Metal ions soluble agents

In a study of the synthesis of transition metal doped carbon xerogels by solubilization of metal salts in resorcinol-formaldehyde aqueous solution. Job et al. [50] used complexing agents when necessary to render the metal ions soluble. Indeed,... 

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... 

The product is equal to the equilibrium constant X for the reaction shown in equation 30. It is generally considered that a salt is soluble if > 1. Thus sequestration or solubilization of moderate amounts of metal ion usually becomes practical as X. approaches or exceeds one. For smaller values of X the cost of the requited amount of chelating agent may be prohibitive. However, the dilution effect may allow economical sequestration, or solubilization of small amounts of deposits, at X values considerably less than one. In practical appHcations, calculations based on concentration equihbrium constants can be used as a guide for experimental studies that are usually necessary to determine the actual behavior of particular systems. 

Precipitation involves the alteration of the ionic equilibrium to produce insoluble precipitates. To remove the sediment, chemical precipitation is allied with solids separation processes such as filtration. Undesirable metal ions and anions are commonly removed from waste streams by converting them to an insoluble form. The process is sometimes preceded by chemical reduction of the metal ions to a form that can be precipitated more easily. Chemical equilibrium can be affected by a variety of means to change the solubility of certain compounds. For e.xample, precipitation can be induced by alkaline agents, sulfides, sulfates, and carbonates. Precipitation with chemicals is a common waste stream treatment process and is effective and reliable. The treatment of sludges is covered next. 

Let us apply these ideas to the third-row elements. On the left side of the table we have the metallic reducing agents sodium and magnesium, which we already know have small affinity for electrons, since they have low ionization energies and are readily oxidized. It is not surprising, then, that the hydroxides of these elements, NaOH and Mg(OH)z, are solid ionic compounds made up of hydroxide ions and metal ions. Sodium hydroxide is very soluble in water and its solutions are alkaline due to the presence of the OH- ion. Sodium hydroxide is a strong base. Magnesium hydroxide, Mg(OH)2, is not very soluble in water, but it does dissolve in acid solutions because of the reaction... 

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... 

Various polyphosphates are effective sequestering agents under appropriate conditions. The best known of these is sodium hexametaphosphate (10.14), the cyclic hexamer of sodium orthophosphate. Further examples are the cyclic trimer sodium trimetaphosphate (10.15), as well as the dimeric pyrophosphate (10.16), the trimeric tripolyphosphate (10.17) and other linear polyphosphates (10.18). All of these polyanions function by withdrawing the troublesome metal cation into an innocuous and water-soluble complex anion by a process of ion exchange as shown in Scheme 10.7 for sodium hexametaphosphate. Hence these compounds are sometimes referred to as ion-exchange agents. 

---