Increasing Solubility with Complexing Agent

The key to the high solubility of the copper ions in the NH4OH slurry is the complexing action of the dissolved NH, gas that is prevalent in aqueous solutions of NH4OH. In pure water, copper ions are surrounded by polar water molecules which form coordination bonds with the ion. The solubility of copper in pure water is determined by the ability of the water molecules to shield the charge on a given copper ion firom other copper ions in solution. Complexing of copper occurs with NHj because the polar 

For NH4OH whether the slurry is flowing or not, the difference in polish rate is negligible because the NHj complexes the abraded copper entering the slurry. In both the 1 vol% NH4OH and 1.4 wt% NH4NO3 slurries, we hypothesize that the abraded copper is first dissolved as Cu and then complexed to Cu(NHj)2 Thus, dissolution of the abraded material occurs by the following sequence  

The rate of dissolution, i.e., the rate that reaction (7.2) proceeds to the right, depends upon the concentration of Chi, not the total dissolved copper concentration. Complexing, which converts Cu to Cu(NH3)2, increases the dissolution rate by lowering the CIu ion concentration. As discussed in Section 7.3.3, reaction (7.2) appears to be the rate limiting step. 

The difference between die two slurries in polish rate behavior with slurry flow is a result of the degree of copper complexing obtained in each slurry. The ratio of ion concentration to Cu(NH3)2 ion concentration at equilibrium may be determined from the equilibrium constant for reaction (7.2)  

A similar analysis for the NH4NO3 sliury gives pH = 4.7, [OHl = 5.0x10  

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Complex Ion Formation. Phosphates form water-soluble complex ions with metallic cations, a phenomenon commonly called sequestration. In contrast to many complexing agents, polyphosphates are nonspecific and form soluble, charged complexes with virtually all metallic cations. Alkali metals are weakly complexed, but alkaline-earth and transition metals form more strongly associated complexes (eg, eq. 16). Quaternary ammonium ions are complexed Htde if at all because of their low charge density. The amount of metal ion that can be sequestered by polyphosphates generally increases... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Yang and De Villiers (2005) found that both 4-sulfonato-carf]afenes and HP-ft-CD can form complexes with niclosamide, and the increase in solubility by both agents was additive. The greatest increase in the aqueous solubility of niclosamide was observed when both agents were used together. 

The highly water-soluble 2-hydroxypropyl-/i-cyclodextrin (2-HP-/1-CD) is a commercially useful general complexing agent. Inclusion complexes of poorly water-soluble Naproxen with 2-HP-/1-CD were useful to increase its solubility and dissolution rate, and resulted in an enhancement of bio-availability and minimized the gastrointestinal toxicity of the drug [69]. The water solubility of melatonin, which is an indole amide neurohormone, was also enhanced in a complex with 2-HP-/J-CD [70]. 

The choice of mobile phase is largely empirical but general rules can be formulated. A mixture of an organic solvent and water with the addition of acid, base or complexing agent to optimize the solubility of the components of a mixture can be used. For example, good separations of polar or ionic solutes can be achieved with a mixture of water and n-butanol. Addition of acetic acid to the mixture allows more water to be incorporated and increases the solubility of basic materials, whilst the addition of ammonia... 

Other factors also affect the mobility of uranium in soil. A field study performed near an active carbonate leach uranium mill showed that uranium in an alkali matrix can migrate to the groundwater (Dreesen et al. 1982). Uranium mobility may also be increased due to the formation of soluble complexes with chelating agents produced by microorganisms in the soil (Premuzie et al. 1995). 

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