Carbonate ion concentration

Since the effluent from a softening unit is usually supersaturated with calcium carbonate at the usually high pH values, it is necessary to reduce the pH to a value that allows the solution to be exactiy saturated for the calcium-ion and carbonate-ion concentrations present. The relationship is... 

In both cases, the carbonate ion concentration increases and eventually equiUbrates in the system, releasing carbon dioxide in the stripping column and thereby reducing product purity. Hence, a small caustic wash tower is employed to remove any carbon dioxide that is Hberated in the stripper. 

The higher the pH of the seawater the greater will be the proportion of carbonate ions present (since as the pH increases, so the H concentration decreases, thereby moving the equilibrium to the right). It follows that at a surface under cathodic protection, the hydroxyl ion produced has the effect of increasing the local carbonate ion concentration. 

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. 

In experiments where Mono Lake water was acidified to remove carbonate and bicarbonate ions and again adjusted to pH 10, more than 90 percent of the soluble plutonium moved to the sediment phase. When carbonate ion concentration was restored, the plutonium returned to solution—strong evidence of the importance of inorganic carbon to solubility in that system(13). Early studies with Lake Michigan water, which has low DOC, had also implicated bicarbonate and carbonate as stabilizing ligands for plutonium at pH 8(14). This latter research characterized the soluble species as mainly anionic in character. 

The body maintains blood pH by two primary mechanisms respiration and excretion. Carbonic acid concentration is controlled by respiration as we exhale, we deplete our system of CO, and hence deplete it of H2C03, too. This decrease in acid concentration raises the blood pH. Breathing faster and more deeply increases the amount of C02 exhaled and hence decreases the carbonic acid concentration in the blood, which in turn raises the blood pH. Hydrogen carbonate ion concentration is controlled by its rate of excretion in urine. 

Figure 6-6 also shows the variation in the partial pressure of carbon dioxide in equilibrium with the lagoon s waters. The average value of this pressure exceeds the atmospheric value, 1, so on average, carbon dioxide is evaporating from the lagoon. The evaporation rate is greatest at times of maximum alkalinity and bicarbonate concentration and minimum carbonate ion concentration. 

Figure 6-10 shows how the various elements of the carbon system respond to the seasonal change in evaporation rate. Although the fluctuations in carbonate ion concentration cannot be seen on the scale of this figure, examination of the numbers shows that the amplitude of the carbonate fluctuation is about 3 percent, comparable to the amplitude of the... 

Broecker, W. S. Takahashi, T. (1978). The relationship between lysocline depth and in situ carbonate ion concentration. Deep Sea Res., 25, 65-95. 

O A sample of blood was taken from a patient and sent to a laboratory for testing. Chemists found that the blood pH was 7.40. They also found that the hydrogen carbonate ion concentration was 2.6 x 10 mol/L. What was the concentration of carbonic acid in the blood ... 

The degree of saturation is also commonly expressed as a saturation index (SI)=logn and as excess carbonate ion concentration, ACO = [COj... 

Calculation of Carbonate Ion Concentrations from Measurements of pH and Alkalinity... 

The carbonate ion concentration can be estimated from the following expression... 

In the surface waters, geographic variability in 2CO2 and TA. are caused by the effects of temperature on CO2 solubility and by variations in the local rates of photosynthesis and biogenic calcification. In general, surfece water 2CO2 concentrations are lowest in warm surface waters due to the low solubility of CO2 at higher temperatures. The lower influx of CO2 also causes warm surface waters to have a higher carbonate ion concentration as compared to cold surfece waters. Carbonate ion concentrations are... 

Of particular concern are the impacts of seawater acidification on biocalcification and the burial rates of sedimentary carbon. Carbonate ion concentrations in the surface waters have already declined by 16%. Thus, it is not surprising that the abundance of tropical/subtropic planktonic foraminiferan species appears to have declined since the 1960s. This information was obtained by studying the rapidly accumulating sediments of the Santa Barbara Basins off the coast of California. 

Carbonate compensation The ocean s response to perturbations through shifts in its carbonate chemistry. These shifts require changes in the carbonate ion concentration that change the depth of the calcium carbonate compensation depth and hence lead to changes in the burial rate of carbon as biogenic calcium carbonate. 

According to this equation an 0 increase of 0.26%c in carbonate represents a 1°C temperature decrease. Bemis et al. (1998) have re-evaluated the different temperature equations and demonstrated that they can differ as much as 2°C in the temperature range between 5 and 25°C. The reason for these differences is that in addition to temperature and water isotopic composition, the 5 0 of a shell may be affected by the carbonate ion concentration in sea water and by photosynthetic activity of algal symbionts. 

