Citric Acid Complexes

Citrate ions are involved in enormous number of complexation reactions. Devoted to this subject literature is so extensive that caimot be adequately covered in this book considering that citric acid forms complexes with almost all known metal ions. Thus, this topic should be covered by a special and separate treatment. Nevertheless, in spite that citrate complexes are not considered in this book, for convenience of the readers in Table 3.8 are compiled a number of references associated with the formation, stability and stmcture of citrate complexes in a solid state and in aqueous solutions. These references will be of help when information about particular metal-citrate systems is desired. Besides, they often include summary of previous works on the subject. There is also a number of reviews that are partially dedicated to formation of citrate complexes of different types [21, 73-78] and tabulations of formation constants [79-85]. 

Citric acid having its hydroxyl group and three carboxylic groups is a multi-dentate ligand able to form quite stable mononuclear and polynuclear complexes with cations of almost all elements [85]. It forms mixed-metal complexes and also mixed-ligand complexes. In some cases it was possible to isolate complexes in a solid form and their crystal stmctures were established. 

Citric acid complexes are soluble in water, but exist also water-insoluble complexes, their speciation and properties are depending on the oxidation state of metal cation, ionic strength, pH and temperature. Citrate complexes were investigated by a variety of experimental methods but evidently the potentiometric titration technique and spectroscopic methods prevailed. Available in the hterature investigations were directed mainly to citrate complexes with alkali metals and alkaline earth metals, Na, K+Ca and Mg % considering that they are major components of natural waters and biological fluids. Other important group of complexes is these with divalent and trivalent metal cations (Cu Ni +, Cu, Ni, Fe % Tl ). 

They were studied in the context of chemical processes occurring in soils and aquatic systems, but also considering their importance in other circumstances. Citric acid as a complexing agent is often used in the separation of actinides, lanthanides and other toxic metals from wastes, sediments and contaminated soils. Citric complexes of molybdenum and other metals are components of electrolytic baths used for electrodeposition and cleaning of corrosion resistant alloys. 

3 Dissociation Equilibria in Solutions with Citrate Ions 

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Manufacture of vitamin C starts with the conversion of sorbitol to L-sorbose. Sorbitol and xyHtol have been used for parenteral nutrition following severe injury, bums, or surgery (246). An iron—sorbitol—citric acid complex is an intramuscular bematinic (247). Mannitol administered intravenously (248) and isosorbide administered orally (249) are osmotic diuretics. Mannitol hexanitrate and isosorbide dinitrate are antianginal dmgs (see Cardiovascular agents). 

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. 

The mechanisms by which Pu(IV) is oxidized in aquatic environments is not entirely clear. At Oak Ridge, laboratory experiments have shown that oxidation occurs when small volumes of unhydrolyzed Pu(IV) species (i.e., Pu(IV) in strong acid solution as a citric acid complex or in 45 percent Na2Coj) are added to large volumes of neutral-to-alkaline solutions(23). In repeated experiments, the ratios of oxidized to reduced species were not reproducible after dilution/hydrolysis, nor did the ratios of the oxidation states come to any equilibrium concentrations after two months of observation. These results indicate that rapid oxidation probably occurs at some step in the hydrolysis of reduced plutonium, but that this oxidation was not experimentally controllable. The subsequent failure of the various experimental solutions to converge to similar high ratios of Pu(V+VI)/Pu(III+IV) demonstrated that the rate of oxidation is extremely slow after Pu(IV) hydrolysis reactions are complete. 

Todorovsky, D. S., Dumanova, D. G., Todorovska, R. V., Getsova, M. M., Preparation and characterization of yttrium-iron citric acid complexes, Croat. Chem. Acta 75, 155-164(2002). 

Fig. 6 presents the data for citric acid complexes and an extraction of ions, plotted as recommended by Fidelis etSiekerski (23),Peppard (24) and Sinha (25), who introduced different names for the deviations from a smooth log/ = f(Z) variation which occur for a quarter, a half and three-quarters of a filled f subshell. Fig. 6 effectively illustrates this accidents for the 4 f 3-4, 4 f7,4 fIdeviations from a smooth variation) is believed to be a thermodynamic consequence of the nephelauxetic effect. 

