Seawater dissolved ions

Up to this point, we have focused on aqueous equilibria involving proton transfer. Now we apply the same principles to the equilibrium that exists between a solid salt and its dissolved ions in a saturated solution. We can use the equilibrium constant for the dissolution of a substance to predict the solubility of a salt and to control precipitate formation. These methods are used in the laboratory to separate and analyze mixtures of salts. They also have important practical applications in municipal wastewater treatment, the extraction of minerals from seawater, the formation and loss of bones and teeth, and the global carbon cycle. 

Hioki et al. [215] have described an on-line determination of dissolved silica in seawater by ion exclusion chromatography in combination with inductively coupled plasma emission spectrometry. 

One of the most notable features of seawater is its high degree of saltiness. In previous chapters, we have discussed various sources of this salt, these being rivers, volcanic gases, and hydrothermal fluids. These elements have ended up in one of four places (1) as dissolved ions in seawater, (2) as sedimentary minerals, (3) as hydrothermal minerals, and (4) as volatiles that reside in the atmosphere. The minerals are recycled via geologic uplift and subduction. Upon return to Earth s surface, these minerals are chemically weathered via acid attack by the atmospheric volatiles remobilizing the salts for return to the ocean in river runoff. 

Chemical equilibrium and analysis of a mixture. A remote optical sensor for C02 in the ocean was designed to operate without the need for calibration.21 The sensor compartment is separated from seawater by a silicone membrane through which C02, but not dissolved ions, can diffuse. Inside the sensor, C02 equilibrates with HCO3 and CO3-. For each measurement, the sensor is flushed with fresh solution containing 50.0 pM bromothymol blue indicator (NaHIn) and 42.0 pM NaOH. All indicator is in the form HIn-or In2 near neutral pH, so we can write two mass balances (1) [HIn ] + [In2-] = Fln = 50.0 pM and (2) [Na"] = FNa = 50.0 pM + 42.0 pM = 92.0 pM. HIn- has an absorbance maximum at 434 nm and In2 has a maximum at 620 nm. The sensor measures the absorbance ratio RA = A620/A434 reproducibly without need for calibration. From this ratio, we can find C()2( [Pg.420]

Cotecchia et al. (1974) studied the salinization of wells on the coast of the Ionian Sea. A fingerprint diagram (Fig. 6.23) served to define a conceptual model. The lowest line (MT) is of a fresh water spring and the uppermost line (I.S.) is of the Ionian Sea water. The lines in between (SR and CH) are of groundwaters with increasing proportions of seawater intrusion. The CH well met the nondiluted seawater at a depth of 170 m. This interpretation seems to be well founded as it is based on six dissolved ions. The whole story is condensed into one fingerprint diagram. 

The solubility of the noble gases depends on a third parameter the concentration of dissolved ions in the water. The data in Fig. 13.1 are for salt-free water. Seawater, for example, dissolves 30% less. This effect has only seldom to be regarded in groundwater tracing, since recharge is in most cases relatively fresh. 

The main mechanisms for delivery of dissolved constituents to the ocean are river inflow and atmospheric input. Formation of authigenic minerals (those minerals that form in situ) is the ultimate sink (Fig. 2.1) for these constituents. Authigenesis primarily involves precipitation of plant and animal shells, chemical reactions in sediments, and high-temperature reactions at hydrothermal regions. We begin with a brief review of the chemical reactions influencing the dissolved ion concentrations of rivers, and end with an attempt to balance the river sources with plausible sinks for the major seawater ions. 

Key nuclear and physical transformations that and its longer-lived daughters undergo in the ocean and atmosphere are illustrated in Fig. 5.20. This cartoon is a simplification that excludes some physical sources and sinks, does not specify the chemical forms of the elements, and ignores daughters vith half lives less than a day. Like most seawater elements, is weathered out of continental rocks and carried by rivers to the ocean, where it occurs in a highly soluble dissolved form or in detrital sedimentary minerals. Because uranium is strongly complexed by CO3 ions, it is relatively inert to particle adsorption, is not readily used by marine biota, and behaves conservatively in seawater. Dissolved which does decay in... 

