Mei 2015 - Jejaring Kimia

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Mei 25, 2015

Magnesium [Mg]

Mei 25, 2015

Characteristics

An: 12 N: 13
Am: 24.3050 g/mol
Group No: 2
Group Name: Alkaline earth metal
Block: s-block Period: 3
State: solid at 298 K
Colour: silvery white Classification: Metallic
Boiling Point: 1363K (1090oC)
Melting Point: 923K (650oC)
Density: 1.738g/cm3
Availability: Magnesium is the eight most abundant element in the Earth’s crust, about 2% by weight.

Discovery Information

Who: Sir Humphrey Davy
When: 1808
Where: England

Name Origin

From Magnesia ancient city in Asia Minor. " Magnesium" in different languages.
Some crystals of dolomite from the Tri-State mining district near Joplin, Missouri USA.
 
Dolomite is an commercially important source of magnesium.

Sources

Although magnesium is found in over 60 minerals, only dolomite (CaMg(CO3)2), magnesite (MgCO3), brucite (Mg(OH)2), carnallite (KMgCl3 - 6(H2O)), talc, and olivine ((Mg,Fe)2SiO4) are of commercial importance. Usually obtained by electrolysis of melted magnesium chloride found in sea water.
China produces around 60% of the world’s magnesium, other important producers are Austria, Poland, Russia, the USA, India, Greece and Canada. Annual production is around 350 thousand tons.

Abundance

Universe: 600 ppm (by weight)
Sun: 700 ppm (by weight)
Carbonaceous meteorite: 1.2 x 105 ppm
Earth’s Crust: 23000 ppm
Seawater: 1200 ppm
Human: 270000 ppb by weight; 70000 ppb by atoms

Uses

Used in alloys to make airplanes, missiles, car engine parts, racing bikes and other things that need light metals. Also used in fireplace bricks, flashbulbs, pigments and filters. Magnesium powder is still used in the manufacture of fireworks and marine flares where a brilliant white light is required.
Magnesium oxide and other compounds also are used in agricultural, chemical, and construction industries. As a metal, this element’s principal use is as an alloying additive to aluminium with these aluminium-magnesium alloys being used mainly for beverage cans.
Magnesium carbonate (MgCO3) powder is also used by athletes, such as gymnasts and weightlifters, to improve the grip on objects - the apparatus or lifting bar.
Magnesium stearate is a slightly flammable white powder with lubricative properties. In pharmaceutical technology it is used in the manufacturing of tablets, to prevent the tablets from sticking to the equipment during the tablet compression process (i.e., when the tablet’s substance is pressed into tablet form).

History

The name originates from the Greek word for a district in Thessaly called Magnesia. It is related to magnetite (Fe3O4) and manganese, which also originated from this area, and required differentiation as separate substances. See manganese for this history.
Magnesium is the eighth most abundant element in the earth’s crust. It is found in large deposits of magnesite, dolomite, and other minerals, and in mineral waters, where magnesiumion is soluble. In 1618 a farmer at Epsom in England attempted to give his cows water from a well. This they refused to drink because of the water’s bitter taste. However the farmer noticed that the water seemed to heal scratches and rashes. The fame of Epsom salts spread. Eventually they were recognised to be hydrated magnesium sulphate, MgSO4.
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Sir Humphry Davy electrolytically isolated pure magnesium metal in 1808 from a mix of magnesia and HgO, and A. A. B. Bussy prepared it in coherent form in 1831. Davy’s first suggestion for a name was magnium, but the name magnesium is now used.

Notes

Organic magnesium is important in both plant and animal life.

Hazards

Magnesium metal and alloys are highly flammable in their pure form when molten, as a powder, or in ribbon form. Burning or molten magnesium metal reacts violently with water. Once ignited it is difficult to extinguish, being able to burn in both nitrogen (forming magnesium nitride), and carbon dioxide (forming magnesium oxide and carbon). Magnesium powder is an explosion hazard. Magnesium reacts violently with halogens.

Magnesium Compounds

Magnesium carbonate MgCO3
Used in flooring, fireproofing, fire extinguishing compositions, cosmetics, dusting powder, and toothepaste. Other applications are as filler material, smoke suppressant in plastics, a reinforcing agent in neoprene rubber, a drying agent, and colour retention in floods. In addition, high purity magnesium carbonate is used as antacid and as an additive in table salt to keep it free flowing.

Magnesium chloride MgCl2
It is used in the manufacture of textiles, paper, fireproofing agents, cements and refrigeration brine.
Magnesium chloride is an important coagulant used in the preparation of tofu from soy milk. In Japan it is sold as nigari (the term is derived from the Japanese word for "bitter"), a white powder produced from seawater after the sodium chloride has been removed, and the water evaporated. Nigari consists mostly of magnesium chloride, with some magnesium sulfate and other trace elements.

