Organic chelates, molecular weight

Sca.Ie nd Sta.in Controllers. Polyacrjiates (low molecular weight) and organic phosphonates, eg, (l-hydroxyethyhdene)diphosphonic acid, prevent or control precipitation of CaCO by acting as chelating agents (qv) or dispersants (qv) to prevent excessive formation of hard scale by promoting crystal distortion. 

The literature of polyimines is extensive [164-173]. A number of researchers have tried to synthesize high molecular weight polymers but failed due to poor solubility in organic solvents. Polyimines are of great interest because of their high thermal stability [174-176], ability to form metal chelates [174-177], and their semiconducting properties [178-181]. Due to insolubility and infusibility, which impeded characterization of the molecular structure, the application of these polymers is very limited and of little commercial importance. 

In the rhizosphere, microorganisms utilize either organic acids or phytosiderophores to transport iron or produce their own low-molecular-weight metal chelators, called siderophores. There are a wide variety of siderophores in nature and some of them have now been identified and chemically purified (54). Pre.sently, three general mechanisms are recognized for utilization of these compounds by microorganisms. These include a shuttle mechanism in which chelators deliver iron to a reductase on the cell surface, direct uptake of metallated siderophores with destructive hydrolysis of the chelator inside the cell, and direct uptake followed by reductive removal of iron and resecretion of the chelator (for reviews, see Refs. 29 and 54). 

Free metal ions Inorganic ion pairs complexes Organic complexes chelates Metals species bound to high molecular weight DOM Metals in colloids Metals sorbed onto colloids Precipitates... 

Type 3 metal complexes involve the physical interaction of a metal complex, chelates, or metal cluster with an organic polymer or inorganic high molecular weight compound. The preparation of type 3 compounds differs from those of type 1 and type 2, as they are ultimately achieved through the use of adsorption, deposition by evaporation, microencapsulation, and various other methods. 

Microbes acquire iron by utilizing very specific, low molecular weight iron chelators called siderophores. The resistance of bacteria to previously effective antibiotics can be circumvented in part by covalent coupling of antibiotics to siderophores. Some synthetic siderophores have been found to possess significant antibacterial activity themselves. They have also been found to have considerable potential as nontoxic, organ selective MRI contrast agents (27). 

GC has been used extensively for the separation and determination of volatile organic molecules, and most aspects of this application area are fully documented in monographs on this technique. In the inorganic trace analysis area, however, fewer species possess the required volatility, and applications tend to be limited to the separation of volatile species of low molecular weight (such as methyl derivatives of As, Se, Sn, Hg) and the separation of semi-volatile organo-metals, metal halides, metal hydrides, metal carbonyls and metal chelates. For organo-metal species, the type of detection system required varies with the nature of the analyte, and the options include electron capture detection, flame photometric detection (sometimes ICP), AAS and MS. 

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