Equilibrium calcium carbonate

Ca/HCOa by calcium carbonate equilibria and control of carbon. 

We may now examine specific information from chemical analyses of the Great Lakes (7, 8, 13) to determine to what degree the variations of the proposed model fit the actual data. Rather than consider all of the variables at once, it is simpler to consider smaller portions to get a better idea of what actually is happening. We shall look at calcium carbonate equilibria, dolomite equilibria, phosphate equilibria, and silicate equilibria. 

Calcium Carbonate Equilibria. Values of the ion product, (Ca2+) (CO32 ), are plotted in Figure 1 as a function of temperature. These values represent samples from Lake Erie and Lake Ontario for all depths... 

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]. 

The content of hydrogen carbonates is quite low because of the intensive ventilation of coohng water in an open circulation system and thus, the majority of free CO2 is removed, so that the foUowing calcium carbonate equilibrium shifts to the left ... 

Microbial reactions also change the chemical composition of the groundwater e.g. due to production of carbonate and sulfide. If the inorganic composition of groundwater is to be used to estimate microbial degradation, chemical reactions have to be considered, too. In this example a simplified geochemistry is simulated, consisting of a calcium-carbonate equilibrium system and precipitation of iron sulfide. 

In practice, the effect in systems is far more complex because the temperature also affects the calcium carbonate equilibrium. Temperature can shift the equilibrium toward... 

Note on calcium carbonate equilibrium and the Ryznar index... 

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. 

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. 

An increase in carbonate-ion concentration moves the equilibrium in favour of calcium carbonate deposition. Thus one secondary effect of cathodic protection in seawater is the production of OH , which favours the production of CO, , which in turn promotes the deposition of CaCOj. Cathodically protected surfaces in seawater will often develop an aragonite (CaCOj) film. This film is commonly referred to as a calcareous deposit. 

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). 

Consequently, a loss of free carbon dioxide in the water, because of either a rise in temperature (as occurs in a FW heater or boiler) or an increase in pH (all boilers operate at an alkaline pH) leads to a change of equilibrium and the resultant rapid and troublesome precipitation of insoluble calcium carbonate scale on heat transfer surfaces. The reaction is as shown here ... 

Removing a product from a system at equilibrium also makes Q < and leads to the formation of additional products. This behavior is commonly used to advantage in chemical synthesis. For example, calcium oxide (lime), an important material in the construction industry, is made by heating calcium carbonate in a furnace to about 1100 K CaCOs ( ) CaO (s) + CO2 (g) = (pco2 = 10 at 1100 K... 

Over the eons, the flow and evaporation of water inside a cavern creates a stunning array of rock sculptures. Stalagmites grow upward from the floor, sometimes joining stalactites to form massive columns. Limestone dams create beautiful pools of water. Limestone draperies fall like curtains from water flowing around overhanging rock. Delicate mineral flowers sprout from the walls. All these features result from the aqueous solubility equilibrium of calcium carbonate. 

The limestone deposits that decorate Carlsbad and other caverns are the result of the solubility equilibrium of calcium carbonate in groundwater, as described in Chapter 16 ... 

Calcium carbonate is insoluble in pure water but dissolves in weakly acidic water. The role of this solubility phenomenon in the geochemistry of caverns is described in Box. We can understand this dependence on pH by examining the acid-base properties of the species involved in the solubility equilibrium. 

The analysis can be made quantitative by writing the various equilibria and their values. The reactions can be added to obtain the net reaction that occurs when calcium carbonate is exposed to acidic water, and the equilibrium constant for the net reaction is the product of the individual. S eq values ... 

The equilibrium constant expression derived for calcium carbonate dissolving in acidic solution is a general one... 

The introduction of carbon dioxide in the pipes regulates a state of equilibrium between dissolved bicarbonates, calcium carbonate inlaid and the C02 added. 

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). 

Morse, J.W. 1978. Dissolution kinetics of calcium carbonate in sea water VI, The near-equilibrium dissolution kinetics of calcium carbonate-rich deep sea sediments. American Journal of Science, 278, 344-353. 

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... 

Many chemical reactions in seawater do not achieve equilibrium. The most notable are ones that involve marine organisms. Since organisms require energy, they cannot survive if their constituent biochemicals are at equilibrium. Equilibrium is also not likely to be achieved if some other process is adding or removing a chemical faster than equilibrium can be reattained. For example, calcium carbonate shells should spontaneously dissolve in deep ocean water, but some sink so fest that they can reach the sediments where they eventually become buried and, hence, preserved. In other words, the equilibrium approach is most applicable to reactions that attain equilibrium fester than any other competing processes acting on the chemical of interest. 

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. 

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