Citrate chelation

Chelation is an equiUbrium reaction. There are always some free-metal ions present as well as chelated metal ions. In a system where a metal salt is being reduced, such as in metal plating, the rate of the reaction forming the metal can be controlled by using the metal citrate chelate. 

A clear solution of aluminum citrate neutralized to pH 7 is used for in situ gelling of polymers in polymer flooding and well stimulation in enhanced oil recovery techniques (128—132). The citrate chelate maintains aluminum ion solubiUty and controls the rate of release of the aluminum cross-linker. 

There is a direct correlation between the concentration of diffusible Ca (and diffusible Mg) and the concentration of diffusible citrate (Figure 5.6b) this correlation, which is very good at constant pH, exists because citrate chelates Ca2 + more strongly than phosphate to form soluble unionized salts. 

Sodium dithionite reduction, sodium citrate chelation, determination by atomic absorption spectrometry 23,24, ... 

Sodium citrate solution, at a concentration of 34 to 38 g/L in a ratio of 1 part to 9 parts of blood, is widely used for coagulation studies because the effect is easily reversible by addition of Ca. Because citrate chelates calcium, it is unsuitable as an anticoagulant for specimens for measurement of this element. It also inhibits aminotransferases and alkaline phosphatase but stimulates acid phosphatase when phenylphosphate is used as a substrate. Because citrate complexes molybdate, it decreases the color yield in phosphate measurements that involve molybdate ions and produces low results. 

As indicated above, citrate influences both metal and ligand chemistry in the rhizosphere via aqueous complexation reactions and reactions at the solid-solution interface. Citrate is an important biomolecule in both plant and animal systems, particularly with respect to Fe(III) bioavailability and internal transport and in the amelioration of Al toxicity (Jones, 1998). Thus, this ligand has received considerable study. Yet despite this importance and relationship to citrate chelation capabilities, the aqueous Fe(III)-and Al-citrate complexation chemistry remains ambiguous. Consequently, in this chapter we focus on an examination of citrate aqueous metal complexation, with specific emphasis on the complexation of Fe(III) and Al. 

The application of well-dispersed nickel nanoparticles inside the mesopores of MCM-41 by use of a nickel citrate chelate as precursor... 

Crocidolite, Closed circular In vitro DNA single 32 % DNA damage induced by crocido- Aust et al. Citrate-chelated... 

Citrate-chelated iron, reduced by ascorbic acid to Fe acts as a Fenton catalyst to generate HO which induces DNA singlestrand breaks. 

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

Also reported as commercial products are selenium amino acid chelate, selenium ascorbate, selenium aspartate, and selenium citrate triturations, as well as selenium dibromide, selenium dichloride, selenium lysinate, and selenium sulfide bentonite. 

Ethylenediamine tetraacetic acid (EDTA) [60-00-4] (Sequestrene), an anticoagulent at 1 mg of the disodium salt per mL blood, complexes with and removes calcium, Ca ", from the blood. Oxalate, citrate, and fluoride ions form insoluble salts with Ca " and chelate calcium from the blood. Salts containing these anticoagulants include lithium oxalate [553-91-3] 1 mg/mL blood sodium oxalate [62-76-0]2 mg/mL blood ... 

The Ca(Il) coaceatratioa ia blood is closely coatroUed aormal values He betweea 2.1 and 2.6 mmol/L (8.5—10.4 mg/dL) of semm (21). The free calcium ion concentration is near 1.2 mmol/L the rest is chelated with blood proteias or, to a lesser extent, with citrate. It is the free Ca(Il) ia the semm that determines the calcium balance with the tissues. The mineral phase of bone is essentially ia chemical equiUbrium with calcium and phosphate ions present ia blood semm, and bone cells can easily promote either the deposition or dissolution of the mineral phase by localized changes ia pH or chelating... 

Chelate Formation. Citric acid complexes with many multivalent metal ions to form chelates (9,10). This important chemical property makes citric acid and citrates useful in controlling metal contamination that can affect the color, stabiUty, or appearance of a product or the efficiency of a process. 

