Chelant

CX-Aminonitriles are compounds containing both cyano and amine substituents attached to the same carbon atom. They are versatile synthetic intermediates that are used to make aminoacids, agrichemicals, chelants, radical initiators, and water-treatment chemicals. In some cases, aminonitriles produced as intermediates are not isolated, but immediately further reacted, for example by hydrolysis, as is the case in producing... 

Chelants at concentrations of 0.1 to 0.2% improve the oxidative stabiUty through the complexation of the trace metal ions, eg, iron, which cataly2e the oxidative processes. Examples of the chelants commonly used are pentasodium diethylenetriarninepentaacetic acid (DTPA), tetrasodium ethylenediarninetetraacetic acid (EDTA), sodium etidronate (EHDP), and citric acid. Magnesium siUcate, formed in wet soap through the reaction of magnesium and siUcate ions, is another chelant commonly used in simple soap bars. 

In lower pressure boilers a variety of additional treatments may be appropriate, particularly if the steam is used in chemical process or other nonturbine appHcation. Chelants and sludge conditioners are employed to condition scale and enable the use of less pure feedwater. When the dmm pressure is less than 7 MPa (1015 psia), sodium sulfite may be added direcdy to the boiler water as an oxygen scavenger. It has minimal effect on the oxygen concentration in the system before the boiler. 

Scale and deposits are controlled through the use of phosphates, chelants, and polymers. Phosphates are precipitating treatments, and chelants are solubilizing treatments. Polymers are most widely used to disperse particulates but they are also used to solubilize contaminants under certain conditions. 

Chelant Control. Chelants are the prime additives in a solubilizing boiler water treatment program. Chelants have the abihty to complex many cations (hardness and heavy metals under boiler water conditions). They accomplish this by locking metals into a soluble organic ring stmcture. The chelated cations do not deposit in the boiler. When apphed with a dispersant, chelants produce clean waterside surfaces. 

A chelant—polymer combination is an effective approach to controlling iron oxide. Adequate chelant is fed to complex hardness and soluble iron, with a slight excess to solubilize iron contamination. Polymers are then added to condition and disperse any remaining iron oxide contamination. 

A chelant—polymer program can produce clean waterside surfaces, contributing to much more rehable boiler operation. Out-of-service boiler cleaning schedules can be extended and, in some cases, eliminated. This depends on operational control and feed-water quaUty. Chelants with high complexing stabiUties are "forgiving" treatments they can remove deposits that form when feed-water quaUty or treatment control periodically deviates from standard. 

Phosphate—Chelant—Polymer Combinations. Combinations of polymer, phosphate, and chelant are commonly used to produce results comparable to chelant—polymer treatment in boilers operating at 4137 x 10 Pa or less. Boiler cleanliness is improved over phosphate treatment, and the presence of phosphate provides an easy means of testing to confirm the presence of treatment in the boiler water. 

Polymer-only Treatment. Polymer-only treatment programs are also used with a degree of success. In this treatment, the polymer is usually used as a weak chelant to complex the feed-water hardness. These treatments are most successful when feed-water hardness is consistently low. 

Clean boiler water surfaces reduce potential concentration sites for caustic. Deposit control treatment programs, such as those based on chelants and synthetic polymers, can help provide clean surfaces. 

If deposits are minimized, the areas where caustic can be concentrated is reduced. To minimize the iron deposition in 6.895-12.07 x 10 Pa boilers, specific polymers have been designed to disperse the iron and keep it in the bulk water. As with phosphate precipitation and chelant control programs, the use of these polymers with coordinated phosphate—pH treatment improves deposit control. 

Chelation is an equilibrium system involving the chelant, the metal, and the chelate. Equilibrium constants of chelation are usually orders of magnitude greater than are those involving the complexation of metal atoms by molecules having only one donor atom. 

Eig. 1. Types of chelates where (1) represents a tetracoordinate metal having the bidentate chelant ethylenediamine and monodentate water (2), a hexacoordinate metal bound to two diethylenetriamines, tridentate chelants (3), a hexacoordinate metal having triethylenetetramine, a tetradentate chelant, and monodentate water and (4), a porphine chelate. The dashed lines iadicate coordinate bonds. 

Table 1 Hsts a number of chelating agents, grouped according to recognized stmctural classes. Because systematic nomenclature of chelating agents is frequently cumbersome, chelants are commonly referred to by common names and abbreviations. For the macrocyclic complexing agents, special systems of abbreviated nomenclature have been devised and are widely used. Some of the donor atoms involved ia chelation and the many forms ia which they can occur have been reviewed (5).

