Precipitation reactions defined

It is necessary to consider a number of equilibrium reactions in an analysis of a hydrometallurgical process. These include complexing reactions that occur in solution as well as solubility reactions that define equilibria for the dissolution and precipitation of solid phases. As an example, in analyzing the precipitation of iron compounds from sulfuric acid leach solutions, McAndrew, et al. (11) consider up to 32 hydroxyl and sulfate complexing reactions and 13 precipitation reactions. Within a restricted pH range only a few of these equilibria are relevant and need to be considered. Nevertheless, equilibrium constants for the relevant reactions must be known. Furthermore, since most processes operate at elevated temperatures, it is essential that these parameters be known over a range of temperatures. The availability of this information is discussed below. 

Table 4 summarizes some experimentally determined Ka values for a variety of apatites and contaminants. Some values are veiy large (> 10,000), indicating the propensity for apatites to sorb and/or induce surface precipitation reactions. These values are useful in terms of understanding the broad affinity that apatites have for various solutes. However, they reflect the operationally defined conditions particular to each partition study (pH, I, solid-to-liquid ratio, and apatite type). 

Dissolution-precipitation reactions often exhibit characteristic time scales that are much larger than those for complexation reactions in aqueous solution. When this is true, the aqueous species in a soil solution will come to mutual equilibrium long before they equilibrate with the solid phase via the reaction in l-q. 3.1. It is possible under these circumstances to define two useful criteria for dissolution precipiialionequilibrium, the ion activity product (IAP) ... 

Titration curves used in precipitation reactions usually use a concentration-dependent variable called the p function rather than the concentration itself The p function for a species X is defined as follows ... 

The Snake River Plain aquifer is also an example of an aquifer in which groundwater chemistry is not strongly affected by directions and rates of groundwater flow. There are no well-defined changes in solute concentrations along the aquifer flow path (Table 1). As we shall see in subsequent examples, this insensitivity to directions of groundwater flow is unusual, and is due to the relatively rapid nature of ongoing dissolution and precipitation reactions in the Snake River Plain aquifer. 

Standard substances in clinical chemistry include primary standards, which can be obtained sufficiently pure to be used for the preparation of solutions by weighing or by reference to other definable physical characteristics (e.g., constant-boiling hydrochloric acid). Primary standard chemicals are available for acid-base reactions, precipitation reactions, oxidation-reduction reactions, etc. (V3), and are used in these various categories of analytical determination to validate the preparation of solutions of other chemical substances which cannot be obtained in a form suitable to meet the criteria demanded for a primary standard. Following their calibration in terms of a primary standard, these other chemieals can act as secondary standards. 

Reagents are frequently used for precipitation reactions and the defined unit of reaction is the amount which provides Aa positive (or negative) charges. For example, for AgN03 one equivalent is the same as one mole, but for BaC one equivalent is one-half mole. 

Usually when reaction paths are simulated, the irreversible reactant is an unstable mineral or a suite of unstable minerals that is, the stoichiometry of the irreversible reaction is fixed. Evaporation poses a special problem in reaction path simulation because the stoichiometry of the irreversible reaction (defined by the aqueous solution composition) continually changes as other minerals precipitate (or dissolve). In the second problem (above) evaporation of seawater was simulated by irreversible addition of "sea salt", that is, a hypothetical solid containing calcium, magnesium, sodium, potassium, chloride, sulfate and carbon in stoichiometric proportion to seawater. The approach used was valid as long as intermediate details of the reaction path are not required. The reaction path during evaporation could be solved in PHRQPITZ by changing the stoichiometry of the irreversible reactant (altered "sea salt") incrementally between phase boundaries, but this method would be extremely laborious. 

Chemical reactions involving precipitation of elemental (metals) or binary phases (metal oxides, nitrides, chalco-genides, etc.) are relatively straightforward. The process becomes more complicated in the simultaneous precipitation of various components from the reaction mixture this is especially challenging when several stable compositions exist in a multi-component system. The products of room-temperature precipitation reactions are usually amorphous, and calcination or annealing steps are inevitable to obtain a defined material. Since the nature of the amorphous intermediates is difficult to determine by experimental techniques, any inhomogeneity with respect to the elemental distribution shows up, in the form of constituent segregation and secondary products, in the final material. 

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

This section presents the governing equations for fluid flow in porous media with precipitation reactions, dissolution of minerals, and laminar premixed combustion, as well as similarity parameters. The model is based on Navier-Stokes equations. For modeling precipitation and dissolution, we used the Boussinesq approximation and Darcy s law, which wiU not be considered in the case of combustion in porous media. Darcy s law, in general, defines the permeability or the ability of a fluid to flow through a porous medium [29]. Another difference from the model of combustion lies in the equations for species, which are based on concentrations. 

In a glovebox, [Mo(=CHt-Bu)Cp(pyridine)(NO)] (102 mg, 0.30 mmol) and PhjSiOH (83 mg, 1.0 equiv) were weighed into the reaction vessel. Benzene (20 mL) was vacuum-transferred onto the solids. The mixture was then warmed to rt and stirred for 1.5 h. Over the course of the reaction a color change from amber to dark red-brown occurred. The solvent was removed from the final mixture in vacuo, and the residue was extracted with EtaO (2x25 mL). The extracts were filtered through Celite and the filtrate was concentrated under reduced pressure to incipient precipitation. Well-defined red blocks formed overnight and were isolated by cannulation yield 121 mg (75%) IR (Nujol) Vmaxi (NO) 1607 (vs) cm HNMR (benzene-dg, 8) 3.79 (d, IH, CHH, Jhh = 9.9Hz), 0.99 (d, IH, CHH, Jhh 9.9 Hz). 

Zn +, Cd +, U02, Pb +, Ni +, and Mn + ions with ferrocyanide ion. Numerous poorly soluble ferrocyanides indeed have a well-defined composition in some given conditions. At the equivalence point, the ferrocyanide ion in excess is detected by an external indication. For example, Zn + is determined in diluted strongly acidic medium by the following precipitation reaction ... 

Because many redox reactions involve H, OH , or insoluble ionic compounds, it is easy to confuse a redox reaction with an acid-base or a precipitation reaction. It is important that you remember the defining features of each type of reaction. Precipitation reactions involve the combination of ions in solution to produce an insoluble precipitate, acid-base reactions involve proton (H ) transfer, and redox reactions involve changes in oxidation states. 

Ternary Alloys. Almost ah commercial ahoys are of ternary or higher complexity. Ahoy type is defined by the nature of the principal ahoying additions, and phase reactions in several classes of ahoys can be described by reference to ternary phase diagrams. Minor ahoying additions may have a powerflil influence on properties of the product because of the influence on the morphology and distribution of constituents, dispersoids, and precipitates. Phase diagrams, which represent equhibrium, may not be indicative of these effects. 

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