Supersaturation of carbon

In areas where supersaturation of carbonate minerals exists, it was found that the total amount of magnesium plus sulfate was greater than the amount of calcium. In that case, we concluded that calcite was precipitating, and this equivalent amount of entropy change was added to the total chemical entropy production. 

Among Fe, Cu, Ti, Ni, Mo andNb, Fe shows the best catalytic effect on diamond nucleation. Pd has also a positive catalytic effect on diamond nucleation, while Co suppresses diamond nucleation by promoting soot formation. The strong reactivity of these metals with carbon, the formation of metal carbides, the supersaturation of carbon in/on the metals and/or the deformation of graphite sheets by metal atoms to form diamond structure have been proposed as possible mechanisms governing the catalytic effects. 

Aragonite. Calcium carbonate is a common deposit in shallow tropical waters as a constituent of muds, or in the upper part of coral reefs where it precipitates from carbon dioxide-rich waters supersaturated with carbonate from intense biological photosynthesis and solar heating. Deposits of ooHtic aragonite, CaCO, extending over 250,000 km in water less than 5 m deep ate mined for industrial purposes in the Bahamas for export to the United States (19). 

If small specimens are prepared in which the austenite can be cooled to 250—500°C sufficiendy rapidly to avoid the above microconstituents, and transformed at temperatures in this range, the formation of a completely different phase, a bcc a-phase supersaturated with carbon and containing small cementite particles (bainite), which is both strong and tough, occurs. Bainite is rarely found in plain carbon steels, but it can be obtained in commercial practice by judicious alloying and is increasing in importance. 

You can observe heterogeneous nucleation easily in carbonated drinks like "fizzy" lemonade. These contain carbon dioxide which is dissolved in the drink under pressure. When a new bottle is opened the pressure on the liquid immediately drops to that of the atmosphere. The liquid becomes supersaturated with gas, and a driving force exists for the gas to come out of solution in the form of bubbles. The materials used for lemonade bottles - glass or plastic - are poor catalysts for the heterogeneous nucleation of gas bubbles and are usually very clean, so you can swallow the drink before it loses its "fizz". But ordinary blackboard chalk (for example), is an excellent former of bubbles. If you drop such a nucleant into a newly opened bottle of carbonated beverage, spectacular heterogeneous nucleation ensues. Perhaps it is better put another way. Chalk makes lemonade fizz up. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Above the eutectic temperature in the iron-FcsC system (1130°C)12, growth of large graphite plates and flakes occurs from the liquid phase. Carbon precipitates in the form of highly ordered graphite crystals from molten iron supersaturated with carbon. The Raman spectrum for chlorination at 1200°C is shown in Fig. 2c. A very strong and narrow... 

Figure 8-10 shows the first 200 years of evolution of the concentrations at the same depths as plotted in Figure 8-9. The concentrations of both total carbon and calcium at a 500-centimeter depth decrease at first and then increase. This decrease occurs because I used starting values equal to seawater values. The waters were initially supersaturated and started out by precipitating calcium carbonate. This initial precipitation was overwhelmed at the shallower depths by the rapid addition of carbon as a result of respiration.

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

Iron and Potassium. — Decompose 10 gm. of sodium oxalate by igniting moderately in a platinum crucible, removing the last traces of carbon by cautious ignition with a blast lamp. The residue, when treated with warm water in a platinum dish, should be completely soluble, and should leave at most a scarcely weighable trace of undissolved iron oxide. The solution is filtered if necessary, and supersaturated with hydrochloric acid as free from iron as possible. It is then evaporated in a platinum dish on the water-bath, and the residue dried for two hours in the drying oven at 120° C. The residue must dissolve clear in water and the solution should give ... 

Potassium niobate, 8K20.7Nb20B.82H2Q, predpitates out on slow evaporation of solutions of either the 4 3 salt or the 7 6 salt. It forms rhombic bipyramids which can be recrystallised unchanged a b c=0 9584 1 0 7083. Twenty-three molecules of water are lost at 100° C. It readily yields supersaturated solutions. When its aqueous solutions are treated with a current of carbon dioxide they predpitate salts which contain a larger proportion of niobic add.9... 

The increased amount of carbonate ion present causes the supersaturation of calcium carbonate, which comes out of the solution. As the carbon dioxide is least soluble at the temperature of the hot metal surface, the calcium carbonate has its greatest supersaturation at the surface and therefore tends to deposit there. 

Scaling occurs if a dissolved salt exceeds its saturation point. Most cooling systems can tolerate a slight degree of supersaturation of (for example) calcium carbonate for short periods however, the degree of scaling and rate of deposition are dependent on a number of critical factors. The primary factors are degree of supersaturation, residence time, temperature, active deposition sites, pH, and the DCA product employed. 

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