Cation-Selective Reagents for Conservation Treatments
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CATION-SELECTIVE REAGENTS FOR CONSERVATION TREATMENTS DUANE R. CHARTIER ConservArt Associates, 826 North Sweetzer Avenue, Los Angeles, CA 90069 ABSTRACT Crown ethers, cryptands, polyphenols, and several other reagents are known to have high specificity for various cationic species. These reagents have great potential for application in general conservation problems of selective cleaning, desalination, stain removal and reversal of treatments in conservation. They are remarkably more effective in metal ion complexation than the more common chelates (usually only EDTA is used for conservation applications) which are used generally with little regard for selectivity. The advantages of a selective reagent are both its minimal effect on other species and its maximal cost-effectiveness. Most of the macrocyclic chelates are noutral sp • dýarp lipophilic and solubilize metal ions (of the correct size) in non-aqueous media as well as water. Therefore, polar organic solvents may be used for water-sensitive treatments. Research has been designed to assess the use of cationselective materials both in the laboratory and in situ. The potential of these compounds to expand the range and the safe execution of existing and new conservation treatments is excellent. BACKGROUND To improve the efficiency of chemical reactions (to reduce non-productive side reactions) and to minimize damaging secondary effects, the selectivity of reagents for any target species must be high. Standard means to achieve particular products or results in a chemical reaction usually only involve forcing the equilibrium towards products by mass balance or improving the kinetics by catalysis, thermal controls, and other means. Highly selective reagents have only become industrially important in the last 15 to 20 years, especially with the advent of biotechnologies. Nature provides stunning examples of complex and highly efficient reaction systems. There is a very high degree of chemical/physical selectivity in most biochemical reaction systems, especially in enzyme reactions. Varying degrees of selectivity for cations are achieved by several types of reagents. Unfortunately, there are very few inorganic materials that are cation-selective. There has been some success in Cs+ complexation with potassium hexacyanocobalt(II) ferrate(Il) [1] as well as some ion scavenging in other systems such as zirconium phosphate [2]. In marked contrast, there are many naturally occurring organic molecules that are extremely effective and selective in cation complexation. These molecules are mostly chelates. A chelate is a ligand that forms heterocyclic rings by multiple attachment to a metal ion. The resulting metal complex is stable and usually isolable. The term "chelate" was first used by Morgan and Drew in 1920 [3] and derives from the Greek Mat. Res. Soc. Symp. Proc. Vol. 185. @1991 Materials Research Society
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root "chela" meaning claw. However, the essence of chelation was already well realized by Werner in his 1893 work on Ptethylenediamine complexes [4]. The so-cal
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