Stability constants ammonia complexes

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

Table 1.8 Consecutive stability constants, expressed as logXMX., of complexes of ammonia (A), ethylene diamine (B), diethylenetriamine (C) and the anion of ethylenediaminetetraacetic acid (D4 ) at 20°C and 0.1 mKN03 as indifferent electrolyte. (According to J. Bjerrum and G.

MePO2- or PME2- (Table XIX), but the open closed equilibrium lies very much on the side of the chelated form of the complex (87% for the Ca2+ complex - compare 15% for [Ca(atp)]2 and just 7% for [Ca(amp)] (695)). The availability of stability constants both for methylphosphonate and for benzimidazole (a purine model) complexes means that the chelate effect for complexes of (1H-benzimidazol-2-yl-methyl)phosphonate can be discussed without the usual complications, such as the differences between ethane-1,2-diamine and two ammonia or two methylamine ligands and disparities between units (704). 

If ammonium hydroxide (ammonia in water)—a common complexant for Cd in CD—is added to a suspension of Cd(OH)2, the Cd(OH)2 will redissolve, assuming enough ammonia has been added. How much is enough ammonia This can be calculated from the stability constant of the complex between ammonia and Cd. The equilibrium of this reaction to form the cadmium tetraamine complex is given by... 

The participation of Cd(OH)2 in the deposition of CdS (and other metal chalcogenides) has been demonstrated or suggested on many occasions. Kitaev et al. presented a theoretical thermodynamic treatment of the Cd " /ammonia/ thiourea system to show when Cd(OH)2 should be present as a solid phase in the deposition solution [36]. A graphical representation of this analysis is shown in Eigure 3.1. This graph is based on two equilibria the solubility product of Cd(OH)2 and the stability constant of the ammonia (ammine) complex of Cd. Consider first the former ... 

A similar (somewhat more complicated) calculation can be made based on the stability constant of the Cd-tetrammine complex [see Eq. (1.27)] and using the hydrolysis of ammonia (see Refs. 34 and 36 for details of the calculation), which... 

Reference 85 presents the thermodynamic side of the previous paper. It is pointed out that although both ammonia and thiourea are present in the solution, because of the much higher stability constant of the Ag-thiourea complexes compared to the Ag-ammines, essentially all the Ag will be present as a thiourea complex. In this case, it can be assumed that the role of ammonia is only to control pH. 

The stability of a complex ion is measured by its formation constant Kf (or stability constant), the equilibrium constant for formation of the complex ion from the hydrated metal cation. The large value of Kf for Ag(NH3)2+ means that this complex ion is quite stable, and nearly all the Ag+ ion in an aqueous ammonia solution is therefore present in the form of Ag(NH3)2+ (see Worked Example 16.12). 

The papers cited above include studies of La and Lu " EDTA-type complexes, (308, 309, 311, 312) Cu complexes, (314, 315) Fe " complexes, (310) and Sm " complexes. (316, 317) Kinetic parameters and stability constants have been calculated in many cases. In one study (315) the variation in the relaxation time of [Cu(EDTA)(H20)] was followed on gradual addition of ammonia at pH 9. Values were found to shorten and then remain constant after addition of a ten-fold excess of ammonia. This is due to the formation of [Cu(EDTA)(NH3)2] " with a formation constant of 18-8. 

Since ammonia forms stable, water-soluble complexes with many metals, leaching can be carried out under alkaline conditions to give these metals in solution. Of particular interest are the metals copper, nickel and cobalt, which form particularly stable amines lliat have been well characterized as having the following approximate stability constants (at high ionic strength) Cu, 2 = Cu , 4 = 13 Ni , 6 = 9 = 5 Fe ,j52 < 2. 

For the copper complexes the residual effect is positive and the successive stability constants decrease in regular order. For the zinc-ammonia complexes, the residual effect is negative and the first three stability constants are about equal. 

It is observed in the experiment that the iron nail immediately creates a copper deposit in a blue colored copper sulfate solution (see E8.1), whereby this does not happen in the violet colored ammine complex solution. A trace of copper deposit can only be observed after it has been dipped into the complex solution for a while (see E9.6). It is possible to verify this hypothesis with the help of a second reaction, the metal hydroxide precipitation (see E9.6) a greenish blue deposit is commonly observed in the blue solution of hexaaquacopper ions, but not in the solution of tetraamminecopper ions. Apparently, copper ions and water molecules are not very tightly bonded in aqua complexes, but copper ions and ammonia molecules in ammine complexes are there is a weak stability of aquacopper ions, but a great stability of tetraamminecopper complexes. The stability constants can be taken and interpreted if one wants a quantitative explanation of these phenomena. 

As addition of the ammonia solution is continued, the free NH3 molecules displace the water molecules from the pale-blue Cu(H20) + complex ion to form the royal-blue Cu(NH3)4 complex, which is more stable than the water complex (larger stability constant),... 

Table 5.1 The influence of metal ion charge, size and (for d-block elements) crystal field effects on experimentally-determined stability constants of their complexes, illustrated with hydroxide and ammonia ligands. ...

The stability constant of a complex of pyrophosphate with Ni (presumably NiPjO though this is not specified by the authors) was measured by an amperometric titration procedure. Small amounts of the nickel nitrate salt were titrated with the pyrophosphate in an ammonium nitrate buffer solution (0.1 M), adjusted to a pH value of 8 with aqueous ammonia. A Pb02 indicating electrode was used with a platinum foil coimter electrode and a saturated calomel reference electrode. The procedure was found to be very sensitive to pyrophosphate even in the presence of phosphate. 

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