Magnesium diboride MgB2
An inexpensive and simple superconductor that can be synthesized by high temperature reaction between boron and magnesium.

Magnesium hydroxide Mg(OH)2
Otherwise known as milk of magnesia, is commonly used as an antacid or a laxative. The mineral form of magnesium hydroxide is known as brucite. Magnesium hydroxide interferes with the absorption of folic acid and iron. The diarrhea caused by magnesium hydroxide carries away much of the body’s supply of potassium, and failure to take extra potassium will lead to muscle cramps.

Magnesium oxide Mg(OH)2
It is used by many libraries for preserving/deacidifying books, and as an insulator in industrial cables. It is also used as a medication to relieve heartburn, sour stomach, or acid indigestion. Medical uses of Magnesium oxide also include using it as a short-term laxative, or used to supplement a diet with magnesium. Magnesium oxide is a principle ingredient in construction materials used for fireproofing.

Magnesium peroxide MgO2
Magnesium peroxide being environmentally benign and its stable oxygen release are used widely in the cosmetic, agricultural, pharmaceutical, and environmental industries. It is used to reduce contaminant levels in groundwater. Magnesium peroxide is used in the bioremediation of contaminated soil and can improve the soil quality for plant growth and metabolism. It also used in the aquaculture industry for bioremediation.

Magnesium stearate C36H70MgO4
It is often used as a filling agent in the manufacture of medical pills. In this regard, the substance is also useful because it has lubricating properties, preventing ingredients from sticking to manufacturing equipment during the compression of chemical powders into solid pills. It is also a common ingredient in baby powders. In pure powder form, the substance can be a dust explosion hazard, although this issue is effectively moot beyond the manufacturing plants using it.

Magnesium sulfate (Epsom Salt) MgSO4.7H2O
Epsom salt was originally prepared by boiling down mineral waters at Epsom, England and afterwards prepared from sea water.
In agriculture and gardening, magnesium sulfate is used to correct magnesium deficiency in soil (magnesium is an essential element in the chlorophyll molecule). It is most commonly applied to potted plants, or to magnesium-hungry crops, such as potatoes, roses, and tomatoes. The advantage of magnesium sulfate over other magnesium soil amendments (such as dolomitic lime) is its high solubility.
Locally it may be used as a treatment of an ingrown nail. Oral magnesium sulfate, or magnesium oxide, is used as a laxative. Epsom salts are also available in a gel form for topical application in treating aches and pains. Intravenous use is broadening, as magnesium sulfate reduces striated muscle contractions and blocks peripheral neuromuscular transmission by reducing acetylcholine release at the myoneural junction, as well as other effects.

Reactions of Magnesium

Reactions with water
Although magnesium does not react with water to any significant extent it does react with steam to yield magnesium oxide, or magnesium hydroxide with excess steam, and hydrogen gas.
Mg (s) + H2O (g) --> MgO (aq) + H2 (g)
Mg (s) + excess 2H2O (g) --> Mg(OH)2 (aq) + H2 (g)
 
Reactions with air
A thin layer of oxide helps to protect magnesium from attack by air. However, once ignited magnesium burns in air with its characteristic intensely bright white flame. This yields magnesium oxide and magnesium trioxide.
2Mg (s) + O2 (g) -- >2MgO (s)
3Mg (s) + N2 (g) --> Mg3N2 (s)
 
Reactions with halogens
Magnesium will react with chlorine and bromine to form magnesium(II) dihalides.
Mg (s) + Cl2 (g) --> MgCl2 (s)
Mg (s) + Br2 (g) --> MgBr2 (s)
Reactions with acids
Magnesium will dissolve readily in dilute sulphuric acid, as will as other acids, to form solutions containing the Mg(II) ion and hydrogen gas.
Mg (s) + H2SO4 (aq) --> Mg2+(aq) + SO42-(aq) + H2 (g)
Mg (s) + 2HCl (aq) --> Mg2+ (aq) + 2Cl- (aq) + H2 (g)
Reactions with bases
Magnesium appears not to react with dilute alkalis.

