Citric acid metal complexes

Table 3 Some Recently Determined Log Formation Constants of Metal-Citric Acid (HjL) 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]. 

Maslowska J, Bielawski M, Baranowska A (1985) Thermoanalytical investigation of citric acid and complexe salts of transition metals with citric acid. Thermochim Acta 92 235-239 Wu S, Chang Z, Wang K, Xiong W (1995) Preparation and thermal behaviour of rare earth citrate hydrates. J Therm Anal 45 199-206... 

Certain compounds, known as chelating agents (qv), react synergisticaHy with many antioxidants. It is beheved that these compounds improve the functional abiUties of antioxidants by complexing the metal ions that often initiate free-radical formation. Citric acid and ethylenediaminetetraacetic acid [60-00-4] (EDTA), C2QH2gN20g, are the most common chelating agents used (22). 

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. 

Complexing agents, which act as buffers to help control the pH and maintain control over the free metal—salt ions available to the solution and hence the ion concentration, include citric acid, sodium citrate, and sodium acetate potassium tartrate ammonium chloride. Stabilizers, which act as catalytic inhibitors that retard the spontaneous decomposition of the bath, include fluoride compounds thiourea, sodium cyanide, and urea. Stabilizers are typically not present in amounts exceeding 10 ppm. The pH of the bath is adjusted. 

Inorganic heavy metals are usually removed from aqueous waste streams by chemical precipitation in various forms (carbonates, hydroxides, sulfide) at different pH values. The solubiUty curves for various metal hydroxides, when they are present alone, are shown in Figure 7. The presence of other metals and complexing agents (ammonia, citric acid, EDTA, etc) strongly affects these solubiUty curves and requires careful evaluation to determine the residual concentration values after treatment (see Table 9) (38,39). 

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. 

Citric acid is utilized in a large variety of food and industrial appHcations because of its unique combination of properties. It is used as an acid to adjust pH, a buffer to control or maintain pH, a chelator to form stable complexes with multivalent metal ions, and a dispersing agent to stabilize emulsions and other multiphase systems (see Dispersants). In addition, it has a pleasant, clean, tart taste making it useful in food and beverage products. 

Example. The Pechini method for fuel cell electrode preparation. La, Ba, Mn niU ates - - CgHgO — citrate complex - - C2FI6O2 — gel. Metal nitrates are complexed with citric acid, and then heated with ethylene glycol to form a transparent gel. This is then heated to 600 K to decompose the organic content and then to temperatures between 1000 and 1300K to produce tire oxide powder. The oxide materials prepared from the liquid metal-organic procedures usually have a more uniform particle size, and under the best circumstances, this can be less than one micron. Hence these particles are much more easily sintered at lower temperatures than for the powders produced by tire other methods. 

For generation of acidic conditions, a non-volatile acid such as citric acid (12.7), or an acid donor such as ammonium tartrate (12.8) or ammonium sulphate, is preferred. An acid or acid donor is not used with 1 2 metal-complex dyes of high neutral-dyeing affinity,... 

Synergy between primary and secondary anti-oxidants occurs and often a mixture is employed. Also included are metal complexing agents, e.g., EDTA (ethylenediaminetetraacetic acid), citric acid, the purpose of which is to deactivate extraneous metal ions that catalyse polymer oxidation. 

A common form of EDTA used as a preservative is calcium disodium EDTA (CaNa2EDTA). What metals will this form of the sequestrant scavenge effectively The dissolution of the solid will yield calcium ions, sodium ions, and the EDTA anion. Any metal more effectively complexed than calcium will be readily scavenged, including all ions listed in Table 9.1 except silver (Ag+) and magnesium (Mg2+). (In the absence of the calcium counterion, as in the case of the acid form of EDTA, chelation of calcium in the body can occur. In fact, EDTA administered orally is an FDA-approved treatment for calcium deposits in the bloodstream that lead to cardiovascular disease.) Citric acid (Fig. 9.3.3) is another sequestrant of metal ions in foodstuffs. 

Y3Fe50i2 [7], These complexes have been obtained at 110-120°C, isolated, and studied in the crystalline form and in an aqueous solution. IR spectra suggest that these are monodentate coordinated to the metal ions. NMR data are in accord with an assumption that hydroxy and carboxylate groups attached to the central carbon atom of the citric acid anion are coordinated to the metal cations forming a stable five-membered ring. However, these important data are obtained at conditions that do not correspond to the conditions of the synthesis of precursors. 

The removal of Pb by Brevibacterium sp strain PBZ was markedly enhanced by the presence of glucose (Simine et al. 1998). Desorption of the metal by EDTA restored the binding capacity of the cells. U(VI) could be desorped from the cell surface of B. cereus by citric acid or sodium bicarbonate with the formation of water-soluble complexes although U(VI) was strongly bound on the cell surface of the bacteria. However, uranyl in... 

When using a scrubber, an additional acid treatment must be implemented, preferably afterward. If an HF bath is used prior to the scrubber to improve global particle removal [24], the metallic cleanliness obtained can be preserved if a complexing agent is used in the scrubber process—EDTA in water or choline instead of ammonia. The acid treatment can be introduced in the second scrubber station citric acid or HF. In the latter case, the scrubber has to be especially designed for corrosion and security reasons. 

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