Iron trivalent, parenteral preparations iron-sorbitol-citric acid complex... 

Parenteral preparations Iron dextran (IMFERON) Iron-sorbitol citric acid complex (JECTOFER). 

Iron sorbitol-citric acid complex Isatidine... 

Dodge CJ, Francis AJ. 1997. Biotransformation of binary and ternary citric acid complexes of iron and uranium. Environ Sci Technol 31(ll) 3062-3067. 

FIGURE 5.2 Single crystal of water-soluble titanium complex ammonium salt (left) citric acid complex (right) lactic acid complex. 

Iron compounds for intramuscular administration are iron sorbitol-citric acid complex (iron sorbitex), iron dextran, iron glycerin-citric acid complex, and iron poljdso-maltose. The work on these formulations is largely old and has been reviewed in previous volumes in this series. 

Iron toxicity can be expected if the amount of free iron released into the plasma exceeds the plasma iron-binding capacity. This is more likely to occur when using iron sorbitol-citric acid complex (iron sorbitex), since the iron is less firmly bound than with iron dextran. Several conditions associated with low iron-binding capacity, such as malnutrition (kwashiorkor, malnutrition syndrome) and previous or simultaneous oral iron therapy appear to predispose to the development of these toxic reactions. In addition, folic acid deficiency has been reported to be a predisposing factor (SED-9, 516), the likely mechanism being altered iron utilization secondary to folic acid deficiency, which results in an increased saturation of ironbinding capacity. 

Urine can turn dark in color in the first 24 hours after parenteral iron injection. The reddish-brown color, which has been observed after the intramuscular injection of iron sorbitol-citric acid complex (SED-8, 515), is due to urinary excretion of part of the iron compound. It has to be distinguished from the black discoloration that may develop if urine of patients who have received iron-sorbitex is allowed to stand, assumed to be due to production of iron sulfide by bacterial growth. Phenomena of this kind are unhkely to occur after the... 

Fig. 7.2. a Schematic of the reduction of chloroplatinic acid by citric acid complex showing the Pt phase separation redox process and subsequent growth of the cluster, b Schematic representation of the hydrogen bonding interaction between the ether oxygens in polyoxiethylene and the surface OH groups, c Correlation of hydrogen evolution rates for Carbowax 20-M with the concentration of MV+ and the ratio of Carbowax 20-M to Pt in the stationary state... 

Roth HP and Kirchgessner M (1985) Utilization of zinc picolinic or citric acid complexes in rdation to dietary protein sources in rats. J Nutr 115 1641 -1649. 

Figure 12.18 V-I characteristics of Citric acid complex of Arabica (specimen size A=1 cm d = 0.3 mm) [34],...

Y. Nakamura. Polymerized complex synthesis of perovskite lead titanate at reduced temperatures Possible formation of heterometallic (Pb,Ti)-citric acid complex. Chem. Mater, 9 451 56, 1997... 

Tile Eu(III)-citric acid complex is reduced at 1/2= -0.89 V SCE and can be used to determine Eu in rare earth phosphates which contain titanium (Ignatova, 1977). Eu(II) and Eu(III) also form complexes with ethylenediamine, inalonic acid, succinic acid, glutaric acid, etc., and yield well-defined waves suitable for analysis. 

Arima M., Kakihana M., Nakamura Y., Yashima M., Yoshimura M. Polymerized conplex route to barium titanate powders using barium-titanium mixed-metal citric acid complex. J. Am. Ceram Soc. 1996 79 2847-2856... 

In the chemical modeling approach, the simultaneous extraction of citric acid and water by alcohols is interpreted by the solvation and hydration processes in the alcoholic phase. In a simplest molecular model, it is assumed that only one predominant citric acid complex (solvate) is formed, and the partition process can be treated as the following chetnieal reaction... 

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