Occurrence and Production of Magnesium

From the ground
Although magnesium is found in over 60 minerals, but only dolomite (CaMg(CO3)2), magnesite (MgCO3), brucite (Mg(OH)2), carnallite (KMgCl3.6(H2O)), talc (Mg3Si4O10(OH)2), and olivine ((Mg,Fe)2SiO4) are of commercial importance.
In the United States this metal is principally obtained by electrolysis of fused magnesium chloride from brines, wells, and sea water:
cathode: Mg2+ + 2e- --> Mg
anode: 2Cl- --> Cl2 (g) + 2e-
The United States has traditionally been the major world supplier of this metal, supplying 45% of world production even as recently as 1995. Today, the US market share is at 7%, with a single domestic producer left, US Magnesium, a company born from now-defunct Magcorp. As of 2005 China has taken over as the dominant supplier, pegged at 60% world market share, which increased from 4% in 1995. Unlike the above described electrolytic process, China is almost completely reliant on a different method of obtaining the metal from its ores, the silicothermic Pidgeon process (the reduction of the oxide at high temperatures with silicon).
From the Sea
Sea water contains magnesium salts, such as magnesium chloride. To extract the magnesium, calcium carbonate is added to sea water to form magnesium carbonate.
MgCl2 + CaCO3 --> MgCO3 + CaCl2
Magnesium carbonate is insoluble in water so it can be filtered out, and reacted with hydrochloric acid to obtain concentrated magnesium chloride.
MgCO3 + 2HCl --> MgCl2 + CO2 + H2O
From magnesium chloride, electrolysis produces magnesium.

Isotopes of Magnesium

24Mg [12 neutrons]
Abundance: 78.99%
Stable with 12 neutrons

25Mg [13 neutrons]
Abundance: 10%
Stable with 13 neutrons

26Mg [14 neutrons]
Abundance: 11.01%
Stable with 14 neutrons

Large enrichments of stable 26Mg have been observed in the Ca-Al-rich inclusions of some carbonaceous chondrite meteorites. The anomalous abundance of 26Mg is attributed to the decay of its parent 26Al in the inclusions.
Therefore, the meteorite must have formed in the solar nebula before the 26Al had decayed. Hence, these fragments are among the oldest objects in the solar system and have preserved information about its early history.
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Rino Safrizal
Jejaring Kimia Updated at: Mei 25, 2015

Mei 21, 2015

SMITHSONITE/ZINC CARBONATE [ Carbonates : Calcite ]

Mei 21, 2015
ZnCO3, zinc Carbonate
Minor ore of zinc and as mineral specimens
Smithsonite is named for James Smithson, the founder of the Smithsonian Institution. The luster of smithsonite sets it apart from other minerals: it has a silky to pearly luster giving natural specimens a certain play of light across its surface that resembles the fine luster of melted wax glowing under a candle flame. It is easy to wax poetically when discussing smithsonite’s unique luster. It is really unusual and captivating and collectors can easily get hooked.

Smithsonite in addition to wonderful luster also has a varied colour assortment. The apple green to blue-green colour is probably smithsonite’s most well known colour, but it is its purple to lavender colour that is probably its most sought after hue. There also exists attractive yellow, white, tan, brown, blue, orange, peach, colourless, pink and red smithsonite specimens and all of them are a credit to this mineral.

The typical crystal habit of smithsonite is an interesting form called botryoidal. This form has the appearance of grape bunches and is the result of radiating fibrous crystals that form from central attachment points and grow outward and into each other. The result is a rounded, bubbly landscape for which smithsonite is considered the classic example. There are also other habits more typical of calcite Group minerals including rounded rhombohedrons and scalenohedrons. Most of these come from the famous mines of Tsumeb, Namibia and the Broken Hill Mine in Zambia. The Tsumeb specimens are coloured by trace amounts of cobalt and can have some real exotic colours. The Kelly Mine, Magdalena, New Mexico has produced the absolute finest blue-green botryoidal masses of smithsonite. But there are many localities that have or are producing excellent specimens.

Smithsonite has been and is still being used as an important, although rather minor ore of zinc. At Leadville, Colorado the smithsonite deposits were largely overlooked until their profit potential was finally realized. Many other zinc ore minerals may have been originally smithsonite before metamorphism or other altering processes, formed new minerals. Smithsonite forms in dry climates as a weathering product of primary sulfide zinc ores such as sphalerite.

Smithsonite is not easy to confuse with many other minerals. Hemimorphite has a similar botryoidal habit and blue-green colour, but the fracture edges of smithsonite’s specimens have a plastic-like look while hemimorphite reveals minute, radiating crystals. Prehnite has similar colour and habit as well, but is much lighter and harder. Both of these minerals lack the melted wax luster of smithsonite. Its high density, good cleavage, crystal habit, luster, its reaction to hot HCl acid and its high hardness for a carbonate are all quite conclusive for smithsonite to be differentiated from all other minerals. With its lovely luster, many beautiful colours and interesting habits, smithsonite specimens are a source of real pleasure for collectors around the world.

