Metal citrates

The nature of the highly dispersed oxides obtainable from the decomposition of metal salts of hydroxypolycarboxylic acids has been discussed by Marcilly et al. [285]. 

---

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. 

Stabihty constants are measured at thek optimum pH. Conditional stabiUty constants are measured at a specific pH. In general, stabiUty constants for metal citrates are very low below pH 2—3, high at pH 3—10, and low above pH 10—12. 

Joshi-Tope G, AJ Francis (1995) Mechanisms of biodegradation of metal-citrate complexes by Pseudomonas fluorescens. J Bacterial 177 1989-1993. 

Uptake of small organic metal complexes over transport systems of organic metabolites may be possible, for example, of small organic acids like citrate or amino acids. However, only few examples of such processes have been studied so far. Increased uptake of cadmium by an alga has been observed in the presence of citrate and has been attributed to accidental transport of the metal-citrate complex over a citrate transporter [212]. Transport systems of inorganic anions may also play a role in metal transport. Silver uptake by algae was enhanced in the presence of thiosulfate. In this case, the silver thiosulfate complex was transported over a sulfate uptake system [213]. It remains to be demonstrated how widespread these processes may be for metal uptake in the aquatic environment [12]. 

Brookhaven National Laboratory s (BNL s) biochemical recovery of radionuclides and heavy metals is a patented biochemical recovery process for the removal of metals and radionuclides from contaminated minerals, soil, and waste sites. In this process, citric acid, a naturally occurring organic complexing agent, is used to extract metals and radionuclides from solid wastes by the formation of water-soluble, metal-citrate complexes. The complex-rich extract is then subjected to microbiological biodegradation that removes most of the extracted heavy metals. 

IV.39 CITRATES, C6H50 " Solubility Citric acid, HOOC.CH2.C(OH)-CO2H.CH2.COOH.H2O or H3.CgH5O7.H2O is a crystalline solid which is very soluble in water it becomes anhydrous at 55° and melts at 160°. It is a tribasic acid, and therefore gives rise to three series of salts. The normal citrates of the alkali metals dissolve readily in water other metallic citrates are sparingly soluble. The acid citrates are more soluble than the acid tartrates. 

An impure hydrosol of copper was first prepared by Lottermoser 2 as an adsorption-compound with stannic oxide by heating a slightly alkaline solution of a cupric salt with a similar solution of stannous chloride in presence of an alkali-metal citrate or tartrate. With water the black precipitate yields a reddish-brown liquid, rapidly oxidized by atmospheric oxygen, with production of a greenish coloration which soon becomes yellow. 

Chemical models of metal-citrate aqueous complexation 382... 

CHEMICAL MODELS OF METAL-CITRATE AQUEOUS COMPLEXATION... 

The possible combinations and stoichiometries of AP" " or Fc +, cit -, H+, and OH that may occur in a metal-citrate species are numerous. In general, the formation of soluble metal—citrate complexes can be described by... 

Species are denoted by the metal, citrate, and proton or hydroxide stoichiometry of each complex mnj), as delined in Eq. (10.7). 

Jordan, S.L., I.R. McDonald, A.J. Kraczkiewicz-Dowjat, D.P. Kelly, F.A. Rainey, J-.C. Murrell, and A.P. Wood. 1997. Autotrophic growth on carbon disulfide is a property of novel strains of Paracoccus denitrificans. Arch. Microbiol. 168 225-236. Joshi-Tope, G. and A.J. Francis. 1995. Mechanisms of biodegradation of metal-citrate complexes by Pseudomonas fluorescens. J. Bacteriol. 177 1989-1993. 

Krom bp, Warner JB, Konings WN and Lolkema J S (2000) Complementary metal ion specificity of the metal-citrate transporters CitM and CitH of Bacillus subtilis. ] Bacteriol 182 6374—6381. 

---