Dissolved species

Preferential deposition of weaMy adsorbed dissolved components on the outer surface of the catalyst body is a similar phenomenon but can occur after impregnation during removal of the solvent. When the impregnated particle is heated, the Hquid phase expands and coats the exterior surfaces with dissolved species as the solvent evaporates. In particularly severe cases, the entire outer surface of the catalyst can become completely coated with a soHd that blocks access to the underlying pore stmcture. 

There are two general theories of the stabUity of lyophobic coUoids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these coUoids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of coUoids. 

In each form of attack, solute concentration differences arise primarily by diffusion-related processes. As a consequence, stagnant conditions may promote attack, since concentration gradients near affected areas are reduced by flow and these concentration gradients supply the energy that drives diffusion. Similarly, high concentrations of dissolved species increase attack. Elevated temperature usually stimulates attack by increasing both diffusion and reaction rates. 

Components in which water temperature changes abruptly with distance, such as heat exchangers, tend to accumulate precipitates. Heater surfaces also accumulate precipitates if the dissolved species have inverse temperature solubilities. Systems in which pH excursions are frequent may accumulate deposits due to precipitation processes. Plenum regions, such as heat exchanger headboxes, tend to collect deposits. 

Whatever the water composition, corrosivity can be increased by evaporation, which may elevate pH by increasing concentrations of ions in the remaining liquid. This is the reason that cooling systems that experience boiling and/or large evaporative losses without sufficient makeup water additions may be especially prone to attack. Cycling may also increase dissolved species concentrations. 

Repeat the above steps for dissociation of HCl in waU (label the vertical axis aqueous phase energy). T1 energies contained in this sequence have been obtaine by calculating the effect a polar medium like water won have on the dissolved species. How many energy minin are there What species do these minima correspond t(... 

A wide range of structural techniques may be utilized for the study of ionic liquids and dissolved species. Overall, in both high-temperature and low-temperature ionic... 

If a fixed volume of the solution is chosen as the basis, the concentration of a component (or dissolved species) can be expressed in one of the following ways ... 

It is worth emphasising too, that the position of those lines representing equilibria with the dissolved species, M, depend critically on the solubility of the ion, which is a continuous function of pH. For example, iron in moderately alkaline solution is expected to be very passive and so it is in borate solutions (in the absence of aggressive ions). However, the anodic polarization curve still shows a small active loop at low potential. 

Equation 10.2, which involves consumption of the metal and release of electrons, is termed an anodic reaction. Equation 10.3, which represents consumption of electrons and dissolved species in the environment, is termed a cathodic reaction. Whenever spontaneous corrosion reactions occur, all the electrons released in the anodic reaction are consumed in the cathodic reaction no excess or deficiency is found. Moreover, the metal normally takes up a more or less uniform electrode potential, often called the corrosion or mixed potential (Ecotr)-... 

Click Coached Problems for a self-study a dissolved species or its percentage in a solid mixture. This is done by carrying out a titra-... 

In reaction (7), all of the molecular species involved in the equilibrium are in the solution as dissolved species. Though the equilibrium relationship that exists among the concentrations is a little more complicated than in the solubility product expressions, the guiding principles are the same. 

The complexation of Pu(IV) with carbonate ions is investigated by solubility measurements of 238Pu02 in neutral to alkaline solutions containing sodium carbonate and bicarbonate. The total concentration of carbonate ions and pH are varied at the constant ionic strength (I = 1.0), in which the initial pH values are adjusted by altering the ratio of carbonate to bicarbonate ions. The oxidation state of dissolved species in equilibrium solutions are determined by absorption spectrophotometry and differential pulse polarography. The most stable oxidation state of Pu in carbonate solutions is found to be Pu(IV), which is present as hydroxocarbonate or carbonate species. The formation constants of these complexes are calculated on the basis of solubility data which are determined to be a function of two variable parameters the carbonate concentration and pH. The hydrolysis reactions of Pu(IV) in the present experimental system assessed by using the literature data are taken into account for calculation of the carbonate complexation. 

The dissolved species are considered to be a composite of numerous hydroxides, carbonates, and hydroxocarbonates of Pu(IV), which can be expressed by... 

CO2 ) The ocean to atmosphere flux, which is dependent on the concentration of the dissolved species C02(aq), is related to the total carbon content in the surface layer (Ms) by... 

Finally, it must be stressed that diffusion of dissolved species in solutions is a key physicochemical process for the sea/sediment interaction and energy exchange at the sediment-water interface. The reader is referred to Cussler (1984) for a more comprehensive presentation of diffusion in fluid systems. 

In the case of substances having at least some solubility (e.g., 10 mol/L), the reaction often follows a scheme involving dissolved species ... 

Reactions of this type can also occur when the conductivity of one of the phases is very low or practically zero. In these reactions, the sites of reactant lattice destruction and product lattice formation are spatially separated. During the reaction, dissolved species diffuse from the dissolution sites to sites where they undergo further reaction and form the nuclei of the new phase. The length of the diffusion pathway in the sofution depends on the degrees of dispersion of the originaf reactant and resufting product, and most often is between 10 and 10 m. 

This reaction means that higher /hjS causes higher concentration of Au(HS)2 as well as higher fo. Higher fujS, /ss and /o, of back-arc hydrothermal fluids can explain higher Au, Hg, As and Sb who.se dominant dissolved species are probably thio complexes. 

Potential cycling has been found to accelerate Pt dissolution compared with poten-tiostatic conditions. The dissolution mechanisms and dissolved species involved in this process are unclear [Johnson et al., 1970 Kinoshita et al., 1973 Ota et al., 1988 Rand and Woods, 1972]. Darling and Meyers have developed a mathematical model based on (9.5)-(9.7) to smdy Pt dissolution and movement in a PEMFC during potential cycling from 0.87 to 1.2 V [Darling and Meyers, 2003, 2005]. Severe Pt dissolution occurs when the potential switches to the upper limit potential (1.2 V), and then stops once a monolayer of PtO has formed. The charge difference between the anodic and cathodic cycles was found to be consistent with the amount... 

Unlike solid electrodes, the shape of the ITIES can be varied by application of an external pressure to the pipette. The shape of the meniscus formed at the pipette tip was studied in situ by video microscopy under controlled pressure [19]. When a negative pressure was applied, the ITIES shape was concave. As expected from the theory [25a], the diffusion current to a recessed ITIES was lower than in absence of negative external pressure. When a positive pressure was applied to the pipette, the solution meniscus became convex, and the diffusion current increased. The diffusion-limiting current increased with increasing height of the spherical segment (up to the complete sphere), as the theory predicts [25b]. Importantly, with no external pressure applied to the pipette, the micro-ITIES was found to be essentially flat. This observation was corroborated by numerous experiments performed with different concentrations of dissolved species and different pipette radii [19]. The measured diffusion current to such an interface agrees quantitatively with Eq. (6) if the outer pipette wall is silanized (see next section). The effective radius of a pipette can be calculated from Eq. (6) and compared to the value found microscopically [19]. 

---