videos/vC5kbN0M">The Periodic Table of Videos</a></i> (University of Nottingham)</li> <li><a href="https://www.americanelements.com/rh.html">Rhodium Technical and Safety Data</a></li> <li><a href="https://www.cdc.gov/niosh/npg/npgd0544.html">CDC – NIOSH Pocket Guide to Chemical Hazards</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Wollaston, W. H. (1804). "On a New Metal, Found in Crude Platina". Philosophical Transactions of the Royal Society of London. 94: 419–430. doi:10.1098/rstl.1804.0019. <a href="/wiki/William_Hyde_Wollaston" target="_blank">/wiki/William_Hyde_Wollaston</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Griffith, W. P. (2003). "Rhodium and Palladium – Events Surrounding Its Discovery". Platinum Metals Review. 47 (4): 175–183. doi:10.1595/003214003X474175183. <a href="http://www.platinummetalsreview.com/dynamic/article/view/47-4-175-183" target="_blank">http://www.platinummetalsreview.com/dynamic/article/view/47-4-175-183</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Wollaston, W. H. (1805). "On the Discovery of Palladium; With Observations on Other Substances Found with Platina". Philosophical Transactions of the Royal Society of London. 95: 316–330. doi:10.1098/rstl.1805.0024. <a href="/wiki/William_Hyde_Wollaston" target="_blank">/wiki/William_Hyde_Wollaston</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Usselman, Melvyn (1978). "The Wollaston/Chenevix controversy over the elemental nature of palladium: A curious episode in the history of chemistry". Annals of Science. 35 (6): 551–579. doi:10.1080/00033797800200431. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Lide, David R. (2004). CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Boca Raton: CRC Press. pp. 4–26. ISBN 978-0-8493-0485-9. <a href="978-0-8493-0485-9" target="_blank">978-0-8493-0485-9</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1113. ISBN 978-0-08-037941-8. <a href="978-0-08-037941-8" target="_blank">978-0-08-037941-8</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Griffith, W. P. (2003). "Bicentenary of Four Platinum Group Metals: Osmium and iridium – events surrounding their discoveries". Platinum Metals Review. 47 (4): 175–183. doi:10.1595/003214003X474175183. <a href="https://doi.org/10.1595%2F003214003X474175183" target="_blank">https://doi.org/10.1595%2F003214003X474175183</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Hulett, G. A.; Berger, H. W. (1904). "Volatilization of Platinum". Journal of the American Chemical Society. 26 (11): 1512–1515. Bibcode:1904JAChS..26.1512H. doi:10.1021/ja02001a012. Archived (PDF) from the original on 24 January 2024 – via Zenodo. <a href="https://zenodo.org/record/1428894" target="_blank">https://zenodo.org/record/1428894</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>ASTM Committee E.2.0. on Temperature Measurement (1993). "Platinum Type". Manual on the use of thermocouples in temperature measurement. ASTM Special Technical Publication. ASTM International. Bibcode:1981mutt.book.....B. ISBN 978-0-8031-1466-1.[permanent dead link] <a href="978-0-8031-1466-1" target="_blank">978-0-8031-1466-1</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>J. V. Pearce, F. Edler, C. J. Elliott, A. Greenen, P. M. Harris, C. G. Izquierdo, Y. G. Kim, M. J. Martin, I. M. Smith, D. Tucker and R. I. Veitcheva, A systematic investigation of the thermoelectric stability of Pt-Rh thermocouples between 1300 °C and 1500 °C, METROLOGIA, 2018, Volume: 55 Issue: 4 Pages: 558-567 <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Kushner, Joseph B. (1940). "Modern rhodium plating". Metals and Alloys. 11: 137–140. <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Amatayakul, W.; Ramnäs, Olle (2001). "Life cycle assessment of a catalytic converter for passenger cars". Journal of Cleaner Production. 9 (5): 395–403. Bibcode:2001JCPro...9..395A. doi:10.1016/S0959-6526(00)00082-2. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Heck, R.; Farrauto, Robert J. (2001). "Automobile exhaust catalysts". Applied Catalysis A: General. 