Freshwater chlorides

Chloride. Chloride is common in freshwater because almost all chloride salts are very soluble in water. Its concentration is generally lO " to 10 M. Chloride can be titrated with mercuric nitrate. Diphenylcarbazone, which forms a purple complex with the excess mercuric ions at pH 2.3—2.8, is used as the indicator. The pH should be controlled to 0.1 pH unit. Bromide and iodide are the principal interferences, whereas chromate, ferric, and sulfite ions interfere at levels greater than 10 mg/L. Chloride can also be deterrnined by a colorimetric method based on the displacement of thiocyanate ion from mercuric thiocyanate by chloride ion. The Hberated SCN reacts with ferric ion to form the colored complex of ferric thiocyanate. The method is suitable for chloride concentrations from 10 to 10 M. 

In addition to freshwater, seawater is also a source for sodium, magnesium, chlorides, iodine, bromine, and magnesium metal (see Sodium coLD>ouNDS Magnesium coLD>ouNDS Iodine Bromine Magnesiumand magnesium alloys). Many other elements are certain to be economically obtained from the ocean as technology for the recovery improves. 

Calcium chloride is found in the marine environment. The elemental composition of seawater is 400 ppm calcium, 18,900 ppm chlorine, and many organisms and aquatic species are tolerant of these concentrations. Toxicity arises either from the invasion of freshwater in otherwise saltwater environments or possible toxic doses of calcium chloride from spills, surface mnoff, or underground percolation into typically freshwater streams or aquifers. Various agencies have guidelines for calcium and chloride in potable water (41). The European Economic Community (EEC) is the only agency to have a minimum specification for calcium in softened water. 

The liquid phase of saturated saltwater muds is saturated with sodium chloride. Saturated saltwater muds are most frequently used as workover fluids or for drilling salt formations. These muds prevent solution cavities in the salt formations, making it unnecessary to set casing above the salt beds. If the salt formation is too close to the surface, a saturated saltwater mud may be mixed in the surface system as the spud mud. If the salt bed is deep, freshwater mud is converted to a saturated salt water mud. 

The water phase of oil-base mud can be freshwater, or various solutions of calcium chloride (CaCl ) or sodium chloride (NaCl). The concentration and composition of the water phase in oil-base mud determines its ability to solve the hydratable shale problem. Oil-base muds containing freshwater are very effective in most water-sensitive shales. The external phase of oil-base mud is oil and does not allow the water to contact the formation the shales are thereby prevented from becoming water wet and dispersing into the mud or caving into the hole. 

Common salt, or sodium chloride, is also present in dissolved form in drilling fluids. Levels up to 3,000 mg/L chloride and sometimes higher are naturally present in freshwater muds as a consequence of the salinity of subterranean brines in drilled formations. Seawater is the natural source of water for offshore drilling muds. Saturated brine drilling fluids become a necessity when drilling with water-based muds through salt zones to get to oil and gas reservoirs below the salt. 

Using the equilibrium constants below, calculate the concentrations of free (uncomplexed) cadmium ion in a freshwater with a chloride concentration of 15 mg/L, and in seawater containing 17 000 mg/L chloride. Ignore com-plexation with other ions. 

The concentration of chloride ion in seawater is around 0.55 M. To compare the solubility of Pb freshwater vs. seawater, calculate the solubility in g/L of PbCb in pure water and in 0.55 M NaCl. 

Sometimes it may become necessary to shut-in a gas well when the demand for gas is low. In such instances, the well is shut-in for an indefinite period, after which it is reopened and production is resumed. It often has been found that the production rate of gas from the reopened well is substantially less than it was before the well was shut-in. During production, the inner wall of the production tubing will be coated with a film of condensed freshwater because of the geothermal gradient. This water flows down when production is interrupted and can cause formation damage. This may occur because clays are normally saturated with brine water and not with freshwater. This swelling can be prevented with the injection of some additive, for example, sodium chloride, potassium chloride, calcium chloride, or an alcohol or a similar organic material [1853]. 

The cementing technology can be improved in wells with zones containing clays or shales that are sensitive to freshwater cement filtrate. Sodium chloride (i.e., natural salt) and potassium chloride (KCl) have been the primary materials of choice to yield a filtrate that damages these zones less. However, the unfavorable effects of salt on cement have been extensively documented, in particular. 

In aquatic environments, radiocerium readily forms chemical complexes in seawater and associates with particles by adsorption (Mauch-line and Templeton, 1963). When radiocerium was added to natural seawater, it became associated with suspended matter, especially that with apparent particle diameters of 0.02 to 0.1 fim (Carpenter and Grant, 1967). When ionic radiocerium was added to filtered seawater at pH > 6.0, it hydrolyzed and formed complexes with hydroxide, chloride, or other anions in seawater and went on to form particles (Hirano et al., 1973). Adsorption of radiocerium onto suspended particles has also been noted after its release to freshwater ecosystems (Beninson et al., 1966). 

