Calcium carbonate equilibrium with

First, situating the water on the corrosive side of its calcium carbonate equilibrium with a pH of about 6.5 to 7. As a result, no CaC03 scale can be formed. 

The CO2 concentration in the earth s atmosphere is ultimately governed by the calcium carbonate equilibrium in the ocean (e.g., Berner et al. 1983). If the oceans are in equilibrium with calcite, which is usually the case, then to a reasonable approximation, the PCO2 of the atmosphere is defined by the equilibrium ... 

We now consider the calcium carbonate equilibrium and aggressive carbon dioxide. Of the various chemical equilibria in natural and service waters, the calcium carbonate equilibrium is of the greatest theoretical and practical importance. It is concerned with the evaluation of water aggressivity, control of deacidiflcation processes, limnology, evaluation of buffering capacity of water, etc. 

Many authors discuss the theoretical and experimental solution of the calcium-carbonate equilibrium [14]. The methods are based on various simplifying assumptions and they solve calcium-carbonate equilibrium and thus also aggressivity of water with greater or lesser success. In the majority of cases the aggressivity of water can be evaluated only semi-quantitatively. 

None of these methods of pH, calculation and water oversaturation by CaC03 take into consideration the formation of ioiiic associates in the calculation of ionic strength as well as in the calculation of actual and equilibrium concentrations of calcium and hydrogen carbonates which are primarily used for the calculation of calcium-carbonate equilibrium. More exact calculations of calcium-carbonate equilibrium, more exact calculations with regard to the formation of ionic associates may be found elsewhere [16, 17]. 

Soft upland waters are aggressive to most metals, their behaviour depending to some extent on pH values as discussed on p. 2.53. They are inevitably unsaturated with respect to calcium carbonate and it is not usually practicable to modify the carbonate equilibrium to make them non-aggressive. 

The most important property of the dissolved solids in fresh waters is whether or not they are such as to lead to the deposition of a protective film on the steel that will impede rusting. This is determined mainly by the amount of carbon dioxide dissolved in the water, so that the equilibrium between calcium carbonate, calcium bicarbonate and carbon dioxide, which has been studied by Tillmans and Heublein and others, is of fundamental significance. Since hard waters are more likely to deposit a protective calcareous scale than soft waters, they tend as a class to be less aggressive than these indeed, soft waters can often be rendered less corrosive by the simple expedient of treating them with lime (Section 2.3). 

The effect of pH on the corrosion of zinc has already been mentioned (p. 4.170). In the range of pH values from 5 -5 to 12, zinc is quite stable, and since most natural waters come within this range little difficulty is encountered in respect of pH. The pH does, however, affect the scale-forming properties of hard water (see Section 2.3 for a discussion of the Langelier index). If the pH is below the value at which the water is in equilibrium with calcium carbonate, the calcium carbonate will tend to dissolve rather than form a scale. The same effect is produced in the presence of considerable amounts of carbon dioxide, which also favours the dissolution of calcium carbonate. In addition, it is important to note that small amounts of metallic impurities (particularly copper) in the water can cause quite severe corrosion, and as little as 0-05 p.p.m. of copper in a domestic water system can be a source of considerable trouble with galvanised tanks and pipes. 

Saturation Index an index which shows if a water of given composition and pH is at equilibrium, supersaturated or unsaturated with respect to calcium carbonate (or to magnesium hydroxide). 

In addition to climate change, the increased atmospheric concentration of C02 is likely to have wide-spread ecological effects in various environments, since C02 is a physiologically active gas, in plants as well as animals. The acidic nature of C02 will also lead to changes in the chemistry of the ocean s surface, which is in equilibrium with the atmosphere. Once the shift in the oceanic chemical balance becomes significant, it will affect ecosystems. It has been shown, for example, that doubling C02 concentration in the atmosphere will reduce the rate of calcium carbonate deposition in coral reefs by 30-40% (Langdon et al., 2000). 

Figure 29.2 shows the mineralogic results of the calculation. Dolomite dissolves, since it is quite undersaturated in the waste fluid. The dissolution adds calcium, magnesium, and carbonate to solution. Calcite and brucite precipitate from these components, as observations from the wells indicated. The fluid reaches equilibrium with dolomite after about 11.6 cm3 of dolomite have dissolved per kg water. About 11 cm3 of calcite and brucite form during the reaction. Since calculation... 

Egg shells are made of calcium carbonate, CaCCE. The chicken ingeniously makes shells for its eggs by a process involving carbon dioxide dissolved in its blood, yielding carbonate ions which combine chemically with calcium ions. An equilibrium is soon established between these ions and solid chalk, according to... 

Equilibrium with precipitation. The previous example calculated carbonate speciation admitting unrestricted solubility of all species. Actually, it is easily verified that the calculated calcium and carbonate concentrations exceed calcium carbonate solubility as measured by the solubility product... 

As a water mass ages, the ratio of [CO3 ] to [HCO3] declines because the continuing generation of CO2 from the remineralization of POC pushes the equilibrium reaction in Eq. 15.18 further toward the products. Thus, as a deep water mass ages, it becomes increasingly more undersaturated with respect to biogenic calcium carbonate. 

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. 

Calcium is also found in the rigid structural components of living things, either as the calcium carbonate of the shells of shellfish or the calcium phosphate of bone. About a kilogram of calcium is present in an adult human body, mostly in the form of insoluble calcium phosphate, but also as Ca2T ions in other fluids inside our cells. The calcium in newly formed bone is in dynamic equilibrium with the calcium ions in the body fluids, so calcium must be part of our daily diet to maintain bone strength. 

The spring waters of the Sierra Nevada result from the attack of high C02 soil waters on typical igneous rocks and hence can be regarded as nearly ideal samples of a major water type. Their compositions are consistent with a model in which the primary rock-forming silicates are altered in a closed system to soil minerals plus a solution in steady-state equilibrium with these minerals. Isolation of Sierra waters from the solid alteration products followed by isothermal evaporation in equilibrium with the eartKs atmosphere should produce a highly alkaline Na-HCO.rCOA water a soda lake with calcium carbonate, magnesium hydroxy-silicate, and amorphous silica as precipitates. 

Dolomite Equilibrium. Figure 2 is a plot of the ion product, (Ca2+) (Mg2+) (C032 )2, as a function of temperature. Saturation of lake waters with respect to dolomite is similar to the results for calcium carbonate. The same general conclusions are apparent. Dolomite tends to be saturated with respect to lake water at 14°C. 

In the. first run, acid feed rate was gradually reduced without immediate evidence of scale formation and then was cut off entirely. Heat transfer coefficients started to drop immediately and at the end of the week the tubes were coated with calcium carbonate scale. In this first run, the pH entering the evaporator was 8.3 and leaving it was 8.4. Langelier, Caldwell, and Lawrence have measured the equilibrium pH above which a sea water concentrate is supersaturated with respect to calcium carbonate and magnesium hydroxide (4). Under the conditions of this test, the following conditions were encountered ... 

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