The evidence from field studies is somewhat contrary to the predictions based on equilibrium chemistry. Abiological precipitation of CaC03 seems to be very limited, restricted to geographically and geochemically unusual conditions. The reasons why carbonate minerals are reluctant to precipitate from surface seawater are still poorly understood, but probably include inhibiting effects of other dissolved ions and compounds. Even where abiological precipitation is suspected—for example, the famous ooid shoals and whitings of the Bahamas (Box 6.5)—it is often difficult to discount the effects of microbial involvement in the precipitation process. 

In high-salinity waters such as seawater, both ion-pairing and activity-coefficient effects (see Chap. 4) increase the concentrations of species limited by the solubility of minerals. For example, in pure water saturated with respect to calcite, the molal solubility product ZmCa x ZmCOf" = 10 whereas in seawater this product equals 10 If the concentration of carbonate is constant, this corresponds to a 250-fold increase in the concentration of dissolved calcium in seawater relative to that in pure water. 

Seawater is approximately 4.0% by mass dissolved ions. About 85% of the mass of the dissolved ions is from NaCl. 

Temperature, however, is not the only consideration ocean-water density is also influenced by salinity. Seawater contains dissolved ions, mainly sodium and chlorine, and, as a result, is denser than freshwater (typically 1035 kg m 3 compared with 1000 kg m-3). The salinity of seawater is not uniform everywhere in the ocean. Evaporation at the sea surface increases the salinity (as water is evaporated off but dissolved ions are left behind), whereas inputs from rain or river runoff lead to decreased salinity. Salinity is also increased by the formation of sea ice as ice forms, salt is rejected from the freezing water, and the remaining liquid water is enriched in salt. 

The majority of the salt ions in the ocean are sodium and chloride— these make up about 90% of dissolved ions in seawater. The remainder of the ions present are mainly magnesium, sulfate, and calcium. The concentration of salts in seawater is fairly high, and, on average, seawater is about 3.5% salt by weight. 

Because seawater covers 72% of Earth, it is not surprising that it is considered a water source for areas where freshwater supplies aren t sufficient to meet the demand. The oceans contain an average 3.5% (35,000 ppm) dissolved salts by weight, a concentration too high to make ocean water useful for drinking, washing, or agricultural use (Table 11.9). The total of dissolved ions must be reduced to below 500 ppm before the water is suitable for human consumption. 

The dissolved ion chemistry of seawater is dominated by eight ions all present at millimolar concentrations or more, with sodium and chloride as the overwhelmingly dominant ions (Table 1). The ratio of these eight ions one to another is very constant in ocean waters because the long residence time of these ions ( 10 years) allows for complete mixing of the waters at fast rates compared to internal mixing. 

Despite the fact that the major ions are present in seawater at relatively high concentrations (see Chapter 11), difficulties in storage have been observed by several workers. For example, most types of glass contain alkali and alkaline earth metals capable of ion-exchange reactions with the dissolved ions of seawater and thus are unsuitable materials for... 

Since the end of the 19th century it has been known that the composition of seawater is almost constant in space and time (the concept of conservatism , see Chapter 11). Therefore, to a good first approximation, oceanographers assume that seawater consists of just two components, the first one being pure water and the second one representing all dissolved ions that contribute to the mass of seawater, namely salinity. By this two-component assumption, three thermodynamic parameters are needed to derive the state of seawater. Besides salinity, it is convenient to choose temperature (T) and pressure (P) since they are relatively easy to measure at the required accuracies and, together with salinity, are also valuable for water mass analysis. 

In seawater silicon occurs predominantly as reactive silicate, the dissolved ions of orthosi-licic acid, but also as inorganic and organic fractions of suspended material, particularly originating in plankton diatoms (see Section 10.1). Die concentration of suspended silicon is very small, usually not exceeding a few /imol/L. Deating with an alkaline persulphate solution will break down these silicon complexes and convert them into reactive silicate. Only when large amounts of clay material are present a carbonate fusion is needed. 

Seawater is approximately 4.0% by mass dissolved ions, 85% of which are from NaCl. (a) Find the mass % of NaCl in seawater. (b) Find the mass % of Na ions and of Cl ions in seawater,... 

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