Copper compounds are used routinely and widely to control freshwater snails that serve as intermediate vectors of schistosomiasis and other diseases that afflict humans (Hasler 1949 NAS 1977 Rowe and Prince 1983 Winger etal. 1984 Al-Sabri etal. 1993). These compounds include copper sulfate, copper pentachlorophenate, copper carbonate, copper-tartaric acid, Paris green (copper arsenite-acetate), copper oxide, copper chloride, copper acetyl acetonate, copper dimethyl dithiocar-bamate, copper ricinoleate, and copper rosinate (Cheng 1979). Also, many species of oyster enemies are controlled by copper sulfate dips. All tested species of marine gastropods, tunicates, echinoderms, and crabs that had been dipped for 5 seconds in a saturated solution of copper sulfate died if held in air for as little as a few seconds to 8 h mussels, however, were resistant (MacKenzie 1961). 

Jha, B.S. and M.M. Jha. 1995. Biochemical effects of nickel chloride on the liver and gonads of the freshwater climbing perch, Anabas testudineus (Bloch). Proc. Nat. Acad. Sci. India, Sec. B. 65 39 4-6. 

In seawater, silver nitrate is less toxic than in freshwater (Wood et al. 1995 Wood et al. 1999). This difference is probably due to the low concentration of free Ag+ (the toxic moiety in freshwater) in seawater, the high levels of chloride, and the predominance of negatively charged silver-chloro complexes. However, high levels of silver nitrate are toxic to marine invertebrates despite the absence of Ag+, and this is attributed to the bioavailability of stable silver-chloro complexes (Wood et al. 1995 Ratte 1999). In seawater, in contrast to freshwater, plasma Na+ and Cl- rise rather than fall, and death may result from the elevated Na+ and Cl concentrations combined with dehydration (Hogstrand and Wood 1998). Osmoregulatory failure occurs in marine teleosts exposed to high concentrations of Ag+, and the intestine is the main toxic site of action (Wood et al. 1999). 

Inorganic ligands in aqueous solutions, and in particular in natural freshwaters, include, in addition to H2O and OH, the major ions carbonate and bicarbonate, chloride, sulfate and also phosphate [29], The distribution of metal ions between these ligands depends on pH and on the relative concentrations of the ligands. The pH is a master variable with regard to the occurrence of hydrolysed species and to the formation of carbonate and bicarbonate complexes. 

Under typical freshwater conditions, at pH 7-9 and in presence of millimolar concentrations of carbonate, most transition metals in solution (Cu(II), Zn(II), Ni(II), Co(II), Cd(II), Fe(TII), etc.) occur predominantly as hydroxo or carbo-nato complexes. For a few metals, chloro complexes may be predominant (Ag(I), Hg(II)), if chloride is in the range 10-4—10-3 mol dm-3 or higher. Alkali and alkali-earth cations occur predominantly as free aquo metal ions [29], At lower pH values, the fraction of free aquo metal ions generally increases. Strong sulfide complexes of several transition metals have recently been shown to occur even under oxic conditions [32,33]. 

Figure 2. Sodium and chloride uptake across an idealised freshwater-adapted gill epithelium (chloride cell), which has the typical characteristics of ion-transporting epithelia in eukaryotes. In the example, the abundance of fixed negative charges (muco-proteins) in the unstirred layer may generate a Donnan potential (mucus positive with respect to the water) which is a major part of the net transepithelial potential (serosal positive with respect to water). Mucus also contains carbonic anhydrase (CA) which facilitates dissipation of the [H+] and [HCO(] to CO2, thus maintaining the concentration gradients for these counter ions which partly contribute to Na+ import (secondary transport), whilst the main driving force is derived from the electrogenic sodium pump (see the text for details). Large arrow indicates water flow...

Pisam, M., Carott, A. and Rambourg, A. (1987). Two types of chloride cells in the gill epithelium of a freshwater adapted euryhaline fish Lebistes reticularis. Their modifications during adaptation to saltwater, Am. J. Anat., 179, 40-50. 

The aqua Zn ion is dominant in organic matter-free freshwater while the free Zrf+ ion and chloride complexes dominate in seawater (Stanley and Byrne 1990 Millero 1996). The free Cu + ion is dominant in freshwater, while the carbonate complexes CuCO, and [Cu(C03)2] are preponderant in seawater. Speciation and solubility of Zn in Cl-rich hydrothermal solutions has been investigated by Wesolowski et al. (1998). Speciation and solubility of Cu have been investigated by Mountain and Seward (1999) for hydrothermal solutions dominated by sulfides and by Xiao et al. (1998), Liu et al. (2002), and Archibald et al. (2002) for solutions dominated by chlorides. 

A full imderstanding of the speciation of dissolved iron requires consideration of ligands other than water and hydroxide. The most important ones are listed in Table 5.6 along with their concentration ranges in seawater and freshwater. For Fe(III) in seawater at pH > 4, the formation of complexes with hydroxide is most important, but at pH <4, sulfete, chloride, and fluoride pairing predominates (Figure 5.15b). To predict the equilibrimn speciation at low pH, these anions need to be added to the mass balance equation fiar Fe(III) (Eq. 5.20). Seawater with low pH tends to have low O2 concentrations. Under these conditions, most of the dissolved iron is present as Fe( II), which undergoes complexation with sulfide and carbonate. 

The continuous wastewater stream from a desalter contains emulsified oil (occasionally free oil), ammonia, phenol, sulfides, and suspended sohds, all of which produce a relatively high BOD and COD concentration. It also contains enough chlorides and other dissolved materials to contribute to the dissolved solids problems in discharges to freshwater bodies. Finally, its temperature often exceeds 95°C (200°F), thus it is a potential thermal pollutant. 

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