Seawater acid-based measurements

Willason and Johnson [53] have described a modified flow-injection analysis procedure for ammonia in seawater. Ammonium ions in the sample were converted to ammonia which diffused across a hydrophobic membrane and reacted with an acid-based indicator. Change in light transmittance of the acceptor steam produced by the ammonia was measured by a light emitting diode photometer. The automated method had a detection limit of 0.05 xmol/l and a sampling rate of 60 or more measurement per hour. 

Alternatively, over very long time periods, L-amino acids can racemize to produce D-amino acids. However, measured d/l ratios for certain amino acids cannot be achieved based on known racemization rates of amino acids in seawater. For example, Lee and Bada (1977) calculated d/l ratios of 0.01 and 0.004 for aspartic acid and alanine, respectively, assuming an oceanic residence time of 3,400 years for these amino acids. These calculated values are much lower than the measured values and led Lee and Bada (1977) to conclude that the enhanced D-amino acid concentrations in marine DOM must be derived from a bacterial source. In a later paper, Bada et al. (1982) suggested that the near-racemic mixture (50% each of the d and l enantiomer) of alanine at depth in the ocean was a result of the dehydration of serine or threonine to produce racemic alanine. These authors also detected near racemic a-amino- -butyric acid (ABA), which can be produced from the dehydration of threonine. This mechanism of D-alanine formation... 

Ammonium is present at very low concentrations (0.03—0.5 iM) in oceanic surface waters, at higher concentrations in coastal and estuarine waters (Sharp, 1983), and at concentrations orders of magnitude higher in sediment pore waters. In seawater, NH4+ exists as the acid base pair NH4+-NH3 (ammonia) the pFC of the pair is 9.3. The methods discussed here measure the sum of NH4+, the form that dominates at the pH of seawater ( <8.3), and NH3, the volatile form that dominates under more alkaline conditions. There are many approaches to measuring NH4+, but we win focus on the two most widely used— phenol-hypochlorite and orthophtal-dialdehyde (OPA). 

Measurements of a number of acids have been made in seawater, and equations are available to determine the elfect of salinity, temperature, and pressure on the dissociation constants (Millero, 2001). The most widely studied acids in seawater are those related to the carbonate system. Most of these studies have been made in artihcial seawater that contains the major ionic components (Na, Mg +, Ca +, K+, Cr, SO, F ). It was thought that constants determined in this artificial seawater could be used in real seawater. Although this is the case for most acid-base systems, as will be discussed below, this is not the case for the carbonate system (Mojica and Millero, 2002). 

The pH values so measured are on a different activity scale they may be used to characterize and compare seawater samples and so serve as an index of acid-base balance and speciation. ... 

The pH accordingly acquires primary importance as an index of the state of the many interactive acid-base systems of which seawater is composed. Two factors are essential to the most fruitful application of this acid-base parameter. The first is that pH measurements possess reproducibility of a high order, and the second is that the numbers obtained have a clear meaning in terms of the processes of interest. To achieve the necessary reproducibility, uniform procedures and standards for the measurement must be accepted by all workers in the field. 

Analytical chemistry has found great utility in conductimetric measurements in spite of its apparent nonspecificity. Rapid quantitative accuracy of a few tenths of a percent may be quickly accomplished by direct conductimetric determination of binary electrolytic solutions such as aqueous acids, bases, or salts. A nearly linear increase in conductivity is observed for solutions containing as much as 20% of solute. The concentration of strong solutions, such as the salinity of seawater, may be determined from conductance measurements traces of electrolyte impurities, such as the impurity in ultrapure water, may be reported at the pgl level. Conductimetric titrations may increase the accuracy of endpoint detection and permit titrimet-ric analysis of weak electrolytes, such as boric acid, which is not feasible by potentiometric or colorimetric... 

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. 

The alkalinity in a mixed electrol5d e solution is the excess in bases (proton acceptors) over acids (proton donors) in the solution. The alkalinity is measured by adding acid to seawater to an end point where most all proton acceptors have reacted. When one adds acid the hydrogen ion concentration does not increase as much as it would in the absence of alkalinity because some of the added hydro-... 

The method described here is based on the difference between measurements of total alkaline earths by complexometric titration with EDTA (ethylenediamine-N,N,N, N -tetra-acetic acid) and selective measurement of calcium described in Section 11.2.1. The simultaneous EDTA titration of calcium, strontium and magnesium involves Eriochrome Black T (EBT) as indicator and was originally applied to seawater analysis by Voipio (1959) and Pate and Robinson (1961). To eliminate subjective errors in the determination of the endpoint, Culkin and Cox (1966) used photometric endpoint detection. A slight modification of this procedure, including the standardization of EDTA by magnesium is reported here. 

An alternative method has been described by Hartmann et al. (1989). It is based on preconcentration by co-precipitation of manganese hydroxide with Mg(OH)2 (fi om the seawater magnesium) at a pH of 10. After separation from the bulk of the seasalts by centrifugation and redissolution of the precipitate with nitric acid, Mn is measured by ETAAS. When applying a concentration factor of 40, the authors reported a detection limit for Mn of 0.02-0.04 nmol/kg. 

The determination of chromium is based on adsorptive collection of complexes with diethylenetriaminepentaacetic acid (DTPA) (Golimowski et al, 1985 Boussemart et ai, 1992). The measurement of chromium in seawater is carried out at a pH between 5.0 and 5.3, whereas measurements in freshwater are carried out at a slightly higher pH of 6.2-6.4. 

The automation of the diacetyl monoxime reaction was proposed for Technicon Autoanalyzer by DeManche et al. [186], reducing the incubation time to 13 min at the temperature of 95°C. The method was subsequently modified and revised in the following studies [90,184,210]. An automated method for determination of urea based on Alpkem Autoanalyzer (flow solution HI Ol-analytical) was proposed by Cozzi [186]. In this study, the surfactant generally used in these applications (i.e., Brij-35) was substituted to avoid its interference on the colorimetric measure in the acidic conditions of the diacetyl monoxime reaction. On the whole, the autoanalyzer technique appears to be suitable for the determination of urea in the natural waters as it permits the analysis of a large number of samples, with detection limits (ss0.04 pmol N L ) and precision (RSD 1—2% at the concentration 1.0 pmol N L ) comparable to the manual methods [90,184,187]. FIA technique was used for the automation of the diacetyl monoxime reaction for the determination of urea in soil extracts [183] and in brackish and seawater samples [193] in concentration ranges of 0.7-571 and 0.7—28 pmol N L , respectively. 

Sea spiay Near the ocean, some of the sulfate in precipitation comes from sea spray, not pollution. This sulfur is already fully oxidized and contributes no acidity to precipitation. (In fact, sea water is slightly alkaline.) A technique for distinguishing between the sulfate derived from sea spray and the sulfate from man-made pollution was developed in the middle of the nineteenth century. It is based on the fact that the ratio of chloride to sulfate in seawater is constant. Since the major source of chloride in precipitation near the coast is sea spray, measuring the amount of chloride in precipitation allows a determination of the amount of sulfate in precipitation contributed by sea-spray. In this book, the values given for sulfate in precipitation do not include the "excess" sulfate contributed by sea-spray. 

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