221 (1–2): 443–457. Bibcode:2001AppCA.221..443H. doi:10.1016/S0926-860X(01)00818-3. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Heck, R.; Gulati, Suresh; Farrauto, Robert J. (2001). "The application of monoliths for gas phase catalytic reactions". Chemical Engineering Journal. 82 (1–3): 149–156. Bibcode:2001ChEnJ..82..149H. doi:10.1016/S1385-8947(00)00365-X. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Cramer, Stephen D.; Covino, Bernard S. Jr., eds. (1990). ASM handbook. Materials Park, OH: ASM International. pp. 393–396. ISBN 978-0-87170-707-9. <a href="978-0-87170-707-9" target="_blank">978-0-87170-707-9</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Emsley, John (2001). Nature's Building Blocks ((Hardcover, First Edition) ed.). Oxford University Press. p. 363. ISBN 978-0-19-850340-8. <a href="978-0-19-850340-8" target="_blank">978-0-19-850340-8</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1985). Lehrbuch der Anorganischen Chemie (91–100 ed.). Walter de Gruyter. pp. 1056–1057. ISBN 978-3-11-007511-3. <a href="978-3-11-007511-3" target="_blank">978-3-11-007511-3</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Da Silva Santos, Mayara; Stüker, Tony; Flach, Max; Ablyasova, Olesya S.; Timm, Martin; von Issendorff, Bernd; Hirsch, Konstantin; Zamudio-Bayer, Vicente; Riedel, Sebastian; Lau, J. Tobias (2022). "The Highest Oxidation State of Rhodium: Rhodium(VII) in [RhO3]+". Angewandte Chemie International Edition. 61 (38): e202207688. doi:10.1002/anie.202207688. PMC 9544489. PMID 35818987. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9544489" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9544489</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>Reisner, B. A.; Stacy, A. M. (1998). "Sr3ARhO6 (A = Li, Na): Crystallization of a Rhodium(V) Oxide from Molten Hydroxide". Journal of the American Chemical Society. 120 (37): 9682–9989. doi:10.1021/ja974231q. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>Griffith, W. P. (1976). The Rarer Platinum Metals. New York: John Wiley and Sons. <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001 <a href="/wiki/Aaldert_Wapstra" target="_blank">/wiki/Aaldert_Wapstra</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes. <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>Barbalace, Kenneth, "Table of Elements[permanent dead link]". Environmental Chemistry.com; retrieved 2007-04-14. <a href="http://environmentalchemistry.com/yogi/periodic/Periodic" target="_blank">http://environmentalchemistry.com/yogi/periodic/Periodic</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>D.E.Ryan, J.Holzbecher and R.R.Brooks, Chemical Geology, Volume 85, Issues 3–4, 30 July 1990, Pages 295-303 <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> <li id="fn:25"><p>Orecchio, Santino; Amorello, Diana (2019). "Platinum and Rhodium in Potato Samples by Using Voltammetric Techniques". Foods. 8 (2): 59. doi:10.3390/foods8020059. hdl:10447/349662. PMC 6406736. PMID 30764564. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6406736" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6406736</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></p></li> <li id="fn:26"><p>Loferski, Patricia J. (2013). "Commodity Report: Platinum-Group Metals" (PDF). United States Geological Survey. Retrieved 16 July 2012. <a href="https://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2012-plati.pdf" target="_blank">https://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2012-plati.pdf</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></p></li> <li id="fn:27"><p>Kolarik, Zdenek; Renard, Edouard V. (2005). "Potential Applications of Fission Platinoids in Industry" (PDF). Platinum Metals Review. 49 (2): 79–90. doi:10.1595/147106705X35263. <a href="http://www.platinummetalsreview.com/pdf/79-90-pmr-apr05.pdf" target="_blank">http://www.platinummetalsreview.com/pdf/79-90-pmr-apr05.pdf</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></p></li> <li id="fn:28"><p>Kolarik, Zdenek; Renard, Edouard V. (2003). "Recovery of Value Fission Platinoids from Spent Nuclear Fuel. Part I PART I: General Considerations and Basic Chemistry" (PDF). Platinum Metals Review. 