Although the alkalinities of the secondary wastewater tested were fivefold higher than for potable water, the OH scavenging was approximately 2.5 times higher in the potable water based on the HC03 /C032 equilibrium therefore, high removal efficiency results from either a low pH and/or a low bicarbonate/carbonate ion concentration. 

There is one rather nasty twist in the ion pair evaluation where a negative value for free carbonate ion concentration results, because the initial value which must be used for the carbonate ion activity coefficient is much larger than it is under the new conditions. This demands some maneuvers with both the calcite solubility constant and the calcium carbonate ion pair association constant. These will not be gone into here. 

Thus, the fossil fuel "neutralizing capacity of seawater is largely dependent on the carbonate ion concentration and that of marine sediments depends on the calcium carbonate which is near enough to the sediment-water interface to react with the overlying water. 

Here we will use a simplified example to illustrate some basic aspects of the mass transport process for carbonates that avoids most of the more complex relationships. In this example, the calcium and carbonate ion concentrations are set equal, and values of the activity coefficients, temperature, and pressure are held constant. The carbonate ion concentration is considered to be independent of the carbonic acid system. The resulting simple (and approximate) relation between the change in saturation state of a solution and volume of calcite that can be dissolved or precipitated (Vc) is given by equation 7.4, where v is the molar volume of calcite. 

The primary focus of research on secondary porosity formation has been on mechanisms for generating undersaturated formation waters. Because reactions that may result in undersaturation of waters with respect to carbonate minerals by consumption of calcium are unlikely to be quantitatively important, emphasis has been placed on reactions that may lower the carbonate ion concentration. Although not clearly documented in deep subsurface environments, mixing of waters of dissimilar composition can result in undersaturation with respect to calcite (see Chapter 7), and lead to secondary porosity formation. Acidic waters associated with igneous intrusions and thermal metamorphism can also cause carbonate dissolution that results in secondary porosity (e.g., deep Jurassic carbonates in Mississippi, U.S.A. Parker, 1974). 

Figure 8. Distribution of carbonate ion concentration in limestone DA pilot plant.

The precipitate is slightly soluble in solutions of ammonium salts of strong acids this is because the ammonium ion, being a strong acid, reacts with the base, the carbonate ion, C03, leading to the formation of the hydrogen carbonate ion, HCOJ, and hence the carbonate-ion concentration of the solution is decreased. 

BaCr04 as in Table V.26 and the strontium and calcium are precipitated as carbonates by (NH4)2C03 solution in the presence of a little aqueous ammonia. It is better to employ Na2C03 to precipitate the carbonates the influence of ammonium salts in tending to reduce the carbonate-ion concentration is thus eliminated. 

Sodium or ammonium carbonate solution white precipitate of lithium carbonate, Li2C03, from concentrated solutions and in the presence of ammonia solution. No precipitation occurs in the presence of high concentrations of ammonium chloride since the carbonate-ion concentration is reduced to such an extent that the solubility product of Li2C03 is not exceeded ... 

The calculation of the total ion molal carbonate ion concentration is more complex than for calcium because it is part of the carbonic acid system. The following reactions take place between CO and carbonic acid in seawater ... 

The Geochemical Ocean Section Program (GEOSECS) has produced data from which it is possible to profile the saturation state of seawater with respect to calcite and aragonite in the Atlantic and Pacific oceans. Representative north-south calcite saturation profiles for the Western Atlantic and Central Pacific oceans are presented in Figures 5 and 6 (based on 39). It was observed that the saturation state of seawater with respect to calcite at the CCD was close to constant ( 2 = 0.70 I" 0,05) except in the southern extremes (39). Broecker and Takahashi (31) have recently found that the carbonate ion concentration is close to constant at the FL, when appropriate corrections are made for pressure. The saturation state of seawater at the FL, calculated by the method presented in this paper, is 0.80 0.05. Berger (40) has presented profiles for Rq, FL, CCD and CSL (calcite saturation level) in the eastern and western Atlantic ocean (see... 

Figure 17. Log of the dissolution rate vs. total carbonate ion concentration for synthetic aragonite, pteropods, calcitic Pacific Ocean sediment, and foraminifera in the 125-500 iim size fraction. (A) indicates ihe aragonite equilibrium total carbonate ion concentration at 25°C, 1 atm (26). (C) indicates the calcite equilibrium total carbonate ion concentration at 25°C, 1 atm (25).

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