السيريوم بالإنجليزية Cerium ، هوعنصر كيميائي له الرمز Ce والعدد الذري 58 في الجدول الدوري للعناصر.

الخصائص المميزة

يتميز السيريوم بأنه معدن فضي اللون ، وينتمي لمجموعة اللانثنيدات. يشبه الحديد من حيث اللون واللمعان ، وطيع وقابل للطرق أيضا.

وينتمي السيريوم أيضا لمجموعة من العناصر الكيميائية المسامة الأتربة النادرة ، مع أنها ليست كلها نادرة، إذ إذا بعضها أكثر انتشاراً في الطبيعة من الرصاص، وقد احتفظت بتسميتها هذه مع أنها تصنف اليوم باسم العناصر اللانتانيدية نسبة إلى أول عناصرها وهواللانتانيوم La كما يشار إليها أحياناً باسم العناصر الانتنطقية الداخلية. وقد وسمت قبل بداية القرن التاسع عشر بالندرة وارتفاع الثمن، إذ لم تكن تسترعي اهتمام إلا قلة من الفهماء لم يكن عددهم يتجاوز في العالم آنذاك مئتي عالم، ثم تضافرت الجهود تدريجياً، وازداد توجه الأنظار إلى هذه الأتربة لسبر أغوار عناصرها على الصعيد التجريبي بغية فصل بعضها عن بعض من جهة، وتبين خواصها الفيزيائية والكيمياوية من جهة أخرى.

التطبيقات

إستعمالات السيريوم:

• في فهم المعادن:
  • يستعمل السيريوم في عمل سبائك الألومنيوم.
  • يمكن الحصول على الحديد المطاوع بإضافة السيريوم إلى الحديد الزهر.
  • يستخدم السيريوم عند صناعة الحديد الصلب للحد من تكون أكسيد الحديد وكبريتيد الحديد.
  • يستخدم السيريوم في صناعة الحديد المقاوم للصدأ كعنصر لزيادة الصلابة.
  • 3 to 4% cerium added to magnesium alloys, along with 0.2 to 0.6% zirconium, helps refine the grain and give sound casting of complex shapes. It also adds heat resistance to magnesium castings.
  • Cerium is used in alloys that are used to make permanent magnets.
  • Cerium is used as an alloying element in tungsten electrodes for gas tungsten arc welding.
  • Cerium is a major component of ferrocerium, also known as "lighter flint". Although modern alloys of this type generally use Mischmetal rather than purified cerium, it still is the most prevalent constituent.
  • Cerium is used in carbon-arc lighting, especially in the motion picture industry.
• Cerium oxalate is an anti-emetic drug.
• Cerium(IV) oxide
  • The oxide is used in incandescent gas mantles, such as the Welsbach mantle, where it was combined with thorium, lanthanum, magnesium or yttrium oxides .
  • The oxide is emerging as a hydrocarbon catalyst in self cleaning ovens, incorporated into oven walls.
  • Cerium(IV) oxide has largely replaced rouge in the glass industry as a polishing abrasive.
  • Cerium(IV) oxide is finding use as a petroleum cracking catalyst in petroleum refining.
  • Cerium(IV) additives to diesel fuel cause that to burn more cleanly, with less resulting air-pollution.
  • In glass, cerium(IV) oxide allows for selective absorption of ultraviolet light.
• Cerium(IV) sulfate is used extensively as a volumetric oxidizing agent in quantitative analysis.
• Ceric ammonium nitrate is a useful one-electron oxidant in organic chemistry, used to oxidatively etch electronic components, and as a primary standard for quantitative analysis.
• Cerium compounds are used in the manufacture of glass, both as a component and as a decolorizer.
• Cerium in combination with titanium gives a golden yellow color to glass.
• Cerium compounds are used for the coloring of enamel.
• Cerium(III) and cerium(IV) compounds such as cerium(III) chloride have uses as catalysts in organic synthesis.

التاريخ

أكتشف السيريوم في باستنس في السويد بواسطة جونز ياكوب بيرزيليوس ووليهيلم هيسنجر ، وإستخلص في ألمانيا بواسطة مارتين هينريس كلابروث وكان ذلك في عام 1803.

التواجد

يعتبر السيريوم من أكثر العناصر وفرة من مجموعة الأتربة النادرة ، حيث تصل نسبته 0.0046% في القشرة الأرضية من حيث الوزن. وقد عثر مجموعة من المعادن تضم ألانيت والذي يعهد بإسم أورثيت ، ومعدن المونازيت والباستناسيت وهذين الأخيرين من أبرز المصادر لإستخلاص السيريوم.

عادة ما يحضر السيريوم بواسطة عملية التبادل الأيوني التي تستعمل رمال المونازيت كمصدر للسيريوم.

