Copper ions, aqueous stability

Van den Berg, C. M. G., and J. R. Kramer (1979), "Conditional Stability Constants for Copper Ions with Ligands in Natural Waters", in E. Jenne, Ed., On Chemical Modeling Speciation, Sorption, Solubility and Kinetics in Aqueous Systems, ACS Symp. Series. 

Type IV includes chiral phases that usually interact with the enantiomeric analytes through the formation of metal complexes. There are usually used to separate amino acid enantiomers. These types of phases are also called ligand exchange phases. The transient diastereomeric complexes are ternary metal complexes between a transitional metal (usually Cu +), an amino acid enantiomeric analyte, and another compound immobilized on the CSP which is able to undergo complexation with the transitional metal (see also the ligand exchange section. Section 22.5). The two enantiomers are separated based on the difference in the stability constant of the two diastereomeric species. The mobile phases used to separate such enantiomeric analytes are usually aqueous solutions of copper (II) salts such as copper sulfate or copper acetate. To modulate the retention, several parameters—such as the pH of the mobile phase, the concentration of the copper ion, or the addition of an organic modifier such as acetonitrile or methanol in the mobile phase—can be varied. 

The last step is dominated by the metallic-copper cluster formation. We assume that in the initial process, all the accessible sulfur sites of PMeT are saturated. Then, in the absence of a stabilizing agent in aqueous solution, the monovalent copper ions undergo disproportionation to produce Cu2+ ions and metallic copper. Additional Cu2+ is then drained from the solution, resulting in an increase in the absolute copper content. 

The latter two reactions proceed via the inner-sphere mechanism (see below), that is, they require access of the substrate to the central Cu(I) ion. The disproportionation reaction requires the contact of the central copper ion with a smface, preferably a Cu°(s) surface, as the formation of a Cu° atom is extremely endothermic due to the lattice energy of copper, - 301.4 kJmol (5). Thus ligands that block sterically the approach of a substrate or of a surface to the central copper ion stabilize it (19). An extreme example is 1,4,5,7.7,8,11,12,14,14-decamethyl-l,4,8,ll-tetraazacyclotetradecane, (27). Thus [Cu(I)L ] is stable even in aerated aqueous solutions (27). In analogy, some enzymes with Cud) as the active site, for example, CuSOD, inhibit disproportionation or the reaction with O2 by inhibiting the approach of two Cu(I) central ions to each other which is required for these reactions which are thermodynamically exothermic. 

Similarly, on the addition of strong ammonia, a base (in water, NH3 -h OH2 NH4 -h OH all four species are present), little by little to a blue aqueous solution of cop-per(II) sulfate, the final soluble species is the complex tetraammine-copper(II) ion its stability is clear from the fact that it is formed by the dissolution [Eq. (2)] of the intermediate solid [Eq. (1)]. This solid is basic copper(II) sulfate, known also as several minerals (brocchantite, lan-gite, wroewulfite) in oxidized sulfide ore zones. The word basic in the name simply reflects the presence of the hydroxide ion. OH , the basic constituent of water. 

The addition of 2 equivalents of copper ions (Cu(N03)2, Cul, or CuBr) to an aqueous solution of (41) + produced a pale blue solution, from which blue crystals were obtained. An x-ray crystal structure analysis indicated a 2 4 assembly of (41) + and Cu, namely, a 4 4 4 complex of (31)" +, CA -and Cu, as shown in Figure 3.11 and designated 42 (Zn2U)4-(CA ")4-[ l-Cu2(OH)2]2 This structure is stabilized by N -Zn + coordination bonds between Zn + and CA , 7t-7t stacking between bpy units, and hydrogen bonds between two CA - function to stabilize (42) 2-. ... 

The Cu complex again oxidizes to Cu + in air. It is interesting fliat oxidation of Cu bound to poly(l-vinylimidazole) is characterized by higher enthalpy and lower entropy compared to the low-molecular-weight imidazole complex. These differences are attributed to the changes in polymer hgand conformation as a result of Cu oxidation to Cu and is evidenced by viscosity measurements. It has been noted that copper ion doping in poly(ihioether) stabilizes the oxidation state of Cu and results in the formation of a Cu+-Cu + system with a potential of 1V in aqueous solution. ... 

Efficient metal (ion) deactivators are based on their ability to form stable complexes with the metal, especially with copper ions. Mainly polyolefins and mbbers need to be stabilized by those metal (ion) deactivators that must resist extraction in aqueous surroundings. An important commercial product is 2, 3-bis[[3-[3,5-di- ert-butyl-4-hydroxyphenyl]propionyl]] pro-pionohydrazide (Irganox MD-1024, BASF). 

The concentration of copper(I) ion remaining ia solution is not appreciable. However, aqueous copper(I) ion can be stabilized by complex formation with various agents such as chloride, ammonia, cyanide, or acetonitrile. 

It is important to recognize some of the limitations of the Pourbaix diagrams. One factor which has an important bearing on the thermodynamics of metal ions in aqueous solutions is the presence of complex ions. For example, in ammoniacal solutions, nickel, cobalt, and copper are present as complex ions which are characterized by their different stabilities from hydrated ions. Thus, the potential-pH diagrams for simple metal-water systems are not directly applicable in these cases. The Pourbaix diagrams relate to 25 °C but, as is known, it is often necessary to implement operation at elevated temperatures to improve reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 °C are no longer valid. 

It has been recognized that sulfur donors aid the stabilization of Cu(i) in aqueous solution (Patterson Holm, 1975). In a substantial study, the Cu(ii)/Cu(i) potentials and self-exchange electron transfer rate constants have been investigated for a number of copper complexes of cyclic poly-thioether ligands (Rorabacher et al., 1983). In all cases, these macrocycles produced the expected stabilization of the Cu(i) ion in aqueous solution. For a range of macrocyclic S4-donor complexes of type... 

The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free metal ions extracted by a perspiration solution [53]. Histidine (5.67) is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such chromogens by the combined effects of perspiration and sunlight. The absorption of histidine by cellophane film from aqueous solution was measured as a function of time of immersion at various pH values. On addition of histidine to an aqueous solution of a copper-complex azo reactive dye, copper-histidine coordination bonds were formed and the stability constants of the species present were determined [54]. Variations of absorption spectra with pH that accompanied coordination of histidine with copper-complex azo dyes in solution were attributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55]. 

The [Cr(en)3]2+ and [Cr(pn)3]2+ salts have reflectance spectra (Table 11) resembling those of the hexaammines, and the six N donor atoms are assumed to complete tetragonally distorted octahedra around the metal. Stability constant measurements (Table 39) have shown that the ions [Cr(en)(aq)]2+ (vmax= 18 300 cm-1, e = 25 dm3 mol-1 cm-1) and [Cr(en)2(aq)]2+ (vma = 17 500 cm-1, e = 17 dm3 mol-1 cm-1) exist in aqueous solution, but that, as in the copper(II) system, the third ethylenediamine molecule is only weakly bound, and care is needed to prevent loss of en from tris(amine) complexes in the preparations. Several bis(amine) complexes, e.g. [CrBr2(en)2], have been isolated, and these are assigned trans structures because of IR spectral resemblances to the corresponding oopper(II) complexes. Since the spectrum of [Cr(S04)(en)2] also shows the presence of bidentate sulfate, this is assigned a trans octahedral structure with bridging anions. 

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