Citric acid, formation constants with

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]. 

We have considered typical examples of lanthanide and actinide solvent extraction by chelate formation, involving complexes with citric acid and with TTA, to prove that the labelling of a stable element by one of its radioactive isotopes can help to produce accurate data on the stability constants for complex formation. The method is applicable to elements with radioisotopes having a half-life allowing an ion concentration of 10 6m or less. Other methods of partition such as radiopolarography and radio-coulometry also result in accurate thermodynamical data when the same procedure of labelling is used (29). 

Also present in many natural waters are humic/fulvic acid, citric acid, and the like. These organics also can complex actinides. In Figure 15.18, we show the relative stability constants for the first complexation reaction of various ligands with actinides of different oxidation states. Clearly, the carbonate and humate ions along with hydrolysis dominate the chemistry. The tetravalent actinide ions will tend toward hydrolysis reactions or carbonate complexation rather than humate/fulvate formation. 

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