Equilibrium precipitation

The chemist, fathoming the whims of attractions. Artfully imposes on them their precise meetings. 

This photograph by Ansel Adams 4n,0L was taken using film coated jfj with water-insoluble silver j J/ Jfi I bromide, AgBr. V  

In this chapter, we consider two types of equilibria, both in water solution- 

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Equilibria such as these have applications in fields as diverse as geology, medicine, and agriculture. In chemistry you are most likely to meet up with precipitation equilibria in the laboratory when you carry out experiments in qualitative analysis.  

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By convention, [HA(s)] = [B(s)] = 1. Eqs. (6.1) represent the precipitation equilibria of the uncharged species, and are characterized by the intrinsic solubility equilibrium constant, Sq. The zero subscript denotes the zero charge of the precipitating species. In a saturated solution, the effective (total) solubility S, at a particular pH is defined as the sum of the concentrations of all the compound species dissolved in the aqueous solution ... 

From a structural point of view Bruno s hypothesis is sound. In chemical terms the hydrolysis proceeds with the formation of hydroxide bridges up to the formation of the trimer. Then at high concentrations trimeric units are fused together to form the higher aggregates, that can be seen as precursors to the structure adopted by Be(OH)2 in the solid state. (See Section IV, C, precipitation equilibria). Speciation diagrams for Bruno s model are shown in Fig. 3. Two points are noteworthy in these diagrams. In the first place the concentrations of the... 

Once precipitation begins, a quasi-steady state will eventually be attained in which the soil pe and pH are poised by the redox and precipitation equilibria operating. In the transition to the steady state, protons will be provided by dissociation of acids in the soil solution—e.g. H2CO3 derived from C02-and by reactions with the soil exchange complex. The course of reduction and the eventual steady state will depend on these reactions and it is therefore necessary to allow for them in predicting what the steady state conditions will be. 

Hedlund, T. Studies of complexation and precipitation equilibria in some aqueous aluminium(III) systems Ph.D. thesis. University of Umea, Umea, Sweden, 1988. 

To construct the potential pH diagrams of the different elements, all their possible redox processes with water, oxygen, and hydrogen have to be taken into account, and the electrochemical potentials have to be calculated. In addition, the dissolution/precipitation equilibria (e.g., hydrolysis) have to be taken into consideration, as well. The main dissolved ions in groundwater (calcium, magnesium, sodium, and potassium cations hydrocarbonate/carbonate, chloride,... 

The dissolution-precipitation equilibria are characterized by the thermodynamic equilibrium constants and similarly to potential-pH diagrams (Figures 1.4 and 1.5), stability diagrams can be constructed. The axes of the stability diagrams show the concentration of the relevant species, including pH (the concentration [activity]... 

Precipitation equilibria are described in part in Chapter 7. An important type of precipitation equilibrium considered here involves a coordination reaction of a metal ion with univalent anions to form an uncharged chelate that is insoluble. In general, it is necessary to consider stepwise equilibria ... 

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