Complex Ion Formation Reactions

Acid-base (neutralization) reactions are only one type of many that are applicable to titrimetric analysis. There are reactions that involve the formation of a precipitate. There are reactions that involve the transfer of electrons. There are reactions, among still others, that involve the formation of a complex ion. This latter type typically involves transition metals and is often used for the qualitative and quantitative colorimetric analysis (Chapters 8 and 9) of transition metal ions, since the complex ion that forms can be analyzed according to the depth of a color that it imparts to a solution. In this section, however, we are concerned with a titrimetric analysis method in which a complex ion-forming reaction is used. 

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Describe how to use precipitation, acid-base, redox, and complex ion formation reactions in qualitative cation analysis. 

Qualitative Cation Analysis—Precipitation, acid-base, oxidation-reduction, and complex-ion formation reactions are all used extensively in qualitative cation analysis. Such an analysis can provide a rapid means of determining the presence or absence of certain cations in an unknown material. 

Aqueous solutions have low conductivities resulting from extensive complex ion formation. The haUdes, along with the chalcogenides, are sometimes used in pyrotechnics to give blue flames and as catalysts for a number of organic reactions. 

Consider now a somewhat different type of complex ion formation, viz. the production of a complex ion with constituents other than the common ion present in the solution. This is exemplified by the solubility of silver chloride in ammonia solution. The reaction is ... 

The processes of complex-ion formation referred to above can be described by the general term complexation. A complexation reaction with a metal ion involves the replacement of one or more of the coordinated solvent molecules by other nucleophilic groups. The groups bound to the central ion are called ligands and in aqueous solution the reaction can be represented by the equation ... 

Many salts and minerals display an enhanced tendency to decompose and dissolve by processes involving complex ion formation and chloride acts as a ligand in these complexes. Lead sulfate has a poor solubility in water. However, in the presence of chloride ions, a complex chloroplumbate ion is formed and thereby solubility is enhanced. The relevant reactions are ... 

Guilbault GG, Scheide EP. 1970. Chemisorption reactions of diisopropyl methyl phosphonate with various metal salts and the effect of complex-ion formation on the phosphorus-oxygen stretching frequency. Journal of Inorganic and Nuclear Chemistry 32(9) 2959-2962. 

From Eqn. (14) it follows that with an exothermic reaction - and this is the case for most reactions in reactive absorption processes - decreases with increasing temperature. The electrolyte solution chemistry involves a variety of chemical reactions in the liquid phase, for example, complete dissociation of strong electrolytes, partial dissociation of weak electrolytes, reactions among ionic species, and complex ion formation. These reactions occur very rapidly, and hence, chemical equilibrium conditions are often assumed. Therefore, for electrolyte systems, chemical equilibrium calculations are of special importance. Concentration or activity-based reaction equilibrium constants as functions of temperature can be found in the literature [50]. 

The understanding of rates of reactions of cations in solution was much advanced by the fast relaxation techniques of Eigen46) and the subsequent discussion of substitution often in terms of the rates of loss of water from the hydration sphere. It is generally possible to write two pathways of complex ion formation... 

Relaxation of complicated ligands may occur as a step in both pathways. Diebler and Eigen 461 indicated the ways in which such mechanisms could be analysed using fast reaction methods. Several studies of Ln(III) complex formation and of the formation of Ln(III) mixed complexes have been analysed. Generally the dissociative mechanism is considered to dominate and we are then concerned with the water exchange rate. Several studies have shown that the rate decreases from La(III) to Lu(III) but there seems to be a minimum rate around Tm(III). This is also seen in the rate of rotation of ligands on the surface of the ions, Fig. 7. There may be a small crystal field term, or another contribution to a tetrad -like effect from the 4f electron core. However in the hydrate the precise relationship between the inner and outer sphere water may also be important as we saw when we discussed the heat and entropy of complex ion formation. 

In presence of chloride ions, Cu+ ions are thus removed and copper can continue to react with Cu++ ions as in (1) until the cupric salt is all reduced to cuprous. Increasing concentration of Cl-favors reaction (3) more than reaction (2), and thus in concentrated HC1 we obtain the soluble H+CuC12". Dilution of the latter reduces the concentration of (Cl-), which favors the complex ion formation in proportion to its second power, and of the... 

The work of Yaillant3 and Lewis 4 has shown that the colour changes cannot be quantitatively interpreted without considering that water plays a definite r61e in the reactions. It follows that if Donnan and Bassett s views on complex ion formation be correct, water is either produced or used up when cobalt chloride and chloride ion interact thus, for example, where the ion CoCl3 is assumed for simplicity ... 

Complex Ion Formation. Formation of complexes of lead with the various anions such as chloride, fluoride, carbonate, bicarbonate, and hydroxide increases the concentration of lead in natural waters by preventing lead from taking part in other chemical reactions, primarily adsorption, that would lower its... 