Physical Characteristics

Colour: commonly apple green, blue-green, lavender, purple, yellow and white as well as tan, brown, blue, orange, peach, colourless, gray, pink and red
Luster: usually pearly to resinous with light play across its surface and sometimes is simply vitreous
Transparency: Crystals are transparent to translucent
Crystal System: trigonal; bar 3 2/m
Crystal Habits: include the rhombohedrons and scalenohedrons with generally curved faces. But more commonly is botryoidal or globular
Cleavage: perfect in three directions forming rhombohedrons
Fracture: uneven
Hardness: 4 - 4.5
Specific Gravity: approx. 4.4 (heavy for nonmetallic minerals)
Streak: white
Other: Effervesces slightly with warm hydrochloric (HCl) acid
Associated Minerals: those found in oxidation zones of zinc sulfide deposits such as hemimorphite, cerussite, wulfenite, limonite, mimetite, dolomite, hydrozincite, aurichalcite, calcite and other carbonate minerals
Major Occurrences: include Tsumeb, Namibia and the Broken Hill Mine in Zambia; the Kelly Mine, Magdalena, New Mexico; Leadville, Colorado; Utah; Idaho and Arizona, USA; Mexico; Laurion, Greece; Bytom, Poland; Moresnet, Belgium and many other localities
Best Indicators: luster, typical botryoidal habit, cleavage, hardness, reaction to hot acids and density
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Rino Safrizal
Jejaring Kimia Updated at: Mei 21, 2015

Mei 19, 2015

Karakteristik Logam Berat Merkuri

Mei 19, 2015
karakteristik merkuriPada dasarnya, merkuri/raksa (Hg) adalah unsur logam yang sangat penting dalam teknologi di abad modern saat ini merkuri adalah unsur yang mempunyai nomor atom 80 serta mempunyai massa molekul relatif 200,59. Merkuri diberikan simbol kimia Hg yang merupakan singkatan yang berasal dari bahasa Yunani Hydrargyricum, yang berarti cairan perak (Alfian, 2006). Kebanyakan merkuri yang ditemukan di alam terdapat dalam bentuk gabungan dengan elemen lainnya, dan jarang ditemukan dalam bentuk elemen terpisah. Komponen merkuri banyak tersebar di karang-karang, tanah, udara, air dan organisme hidup melalui proses-proses fisik, kimia dan bahan biologi yang kompleks.

Baca juga, dampak merkuri terhadap manusia dan lingkungan.

Beberapa sifat merkuri adalah sebagai berikut:
1) Merkuri merupakan satu-satunya logam yang berbentuk cair pada suhu kamar (250C) dan mempunyai titik beku terendah dari semua logam, yaitu -390C.
2) Merkuri mempunyai volatilitas yang tertinggi dari semua logam.
3) Ketahanan listrik merkuri sangat rendah sehingga merupakan konduktor yang terbaik dari semua logam.
4) Banyak logam yang dapat larut di dalam merkuri membentuk komponen yang disebut amalgam (alloy).
5) Merkuri dan komponen-komponennya bersifat racun terhadap semua makhluk hidup (Fardiaz, 1992).

Merkuri terdapat sebagai komponen renik dari banyak mineral, dengan bantuan continental yang rata-rata mengandung sekitar 80 ppb atau lebih kecil lagi. Sinabor, merkuri sulfida, HgS, yang berwarna merah merupakan bijih merkuri utama yang diperdagangkan. Bahan bakar batu bara fosil dan lignit sering mencapai 100 ppb merkuri, bahkan lebih (Achmad, 2004:100).
Hampir semua merkuri diproduksi dengan cara pembakaran merkuri sulfida (HgS) di udara, dengan reaksi berikut:

HgS + O2 ==> Hg + SO2

Baca juga, kasus pencemaran logam berat merkuri yang dikenal dengan tragedi  Minamata dan Teluk Buyat.

Merkuri dilepaskan sebagai uap, yang kemudian mengalami kondensasi, sedangkan gas-gas lainnya mungkin terlepas di atmosfer atau dikumpulkan.
Merkuri di alam terdapat dalam berbagai bentuk sebagai berikut:
1) Merkuri anorganik, termasuk logam merkuri (Hg++) dan garam-garamnya seperti merkuri klorida (HgCl2) dan merkuri oksida.
2) Kompleks merkuri organik atau organomerkuri terdiri dari:
a) Aril merkuri, mengandung hidrokarbon aromatik seperti fenil merkuri asetat
b) Alkoksi alkil merkuri (R – O Hg)
c) Alkil merkuri, mengandung hidrokarbon alifatik dan merupakan paling beracun, misalnya metil merkuri dan etil merkuri.

SUMBER:

Achmad, R. 2004. Kimia Lingkungan. Yogyakarta: ANDI
Alfian, Z. 2006. Merkuri: Antara Manfaat dan Efek Penggunaannya Bagi Kesehatan Manusia dan Lingkungan. [Online]. Avaliable: http://library.usu.ac.id/download/e-book/zul%20alfian.pdf. [7 Mei 2008]
Fardiaz, S. 1992. Polusi Air dan Udara. Yogyakarta:Konisius.
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Rino Safrizal
Jejaring Kimia Updated at: Mei 19, 2015

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