Sulfate-phosphate systems, complexing

Redox potential-pH diagrams can be expanded to cover more complex systems when the concentration of all components are known. For instance, chloride, sulfate, phosphate, and other ions may complex with lead under specified redox potential-pH conditions. The forms of lead in complex water systems can be determined where the concentrations and chemistry of all components are known. However, in natural sediment-water systems, the factors affecting lead chemistry may be in a dynamic state, and the chemistry of all the components is not known. Such is the case with interactions between organic matter and metals. 

Ligand (264) has been prepared and complexed with Ru" to give [Ru(264)(4-Metpy)] + which protonates on the cyclam N atoms to give a series of species up to [Ru(H3264)(4-Metpy)] +. In aqueous solution, the system acts as a selective luminescent sensor for ATP (with respect to phosphate, sulfate, and chloride ions). Oxaaza macrocycles attached to the 4 -position of tpy either directly or with a spacer as in (265) have been synthesized in addition, l,10-diaza-18-... 

Often, with precipitation reactions the starting materials are limited to whatever salts are soluble in the solvent of choice. For water systems this is often limited to metal salts of halides, nitrates, and some sulfates and phosphates. Halides, in particular chlorides, have a pronounced effect on precipitation reactions. Chlorine is able to form bridged complexes much like the hydroxides or oxides of the desired compounds. In addition, acidic environments make possible the oxidation of chloride to chlorine gas, which can further complicate the synthesis. Sulfates and phosphates are typically easier to work with since they do not have the complicated redox behavior of the halides, but they typically have reduced solubilities. Nitrates, although they do not have the solubility concerns of sulfates and phosphates, do have redox complications, which typically result in oxidation of cations. So, the anion, which is expected to act solely as a spectator, in many cases is actually acting as a catalyst. 

Carbonate Complexes. Of the many ligands which are known to complex plutonium, only those of primary environmental concern, that is, carbonate, sulfate, fluoride, chloride, nitrate, phosphate, citrate, tributyl phosphate (TBP), and ethylenediaminetet-raacetic acid (EDTA), will be discussed. Of these, none is more important in natural systems than carbonate, but data on its reactions with plutonium are meager, primarily because of competitive hydrolysis at the low acidities that must be used. No stability constants have been published on the carbonate complexes of plutonium(III) and plutonyl(V), and the data for the plutoni-um(IV) species are not credible. Results from studies on the solubility of plutonium(IV) oxalate in K2CO3 solutions of various concentrations have been interpreted to indicate the existence of complexes as high as Pu(C03) , a species that is most unlikely from both electrostatic and steric considerations. From the influence of K2CO3 concentration on the solubility of PuCOH) at an ionic strength of 10 M, the stability constant of the complex Pu(C03) was calculated (10) to be 9.1 X 10 at 20°. This value... 

Figure 15-2 (left) depicts several titration curves of Fe(II) with permanganate. Beyond the end point the experimental curves differ from the theoretical shape, which is nearly flat beyond the end point (5-equivalent reduction). The essential symmetry of the curves suggests that the potential is determined by the Mn(III)-Mn(II) couple beyond the end point. Evidence for this behavior can be seen in solutions containing sulfate or phosphate, which tend to stabilize Mn(III) (Section 17-1). That sulfuric and phosphoric acids have about the same effect before and after the end point is consistent with the similarity of the behavior of the Mn(III)-Mn(II) and the Fe(III)-Fe(II) systems with respect to changes in activity coefficients as well as with respect to hydrolysis and complex formation. 

---