Precipitation and Complexation

An important result of the concepts discussed in this section and the preceding one is that precipitation and complexation reactions exert joint control over metal ion solubility and transport. Whereas precipitation can limit the dissolved concentration of a specific species (Me ), complexation reactions can allow the total dissolved concentration of that metal to be much higher. The balance between these two competing processes, taking into account kinetic and equilibrium effects, often determines how much metal is transported in solution between two sites. 

Precipitates and complex salts 1. A more reactive ion displaces a less reactive ion in Q1 (A3) 25... 

Because of their multiple oxidation states, the chalcogens, particularly sulfur, can engage in numerous redox couples participating in acid-base, oxidation-reduction, precipitation, and complexation equilibria. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Polarisation titrations are often referred to as amper-ometric or biamperometric titrations. It is necessary that one of the substances involved in the titration reaction be oxidisable or reducible at the working electrode surface. In general, the polarisation titration method is applicable to oxidation-reduction, precipitation and complex-ation titrations. Relatively few applications involving acid/base titration are found. Amperometric titrations can be applied in the determination of analyte solutions as low as ICE5 M to 10-6 M in concentration. 

In principle, any type of titration can be carried out conductometrically provided that during the titration a substantial change in conductance takes place before and/or after the equivalence point. This condition can be easily fulfilled in acid-base, precipitation and complex-formation titrations and also the corresponding displacement titrations, e.g., a salt of a weak acid reacting with a strong acid or a metal in a fairly stable complex reacting with an anion to yield a very stable complex. However, for redox titrations such a condition is rarely met. 

Gran plots for other types of titrations. Gran64 gave the equations for dibasic acid titration and for precipication, complex-formation and redox titrations especially for the precipitation and complex-formation titrations the equations are complicated. 

Of much greater importance, however, is the amperometric method for precipitation and complex-formation titrations. Here the advantages are as follows ... 

Tab. 2.8 Solvent effects on the formation of precipitates and complexes of the silver ion with halide ions (X-)...

Coulometric titrations have been employed for redox, acid-base, precipitation, and complexation titrations of organics and inorganics in both aqueous... 

Kinetic effects are seen when MgC204(s) fails to form at room temperature from a solution having Q > Ksp but does form when the solution is heated with stirring to 90°C. (4) Several Ag(I) precipitates and complex ions are formed when OH", Cl , NH3, Br", S2032 , I , and S2- are successively added to a solution containing Ag+. 

Other examples of potentiometric titrations include acid-base titrations, in which an indicator electrode provides a response to hydronium ions, such as the glass electrode, quinhydione electrode, or antimony electrode. In precipitation and complexation titrations the indicator electrode should provide the response to the active species in the solution. Thus, during the titration of chloride ions by silver nitrate, a silver electrode is an effective indicator electrode. 

Coulometric titration procedures have been developed for a great number of oxidation-reduction, acid-base, precipitation, and complexation reactions. The sample systems as well as the electrochemical intemediates used for them are summarized in Table 4.1, and indicate the diversity and range of application for the method. An additional specialized form of coulometric titration involves the use of a spent Karl Fischer solution as the electrochemical intermediate for the determination of water at extremely low levels. For such a system the anode reaction regenerates iodine, which is the crucial component of the Karl Fischer titrant. This then reacts with the water in the sample system according to the... 

Despite these recent advances, for many determinations the level of interest is so low that the only feasible approach is to separate and perhaps preconcentrate the analyte prior to determination in a flame or furnace. Some of the possible methods of doing this for high purity mineral acids, such as volatilisation, precipitation and complexation, have been discussed by an IUPAC Commission [5]. An alternative way of categorising the possibilities and a discussion is presented below. 

Acid-base, precipitation and complexation reactions are all examples of exchange (metathesis) reactions in which ions in solution exchange partners , for example ... 

In simple acid-base, precipitation and complexation reactions, no change of oxidation number occurs at any of the atoms involved. 

Heavy metals. The behavior of heavy metals in a landfill leachate plume is simultaneously controlled by sorption, precipitation, and complexation, and proper evaluations of metal attenuation must account for this complex system. Generally, heavy metals do not constitute a groundwater pollution problem at landfills (Arneth et al., 1989), because landfill leachates usually contain only modest heavy metal concentrations, and the metals are subject to strong attenuation by sorption and precipitation in the landfill itself (Kjeldsen et al., 2002). Sulfide-producing conditions result in extremely low solubility of... 

Mineral precipitation and complexation with proteins also contribute to fouling considerably. Adsorbed minerals may serve as salt bridges between the protein and the membrane, which aggravates fouling. In physicochemical conditions that promote calcium phosphate precipitation or calcium-protein complexation, membrane fouling in the filtration of nulk or whey is severe,... 

Hwang, Eui-Deog et al.. Effect of precipitation and complexation on nanofiltration of strontium-containing nuclear wastewater. Desalination, 147, 289, 2002. 

Fig. 12 illustrates the titration of sodium chloride with silver nitrate. After all chloride is precipitated, the addition of excess silver nitrate causes a rapid increase in conductivity. The slope of the initial portion of the curve may be either downward or upward, depending on the relative conductance of the ion being determined and the ion of like charge in the reagent that replaces it. Slow reactions and coprecipitation are sources of difficulty with precipitation and complex-formation titrations. 

In this simple classification of reactions, no distinction needs to be made between acid-base, precipitation, and complex formation reactions they are all coordinative reactions, hence phenomenologically and conceptually similar. 

When all reactants and products are stable, these conclusions are valid. However, when one or both produces are not stable, Eq. (37) is continuously displaced to the right, even when thermodynamics predicts that the equilibrium should lie to the left-hand side. This is a situation akin to that usually considered in equilibrium displacements by physical removal (e.g., distillation, precipitation, and complexation) of one of the products. Thus one may have E° < E and nevertheless observe a reaction between Oi and R2 in Eq. (37). Characteristic examples of such situations are given elsewhere in this book, particularly in Chapter 29, since this phenomenon is the basis of redox catalysis. In such a situation the validity of Eq. (39) is not altered, provided the equilibrium remains established. Yet since (Ri) or (O2) approaches zero, because of the follow-up chemical reaction(s), (Oi) and (R2) may be extremely small and the bracketed term on the left-hand side of Eq. (39) remains considerably smaller than unity, fulfilling the condition imposed by E° < E . 

Inorganic reactions can be divided into four categories, namely redox, acid/ base, precipitation and complexation. By considering complexation reactions as the first stage towards precipitation, and further by considering complexation reactions as a type of acid/base reaction, it is possible to reduce this number to two fundamental categories for inorganic reactions. 

The titration curve of iron(II) with cerium(IV) appears as A in Figure 19-3. This plot resembles closely the curves encountered in neutralization, precipitation, and complex-formation titrations, with the equivalence point being signaled by a rapid change in the ordinate function. A titration involving 0.00500 M iron(II) and 0.01000 M cerium(IV) yields a curve that for all practical purposes is identical to the one we have derived, since the electrode potential of the system is independent of dilution. A spreadsheet to calculate iisysiem a function of the volume of Ce(lV) added is shown in Figure 19-4. 

Summary of Coulometric Titrations Involving Neutralization, Precipitation, and Complex-Formation Reactions... 

Titania/silicas with different molar ratios of Ti to Si were prepared using precipitation and complexing-agent assisted sol-gel method [4]. 

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