47 (2): 74–87. doi:10.1595/003214003X4727487. <a href="http://www.platinummetalsreview.com/pdf/pmr-v47-i2-074-087.pdf" target="_blank">http://www.platinummetalsreview.com/pdf/pmr-v47-i2-074-087.pdf</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></p></li> <li id="fn:29"><p>Kolarik, Zdenek; Renard, Edouard V. (2003). "Recovery of Value Fission Platinoids from Spent Nuclear Fuel. Part II: Separation Process" (PDF). Platinum Metals Review. 47 (2): 123–131. doi:10.1595/003214003X473123131. <a href="http://www.platinummetalsreview.com/pdf/pmr-v47-i2-074-087.pdf" target="_blank">http://www.platinummetalsreview.com/pdf/pmr-v47-i2-074-087.pdf</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></p></li> <li id="fn:30"><p>Loferski, Patricia J. (2013). "Commodity Report: Platinum-Group Metals" (PDF). United States Geological Survey. Retrieved 16 July 2012. <a href="https://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2012-plati.pdf" target="_blank">https://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2012-plati.pdf</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></p></li> <li id="fn:31"><p>Shelef, M.; Graham, G. W. (1994). "Why Rhodium in Automotive Three-Way Catalysts?". Catalysis Reviews. 36 (3): 433–457. doi:10.1080/01614949408009468. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></p></li> <li id="fn:32"><p>Murray, Angela Janet (2012). Recovery of Platinum Group Metals from Spent Furnace Linings and Used Automotive Catalysts (PDF) (PhD thesis). University of Birmingham. <a href="https://etheses.bham.ac.uk/id/eprint/7210/1/Murray12PhD.pdf" target="_blank">https://etheses.bham.ac.uk/id/eprint/7210/1/Murray12PhD.pdf</a> <a href="#fnref:32" class="footnote-back-ref">↩</a></p></li> <li id="fn:33"><p>"The Rhodium Market and Rhodium Price". <a href="https://www.sfa-oxford.com/platinum-group-metals/rhodium-market-and-rhodium-price-drivers" target="_blank">https://www.sfa-oxford.com/platinum-group-metals/rhodium-market-and-rhodium-price-drivers</a> <a href="#fnref:33" class="footnote-back-ref">↩</a></p></li> <li id="fn:34"><p>Roth, James F. (1975). "Rhodium Catalysed Carbonylation of Methanol" (PDF). Platinum Metals Review. 19 (1 January): 12–14. doi:10.1595/003214075X1911214. Archived from the original (PDF) on 24 September 2015. Retrieved 5 February 2009. <a href="https://web.archive.org/web/20150924074253/http://www.platinummetalsreview.com/pdf/pmr-v19-i1-012-014.pdf" target="_blank">https://web.archive.org/web/20150924074253/http://www.platinummetalsreview.com/pdf/pmr-v19-i1-012-014.pdf</a> <a href="#fnref:34" class="footnote-back-ref">↩</a></p></li> <li id="fn:35"><p>Hartwig, John (2010). Organotransition Metal Chemistry: From Bonding to Catalysis. New York: University Science Books. p. 1160. ISBN 978-1-938787-15-7. <a href="978-1-938787-15-7" target="_blank">978-1-938787-15-7</a> <a href="#fnref:35" class="footnote-back-ref">↩</a></p></li> <li id="fn:36"><p>C. Elschenbroich (2006). Organometallics. VCH. ISBN 978-3-527-29390-2. <a href="978-3-527-29390-2" target="_blank">978-3-527-29390-2</a> <a href="#fnref:36" class="footnote-back-ref">↩</a></p></li> <li id="fn:37"><p>Heidingsfeldova, M. & Capka, M. (2003). "Rhodium complexes as catalysts for hydrosilylation crosslinking of silicone rubber". Journal of Applied Polymer Science. 30 (5): 1837–1846. doi:10.1002/app.1985.070300505. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:37" class="footnote-back-ref">↩</a></p></li> <li id="fn:38"><p>Halligudi, S. B.; et al. (1992). "Hydrogenation of benzene to cyclohexane catalyzed by rhodium(I) complex supported on montmorillonite clay". Reaction Kinetics and Catalysis Letters. 48 (2): 547–552. Bibcode:1992RKCL...48..505T. doi:10.1007/BF02162706. S2CID 97802315. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:38" class="footnote-back-ref">↩</a></p></li> <li id="fn:39"><p>Fischer, Torkel; Fregert, S.; Gruvberger, B.; Rystedt, I. (1984). "Contact sensitivity to nickel in white gold". Contact Dermatitis. 