The dashed lines ia Figure 4 are plots of equation 22 for Cu " and Mn and iadicate the concentration of the aquo metal ions ia equiUbrium with the sohd hydroxides as function of pH. At any pH where the soHd curve is above the dashed line for the same metal, the EDTA is holding the unchelated metal ion concentration at a value too low for the precipitation of the sohd hydroxide. Relatively large quantities of the metal can thus be maintained ia solution as the chelate at pH values where otherwise all but trace quantities of the metal would be precipitated. In Eigure 4, this corresponds to pH values where pM of the dashed curves is 4 or greater. At the pH of iatersection of the sohd and dashed lines for the same metal, the free metal ion is ia equihbrium with both the sohd hydroxide and the chelate. At higher pH the hydroxyl ion competes more effectively than the chelant for the metal, and only a trace of either the chelate or the aquo metal ion can exist ia solution. Any excess metal is present as sohd hydroxide. 

The more stable the chelate, the higher the pM that it can maintain, and the higher the pH required to precipitate the metal hydroxide. From equation 22 it can be seen that the smaller the solubihty product ie, the more iusoluble the metal hydroxide, the higher the pM that a chelant must maintain to prevent precipitation. The stabiUty constant of the Fe(III)—EHPG complex (/2), is so large (10 ) that iron is not precipitated even ia strongly alkaline solutions. 

Fig. 6. Base titration of H A chelant A, free acid without coordinating metal B, in the presence of a metal of intermediate coordinate strength and C, in...

Another group of chelants that form stable chelates at high pH because of metal—alkoxide coordination are the sugar acids, such as gluconic acid [526-95-4] (1 )- Utility for this group is found in high alkalinity botde washes and other cleansers (19). 

The first two terms of the right-hand side of the equation are sometimes combined and expressed as E which is called the standard oxidation potential for the chelate system. If the chelation is strong and the ligand is in excess, the metal would be almost entirely in the chelated forms, and [M L] and [M g L] would essentially be equal to the total concentrations of the oxidized and reduced forms of the metal. If, as is usual, the oxidized form is the more strongly chelated K > ), the oxidation potential of a system is increased by the addition of the chelant. 

By buffering the metal ion concentration using a chelant, E can be adjusted to and stabilized at values that give desirable properties to the deposit. Selective buffering can sequester the properties of interfering ions or can be used to regulate the potentials of two or more ions to approximately the same value in order to effect codeposition. 

Three features of chelation chemistry are fundamental to most of the appHcations of the chelating agents. The first and probably the most extensively used feature is the control of free metal ion concentration by means of the binding—dissociation equiUbria. The second, often called the preparative feature, is that in which the special properties of the chelate itself provide the basis of the appHcation. The third feature comprises displacement reactions metal by other metal ions, chelant by chelant, and chelant by other ligands or ions. An appHcation may be termed defensive if an undesirable property in a process or product is mitigated, or aggressive if a new and beneficial property is induced. 

Buffering. If addition or removal of an appreciable amount of a metal ion produces only a relatively small change in the concentration of that ion is a solution, the solution is buffered with respect to the ion. Metal ions are buffered by chelants of various strengths, ie, stabiHty constants, in a manner exactiy analogous to the buffering of hydrogen ions by bases of various strengths. 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

DispEcement. In many of the appHcations of chelating agents, the overall effect appears to be a displacement reaction, although the mechanism probably comprises dissociations and recombinations. The basis for many analytical titrations is the displacement of hydrogen ions by a metal, and the displacement of metal by hydrogen ions or other metal ions is a step in metal recovery processes. Some analytical pM indicators function by changing color as one chelant is displaced from its metal by another. 

The pH effect in chelation is utilized to Hberate metals from thein chelates that have participated in another stage of a process, so that the metal or chelant or both can be separately recovered. Hydrogen ion at low pH displaces copper, eg, which is recovered from the acid bath by electrolysis while the hydrogen form of the chelant is recycled (43). Precipitation of the displaced metal by anions such as oxalate as the pH is lowered (Fig. 4) is utilized in separations of rare earths. Metals can also be displaced as insoluble salts or hydroxides in high pH domains where the pM that can be maintained by the chelate is less than that allowed by the insoluble species (Fig. 3). 

The calcium form of EDTA instead of free EDTA is used in many food preparations to stabilize against such deleterious effects as rancidity, loss of ascorbic acid, loss of flavor, development of cloudiness, and discoloration. The causative metal ions are sequestered by displacing calcium from the chelate, and possible problems, such as depletion of body calcium from ingestion of any excess of the free chelant, had it been used, are avoided. 

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