Selective precipitation of ions

Ions can be separated from each other based on the solubilities of their salts. Gsnsider a solution containing both Ag and Cu. If HCl is added to the solution, AgCl (K = 1.8 X 10 ) precipitates, while Cu remains in solution because CUCI2 is soluble. Separation of ions in an aqueous solution by using a reagent that forms a precipitate with one or more (but not all) of the ions is called selective precipitation. 

A solution contains 1.0 X lO MAg and 2.0 X 10 MPb. When Cl is added, both AgCl (Xjp = 1.8 X 10 ) and PbCl2 (K = 1.7 X 10 ) can precipitate. What concentration of Cl is necessary to begin the precipitation of each salt Which salt precipitates first  

Analyze We are asked to determine the concentration of Cl necessary to begin the precipitation from a solution containing Ag and Pb ions, and to predict which metal chloride will begin to precipitate first. 

Plan We are given Kgp values for the two precipitates. Using these and the metal ion concentrations, we can calculate what Cl concentration is necessary to precipitate each salt. The salt requiring the lower Cl ion concentration precipitates first. 

Because [Ag ] = 1.0 X 10 M, the greatest concentration of Cl that can be present without causing precipitation of AgCl can be calculated fiom the K p expression  

Plan We are given values for the two precipitates. Using these and 

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The first fractionation of urinary ampholytes in this way was carried out by Boulanger et al. (BIO) in 1952 with the use of ion-exchange resins. They had designed this procedure previously for the fractionation of ampholytes in blood serum (B8). According to this method, deproteinized urine was subjected to a double initial procedure aiming at the separation of low-molecular weight substances from macro-molecular ones. One of the methods consisted of the fractionation of urinary constituents by means of dialysis, the second was based on the selective precipitation of urinary ampholytes with cadmium hydroxide, which, as had previously been demonstrated, permits separation of the bulk of amino acids from polypeptides precipitated under these circumstances. Three fractions, i.e., the undialyzable part of urine, the dialyzed fraction, and the so-called cadmium precipitate were analyzed subsequently. 

Selective precipitation of metal ions using hydrogen- or sodium sulphides, hydroxides or carbonates... 

Meyer, J.D., J.E. Matsnnra, J.A. Rnth, E. Shelter, S.T. Patel, J. Bansch, E. McGonigle, and M.C. Manning, Selective precipitation of interleukin-4 using hydrophobic ion pairing a method for improved analysis of proteins formulated with large excesses of human serum albumin. Pharm Res, 1994. 11(10) 1492-5. 

The solubility product is the equilibrium constant for the dissolution of a solid salt into its constituent ions in aqueous solution. The common ion effect is the observation that, if one of the ions of that salt is already present in the solution, the solubility of a salt is decreased. Sometimes, we can selectively precipitate one ion from a solution containing other ions by adding a suitable counterion. At high concentration of ligand, a precipitated metal ion may redissolve by forming soluble complex ions. In a metal-ion complex, the metal is a Lewis acid (electron pair acceptor) and the ligand is a Lewis base (electron pair donor). 

Most base-metal sulfides are very insoluble compounds and, in principle, processes based on the differences in their solubilities can be used in the selective precipitation of sulfides. For example, the strong affinity of copper ions for sulfide ion is used to good effect in the removal of traces of copper from leach liquors in the recovery of nickel,406 cobalt407 and manganese.408 For manganese, other impurity metal ions such as cobalt, nickel and zinc are also selectively precipitated by the use of sulfide ions. 

When solutions of soluble ionic compounds are mixed, an insoluble compound will precipitate if the ion product (IP) for the insoluble compound exceeds its fCsp. The IP is defined in the same way as /equilibrium concentrations. Certain metal cations can be separated by selective precipitation of metal sulfides. Selective precipitation is important in qualitative analysis, a procedure for identifying the ions present in an unknown solution. 

Metal ions are generally recovered or removed from aqueous discharges by selective precipitation of their insoluble compounds. One of the most popular precipitating agents is the OH- ion. The addition of commercial bases to wastewaters so as to precipitate and remove insoluble metal hydroxides is a common practice for water treatment. This is a relatively simple operation, and the reagents used (typically lime and soda ash) are inexpensive, safe, and readily available. The approximate relative cost of the commercial bases Ca(OH)2, Na2CC>3 andNaOH is 1 3 5, respectively. 

Selective precipitation of Ag+, Ba2+, and Fe3+ ions. In this schematic representation a double line means that a solid forms, and a single line designates a solution. 

In the carbonate. basic leaching process, both carbonate and bicarbonate ions are required to buffer the solution and prevent precipitation of uranium salts. In both acidic and basic leaching, complex anions are formed. The solutions may therefore be upgraded by anion exchange or anion solvant extraction (SX) processes. The upgraded solution is normally treated by selective precipitation of umaium by pH adjustment. 

EXAMPLE 18-14 Separating Metal Ions by Selective Precipitation of Metal Sulfides... 

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