Metal dissolution isotherm

Agulyansky et al. [492, 493] investigated the complex structure and composition of solid phases precipitated by ammonia solution from experimental and industrial niobium and tantalum strip solutions. Fig. 136 shows isotherms (20°C) of Nb205 content versus pH for solutions prepared by the dissolution of (NH4)3NbOF6 and (NH4)2NbOF5 in water and of Nb metal in... 

The rate-determining step is detachment of the coordination complex at the surface, and the rate of dissolution is proportional to the degree of ligand binding on the mineral surface or the concentration of ligand in bulk solution to fractional power (p), the slope of the Freundich adsorption isotherm for [HA-] on the mineral surface for a 1 1 metal-to-ligand complex ... 

Propylene dissolves in two modes, a physical dissolution mode in the matrix and a binding mode resulting from a reversible chemical reaction with metal complexes. Henry s law commonly expresses the physical dissolution mode for small molecules, while the Langmuir model adequately describes the reversible binding mode, as shown in Eq. (9-2). Mathematically, a Langmuir adsorption isotherm for a small molecule in a porous media is identical to the expression of the olefin concentration bound to the metal complex. 

The adsorption of an inhibitor onto the metal surface slows the rate of corrosion by blocking part of the surface. The extent of inhibition depends on the equilibrium between the dissolved and adsorbed inhibitor species, expressed by the adsorption isotherm. This mechanism which is particularly important in acids will be discussed in the next section. (Sect. 12.4.2). Certain inhibitors promote the spontaneous passivation of a metal and thus drastically reduce the corrosion rate. Oxidizing species such as chromates fall in this category. Buffer agents that maintain a high pH at the metal surface also favor the passive state. Other inhibitors lead to the formation of surface films by precipitation of mineral salts or of weakly soluble organic complexes. These films reduce the ability of oxygen to reach the surface and, in addition, they may impede the anodic dissolution reaction. 

As noted above, the mass transfer kinetics of temperature gradient loops are usually described with reference to dissolution in the hot leg. It is possible to quantitatively study the dissolution step using the rotating cylinder technique. Unlike loop studies, this technique allows one to study dissolution in a system where the hydrodynamic conditions are fully defined. Experimentally, solid cylinders of the test material are rotated at various speeds in an isothermal liquid-metal bath. Changes in the concentration of solid in the liquid and changes in the cylinder radius are determined as a function of time. With these data it is possible to determine the mass transfer coefficient and the rate-controlling step for dissolution. 

In an isothermal system, with no forced convection of the liquid metal, the dissolution process will stop when the solubility limit of the dissolved species is reached. However, in general, the system is anisothermal, the solubility increasing with temperature, dissolution will occur in hot zones and deposition will occur in cold zones of the system. 

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