Precipitation of ionic compounds

The equilibria we have considered thus far in this chapter have involved acids and bases. Furthermore, they have been homogeneous that is, all the species have been in the same phase. Through the rest of the chapter, we will consider the equilibria involved in the dissolution or precipitation of ionic compounds. These reactions are heterogeneous. 

Other references in Table in discuss applications in precipitation of metal.compounds, gaseous reduction of metals from solution, equilibrium of copper in solvent extraction, electrolyte purification and solid-liquid equilibria in concentrated salt solutions. The papers by Cognet and Renon (25) and Vega and Funk (59) stand out as recent studies in which rational approaches have been used for estimating ionic activity coefficients. In general, however, few of the studies are based on the more recent developments in ionic activity coefficients. 

In this chapter, you will continue your study of acid-base reactions. You will find out how ions in aqueous solution can act as acids or bases. Then, by applying equilibrium concepts to ions in solution, you will be able to predict the solubility of ionic compounds in water and the formation of a precipitate. 

R. A. Pacer, "Will a precipitate form Will it dissolve " J. Chem. Educ., Vol. 71,1994,69. Principles of ionic compound precipitate formation are shown in a series of four colorful demonstrations (1) Yellow Pbl2 is formed only when Q > Ksp. (2) Upon addition of aqueous ammonia, green NiC03(s) is dissolved due to the formation of the blue complex ion [Ni(NH3)6]2+. 

The driving force behind double substitution reactions is the formation of a covalent compound (including water or a gaseous compound) or an insoluble ionic compound from ions in solution. A solid formed from ions in solution is called a precipitate. We can thus predict that a reaction will occur if soluble ionic compounds yield at least one insoluble ionic compound or one covalent compound. We need to be familiar with the solubilities of some common ionic compounds in water. Some types of ionic compounds that are soluble or insoluble in water are listed in Table 8.3. A more comprehensive tabulation of solubilities is presented in Table 8.4 for reference, not necessarily to be memorized. 

Falk, R. Randolph, T. Meyer, J. Kelly, R. Manning, M. Controlled release of ionic compounds from poly(L-lactide) microspheres produced by precipitation with compressed antisolvent. J. Control. Release 1997, 44, 77-85. 

In an aqueous solution, ionic compounds are completely dissociated into ions. For example, an aqueous solution of barium nitrate, Ba(N03)2, contains Ba + ions and NO3 ions. If aqueous solutions of ionic compounds are mixed, some ions may interact to form an insoluble product called a precipitate. For example, if aqueous solutions of barium nitrate and sodium sulfate are mixed, insoluble barium sulfate will precipitate. The complete formula equation for this reaction is written as follows. 

Tip-off You are asked to predict whether a precipitation reaction will take place between two aqueous solutions of ionic compounds, and if the answer is yes, to write the complete equation for the reaction. 

From general rules about solubilities of ionic compounds, we can predict whether a precipitate will form in a reaction. 

Aqueous solutions of ionic compounds contain dissolved positive, and negative ions. When two such solutions are mixed, the ions may take part in a double-replacement reaction. One outcome of a double-replacement reaction is the formation of a precipitate. By writing ionic equations and knowing the solubilities of specific ionic compounds, you can predict whether a precipitate will be formed. 

Assume that you have mixed two solutions, and a solid product (a precipitate) forms. How can you find out what the solid is What is its formula There are several possible approaches you can take to answering these questions. For example, we saw in Chapter 7 that we can usually predict the identity of a precipitate formed when two solutions are mixed in a reaction of this type if we know some facts about the solubilities of ionic compounds. 

In order to write these equations, you have to know something about the solubility of ionic compounds. Don t fret. Here you go If a compound is soluble, it will remain in its free ion form, but if it s insoluble, it will precipitate (form a solid). Table 8-2 gives the solubilities of selected ionic compounds. 

Ion-dipole interactions are the dominant interactions between ions and polar molecules and are very important in the solvation of ionic compounds in a polar solvent. For example, it is the attractive ion-dipole interactions of the ions Na and Cl with the polar water molecule that stabilize an aqueous solution of NaCl against precipitation. The potential energy of interaction between an ion of charge at a distance r from a dipole of magnitude p is given by... 

Hypromellose or hydroxypropylmethylcellulose is a partly O-methylated and 0-(2-hydroxypropylated) cellulose [25]. It is used from 10% up to 80% w/w for controlled drug release in solid dosage form and from 2% to 20% w/w as coating solution for tablets and pellets [25,30,31 ]. Hydroxypropylmethylcellulose is a nonionic polymer with pH of a 2% w/w aqueous solution ranging from 5 to 8. It is soluble in water. Being nonionic, it will not form complexes with metallic salts or ionic compounds that can possibly lead to the precipitation of insoluble compounds [32]. Hydroxypropylmethylcellulose matrix hydrates and swells into a gel layer in the direction of matrix surfaces to core when it is contacted with the dissolution medium [14,20,33]. Erosion of gel takes place thereafter and may occur simultaneously with the subsequent phases of matrix hydration and swelling [14]. The swelling and erosion properties of a solid matrix made of... 

Recently, the possibility of using microemulsions as microreactors for the control of polymorphism of organic compounds, speciflcally amino acids and peptides, was demonstrated by Furedi-Milhofer et al. [107,108]. In these experiments, the organic molecules were solubilized in hot microemulsions and crystallized by slow cooling. In some cases, in contrast to the precipitation of ionic clusters (see above), the procedure was most effective when the solute mole-... 

In the previous section, we used a precipitation reaction to illustrate how to convert a molecular equation to an ionic equation. A precipitation reaction occurs in aqueous solution because one product is insoluble. A precipitate is an insoluble solid compound formed during a chemical reaction in solution. To predict whethCT a precipitate will form when you mix two solutions of ionic compounds, you need to know whether any of the potential products that might form are insoluble or not This is another application of the solubility rules (Section 4.1). 

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