However, we are interested in the reaction between NH3 and Ag+ to form complex ions, and since the position of the above equilibrium lies far to the left (Kb for NH3 is 1.8 X 10 5), we can neglect the amount of NH3 consumed in the reaction with water. So before any complex ion formation occurs, the concentrations in the mixed solution are... 

Carefully regulated conditions are needed for reactions of T1 compounds with I2. When a solution of Til and I2 in concentrated HI is evaporated, black crystals, Tl If, are deposited. However, in a solution containing excess I , which stabilizes the -t- 3 state by complex ion formation, the predominant reaction is one of oxidative addition ... 

Five examples are given of the application of the Nemst equation to half-cell reactions. These examples illustrate the influence of ion concentration, pH, precipitate phases, and complex-ion formation on the electrode potential. All of these variables have significance in aqueous corrosion ... 

Next, consider the suggestion that copper corrodes in the concentrated HC1 because of the formation of a soluble chloride complex with an equilibrium constant for the reaction Cu2+ + 4CL = (CuCl4)2- of K = 10+6. If a CuC1 2- = KL4, and the activity of the CL is that given above in the concentrated acid (acr = 5), calculate Ecell and determine whether corrosion will occur due to the formation of the complex ion. Cell reaction ... 

Lewis acid-base reactions in which a metal cation combines with a Lewis base result in the formation of complex ions. Thus, we can define a complex ion as an ion containing a central metal cation bonded to one or more molecules or ions. Complex ions are crucial to many chemical and biological processes. Here we will consider the effect of complex ion formation on solubihty. In Chapter 22 we will discuss the chemistry of complex ions in more detail. 

The complexing of zirconium and hafnium ions by fluoride ions is quite extensive compared to chloro complexing, while complex ion formation w ith bromide and iodide ions is negligible. Formation constants for fluoride complexing with zirconium(IV) and hafnium(IV) calculated from the data of Connick (126), Buslaev (94), and Hume (574), have been summarized graphically by Goldstein (213). Slightly different values have been published by Bukhsh (92). Noren (15a, 401-403) has redetermined the equilibrium constants for the reaction. 

There are several important aspects about which Figures 1 and 2 tell little or nothing (1) the reversibility of the reactions (2) the probability of metastable, rather than the stable (free energy-wise), mineral species formation (3) the rate at which the mineral transformations will occur in response to pH-Eh changes (4) the eflFect of solid solution (both anions and cations) and (5) the eflFect of complex ion formation. All of these aspects are important for a quantitative description of the solution concentration of heavy metals in dynamic systems. The particle size of the hydrous oxides aflFects several of the above items, particularly (2) and (3). Microbiological activity is undoubtedly important with regard to the occurrence of metastable oxide species. 

Cations often interfere with each other in the final tests designed to detect the presence of specific cations. Therefore, cations must first be separated before identification can be accomplished. In fact, as with many chemical mixtures, separation of cations may be considerably more difficult than identification. Careful work is again very important if the separations are not clean, results in identification tests may be masked by interfering cations. Separation of a complex mixture of cations is by no means simple and is generally broken down into several parts. Each part involves a fairly small group of cations which can be isolated from the mixture on the basis of some property which is common to the ions in the group and then studied as a separate set. After isolation, the cations within a group are further resolved by means of a series of chemical reactions into soluble and insoluble fractions which are sufficient to allow identification of each cation by one or more tests specific to that ion once interferences have been removed. Various types of chemical reactions will be used for separations and identifications in this experiment precipitation reactions, acid-base reactions, complex ion formations, and oxidation-reduction reactions. 

We learn how complex ion formation, which is a type of Lewis acid-base reaction, can enhance the solubility of an insoluble compound. (16.10)... 

The solubility of metal salts is also affected by tiie presence of certain Lewis bases tiiat react with metal ions to form stable complex ions. Complex-ion formation in aqueous solution involves the displacement by Lewis bases (such as NH3 and CN ) of water molecules attached to the metal ion. The extent to which such complex formation occurs is expressed quantitatively by the formation conr stant for tiie complex ion. Amphoteric metal hydroxides are those sligjitly soluble metal hydroxides that dissolve on addition of eitiier acid or base. Acid-base reactions involving the OH" or H2O groups bound to the metal ions give rise to tiie amphoterism. 

Solubility Equilibria Another application of the equilibrium concept is the solubility equilibria of sparingly soluble salts, which are expressed as the solubdily product. TTie solubility of such a substance can be affected by the presence of a conunon cation or anion, or the pH. Complex-ion formation, an example of the Lewis acid-base type reaction, increases the solubility of an insoluble salt. 

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