Calcium carbonate conditional solubility product

Calcium carbonate scaling is perhaps the most common type of problem, with the possible exception of microbial fouling, that RO membranes experience. Fortunately, it is fairly easy to detect and handle. Basically, if the ion product (IP) of calcium carbonate in the RO reject is greater than the solubility constant (Ksp) under reject conditions, then calcium carbonate scale will form. If IP < Ksp/ scaling in unlikely. The ion product at any degree of saturation is defined as ... 

The numerator of the right side is the product of measured total concentrations of calcium and carbonate in the water—the ion concentration product (ICP). If n = 1 then the system is in equilibrium and should be stable. If O > 1, the waters are supersaturated, and the laws of thermodynamics would predict that the mineral should precipitate removing ions from solution until n returned to one. If O < 1, the waters are undersaturated and the solid CaCOa should dissolve until the solution concentrations increase to the point where 0=1. In practice it has been observed that CaCOa precipitation from supersaturated waters is rare probably because of the presence of the high concentrations of magnesium in seawater blocks nucleation sites on the surface of the mineral (e.g., Morse and Arvidson, 2002). Supersaturated conditions thus tend to persist. Dissolution of CaCOa, however, does occur when O < 1 and the rate is readily measurable in laboratory experiments and inferred from pore-water studies of marine sediments. Since calcium concentrations are nearly conservative in the ocean, varying by only a few percent, it is the apparent solubility product, and the carbonate ion concentration that largely determine the saturation state of the carbonate minerals. 

How well can we presently determine the saturation-horizon depth (where D = 1) for calcite in the sea If we assume that we know the calcium concentration exactly, then the error in D is determined by the errors in and the measured carbonate ion concentration, [CO ]. Mucci (1983) was able to determine repeated laboratory measurements of the apparent solubility product, p, at 1 atm pressure to — 5%, and the pressure dependence at 4 km is known to 10%. These errors compound to 11% in the value of K sp (4 km). Carbonate ion concentrations in the sea are almost always calculated from Ax and Die. Being slightly conservative about accuracy of these values in ocean surveys ( 4p.eqkg for Ax and 2p.molkg for DIG they can be determined with errors about half these values if conditions are perfect), and assuming we know exactly the value of the... 

Fortunately, calcium carbonate scale does not always precipitate immediately when the solubility product is exceeded. A considerable degree of supersaturation can be tolerated for a while. In fact, the method of contact stabilization (II) for preventing scale in sea water stills is based on this fact. If supersaturation conditions near the membrane are not allowed to prevail long enough, scale will not form there. Thus it becomes very important to study the process of... 

The solubility product constant, K p, for calcium carbonate at room temperature is approximately 3.0 X 10 . Calculate the solubility of CaCOj in grams per liter under these conditions. 

Calcium carbonate scale formation increases with increasing salt concentration and increasing temperature at a pH above 8.0. Below pH 8.0 the salt is quite water-soluble. It is frequently found on neutral sized fine paper machines located in hard water areas, where conditions for scale formation of this type are favourable, since the pH on such a machine is alkaline and the concentration of dissolved calcium and carbonate ions is high. On a paper machine it is not usually detrimental to paper production in itself unless it becomes extensive. However, it does provide an excellent surface for bacterial growth and deposition of organic material and for this reason should be prevented. 

The solubility of the precipitate is lowest between pH 4 and 6. Its solubility product is around 2.3-10 (at 20 °C). If a minor excess of the precipitating agent (Le., sodium tetraphenylborate) is available in solution, the solubility of the precipitate is negligible. Co-precipitation of calcium and magnesium ions may lead to serious errors when they are first precipitated. This interference is minimized, however, as carbonates which, in contrast to potassium tetraphenylborate, are soluble in acetic acid. Potassium tetraphenylborate is crystalline and starts to decompose when heated above 100 °C. Other cations which form stable precipitates with tetraphenylborate under the conditions applied are rubidium, cesium, ammonium, mercury, thallium(/) and silver (see Section 11.2.3.6). In natural seawaters, however, the concentrations of these constituents are so low as to be negligible. 

The solubility product of salt changes with temperature and the concentration of the ions present in solution. In oilfield brines, if the surface solubility produced is smaller than their solubility product under down-hole formation, scale formation would occur. Under such conditions, the water is called unstable water. The StabiUty Index (S/) is used to predict the tendency of ions to form scales from oilfield brines which are highly corrosive and encountered during drilling process. The Stiff-Davis Modification of the Langelier equation is used to predict calcium carbonate precipitation from the brines and is given by... 

The dihydrate is formed by evaporation at ordinary temperature of an ethereal solution of the hexahydrate which has been dried with calcium nitrate or by crystallisation of the hexahydrate from concentrated nitric acid solution. It yields small lustrous plates, thick and square, probably rhombic, and possessing a green fluorescence. It melts at 179-3° C. It is much more stable than the trihydrate, and can be kept in a vacuum desiccator with caustic alkali or i hosphorus pentoxide without any loss of water. It dissolves readily in ether. If the dihydrate is heated in a current of carbon dio.xide at 98° C. a product corresponding very nearly in composition to the nioiiohydrate, U03(N03)3.H20, is obtained at 160° C. under the same conditions the ankydi ffus salt, U02(N03)2, is obtained. The latter may also be obtained by passing a current of dry nitric anhydride over the tri-hydrate carefully heated at 170° to 180° C. It is a yellow amorphous powder, readily soluble in water with c -olution of heat. It reacts violently with ether. When heated to 200° C. it decomposes and leaves a mixture of uranic acid, UO3.H2O, and uranic anhydride. ... 

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