Complex ions formation

The formation of complex ions in aqueous solutions with anionic ligands is an important feature of the aqueous chemistry of Pu. Complex formation and hydrolysis are competing reactions and may be looked upon as the displacement of the HgO molecules in the hydration sphere by the anionic ligand or by OH , respectively. The ability of an ion to form complexes is dependent on the magnitude of the ionic potential which may be defined by the equation 

Cation e Ionic radius (A) Charge Ionic potential 

The relative tendency of Pu ions to form complexes then is PuttV) Pu(ni) Pu(VI) Pu(V) 

Gel man et al. show that the anionic ligand has some effect on this series. 

For example, the positions of Pu(III) and Pu(VI) are interchanged in the case of oxalate complexes. 

A complex ion is an ion containing a central metal cation bonded to one or more molecules or ions. Complex ions are cmcial to many chemical and biological processes. Here we will consider the effect of complex ion formation on solubility. In Chapter 22 we will discuss the chemistiy of complex ions in more detail. 

Copper(II) sulfate (CUSO4) dissolves in water to produce a blue solutioa The hydrated coppeifll) ions are responsible for this color, many other sulfates (e.g., Na2S04) are colorless. Adding a few drops of concentrated ammonia solution to a CUSO4 solution causes the formation of a light-blue precipitate, copper(II) hydroxide  

A measure of the tendency of a metal ion to form a particular complex ion is given by the formation constant Kf) (also called the stability constant), which is the ecpiilibtium constant for the complex ion formation. The larger K[ is, the more stable the complex ion is. Table 17.5 (page 762) hsts the formation constants of a number of complex ions. 

The large value of K in this case indicates that the complex ion is very stable in solution and accounts for the very low concentration of copper(II) ions at eqnihbrinm. 

Recall that K for the sum of two reactions is the product of the individual K values [m Section 15.3]. The dissolution of silver chloride is represented by the equation 

When more concentrated aqueous ammonia solution is added, the Cu(OH)2 is formed. (Right) When more concentrated aqueous ammonia solution is added, the Cu(OH)2 precipitate dissolves to form the dark-blue complex ion Cu(NH3)4.  

TABLE 17.5 1 rormation Constante of Selected Complex Ions in Water sit  

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Hydrolysis and Complex Ion Formation. Hydrolysis and complex ion formation are closely related phenomena (13,14). 

The degree of hydrolysis or complex ion formation decreases in the order > MO2 Presumably the relatively high tendency... 

Complex Ion Formation. Phosphates form water-soluble complex ions with metallic cations, a phenomenon commonly called sequestration. In contrast to many complexing agents, polyphosphates are nonspecific and form soluble, charged complexes with virtually all metallic cations. Alkali metals are weakly complexed, but alkaline-earth and transition metals form more strongly associated complexes (eg, eq. 16). Quaternary ammonium ions are complexed Htde if at all because of their low charge density. The amount of metal ion that can be sequestered by polyphosphates generally increases... 

Evidence foi the anionic complex PuCP is the precipitation of complex halides such as Cs2PuClg from concentrated HCl (aq). The ability of Pu(IV) to form stable nitrate complexes provides the basis for the Purex and ion-exchange (qv) process used in the chemical processing of Pu (107). Pu(VI) is similar to Pu(IV) in its abihty to form complex ions. Detailed reviews of complex ion formation by aqueous plutonium are available (23,94,105). 

Bromine is moderately soluble in water, 33.6 g/L at 25°C. It gives a crystalline hydrate having a formula of Br2 <7.9H2 O (6). The solubiUties of bromine in water at several temperatures are given in Table 2. Aqueous bromine solubiUty increases in the presence of bromides or chlorides because of complex ion formation. This increase in the presence of bromides is illustrated in Figure 1. Kquilibrium constants for the formation of the tribromide and pentabromide ions at 25°C have been reported (11). 

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. 

As may be seen from the potential-pH diagram " (Fig. 6.3) platinum is immune from attack at almost all pH levels. Only in very concentrated acid solutions at high redox potentials (i.e. under oxidising conditions) is there a zone of corrosion. This accounts for the solubility of platinum in aqua regia. Platinum is also prone to complex-ion formation, and this can lead... 

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 ... 

In the application of the polarographic method of analysis to steel a serious difficulty arises owing to the reduction of iron(III) ions at or near zero potential in many base electrolytes. One method of surmounting the difficulty is to reduce iron(III) to iron(II) with hydrazinium chloride in a hydrochloric acid medium. The current near zero potential is eliminated, but that due to the reduction of iron(II) ions at about - 1.4 volts vs S.C.E. still occurs. Other metals (including copper and lead) which are reduced at potentials less negative than this can then be determined without interference from the iron. Alternatively, the Fe3 + to Fe2+ reduction step may be shifted to more negative potentials by complex ion formation. 

Some precipitates dissolve when the temperature is changed. This strategy is used to purify precipitates. The mixture is heated to dissolve the solid and filtered to remove insoluble impurities. The solid is then allowed to re-form as the solution cools and is removed from the solution in a second filtration. Complex ion formation (Section 11.13) can also be used to dissolve metal ions. 

J 13 Calculate molar solubility in the presence of complex ion formation (Example 11.12). 

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. 

