MOLECULAR SIEVE

A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.

Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level. For instance, a water molecule may not be small enough to pass through while the smaller molecules in the gas pass through. Because of this, they often function as a desiccant. A molecular sieve can adsorb water up to 22% of its own weight.[1]

Often they consist of aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons, or synthetic compounds that have open structures through which small molecules, such as nitrogen and water can diffuse.

Molecular sieves are often utilized in the petroleum industry, especially for the purification of gas streams and in the chemistry laboratory for separating compounds and drying reaction starting materials. The mercury content of natural gas is extremely harmful to the aluminum piping and other parts of the liquefaction apparatus - silica gel is used in this case.

Methods for regeneration of molecular sieves include pressure change (as in oxygen concentrators), heating and purging with a carrier gas (as when used in ethanol dehydration), or heating under high vacuum.
Contents

Types of Molecular Sieve :
* 3A (pore size 3 Å): Adsorbs NH3, H2O, (not C2H6), good for drying polar liquids.

* 4A (pore size 4 Å): Adsorbs H2O, CO2, SO2, H2S, C2H4, C2H6, C3H6, EtOH. Will not adsorb C3H8 and higher hydrocarbons. Good for drying nonpolar liquids and gases.

* 5A (pore size 5 Å): Adsorbs normal (linear) hydrocarbons to n-C4H10, alcohols to C4H9OH, mercaptans to C4H9SH. Will not adsorb isocompounds or rings greater than C4.

* 10X (pore size 8 Å): Adsorbs branched hydrocarbons and aromatics. Useful for drying gases.

* 13X (pore size 10 Å): Adsorbs di-n-butylamine (not tri-n-butylamine). Useful for drying HMPA.
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SILICA GEL

Silica gel is a granular, vitreous, highly porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid.

Silica gel is most commonly encountered in everyday life as beads packed in a semi-permeable plastic. In this form, it is used as a desiccant to control local humidity in order to avoid spoilage or degradation of some goods. Because of poisonous dopants (see below) and their very high adsorption of moisture, silica gel packets usually bear warnings for the user not to eat the contents. If consumed, the pure silica gel is unlikely to cause acute or chronic illness, but would be problematic nonetheless. However, some packaged desiccants may include fungicide and/or pesticide poisons. Food-grade desiccant should not include any poisons which would cause long-term harm to humans if consumed in the quantities normally included with the items of food. A chemically similar substance with far greater porosity is aerogel.

Contents
1 History
2 Properties
3 Preparation
4 Applications
4.1 Desiccant
4.2 Chemistry
4.3 Cat litter
5 Hazards
6 References
7 External links



History
The synthetic route for producing silica gel was patented by chemistry professor Walter A. Patrick at Johns Hopkins University, Baltimore, Maryland in 1919. It was used in World War I for the absorption of vapors and gases in gas mask canisters, as part of his patent. The substance was in existence as early as the 1640s as a scientific curiosity.[1]

In World War II, silica gel was indispensable in the war effort for keeping penicillin dry, protecting military equipment from moisture damage, as a fluid cracking catalyst for the production of high octane gasoline, and as a catalyst support for the manufacture of butadiene from ethanol, feedstock for the synthetic rubber program.


Properties
Silica gel's high surface area (around 800 m²/g) allows it to absorb water readily, making it useful as a desiccant (drying agent). Once saturated with water, the gel can be regenerated by heating it to 120 °C (250 °F) for two hours. Some types of silica gel will "pop" when exposed to enough water.


Preparation
A solution of sodium silicate is acidified to produce a gelatinous precipitate that is washed, then dehydrated to produce colorless silica gel.[2] When a visible indication of the moisture content of the silica gel is required, ammonium tetrachlorocobaltate(II) (NH4)2CoCl4 or cobalt chloride CoCl2 is added.[2] This will cause the gel to be blue when dry and pink when hydrated.[2]


Applications

Desiccant
In many items from leather to pepperoni, moisture encourages the growth of mold and spoilage. Condensation may also damage other items like electronics and may speed the decomposition of chemicals, such as those in vitamin pills. By adding packets of silica gel, these items can be preserved longer.