انظر أيضاتصنيف:معادن لانثنيدات

المركبات

Cerium has two common oxidation states, +3 and +4. The most common compound of cerium is cerium(IV) oxide (CeO₂), which is used as "jeweller's rouge" as well as in the walls of some self-cleaning ovens. Two common oxidising agents used in titrations are ammonium cerium(IV) sulfate (ceric ammonium sulfate, (NH₄)₂Ce(SO₄)₃) and ammonium cerium(IV) nitrate (ceric ammonium nitrate or CAN, (NH₄)₂Ce(NO₃)₆). Cerium also forms a chloride, CeCl₃ or cerium(III) chloride, used to facilitate reactions at carbonyl groups in organic chemistry. Other compounds include cerium(III) carbonate (Ce₂(CO₃)₃), cerium(III) fluoride (CeF₃), cerium(III) oxide (Ce₂O₃), as well as cerium(IV) sulfate (ceric sulfate, Ce(SO₄)₂) and cerium(III) triflate (Ce(OSO₂CF₃)₃).

The two oxidation states of cerium differ enormously in basicity: cerium(III) is a strong base, comparable to the other trivalent lanthanides, but cerium(IV) is weak. This difference has always allowed cerium to be by far the most readily isolated and purified of all the lanthanides, otherwise a notoriously difficult group of elements to separate. A wide range of procedures have been devised over the years to exploit the difference. Among the better ones:

1. Leaching the mixed hydroxides with dilute nitric acid: the trivalent lanthanides dissolve in cerium-free condition, and tetravalent cerium remains in the insoluble residue as a concentrate to be further purified by other means. A variation on this uses hydrochloric acid and the calcined oxides from bastnaesite, but the separation is less sharp.
2. Precipitating cerium from a nitrate or chloride solution using potassium permanganate and sodium carbonate in a 1:4 molar ratio.
3. Boiling rare-earth nitrate solutions with potassium bromate and marble chips.

Using the classical methods of rare-earth separation, there was a considerable advantage to a strategy of removing cerium from the mixture at the beginning. Cerium typically comprised 45% of the cerite or monazite rare earths, and removing it early greatly reduced the bulk of what needed to be further processed (or the cost of reagents to be associated with such processing). However, not all cerium purification methods relied on basicity. Ceric ammonium nitrate [ammonium hexanitratocerate(IV)] crystallization from nitric acid was one purification method. Cerium(IV) nitrate (hexanitratoceric acid) was more readily extractable into certain solvents (e.g. tri-n-butyl phosphate) than the trivalent lanthanides. However, modern practice in China seems to be to do purification of cerium by counter-current solvent extraction, in its trivalent form, just like the other lanthanides.

Cerium(IV) is a strong oxidant under acidic conditions, but stable under alkaline conditions, when it is cerium(III) that becomes a strong reductant, easily oxidized by molecular dioxygen (or air). This ease of oxidation under alkaline conditions leads to the occasional geochemical parting of the ways between cerium and the trivalent light lanthanides under supergene weathering conditions, leading variously to the "negative cerium anomaly" or to the formation of the mineral cerianite. Air-oxidation of alkaline cerium(III) is the most economical way to get to cerium(IV), which can then be handled in acid solution.

انظر أيضاتصنيف:مركبات سيريوم

النظائر المشعة

Naturally-occurring cerium is composed of أربعة stable isotopes; ¹³⁶Ce, ¹³⁸Ce, ¹⁴⁰Ce, and ¹⁴²Ce with ¹⁴⁰Ce being the most abundant (88.48% natural abundance). ¹³⁶Ce and ¹⁴²Ce are predicted to be double beta active but no signs of activity were ever observed (for ¹⁴²Ce, the lower limit on half-life is 5×10¹⁶ years). 26 radioisotopes have been characterized with the most long-lived being ¹⁴⁴Ce with a half-life of 284.893 days, ¹³⁹Ce with a half-life of 137.640 days, and ¹⁴¹Ce with a half-life of 32.501 days. All of the remaining radioactive isotopes have half-lives that are less than أربعة days and the majority of these have half-lives that are less thanعشرة minutes. This element also has 2 meta states.

The known isotopes of cerium range in atomic weight from 123 u (¹²³Ce) to 152 u (¹⁵²Ce).

Cerium 144 is a high-yield product of nuclear fission; the ORNL Fission Product Pilot Plant separated substantial quantities of cerium-144 from reactor waste, and it was used in the Aircraft Nuclear Propulsion and SNAP programs.

الإحتياطات

Cerium, like all rare-earth metals, is of low to moderate toxicity. Cerium is a strong reducing agent and ignites spontaneously in air at 65 to 80 °C (150 to 175 °F). Fumes from cerium fires are toxic. Water should not be used to stop cerium fires, as cerium reacts with water to produce hydrogen gas. Workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions. Animals injected with large doses of cerium have died due to cardiovascular collapse.

Cerium(IV) oxide is a powerful oxidizing agent at high temperatures and will react with combustible organic materials. While cerium is not radioactive, the impure commercial grade may contain traces of thorium, which is radioactive. Cerium serves no known biological function.

وصلات خارجية

مشاع الفهم فيه ميديا متعلقة بموضوع Cerium.

• WebElements.com - Cerium
• It's Elemental - The Element Cerium
• Cerium Properties and Applications

المصادر

• Los Alamos National Laboratory - Cerium
• Lattice and spin dynamics of gamma-Ce