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ON THE SPECIES PRESENT IN AQUEOUS SOLUTIONS OF "SALTS1' OF POLYVALENT METALS' 111. Additional Experimental Methods and Quantitative Data LEWIS POKRAS Polytechnic Institute of Brooklyn, Brooklyn, New York
FORMATION OF COMPLEXES IN SOLUTION
To this point our attention has been focused primarily on hydroxyl complexes of the metal ions. We now turn to complexes with other common anions. While complexes of the type called by Biltz "penetration" or "inner" complexes, which usually involve essentially covalent binding, are well known as a result of the extensive and fundamental researches of Werner, there is a broad field of aqueous chemistry not specifically covered by the Werner concepts. There is increasing evidence for the type of ionicallybound complex described as labile in many systems not previously suspected of exhibiting such effects. While labile in the sense that the particular coordinating groups (ligands) are continually exchanging with similar free ions in solution, many of these complexes posThe last in a. series of three papers on this topic. For the 33, 152, 223 (1956). first and second papers, see J. CHEM.EDUO., The numbers of equations, structures, tables, figures, and literature cited in this paper follow consecutively those assigned in the second paper. TABLE 12 Cornplexing Ability of Various Substances with Zirconium(1V) Substance Acetic acid Orthoboric acid Malonio acid Succinic acid Glutaric acid Carbonic acid Fumaric acid Metasilicic acid Maleic acid Orthophosphoric acid Hydrogen peroxide Bisulfate ion Trifluoroacetie acid Hydrofluoric acid Oxalie acid Hydrophlario ac$ Ni!~ic aci;
Molar cone. 1.0 0.1 0.01 0 . 005 0.1 CO, (1 atm.) 0.05 0.01 0.05 0.012 0.015 0.0031 0.11 10-6 0 . 001 1.20n 2 nna 1.2Om 2. 0oa
Replaces part or all of HCIO,.
O/o Zr uneomplezed
100 103 100 100 94 90 88 8i 74 68 63 56 51 22 0.36 29 10 --
30 20
sess surprisingly high stability constants. Appreciable fractions of polyvalent metal ions may be present in solution as complex ions, with marked effects on the physicochemical and analytical properties of such systems. These salts should therefore be described as weak electrolytes since they do not dissociate completely into discrete anions and cations. I t matters little whether the phenomenon is described as ion-pairing or complex formation; the practical results are identical. Much of the recent work in this field, plus an excellent theoretical discus;;ion of the equilibria, was summarized by Jannik Bjermm in a Chemical Reviews paper (65). Bjerrum has shown that complex formation occurs in successive steps of the type illustrated in equations (5-7). In most cases only the over-all equilibrium constants corresponding to equation (7) are found in the literature. However, Bjerrum discusses methods of evaluating the separate equilibria and gives theoretical consideration to the relative magnitudes of the successive eauilibrium constants. Since this review is readily avafiable, the following discussion will be limited to illustration of the magnitudes of the equilibrium constants in some important chemical systems and t o a few aspects of the problem which are of especial interest. THE COMPLEXITY CONSTANTS OF SEVERAL METAL COMPLEX SYSTEMS
The study of ferric complexes by Rahinowitch and Stockmayer (33) yielded the following values of the equilibrium constants for formation of various halide complexes a t 25%. P
=
0
u=l
I n 0.1 molar FeC13, for example, considering only the equilibrium of equation (35),at least 55 mole per cent of the total Fe must be present as complex species. If the additional equilibria as well as the hydrolytic reactions are considered then only a minor fraction of the
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total ion call be preseut as TABLE 13 the simple [Fe(H20),]+3ion Equilibrium Constants for Formation of Uranyl Complexes a t p = 2.000 in this solution. KI at t = 'C. The data of Connick and McVey on Zr(IV) comh'quilibriunz 10" 66' 40' plexes, also cited previ- UO.++ + 2 ~ 1 -= uo2c~ 0.58 0.88 1.14 ously, leads to their data U02+++ NO3- e [UOINOaI 0.30 0.24 0.17 reproduced in Table 12. 580 63 710 76 820 96 All solutions contained uoS++ + HSO&-e [ U O ~ H S O ~ 6.1 6.4 6.5 trace concentrations of Zr U02++ + R F [UO*Fl++ H+ 55 26 21 a t 25°C. and were 2.00 M in HC104. TABLE 14 A similar study, also based on extraction of conlplexes, Effect of Ionic Strength on Complex ( U O I C ) Formation has been made of the uranyl ion by Day and Powers at 25O C. (56). The uranyl ion is of somewhat special interest p = 0.05 0.25 0.50 1.00 since it represents the case of a metal ion with such a K, = 51 37 24 27 great tendency towards covalent biuding that it forms an extremely stable complex with two oxygens, UOz++. The latter species then exhibits many of the properties OTHER EVIDENCE FOR AQUEOUS COMPLEXES of a simple divalent ion. Like many other divalent The application of ion-exchange techniques t o the ions, it can form further complexes with hydroxyl ion study of complexes has become a useful tool in the past or other anions. Thus, we have the interesting case of ten years. For example, King and Dismukes have complexes of a complex ion. The data are summarized shown (59)that the sequence of complexes-[Cr(H20)*, in Tables 13 and 14. (SCN).]+(3-n), (n = 0,1. . .6)--exists in Cr+"(SCN)systems, partly by passing such systems through a SPECTROPHOTOMETRIC EVIDENCE FOR COMPLEX cation-exchange column. FORMATION It has been well established that such resins hold a It has been noted above that spectrophotoinetric investigations have been of great utility in establishing mixture of ions in accordance with ionic charge; in the existence of aqueous complexes and in evaluating general, the higher its charge, the more firmly is an ion the corresponding equilibrium constants. Two addi- bound by the resin. Thus, in the Cr+"(SCN)- sys[Cr(HzO)5(SCN)]++, tional applications of this method appear of interest tem, the cations [C~(HZO)S]+~, and [Cr(H20)r(SCN)2]+were adsorbed by the resin. here. In coutradiction to what would be predicted from (Neutral and anionic complexes mould not, of course. solubility product considerations, the solubility of silver be adsorbed by a cation-exchange resin.) On eluting iodide increases rather markedly with either increasing with acids of increasing concentration it was found (by (Ag+) or (I-) outside a region of low values of these analysis of SCN/Cr ratios in the eluat,e) that the ion of is eluted first, then concentrations. This effect is reported to be consider- lowest charge, [Cr(H20)4(SCN)a]+, and finally [Cr(H20)6]+" ably greater than can be explained by ordinary electro- [Cr(H20)6(SCN)]++, The electromigration technique has also been applied lyte effects and changes in activity coefficients. King, Krall, and Pandow (57) on the basis of spectrophotometric, solubility, and other studies, have shown that these systems contain not only polyiodide complexes like Ag13-- and Ag14-a, but also complexes polynuclear in silver, for example, Ag31,-(n-3'. These authors suggest that similar complexes are likely in other systems involving iodide, as for example with Cu+ and Auf ions. The importance of various complex aud hydroxy species in determining the properties of solutions is further illustrated by the extreme case of pentavalent antimony in HCl, .as studied spectrophotometrically by Nenmann (68). The results of this study, made primarily in the ultraviolet spectral region, are presented in Figure 21. The spectral curve for the complex [SbC16]- was established by study of HSbCls in ether. All of the important species which exist in this system are complex (according to Neumann). Thus [Sb(OH)Cls]- predominates in 8 fMHC1, [Sb(OH),C14]5 7 9 I1 in 6 M acid, and [Sb(OH)zC13]-and more highly hydm[HCI] lyzed forms a t (HC1)
