Boron in seawater

Various chromogenic reagents have been used for the spectrophotometric determination of boron in seawater. These include curcumin [108,109], nile blue [110], and more recently 3,5 di-tert butylcatechol and ethyl violet [111]. Uppstroem [108] added anhydrous acetic acid (1 ml) and propionic anhydride (3 ml) to the aqueous sample (0.5 ml) containing up to 5 mg of boron per litre as H3BO3 in a polyethylene beaker. After mixing and the dropwise addition of oxalyl chloride (0.25 ml) to catalyse the removal of water, the mixture is set aside for 15-30 minutes and cooled to room temperature. Subsequently, concentrated sulfuric-anhydrous acetic acid (1 1) (3 ml) and curcumin reagent (125 mg curcumin in 100 ml anhydrous acetic acid) (3 ml) are added, and the mixed solution is set aside for at least 30 minutes. Finally 20 ml standard buffer solution (90 ml of 96% ethanol, 180 g ammonium acetate - to destroy excess of protonated curcumin - and 135 ml anhydrous acetic acid diluted to 1 litre... 

Marcantoncetos et al. [112] have described a phosphorimetric method for the determination of traces of boron in seawater. This method is based on the observation that in the glass formed by ethyl ether containing 8% of sulfuric acid at 77 K, boric acid gives luminescent complexes with dibenzoylmethane. A 0.5 ml sample is diluted with 10 ml 96% sulfuric acid, and to 0.05-0.3 ml of this solution 0.1ml 0.04 M dibenzoylmethane in 96% sulfuric acid is added. The solution is diluted to 0.4 ml with 96% sulfuric acid, heated at 70 °C for 1 h, cooled, ethyl ether added in small portions to give a total volume of 5 ml, and the emission measured at 77 K at 508 nm, with excitation at 402 nm. At the level of 22 ng boron per ml, hundredfold excesses of 33 ionic species give errors of less than 10%. However, tungsten and molybdenum both interfere. 

Atomic absorption spectrometry has been used for the rapid determination of boron in seawater [113]. 

Traces of boron in seawater have been determined by flow injection analysis with spectrophotometric detection at 415 nm using G 30 methine H. The linear range was 1 -10 mg/1 boron with a detection limit of 0.017 mg/1 [3]. 

Dissolved boron in seawater comes mainly in two forms — as boric acid, B(OH)3, and borate ion, B(0H)4. The boric acid - borate equiUbrium can be written as ... 

Goldschmidt and Peters (1932) were also the first to note the high concentration of boron in seawater (4.5 ppm) as opposed to its low content in fresh waters (10 ppb in average river, Lemarchand et al, 2002). This contrast served as a basis for the numerous attempts to use boron content as a paleosalinity proxy (see Lerman, 1966 Walker, 1968), again primarily utilizing clays as the indicator phase. 

The average concentration of boron in the earth s CTust is 17 ppm, and most soils fall within the range of 3-100 ppm. Boron occurs in high concentrations in sedimentary rocks and in clay-rich marine sediment due to the relatively high concentration of boron in seawater. Deposits of boron are found in association with volcanic activity and where marshes or lakes have evaporated under arid conditions. Coastal soils contain up to 50 times as much boron as do inland soils while, generally, humid soils are lower in boron content than slightly leached soils. The concentration of boron in soil is influenced also by the presence of other minerals, and soil pH and texture (Butterwick et al., 1989 Steinnes, 2009). 

Boron in seawater has a concentration of about 4.5ppm. It is supplied to the ocean by continental weathering and hydrothermal inputs, and has a residence... 

Tsaikov [ 114] has described a coulometric method for the determination of boron in coastal seawaters. This method is based on the potentiometric titra-... 

Chemically, tungsten is rather inert, but it will form compounds with several other elements at high temperatures (e.g., the halogens, carbon, boron, silicon, nitrogen, and oxygen). Tungsten will corrode in seawater. 

Klochko K, Kaufman AJ, Yao W, Byrne RH, Tossell JA (2006) Experimental measurement of boron isotope fractionation in seawater. Earth Planet Sci Lett 248 276-285 Kloppmann Girard NSgrel P (2002) Exotic stable isotope composition of saline waters and brines from the crystalline basement. Chem Geol 184 49-70 Knauth LP (1988) Origin and mixing history of brines, Palo Duro Basin, Texas, USA, Applied Geochemistry 3 455 74... 

In water, boron readily hydrolyzes to form the electrically nentral, weak monobasic acid H3BO3 and the monovalent ion B(OH)4. Waterborne boron may be adsorbed by soils and sediments (USPHS 1991). The predominant boron species in seawater is boric acid (Thompson et al. 1976) concentrations are higher at higher salinities and in proximity to indnstrial waste discharges (Liddicoat et al. 1983 Narvekar et al. 1983). In seawater, borate or boric acid occurs naturally at... 

