Metal ion-free

Chemical Variety. The term species refers to the actual form in which a molecule or ion is present in solution. Eor example, a metal ion may occur in natural waters, as a free metal ion, ie, an aquo complex Me(H20), an inorganic or organic complex, and it may be present in dissolved or... 

The dashed lines ia Figure 4 are plots of equation 22 for Cu " and Mn and iadicate the concentration of the aquo metal ions ia equiUbrium with the sohd hydroxides as function of pH. At any pH where the soHd curve is above the dashed line for the same metal, the EDTA is holding the unchelated metal ion concentration at a value too low for the precipitation of the sohd hydroxide. Relatively large quantities of the metal can thus be maintained ia solution as the chelate at pH values where otherwise all but trace quantities of the metal would be precipitated. In Eigure 4, this corresponds to pH values where pM of the dashed curves is 4 or greater. At the pH of iatersection of the sohd and dashed lines for the same metal, the free metal ion is ia equihbrium with both the sohd hydroxide and the chelate. At higher pH the hydroxyl ion competes more effectively than the chelant for the metal, and only a trace of either the chelate or the aquo metal ion can exist ia solution. Any excess metal is present as sohd hydroxide. 

The presence of a sufficientiy strong chelating agent, ie, one where K in equation 26 is large, keeps the concentration of free metal ion suppressed so that pM is larger than the saturation pM given by the solubiUty product relation (eq. 29) and no soHd phase of MX can form even in the presence of relatively high anion concentrations. The metal is thus sequestered with respect to precipitation by the anion, such as in the prevention of the formation of insoluble soaps in hard water. 

Three features of chelation chemistry are fundamental to most of the appHcations of the chelating agents. The first and probably the most extensively used feature is the control of free metal ion concentration by means of the binding—dissociation equiUbria. The second, often called the preparative feature, is that in which the special properties of the chelate itself provide the basis of the appHcation. The third feature comprises displacement reactions metal by other metal ions, chelant by chelant, and chelant by other ligands or ions. An appHcation may be termed defensive if an undesirable property in a process or product is mitigated, or aggressive if a new and beneficial property is induced. 

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. 

The log function of the ratio of chelated metal ions to free-metal ions is expressed as the stabiUty constant or formation constant as shown in Table 6. The higher the stabiUty constant the greater the percentage of metal ions that are chelated (11). 

Some heavy metals are essential to life at low concentrations but are dangerous to animal and plant life in higher concentrations. Generally, it is the free metal ion that is the most toxic however, with Hg and Sn certain organic-forms have a greater toxicity. 

This reaction will proceed if the metal indicator complex M-In is less stable than the metal-EDTA complex M EDTA. The former dissociates to a limited extent, and during the titration the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the complex M-In to leave the free indicator (In). The stability of the metal-indicator complex may be expressed in terms of the formation constant (or indicator constant) Ku ... 

In a free metal ion without any ligands, all five d orbitals have identical energies, but what happens to the d orbitals when six ligands are placed around a metal in octahedral geometry The complex is stabilized by attractions between the positive charge of the metal ion and negative electrons of the ligands. At the same time,... 

In the presence of a metal ion (M " ), a metal chalcogenide phase M2Sen will be precipitated upon exceeding the solubility product of [M and [Se ] (or [HSe ]). The concentration of free metal ions must be controlled by an excess of complexing agent, determining the applicable solubility of the metal and the overall competitive chemical reaction, in order to prevent the formation of sulfite, sulfate, and... 

The deposition of a metal from a complex also involves equilibria in solution between the free metal ions, the complexed ions and the complexing agent. The formation of the complex MX2+ from the metal ion M2+ and complexing agent X is characterized by the consecutive stability constants (Section 1.4.3)... 

The larger the formation constant, K, the more likely the M - EDTA complex is to form and the fewer free metal ions remain. 

A controlled concentration of free metal ions, the metallome. Note especially free... 

Although we have described them earlier in general terms, it is well worth stressing again that the concentrations of the free metal ions in these early cells... 

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