Much more Zn(OH)2 will dissolve since essentially all of the released Zn2+ ions are incorporated into the soluble Zn(OH)42 complex because Kd is so small. When significant complex ion formation occurs as it does in this case, the molar solubility must include the concentrations of all the Zn2+ species. Also we cannot assume that [OH-] remains at 0.25 M throughout this process. The net concentration of OH- ion decreases slightly For every one formula unit of Zn(OH)2 that dissolves producing 2 OH- ions, one formula unit of Zn(OH)42- will form, removing 4 OH- ions. The net result is that for every 1 mol/L of Zn(OH)2 that dissolves (the molar solubility), approximately 1 mol/L of Zn(OH)42- is produced, but 2 mol/L of OH- is lost. Therefore,... 

Condensed-phase flame retardant mechanisms, 44 484—485 Condensed phosphates, 18 841-852 colloidal properties of, 48 851 complex ion formation in,... 

The iodine cation forms iodine monochloride (IC1) in a medium having sufficiently high concentration of HC1 and the latter is subsequently stabilized by complex ion formation. Thus, we have ... 

In addition to the above four types of amperometric methods cited, there also exist a plethora of titrations involving neutralization and complex ion formation that have been accomplished successfully, for... 

One of the objectives of this paper will be to show some specific examples of these effects in electrolysis and illustrate the substantial need for a better understanding of the thermodynamics of the solution chemistry involved in electrodics. Some of these needs are more obvious and have been indicated previously ( 3) and include such items as AG°, Kg0 and Cp data on the systems of interest. However, much more extensive information is necessary on adsorption phenomena, complex ion formation and the equilibrium concentrations of these influential species. This need has always existed but it is even more important now if the current challenges being imposed by energy and materials shortages and environmental control are to be met. 

As is evident from Eq. (11.4), copper and zinc are very far apart in the standard EMF series, so alloy codeposition seems next to impossible. Fortunately, the difference can be eliminated (even reversed) by changing the values of the activities. This can be achieved by inducing a considerable change in ionic concentrations via complex ion formation, as discussed in detail below. 

Impetus was given to work in the field of selective cation complex-ation by the observation of Moore and Pressman (5) in 1964 that the macrocyclic antibiotic valinomycin is capable of actively transporting K+ across mitochondrial membranes. This observation has been confirmed and extended to numerous macrocyclic compounds. There is now an extensive literature on the selective complexation and transport of alkali metal ions by various macrocyclic compounds (e.g., valinomycin, mo-nactin, etc.) (2). From solution spectral (6) and crystal X-ray (7) studies we know that in these complexes the alkali metal cation is situated in the center of the inwardly oriented oxygen donor atoms. Similar results are found from X-ray studies of cyclic polyether complexes of alkali metal ions (8) and barium ion (9). These metal macrocyclic compound systems are especially noteworthy since they involve some of the few cases where alkali metal ions participate in complex ion formation in aqueous solution. 

The tendencies to complex ion formation and hydrolysis ordinarily increase in the series M+ < An02+ < An+ < An02+ < An+ and with decreasing ion size from Ac to Lr. For complexation with some univalent... 

A white solid, possibly polonium tetrafluoride, is obtained by treating polonium hydroxide or tetrachloride with dilute aqueous hydrofluoric acid treatment of this solid, in suspension in dilute hydrofluoric acid, with sulfur dioxide yields a bluish grey product (possibly PoF2) which rapidly reverts to the original white solid on standing, presumably owing to radio-lytic oxidation 12). The solubility of polonium(IV) in aqueous hydrofluoric acid increases rapidly with acid concentration, indicating complex ion formation (/ft), p. 48). 

The white basic selenate, 2Po02Se03, is obtained by treating polonium V) hydroxide or chloride with selenic acid (0.015 iV-5.0 N) the salt is yellow above 250°C and is stable to over 400°C. It is rather less soluble than the basic sulfate, but the solubility increases a hundredfold in passing from 0.05 N to 5 N selenic acid (10), indicating complex ion formation. 

The white hydrated disulfate Po(S04)2 is obtained in the same way as the basic salt, but with more concentrated acid (>0.5 N). The compound is less soluble than the basic salt and solubility studies indicate complex ion formation when the acid concentration is increased. The deep purple anhydrous salt is obtained by heating the hydrate above 100°C or by washing it with anhydrous ether. It decomposes to the dioxide at 550°C. 

This salt is a white crystalline solid made by treating polonium (IV) hydroxide or chloride with dilute acetic acid. Its solubility in the latter increased from 0.2 mg (of Po210)/liter in 0.1 N acid to 82.5 mg/liter in 2 N acid, indicating complex ion formation. The acetato complex is colorless in solution and appears to be more stable than the hexachloro complex (11). 

This is a white crystalline solid obtained by treating polonium(IV) hydroxide or chloride with aqueous oxalic acid solubility studies indicate complex ion formation (11). 

The characteristic light purple color of [Cr(OH2)6]3+ in chrom alum, K2S04 Cr2(S04)3 12H20, absorption maxima 575 (13.2), 408 (15.5), I = 1.0 M (70), is frequently masked by complex ion formation in other salts, e.g., the green form of CrCl3 6H20, which contains predominantly [Pg.357]

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