Calcium critical values

Chemical scaling is another form of fouling that occurs in NF and RO plants. The thermodynamic solubility of salts such as calcium carbonate and calcium and barium sulfate imposes an upper boundary on the system recovery. Thus, it is essential to operate systems at recoveries lower than this critical value to avoid chemical scaling, unless the water chemistry is adjusted to prevent precipitation. It is possible to increase system recovery by either adjusting the pH or adding an antisealant, or both. 

Molecular theory of caustification.—An excess of solid calcium hydroxide is supposed to be present at the start, so that as fast as calcium hydroxide is removed from the soln. by reacting with the potassium carbonate, more passes into soln. Thus the cone, of the calcium hydroxide in the soln. is kept constant. The. solubility of calcium carbonate is very small, and, in consequence, any calcium carbonate in excess of the solubility will be precipitated as fast as it is formed. The reaction proceeds steadily from right to left because, all the time, calcium hydroxide steadily passes into soln., and calcium carbonate is steadily precipitated but the solubility of calcium carbonate steadily increases with increasing cone, of potassium hydroxide. There is a steady transformation of the potassium carbonate into potassium hydroxide in progress The cone, of the potassium carbonate is steadily decreasing, while the cone, of the potassium hydroxide is steadily increasing. Consequently, when the potassium hydroxide has attained a certain cone, so much calcium carbonate will be present in the soln. that the reaction will cease. Hence the cone, of the potassium carbonate should be such that it is all exhausted before the state of equilibrium is reached. If the cone, of the potassium hydroxide should exceed this critical value, the reaction will be reversed, and calcium carbonate will be transformed into calcium hydroxide. 

As noted before, the membrane performance could be affected by the presence of H2O, CO2 or other volatile hydrocarbons in the gas phase of both compartments. As laid down in patent literature [1-3], the oxygen fluxes through Mg-, Ca-, Sr-, and Ba-doped perovskites deteriorated over time, roughly 30-50% over a time period of about 100 h, if the air used as feed gas contained several percent of H2O and amounts of CO2 on a hundreds of ppm level. It was claimed, that either no deterioration is found or the fluxes can be restored to their initial values if the temperature is raised above certain critical values, 500°C for magnesium, 600°C for calcium, 700°C for strontium and 810°C for barium. Though no explanation was given, it is possible that carbonate formation took place. One may further note that the tendency for carbonate formation increases at lower temperatures. 

A further example of a compound in which the geometrical conditions are favourable for dimorphism is calcium carbonate. This compound occurs naturally as both calcite and aragonite, two distinct minerals with quite different structures (see 10.10 and 10.11). The calcite structure is common to a considerable number of carbonates (and nitrates) in which the radius of the cation is less than about i-o A, but when the radius exceeds this value the aragonite structure is found. Calcium carbonate itself is dimorphous because the radius of the calcium ion (0 99 A) is close to this critical value, but dimorphism is not found in those carbonates in which the cation is either appreciably smaller or appreciably larger. The closely analogous case of the dimorphism of CdTi03 has already been described ( 8.52). 

It has been known for 40 years (H13) that rickets develops in children when the plasma [Ca] [P] product falls below about 30, and is either absent or healing when the plasma product exceeds 40. In adults the critical value of the product is about 20, below which level osteomalacia results. There are, however, exceptions to these general rules and, in particular, osteomalacia has been described in a case of postoperative hypoparathyroidism in which the plasma calcium was low but the plasma phosphate correspondingly high. The [Ca] [P] product in this case was... 

MACAL is a model which can be used to assess the critical load of forest soils (de Vries, 1988). Here, critical values for aluminium concentrations and Ca Al ratios are set at 0.2 mmol 1 and 1.0 respectively and the model calculates the yearly averaged calcium and aluminium concentrations in soil compartments of 10 cm up to 80 cm depth. The model predicts the element flux and water flux in given soil compartments at given levels of acidic deposition ... 

Normally, the number of anions and cations in each fluid compartment are equal. Cell membranes play the critical role of maintaining distinct ICF and ECF spaces which are biochemically distinct. Serum electrolyte concentrations reflect the stores of ECF electrolytes rather than that of ICF electrolytes. Table 24-4 lists the chief cations and anions along with their normal concentrations in the ECF and ICF. The principal cations are sodium, potassium, calcium, and magnesium, while the key anions are chloride, bicarbonate, and phosphate. In the ECF, sodium is the most common cation and chloride is the most abundant anion while in the ICF, potassium is the primary cation and phosphate is the main anion. Normal serum electrolyte values are listed in Table 24—5. 

The in vitro procedure was tested in "critical" experiments designed to make direct comparisons of in vivo and in vitro estimates of exchangeability and potential bioavailability and to test the use of in vitro exchangeability values in in vivo experiments. (8). Three foods which were expected to show different levels of calcium solubility and exchangeability, collards, soybeans and spinach, were intrinsically labeled with 45Ca in nutrient solution culture. They were used together with 47 Ca as an extrinsic label in both in vitro and in vivo experiments. 

The recommended rate of analysis is forty samples per hour, requiring about 0.3 ml of sample. There are two points in the analysis scheme where establishment of the correct pH is quite critical. The usable range of calcium values for blood serum is quite restricted however, with adequate care in reading the relatively flat curve and with a sufficient number of standards, results which compare favorably with the more conventional procedures have been obtained.33... 

Surface treatment also improves adhesion to the polymer matrix and resulting physical properties. The combination of particle size and surface treatment is critical in the selection of precipitated calcium carbonate fillers to obtain desired properties. Often graded combinations of ultrafine precipitated CaC03 and larger CaC03 particles are used for optimum properties and value. 

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