Precipitation, selective

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. 

We can use the fact that salts have different solubilities to separate mixtures of ions. For example, suppose we have an aqueous solution containing the cations Ag+, Ba2+, and Fe3+, and the anion N03-. We want to separate the cations by precipitating them one at a time, a process called selective precipitation. 

How can the separation of these cations he accomplished We can perform some preliminary tests and observe the reactivity of each cation toward the anions Cl-, S042-, and OH-. For example, to test the reactivity of Ag+ toward Cl-, we can mix the AgN03 solution with aqueous KC1 or NaCI. As we have seen, this produces a precipitate. When we carry out tests of this type using all the possible combinations, we obtain the results in Table 4.2. 

After studying these results, we might proceed to separate the cations as follows  

Add an aqueous solution of NaCI to the solution containing the Ag, Ba2+, and Fe3+ ions. Solid AgCI will form and can be removed, leaving Ba2+ and Fe3+ ions in solution. 

The difference In K,p values required for selective precipitation Is a factor of at least 10.  

EXAMPLE 16.13 Finding the Minimum Required Reagent Concentration for Selective Precipitation 

The magnesium and calcium ions present in seawater ([Mg ] = 0.059M and [Ca ] = 0.011M) can be separated by selective precipitation with KOH. What minimum [OH ] triggers the precipitation of the Mg + ion  

The precipitation commences when the value of Q for the precipitating compound just equals the value of K p. Set the expression for Q for magnesium hydroxide equal to the value of K p, and solve for [OH ], This is the concentration above which Mg(OH)2 precipitates. 

If the concentration of Mg in the previous solution was 0.025 M, what minimum [OH ] triggers precipitation of the Mg ion  

Solutions of metal ions are often separated by selective precipitation which is precipitation with a reagent having an anion that forms a solid specie with only one of the metal ions in the solution. One example is a solution with Ba ions as well as Ag ions. Are NaCl added to the solution only AgCl precipitates whereas Ba ions continues to be in the solution, as Bad is easily soluble. This we will look further into in the following example. 

A solution contains 1.0 10 M Cu ions and 2.0 10 M Pb ions. We now slowly add a aqueous solution containing T ions. Hereby, Pbl2(s) precipitates following the reaction scheme below  

The Ksp values are 1.4 10 M for Pbl2 and 5.3 10 M for Cul respectively. We now wish to determine which one of the two solid species that precipitates first. Furthermore we wish to determine the concentration of T being necessary to precipitate each of the two solid species. 

As we know that the concentration of Pb is 2,0 10 M the largest possible concentration of T being able to exist without precipitation may be calculated from  

A concentration of T ions above 2.6 10 M will thereby lead to precipitation of PbECs). Similarly for Cul  

Having considered the precipitation of single substances, we are now ready to examine selective precipitation procedures for separating ions, a subject of even greater interest to the analytical chemist. 

Chloride ion is added to a mixture containing 0.01 M Tl , 0.02 M Pb and 0.03 M Ag . Calculate the order in which precipitation of these metal halides occur and to what extent separations of the three metals take place. 

Using the solubility product expressions for each of the metal chlorides we can calculate the [Cl ] necessary for saturating the solution with respect to each salt. 

the order in which the metal chlorides will precipitate is AgCl, followed by TlCl, with PbCl2 the last to precipitate, in keeping with the increasing concentration of [CP] required for saturation. As you see, this is not the order of increasing solubility product constants (Why not Is there more than one reason ) 

The selectivity of this precipitation is described by how much of each metal ion will have precipitated before the next one begins to precipitate. 

When undesirable metal ions generally need to be removed from a solution without disturbing the other metal ions present, the process of selective precipitation comes into play. This step involves the precipitation of metal ion by using another solution whose anion forms an insoluble salt with the undesirable ion in the mixture. For example, if a NaCl solution is added to a solution containing both Ag and Mg2+ ions, AgCl will be precipitated as a white solid. Since MgCl2 is soluble, Mg2+ ions will remain in the solution. 

If we add an AgN03 solution drop by drop to another solution containing 1 10-2 M NaCl and 1 lQ-2 M Nal, 

If AgNOj salt is added slowly, [Ag ] will increase gradually. When [Ag+l reaches 1.5 10 M, Agl will be at equilibrium. That means the Agl solution will be saturated. If this value is increased slightly, Agl will start to precipitate. When the concentration of silver ions is increased up to 1.6 10 M, AgCl will reach equilibrium (saturated). When the concentration of silver ions is increased, AgCl will start to precipitate. Consequently, F ions will precipitate earlier than CP ions. 

There are 0.05 M Ca and 0.03 M Ba ions in a solution. If a SO anion is added to this solution gradually, what will be the concentration of Ba ions when CaSO starts to precipitate  

Lithium Sodium Potassium Flame tests of alkali metals 

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Selectivity Due to the chemical nature of the precipitation process, precipitants are usually not selective for a single analyte. For example, silver is not a selective precipitant for chloride because it also forms precipitates with bromide and iodide. Consequently, interferents are often a serious problem that must be considered if accurate results are to be obtained. 

Selected Precipitates Used in Turbidimetric and Nephelometric Methods... 

