Formation and dissolution

The formation and dissolution of CaCOa in the ocean plays a significant role in all of these effects (34)- CaCOa is produced by marine organisms at a rate several times the supply rate of CaCOa to the sea from rivers. Thus, for the loss of CaCOa to sediments to match the supply from rivers, most of the CaCOa formed must be redissolved. The balance is maintained through changes in the [COa] content of the deep sea. A lowering of the CO2 concentration of the atmosphere and ocean, for example by increased new production, raises the [COa] ion content of sea water. This in turn creates a mismatch between CaCOa burial and CaCOa supply. CaCOa accumulates faster than it is supplied to the sea. This burial of excess CaCOa in marine sediments draws down the [COa] - concentration of sea water toward the value required for balance between CaCOa loss and gain. In this way, the ocean compensates for organic removal. As a consequence of this compensation process, the CO2 content of the atmosphere would rise back toward its initial value. 

Thus films can be divided into two groups according to their morphology. Discontinuous films are porous, have a low resistance and are formed at potentials close to the equilibrium potential of the corresponding electrode of the second kind. They often have substantial thickness (up to 1 mm). Films of this kind include halide films on copper, silver, lead and mercury, sulphate films on lead, iron and nickel oxide films on cadmium, zinc and magnesium, etc. Because of their low resistance and the reversible electrode reactions of their formation and dissolution, these films are often very important for electrode systems in storage batteries. 

Sostaric JZ, Caruso-Hobson RA, Mulvaney P, Grieser F (1997) Ultrasound-induced formation and dissolution of colloidal CdS. J Chem Soc Faraday Trans 93 1791-1795... 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

For a stably growing PS the reactions and the rates are different on the pore walls and on the pore bottoms. Furthermore, they are different at different positions of a pore bottom due to the difference in the radius of curvature. The current is the largest at the pore tip because there the radius of curvature is the smallest. It decreases from the pore tip to the pore wall as radius of curvature increases. On the other hand, since the reactions depend on the current density, for a given condition, direct dissolution of silicon dominates at a relatively low current range while oxide formation and dissolution dominate at a higher current range. Thus, oxide formation and dissolution tend to occur at the pore tips at a lower potential than at the side of the pore bottom. There is a distribution of the kind of reactions along the pore bottom. 

Rate of direct dissolution of silicon relative to indirect dissolution via oxide formation and dissolution... 

It is doubtful that formation and dissolution of any mineral in low temperature aqueous solutions has been more fully investigated than the magnesian caicite. This mineral is a preponderant carbonate phase, mostly of biogenic origin, in seawater. Fig. 8.8 gives some data on the solubilities of Mg-calcites as a function of MgC03 content. 

If the same experiment is performed with an n-type Si electrode under identical illumination intensity the anodic photocurrent is found to be larger than for the p-type electrode under cathodic conditions. This increase is small (about 10%) for current densities in excess of JPS. Figure 3.2 shows that in this anodic regime injected electrons are also detected at p-type electrodes. This allows us to interpret the 10% increase in photocurrent observed at n-type electrodes as electron injection during anodic oxide formation and dissolution. 

Small deviations in the Ca " ion ratios in seawater are caused by the formation and dissolution of calcareous (CaCOj) hard parts deposited by marine organisms. Despite the great abundance of these organisms, their activities produce a maximum varia-... 

If f) is greater than 1, the water mass is supersaturated and calcium carbonate will spontaneously precipitate until the ion concentrations decrease to saturation levels. When Cl is less than 1, the water mass is undersaturated. If calcium carbonate is present, it will spontaneously dissolve imtil the ion product rises to the appropriate saturation value. Although calcium is a biointermediate element, it is present at such high concentrations that PIC formation and dissolution causes its concentration to vary by less... 

Changes in phosphate, nitrate, ammonia, and silicate concentrations associated with the biogenic production and destruction of POM can alter seawater alkalinities. These effects are usually so small in scale that they can be ignored. Since the largest biotic impact on alkalinity in oxic seawater is exerted by the formation and dissolution of... 

The mineralogy, distribution, and formation of evaporites are the subjects of this chapter. The role of evaporite formation and dissolution in determining the salinity of seawater is discussed in Chapter 21. 

Bio)chemical reactions may take place prior to or after the continuous separation module and are intended to enhance or facilitate mass transfer, detection or both. The earliest and simplest approach to integrated analytical steps in continuous-flow systems involves a combination of chemical reactions and continuous separations [4,5]. Such is the case with the formation of soluble organic chelates of metal ions in liquid-liquid extractions with the ligand initially dissolved in the organic stream [6], the formation and dissolution of precipitates [7], the formation of volatile reaction products in gas difiusion [8] and that of volatile hydrides in atomic absorption spectro-... 

The bisphosphonates are all analogues of pyrophosphate. They inhibit osteoclast resorption of bone and they are able to inhibit the formation and dissolution of hydroxyapatite crystals, however their exact mechanism is not well understood. Other effects which have relevance for bone homeostasis include inhibition of the activities of PTH, prostaglandins and 1,25-dihydroxy vitamin D. Bisphosphonates bind to bone with high affinity. They have therefore a duration of action that continues long after their use has been stopped. 

The bisphosphonates are analogs of pyrophosphate in which the P-O-P bond has been replaced with a nonhydrolyzable P-C-P bond (Figure 42-4). Etidronate, pamidronate, and alendronate have now been joined by risedronate, tiludronate, ibandronate, and zoledronate for clinical use. The bisphosphonates owe at least part of their clinical usefulness and toxicity to their ability to retard formation and dissolution of hydroxyapatite crystals within and outside the skeletal system. They localize to regions of bone resorption and so exert their greatest effects on osteoclasts. However, the exact mechanism by which they selectively inhibit bone resorption is not clear. 

On most corroding metals, the above reactions occur at an oxidized surface and, depending on the peroperties of the surface layer, passivation may occur by which the kinetics of metal dissolution are substantially supressed either by ohmic, ionic, or electronic transport at a surface passivating film or by electrocatalytic hindrance. In passivation phenomena, a steady state with a balance between the formation and dissolution of the surface film takes place. As a result, the ionic flux of metal ions dissolving through the passivating film is highly reduced. 

The kinetics of formation and dissolution of such new phases, passive... 

In view of the importance of macroscopic structure, further studies of liquid crystal formation seem desirable. Certainly, the rates of liquid crystal nucleation and growth are of interest in some applications—in emulsions and foams, for example, where formation of liquid crystal by nonequilibrium processes is an important stabilizing factor—and in detergency, where liquid crystal formation is one means of dirt removal. As noted previously and as indicated by the work of Tiddy and Wheeler (45), for example, rates of formation and dissolution of liquid crystals can be very slow, with weeks or months required to achieve equilibrium. Work which would clarify when and why phase transformation is fast or slow would be of value. Another topic of possible interest is whether the presence of an interface which orients amphiphilic molecules can affect the rate of liquid crystal formation at, for example, the surfaces of drops in an emulsion. 

---