10 (1): 23–24. doi:10.1111/j.1600-0536.1984.tb00056.x. PMID 6705515. S2CID 46626556. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:39" class="footnote-back-ref">↩</a></p></li> <li id="fn:40"><p>"Hit & Run: Ring the changes". The Independent. London. 2 December 2008. Retrieved 6 June 2009. <a href="https://www.independent.co.uk/news/people/hit-and-run/hit--run-ring-the-changes-1044166.html" target="_blank">https://www.independent.co.uk/news/people/hit-and-run/hit--run-ring-the-changes-1044166.html</a> <a href="#fnref:40" class="footnote-back-ref">↩</a></p></li> <li id="fn:41"><p>Cramer, Stephen D.; Covino, Bernard S. Jr., eds. (1990). ASM handbook. Materials Park, OH: ASM International. pp. 393–396. ISBN 978-0-87170-707-9. <a href="978-0-87170-707-9" target="_blank">978-0-87170-707-9</a> <a href="#fnref:41" class="footnote-back-ref">↩</a></p></li> <li id="fn:42"><p>Lide, David R (2004). CRC handbook of chemistry and physics 2004–2005: a ready-reference book of chemical and physical data (85th ed.). Boca Raton: CRC Press. pp. 4–26. ISBN 978-0-8493-0485-9. <a href="978-0-8493-0485-9" target="_blank">978-0-8493-0485-9</a> <a href="#fnref:42" class="footnote-back-ref">↩</a></p></li> <li id="fn:43"><p>Weisberg, Alfred M. (1999). "Rhodium plating". Metal Finishing. 97 (1): 296–299. doi:10.1016/S0026-0576(00)83088-3. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:43" class="footnote-back-ref">↩</a></p></li> <li id="fn:44"><p>Smith, Warren J. (2007). "Reflectors". Modern optical engineering: the design of optical systems. McGraw-Hill. pp. 247–248. ISBN 978-0-07-147687-4. <a href="978-0-07-147687-4" target="_blank">978-0-07-147687-4</a> <a href="#fnref:44" class="footnote-back-ref">↩</a></p></li> <li id="fn:45"><p>McDonagh, C P; et al. (1984). "Optimum x-ray spectra for mammography: choice of K-edge filters for tungsten anode tubes". Phys. Med. Biol. 29 (3): 249–52. Bibcode:1984PMB....29..249M. doi:10.1088/0031-9155/29/3/004. PMID 6709704. S2CID 250873106. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:45" class="footnote-back-ref">↩</a></p></li> <li id="fn:46"><p>Sokolov, A. P.; Pochivalin, G. P.; Shipovskikh, Yu. M.; Garusov, Yu. V.; Chernikov, O. G.; Shevchenko, V. G. (1993). "Rhodium self-powered detector for monitoring neutron fluence, energy production, and isotopic composition of fuel". Atomic Energy. 74 (5): 365–367. doi:10.1007/BF00844622. S2CID 96175609. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:46" class="footnote-back-ref">↩</a></p></li> <li id="fn:47"><p>Stwertka, Albert. A Guide to the Elements, Oxford University Press, 1996, p. 125. ISBN 0-19-508083-1 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:47" class="footnote-back-ref">↩</a></p></li> <li id="fn:48"><p>Leikin, Jerrold B.; Paloucek Frank P. (2008). Poisoning and Toxicology Handbook. Informa Health Care. p. 846. ISBN 978-1-4200-4479-9. <a href="978-1-4200-4479-9" target="_blank">978-1-4200-4479-9</a> <a href="#fnref:48" class="footnote-back-ref">↩</a></p></li> <li id="fn:49"><p>Landolt, Robert R.; Berk Harold W.; Russell, Henry T. (1972). "Studies on the toxicity of rhodium trichloride in rats and rabbits". Toxicology and Applied Pharmacology. 21 (4): 589–590. Bibcode:1972ToxAP..21..589L. doi:10.1016/0041-008X(72)90016-6. PMID 5047055. <a href="/wiki/Toxicology_and_Applied_Pharmacology" target="_blank">/wiki/Toxicology_and_Applied_Pharmacology</a> <a href="#fnref:49" class="footnote-back-ref">↩</a></p></li> <li id="fn:50"><p>"NIOSH Pocket Guide to Chemical Hazards - Rhodium (metal fume and insoluble compounds, as Rh)". CDC. Retrieved 21 November 2015. <a href="https://www.cdc.gov/niosh/npg/npgd0544.html" target="_blank">https://www.cdc.gov/niosh/npg/npgd0544.html</a> <a href="#fnref:50" class="footnote-back-ref">↩</a></p></li> <li id="fn:51"><p>"NIOSH Pocket Guide to Chemical Hazards - Rhodium (soluble compounds, as Rh)". CDC. Retrieved 21 November 2015. <a href="https://www.cdc.gov/niosh/npg/npgd0545.html" target="_blank">https://www.cdc.gov/niosh/npg/npgd0545.html</a> <a href="#fnref:51" class="footnote-back-ref">↩</a></p></li> </ol>