When a metal ion is chelated by a ligand such as citric acid, it is no longer free to undergo many of its chemical reactions. A metal ion that is normally colored may, in the presence of citrate, have Httie or no color. Under pH conditions that may precipitate a metal hydroxide, the citrate complex may be soluble. Organic molecules that are catalyticaHy decomposed in the presence of metal ions can be made stable by chelating the metal ions with citric acid. 

Medical Uses. Citric acid and citrate salts are used to buffer a wide range of pharmaceuticals at their optimum pH for stabiUty and effectiveness (65—74). Effervescent formulations use citric acid and bicarbonate to provide rapid dissolution of active ingredients and improve palatabiUty. Citrates are used to chelate trace metal ions, preventing degradation of ingredients. Citrates are used to prevent the coagulation of both human and animal blood in plasma and blood fractionation. Calcium and ferric ammonium citrates are used in mineral supplements. 

Hard Surface Cleaners. Citric acid and sodium citrate are used in hard surface cleaners as an acid and chelator for dissolving hard water deposits and as a builder to increase the efficacy of the surfactants. 

Agricultural Use. Citric acid and its ammonium salts are used to form soluble chelates of iron, copper, magnesium, manganese, and zinc micronutrients in Hquid fertilizers (97—103). Citric acid and citrate salts are used in animal feeds to form soluble, easily digestible chelates of essential metal nutrients, enhance feed flavor to increase food uptake, control gastric pH and improve feed efficiency. 

Citric acid is used as a chelating agent in catalyst systems for making resins, and citrate esters are used as plasticizers (qv) in PVC film, especially in food packaging (207). 

Ferric ammonium citrate [1185-57-5]—A mixture of complex chelates prepared by the iateraction of ferric hydroxide with citric acid ia the presence of ammonia. The chelates occur ia brown and green forms, are dehquescent ia air, and are reducible by light. 

Fig. 4.1.8 Influence of various calcium chelators on the relationship between Ca2 " concentration and the luminescence intensity of aequorin, at 23-25°C (panel A) in low-ionic strength buffers (I < 0.005) and (panel B) with 150 mM KC1 added. Buffer solutions (3 ml) of various Ca2+ concentrations, pH 7.05, made with or without a calcium buffer was added to 2 pi of 10 pM aequorin solution containing 10 pM EDTA. The calcium buffer was composed of the free form of a chelator (1 or 2mM) and various concentrations of the Ca2+-chelator (1 1) complex to set the Ca2+ concentrations (the concentration of free chelator was constant at all Ca2+ concentrations). The curves shown are obtained with 1 mM MOPS (A), 1 mM gly-cylglycine ( + ), 1 mM citrate (o), 1 mM EDTA plus 2mM MOPS ( ), 1 mM EGTA plus 2 mM MOPS ( ), 2 mM NTA plus 2 mM MOPS (V), and 2 mM ADA plus 2 mM MOPS (A). In the chelator-free buffers, MOPS and glycylglycine, Ca2+ concentrations were set by the concentration of calcium acetate. Reproduced with permission, from Shimomura and Shimomura, 1984. the Biochemical Society.

Citrate conformation and chelation enzymatic implications. J. P. Glusker, Acc. Chem. Res., 1980,... 

Partially hydrolyzed polyacrylamides, carboxymethylcellulose, polysaccharides, and acrylamido methylpropane sulfonate have been screened to investigate the performance of aluminum citrate as a chelate-type crosslinker. An overview of the performance of 18 different polymers has been presented in the literature [1646]. The performance of the colloidal dispersion gels depends strongly on the type and the quality of the polymer used. The gels were mixed with the polymers at two polymer concentrations, at three polymer-to-aluminum ratios, and in different concentrations of potassium chloride. The gels were quantitatively tested 1,7, 14, and 28 days after preparation. 

Boron zirconimn chelates from ammonium hydroxide water-soluble amines sodium or potassimn zirconium and organic acid salts such as lactates, citrates, tartrates, glycolates, malates, gluconates, glycerates, and mandelates with polyols such as glycerol, erythritol, arabitol, xylitol, sorbitol, dulcitol, mannitol, inositol, monosaccharides, and disaccharides [463,464,1592,1593]... 

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