Boron, aluminum and gallium are all highly hydrolysed trivalent elements, but the extent of their hydrolysis in seawater is distinctly different. While all three elements exist as M(OH) and M(OH)4 in seawater, Al is much more weakly hydrolysed than either B or Ga. The free-to-total metal ratio for Al3+ is in the order of 10 9A at 25°C and pH 8.2, while for Ga the free-to-total metal ratio is in the order of 10 15 7. Owing to its high charge and small radius, boron does not form a simple cation (M3+) in aqueous solution and, in fact, forms of boron less... 

Thallium (Tl), which appears to exhibit conservative behaviour in seawater, has two potential oxidation states. As Tl1, thallium is very weakly complexed in solution. In contrast, Tl111 should be strongly hydrolysed in solution ([T13+]/[T13+]t — 10 20 5) with Tl(OH)3 as the dominant species over a very wide range of pH. The calculation of Turner et at. (1981) indicated thatTl111 is the thermodynamically favoured oxidation state at pH 8.2. Lower pH and p()2 would be favourable to Tl1 formation. Within the water column, pH can be considerably less than 8.2 and /)( )2 lower than 0.20 atm. In view of these factors, and the observation that redox disequilibrium in seawater is not uncommon, the oxidation state of Tl in seawater is somewhat uncertain. The existence of Tl in solution as Tl+, a very weakly interactive ion, would reasonably explain the conservative behaviour of Tl in seawater. However, the extremely strong solution complexation of Tl3+ suggests that Tl3+ may be substantially less particle reactive than other Group 13 elements (with the exception of boron). 

The interaction between sodium and alkaline-earth metal ions and borate has attracted recent attention, particularly from the point of view of association of ions in seawater. Several studies (69, 114,168, 169, 340) have shown that the boron content of seawater (4-5 x 10 4 M) is too low to support appreciable concentrations of polyborate species. The increase in acidity of boric acid in the presence of metal ions results from ion-pair formation ... 

This element is widely employed in the production of glass and glass fibers, fluxes, antiseptics, and other products. Boron compounds are also widely used in nuclear technology [264]. Boron is an element that occurs at a relatively high concentration level in seawater (4.5 mg/L). Yet, economically acceptable processes for boron extraction from the sea do not exist, despite the fact that methods for its recovery from highly mineralized brines have been available since the beginning of the 1960s [253]. With the development of such methods, attempts were made to determine the lowest concentration levels of the element, at which economical processes could be developed [256, 266]. This critical concentration of boron was at that time estimated to be around 20 mg/L. Currently, the critical concentration of boron is estimated to be 15 mg/L or even somewhat less. 

There are two stable isotopes of boron, °B and "B. °B is the more abundant of the two, accounting for —80% of all boron atoms. Variations in boron isotope abundance are reported relative to an NIST boric acid standard as 5 B. In seawater there are two dilferent species of boron. The relative proportion of the two species is a function of pH as indicated by the reaction... 

The ratio of the boron isotopes and °B is known to depend on ambient pH in the boron incorporated in the carbonate shells of marine foraminifera (Sanyal et ai, 1996 Sanyal et al., 2001). The use of boron isotopes as an indicator for seawater paleo-pH has been extended to the calculation of past CO2 concentrations in ocean surface waters and in the atmosphere (Spivack, 1993 Sanyal et al., 1997 Pearson and Palmer, 1999, 2000 Sanyal and Bijma, 1999 Palmer and Pearson, 2003). Sources of uncertainty in these estimates include the fractionation of boron isotopes during incorporation in carbonate shells, effects of diagenesis, the assumptions needed to calculate CO2 concentrations from pH, and the influence of changing boron isotope ratios in ambient seawater (Lemarchand et al., 2000 Lemarchand et al., 2002). The latter problem is especially serious for estimates based on samples older than the 15 Myr residence time of boron in the oceans. 

Magnesium is the third most abundant element in seawater, behind sodium and chorine, and has an average concentration of approximately 1300 ppm. Table 3.2 displays the major and some minor elemental constituents of seawater. Eleven major constituent ions account for 99.5% of the total solutes present in seawater. These 11 are chloride, sulfate, bicarbonate, bromide, fluoride, sodium, magnesium, calcium, potassium, strontium, and boron, and they largely determine the chemistry of seawater. 

Figure 3.3 Variation of boron contamination in seawater-precipitated magnesium hydroxide as a function of precipitation pH.

On the other hand, the kinetics of the boric acid-borate equilibrium are less well known. Yet in order to calculate fluxes, pH gradient and boron isotope distribution in the vicinity of a foraminifer, the kinetics (i.e. the speed of the conversion between the two dissolved boron species) is crucial. At the time when we developed the numerical models of the chemical microenvironment of foraminifera there was, to the best of our knowledge, no measured value for this rate constant available in the literature. The problem was eventually solved by considering sound absorption data in seawater, which is described in Zeebe et al. (2001). 

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