PRECIPITATION AND SEPARATION OF IONS

The use of the reaction quotient Q to determine the direction in vdiich a reaction must proceed to reach equilibrium was discussed in Section 15.6. The possible relationships between Q and are as follows  

Does a precipitate form when 0.10 L of 8.0 X 10 MPb(N03)2isadded to0.40Lof 5.0 X 10 MNa2S04 SOLUTION 

Analyze The problem asks us to determine whether a precipitate forms when two salt solutions are combined. 

Plan We should determine the concentrations of all ions just after the and NaN03. Like all sodium salts NaN03 is soluble, but PbS04 has a 

The extent to which an insoluble metal hydroxide reacts with either acid or base varies with the particular metal ion involved. Many metal hydroxides—such as Ca(OH)2, Fe(OH)2, and Fe(OH)3—are capable of dissolving in acidic solution but do not react with excess base. These hydroxides are not amphoteric. 

What is the difference between an amphoteric substance and an amphiprotic substance  

Recall that we used the reaction quotient Q in Section 15.6 to determine the direction in which a reaction must proceed to reach equilibrium. The form of Q is the same as the equilibrium-constant expression for a reaction, but instead of only equilibrium concentrations, you can use whatever concentrations are being considered. The direction in which a reaction proceeds to reach equilibrium depends on the relationship between Q and K for the reaction. If Q /C, the product concentrations are too low and reactant concentrations are too high relative to the equilibriiun concentrations, and so the reaction will proceed to the right (toward products) to achieve equilibrium. If Q K, product concentrations are too high and reactant concentrations are too low, and so the reaction will proceed to the left to achieve equilibriiun. If Q = K, the reaction is at equilibrium. 

For solubility-product equilibria, the relationship between Q and K p is exactly Uke that for other equilibria. For K p reactions, products are always the soluble ions, and the reactant is always the solid. 

For the case of the barium sulfate solution, then we would calculate Q = [B ][S04 ], and compare this quantity to the K p for barium sulfate. 

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PRECIPITATION AND SEPARATION OF IONS We learn how differences in solubility can be used to separate ions through selective precipitation. 

B. Precipitation and separation of hydroxides at controlled hydrogen ion concentration or pH. The underlying theory is very similar to that just given for sulphides. Precipitation will depend largely upon the solubility product of the metallic hydroxide and the hydroxide ion concentration, or since pH + pOH = pKw (Section 2.16), upon the hydrogen ion concentration of the solution. 

Qualitative analysis involves the separation and identification of ions by selective precipitation, complex formation, and the control of pH. 

For many reasons, H2S is an important reagent for the separation of metal ions in solution. H2S is a gas and therefore is easy to obtain from a com-merical cylinder it is easy to saturate a solution with H2S (the saturation concentration is 0.10 M) the enormous range in Ksf> values makes precipitations and separations amenable to control by pH control and most metals form sulfides that are insoluble to some degree. 

There are a number of ways to separate ampholytes from proteins. Electroelution ammonium sulfate precipitation and gel filtration, ion exchange, and... 

The mechanism of the action of carriers depends on the nature of both the trace substance and the carrier involved. The co-precipitation consists in separation of ions coprecipitating from the solution with particles of the carrier formed in the solution. The coprecipitation may be either isomorphous (formation of solid solutions or mixed crystals) or based on adsorption phenomena. 

Chemisorption reactions in soils, which are two-dimensional surface processes, can rarely be separated experimentally from the three-dimensional nucleation and precipitation reactions. It is perhaps best to view the removal of adsorbate ions from solution, broadly termed sorption, as a continuous process that ranges from chemisorption (at the low end of solubility) to precipitation (at the high end of solubility). Unless a new solid phase can be detected, the onset of precipitation and termination of chemisorption during sorption is usually not recognized by experimentalists. For this reason, an understanding of sorption necessitates some knowledge of precipitation reactions, which will be outlined here. 

Cm is recovered from irradiated Pu/Al alloys and Am02(Pu02)/Al cermets by dissolution, extraction of plutonium with TBP in n-dodecane, extraction of americium and curium from the aqueous raffinate with 50 percent TBP in kerosene, purification of the americium and curium fraction by extraction with tertiary amines, and separation of americium by precipitation of the double carbonate K5 Am02 (003)3 A high-pressure ion-exchange system for the separation... 

In the laboratory, students often learn to analyze mixtures of the common positive and negative ions, separating and confirming the presence of the particular ions in the mixture. One of the first steps in such an analysis is to treat the mixture with hydrochloric acid, which precipitates and removes silver ion, lead(II) ion, and mercury(I) ion from the aqueous mixture as the insoluble chloride salts. Write balanced net ionic equations for the precipitation reactions of these three cations with chloride ion. 

Their removal from aqueous media [57-59] is based on the reduction of the ions when they exist in higher-valence states to the lower-valent or zero-valent state where these elements can be precipitated and separated by filtration or flotation. This is effected under reducing conditions, i.e. the oxidizing OH radical is transformed into a reducing free radical by having it react with a suitable organic additive such as ethanol. 

Ion flotation in the presence of surfactants for the treatment of rinses and separation of metal ions is of interest since the sixties [327, 328]. Here, we take only a few examples. The recovery of silver ions from highly diluted solutions is possible by forming a silver-thiourea complex in form of a colloidal precipitate (sublate) followed by sublate flotation with sodium dodecyl benzene sulfonate [329]. Skiylev [330] has developed methods for the removal of non-ferrous metal salts from waste waters. Subject of the investigations were 0.01 - 0.001% solutions of ferrous metal salts. Typical anionic surfactants (alkyl sulfates, alkyl phosphates, alkyl xanthogenates of potassium) or cationic surfactants (quaternary ammonium salts) were used as collectors in ion flotation from diluted solutions. At certain pH, a sublate containing a non-ferrous metal ion was formed, followed by a sublate film formation at the surface due to the rise of the complexes with air bubbles stabilised by the surfactants. 

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. 

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