Silica gel may also be used to keep the relative humidity inside a high frequency radio or satellite transmission system waveguide as low as possible. Excessive moisture buildup within a waveguide can cause arcing inside the waveguide itself, damaging the power amplifier feeding it. Also, the beads of water that form and condense inside the waveguide change the characteristic impedance and frequency, impeding the signal. It is common for a small compressed air system (similar to a small home aquarium pump) to be employed to circulate the air inside the waveguide over a jar of silica gel.

Silica gel is also used to dry the air in industrial compressed air systems. Air from the compressor discharge flows through a bed of silica gel beads. The silica gel adsorbs moisture from the air, preventing damage at the point of use of the compressed air due to condensation or moisture. The same system is used to dry the compressed air on railway locomotives, where condensation and ice in the brake air pipes can lead to brake failure.

Silica gel is sometimes used as a preservation tool to control relative humidity in museum and library exhibitions and storage.


Chemistry

Chromatography column In chemistry, silica gel is used in chromatography as a stationary phase. In column chromatography the stationary phase is most often composed of silica gel particles of 40-63 μm. Different particle sizes are used for achieving a desired separation of certain molecular sizes. In this application, due to silica gel's polarity, non-polar components tend to elute before more polar ones, hence the name normal phase chromatography. However, when hydrophobic groups (such as C18 groups) are attached to the silica gel then polar components elute first and the method is referred to as reverse phase chromatography. Silica gel is also applied to aluminum, glass, or plastic sheets for thin layer chromatography.

Chelating groups have also been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous media. Chelating groups can be covalently bound to polyamines that have been grafted onto a silica gel surface producing a material of greater mechanical integrity. Silica gel is also combined with alkali metals to form a M-SG reducing agent.

Silica gel is not thought to biodegrade in either water or soil [3].


Cat litter
Silica gel is also used as cat litter[4], by itself or in combination with more traditional materials, such as clays including bentonite. It is trackless and virtually odorless, albeit expensive. Silica in this form can be a cost effective way for private people to easily purchase silica gel for application in such things as keeping tools rust free in damp environments, long term storage, and preservation of dried food for long term storage.


Hazards
Silica gel is non-toxic, non-flammable, and non-reactive and stable with ordinary usage. It will react with hydrogen fluoride, fluorine, oxygen difluoride, chlorine trifluoride, strong acids, strong bases, and oxidizers[5]. Silica gel is irritating to the respiratory tract, may cause irritation of the digestive tract, and dust from the beads may cause irritation to the skin and eyes, so precautions should be taken [6]. Some of the beads may be doped with a moisture indicator, such as cobalt(II) chloride, which is toxic and may be carcinogenic. Cobalt (II) chloride is deep blue when dry (anhydrous) and pink when moist (hydrated).

Crystalline silica dust can cause silicosis but synthetic amorphous silica gel is non-friable, and so does not cause silicosis.


References
^ Maryann Feldman and Pierre Desrochers (March 2003). "Research Universities and Local Economic Development: Lessons from the History of the Johns Hopkins University". Industry and Innovation 10 (1, 5–24). http://www.rotman.utoronto.ca/feldman/papers/Research%20Universities%20and%20Local%20Economic%20Development.pdf.
^ a b c Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0-7506-3365-4
^ Environmental Health and Safety (2007-09-10). ""Silica Gel"". http://www.jtbaker.com/msds/englishhtml/S1610.htm. Retrieved on 2008-01-12.
^ Andrew Kantor (2004-12-10). ""Non-Tech High Tech Litters the Landscape"". USA Today. http://www.usatoday.com/tech/columnist/andrewkantor/2004-12-10-kantor_x.htm. Retrieved on 2008-03-02.
^ Environmental Health and Safety (2007-09-10). ""Silica Gel"". http://www.jtbaker.com/msds/englishhtml/S1610.htm. Retrieved on 2008-01-12.
^ Fisher Scientific (1997-02-09). ""Silica Gel Dessicant"". http://www.atmos.umd.edu/~russ/MSDS/silicagel28200.html. Retrieved on 2008-01-12.