Selective Reduction. In aqueous solution, europium(III) [22541 -18-0] reduction to europium(II) [16910-54-6] is carried out by treatment with amalgams or zinc, or by continuous electrolytic reduction. Photochemical reduction has also been proposed. When reduced to the divalent state, europium exhibits chemical properties similar to the alkaline-earth elements and can be selectively precipitated as a sulfate, for example. This process is highly selective and allows production of high purity europium fromlow europium content solutions (see Calcium compounds Strontiumand strontium compounds). 

Fra.ctiona.1 Precipituition. A preliminary enrichment of certain lanthanides can be carried out by selective precipitation of the hydroxides or double salts. The lighter lanthanides (La, Ce, Pr, Nd, Sm) do not easily form soluble double sulfates, whereas those of the heavier lanthanides (Ho, Er, Tm, Yb, Lu) and yttrium are soluble. Generally, the use of this method has been confined to cmde separation of the rare-earth mixture into three groups light, medium, and heavy. 

A number of substances, such as the most commonly used sulfur dioxide, can reduce selenous acid solution to an elemental selenium precipitate. This precipitation separates the selenium from most elements and serves as a basis for gravimetry. In a solution containing both selenous and teUurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 W with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxylamine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and deterrnination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

Chlotobenzoyl)-benzoic acid is nitrated in concentrated sulfuric acid, then reduction of the nitro group, ring closure, and hydrolysis occur simultaneously in concentrated sulfuric acid in the presence of a reducing agent and boric acid. Thus obtained cmde chloro pink is purified by selective precipitation from sulfuric acid in order to separate it from by-produced 2-amino-3-chloto-l-hydroxyanthtaquinone (24) (36). 

Purification. Enzyme purity, expressed in terms of the percent active enzyme protein of total protein, is primarily achieved by the strain selection and fermentation method. In some cases, however, removal of nonactive protein by purification is necessary. The key purification method is selective precipitation of the product or impurities by addition of salt, eg, sodium sulfate, or solvent, eg, ethanol or acetone by heat denaturation or by isoelectric precipitation, ie, pH adjustments. Methods have been introduced to produce crystalline enzyme preparations (24). 

The physical properties of the base pigments produced from both processes are further improved by sluirying in water and selectively precipitating on the finely divided particles a surface coating of Si02, AI2O3. or TiOi itself. 

One way to separate two cations in water solution is to add an anion that precipitates only one of the cations. This approach is known as selective precipitation. To see how it works, consider a simple case, a solution containing Mg2+ and Na+ ions. Referring back to Table 16.1 (p. 433), you can see that Mg2+ forms a couple of insoluble compounds MgC03 (K = 6.8 X 10-6) and Mg(OH)2 (K = 6 X 10-12). In contrast, all of the common compounds of sodium are soluble, including the carbonate and hydroxide. It follows that you could readily separate Mg2+ from Na+ by adding either C032- or OH- ions to the solution. In either case, Mg2+ will precipitate while Na+ remains in solution. 

In the laboratory you will most likely carry out one or more experiments involving the separation and identification of cations present in an "unknown" solution. A scheme of analysis for 21 different cations is shown in Table A. As you can see, the general approach is to take out each group (I, II, III, IV) in succession, using selective precipitation. 

Complex formation, selective precipitation, and control of the pH of a solution all play important roles in the qualitative analysis of the ions present in aqueous solutions. There are many different schemes of analysis, but they follow the same general principles. Let s think through a simple procedure for the identification of five cations by following the steps that might be used in the laboratory. We shall see how each step makes use of solubility equilibria. 

FIGURE 11.21 Steps in the analysis of five cations by selective precipitation. 

Sulfides with widely different solubilities and solubility products can be selectively precipitated by adding S2 ions to the solution removed from the chlorides in the first step (see Fig. 11.20). Some metal sulfides (such as CuS, HgS, and Sb2S3) have extremely small solubility products and precipitate if there is the merest trace of S2" ions in the solution. Such a very low concentration of S2 ions is achieved by adding hydrogen sulfide, H2S, to an acidified solution. A higher hydronium ion concentration shifts the equilibrium... 

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

We wish to separate magnesium ions and barium ions by selective precipitation. Which anion, fluoride or carbonate, would be the better choice for achieving this precipitation Why ... 

Separations of polysaccharides by fractionation on a preparative scale were also examined. Stemming from earlier work in his laboratory on the isolation of acidic polysaccharides by precipitation as their insoluble Cetavlon salts, Stacey and coworkers showed that it was possible to fractionate neutral polysaccharides by selective precipitation with Cetavlon after the formation of borate complexes. 

Total RNA was isolated from mycelia cultured 6 days on PG-inducing and non- inducing conditions (apple pectin and glucose, respectively). The RNA extraction was performed according to the phcnol-SDS method combined with selective precipitation using LiCl. RNA concentration was estimated by absortion at 260nm. 

A specific example where heterogeneous supports provide nanoparticle size-control is the immobilization of homogeneous silver nanoparticles on polystyrene [366]. This work was extended later to the development of a one-pot method for the size-selective precipitation of silver nanoparticles on PVP-protected thiol-functionalized silica. During the immobilization of very small silver nanoclusters both the size of the silver nanoclusters and the thickness of the silver layer on the support could be controlled directly by the reaction parameters applied (Fi re 16) [367]. 

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