Source from wikipidia.

MST
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Chelating Agent EDTA

CHELATING AGENTS

GENERAL

Chelation is a chemical combination with a metal in complexes in which the metal is part of a ring. Organic ligand is called chelator or chelating agent, the chelate is a metal complex. The larger number of ring closures to a metal atom is the more stable the compound. This phenomenon is called the chelate effect; it is generally attributed to an increase in the thermodynamic quantity called entropy that accompanies chelation. The stability of a chelate is also related to the number of atoms in the chelate ring. Monodentate ligands which have one coordinating atom like H2O or NH3 are easily broken apart by other chemical processes, whereas polydentate chelators, donating multiple binds to metal ion, provide more stable complexes. Chlorophyll, green plant pigment, is a chelate that consists of a central magnesium atom joined with four complex chelating agent (pyrrole ring). The molecular structure of the chlorophyll is similar to that of the heme bound to proteins to form hemoglobin, except that the latter contains iron(II) ion in the center of the porphyrin. Heme is an iron chelate. Chelation is applied in metal complex chemistry, organic and inorganic chemistry, biochemistry, and environment protection. It is used in chemotherapeutic treatments for metal poisoning. Chelating agents offers a wide range of sequestrants to control metal ions in aqueous systems. By forming stable water soluble complexes with multivalent metal ions, chelating agents prevent undesired interaction by blocking normal reactivity of metal ions. EDTA (ethylenediamine tetraacetate) is a good example of common chelating agent which have nitrogen atoms and short chain carboxylic groups. The sodium salt of EDTA is used as an antidote for metal poisoning, an anticoagulant, and an ingredient in a variety of detergents. Chelating agents are important in the field of soap, detergents, textile dyeing, water softening, metal finishing and plating, pulp and paper, enzyme deactivation, photo chemistry, and bacteriocides.

APPLICATIONS
Photography, Detergent, Chemical plating, Electroplating without cyanide, cleaning agent, plastic additives, printing of cotton and chemical fiber, industrial desulfation, inhibitor for plant growth, printing ink, medicine, paper and food industry. Water treatment chemical, Agriculture

SPECIFICATION

PROPERTY
1. DTPA
2. EDTA
3. NTA

Appearance
1. White powder
2. White powder
3. White to off-white
crystalline powder

Assay
1. 99 wt% min as H5 DPTA
2. 99 wt% as H4 EDTA
3. 98 wt% min as H3 NTA

Chelation Value
1. 2.5 mmol/g
2. 3.39 mmol/g
3. 5.2 mmol/g

pH
1. 2.1-2.5 (saturated sol.)
2. 2.5-3.0 (saturated sol.)
3. 1.7-2.7 (1% aqueous sol.)

Water Solubility
1. 0.5 wt% max at 25°C
2. 0.1 wt% max at 25°C
3. 0.15 wt% max at 25°C


SYNONYMS

DTPA :
Diethylenetriaminepentaacetic acid; Diethylenetriamine-N,N,N',N',N''-pentaacetic acid; Pentetic acid; N,N-Bis(2-(bis-(carboxymethyl)amino)ethyl)-glycine; Diethylenetriamine pentaacetic acid, [[(Carboxymethyl)imino]bis(ethylenenitrilo)]-tetra-acetic acid

EDTA:
Edetic acid; Ethylenedinitrilotetraacetic acid; EDTA, free base; EDTA free acid; Ethylenediamine-N,N,N',N'-tetraacetic acid; Hampene; Versene; N,N'-1,2-Ethane diylbis-(N-(carboxymethyl)glycine); ETHYLENEDIAMINE TETRA-ACETIC ACID

NTA:
N,N-bis(carboxymethyl)glycine; Triglycollamic acid; Trilone A; alpha,alpha',alpha''-trimethylaminetricarboxylic acid; Tri(carboxymethyl)amine; Aminotriacetic acid; Hampshire NTA acid; nitrilo-2,2',2''-triacetic acid; Titriplex i; Nitrilotriacetic acid


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