Solution seawater

The titanium oxide film consists of mtile or anatase (31) and is typically 250-A thick. It is insoluble, repairable, and nonporous in many chemical media and provides excellent corrosion resistance. The oxide is fully stable in aqueous environments over a range of pH, from highly oxidizing to mildly reducing. However, when this oxide film is broken, the corrosion rate is very rapid. Usually the presence of a small amount of water is sufficient to repair the damaged oxide film. In a seawater solution, this film is maintained in the passive region from ca 0.2 to 10 V versus the saturated calomel electrode (32,33). 

A typical polarographic recording is shown in Fig. 2.1 curve (a) is the po-larogram obtained for chlorinated seawater analysed immediately after chlorination. Identical traces were observed for non-chlorinated seawater and for chlorinated seawater kept in the dark for periods up to 24 h at temperatures up to 40 °C, which indicates a lack of bromate formation under these conditions (BrC>3 < 10 7 M, less than 0.5% conversion of chlorine). Addition of copper sulfate to give a cupric ion concentration in the seawater of 100 parts per billion did not induce measurable bromate production in the dark. Curve (b) was obtained from a chlorinated (4.9 mg/1) seawater solution that was exposed to full sunlight for 70 min. Curve (c), which is offset by 0.4 pA with respect to curves (a) and (b), shows the presence of 1.0 x 10 5 M sodium bromate in seawater. 

Pre-acidified pore water (100 pi, diluted with Millipore Q-water if necessary) was transferred, using an Eppendorf pipette, into a 10 ml volumetric Pyrex flask. To this flask nitric acid (50 pi) was added, and the solution was then brought to volume with Millipore Q-water. Standards were made up by adding various amounts to stock metal solutions (lmg/1), nitric acid (50 pi), and a seawater solution (100 pi) of approximately the same salinity as the samples to be analysed. This final addition ensures that the standards are of approximately the same ionic strength and contain the same salts as the samples. 

Sea anemones (Anemonia viridis) in seawater solutions containing 50 or 200 pg Cu/L regulate copper by expelling zooxanthellae, which are shown to accumulate copper (Harland and Nganro 1990). 

This can be demonstrated by considering the high pressure PVT properties of seawater. For seawater solutions the values of K , A and Bata given temperature are given by (127). 

Comparisons of the predicted and measured values of 63 an u f°r seawater solutions using the additivity method are shown in Table VI. The agreement is quite good and sufficient for most needs. 

COMPARISONS OF THE MEASURED AND CALCULATED VALUES OF B AND u FOR SEAWATER SOLUTIONS AT 25°C S... 

Clllate, Fabrea sallna] held In seawater solution containing radlosllver-110m... 

The chemical shifts in the NMR for the methyl groups on arsenic can vary depending on pH. They are, however, sufficiently different from each other and from other marine arsenic compounds that they are readily distinguishable even in seawater solutions (Fig. 1). Thus, the possibility exists for examining marine arsenic transformations in solutions or cells by NMR spectroscopy. Little work has been done in this area. 

Figure 2.9 Flux and rejection data for a model seawater solution (3.5 % sodium chloride) in a good quality reverse osmosis membrane (FilmTec Corp. FT 30 membrane) as a function of pressure [10]. The salt flux, in accordance with Equation (2.44), is essentially constant and independent of pressure. The water flux, in accordance with Equation (2.43), increases with pressure, and, at zero flux, meets the pressure axis at the osmotic pressure of seawater 350 psi...

McCubbin, D. and Leonard, K.S. (1997) Laboratory studies to investigate short-term oxidation and sorption behaviour of Np in artificial and natural seawater solutions. Mar. Chem., 56, 107-121. 

It is appropriate at this point to discuss the "apparent" pH, which results from the sad fact that electrodes do not truly measure hydrogen ion activity. Influences such as the surface chemistry of the glass electrode and liquid junction potential between the reference electrode filling solution and seawater contribute to this complexity (see for example Bates, 1973). Also, commonly used NBS buffer standards have a much lower ionic strength than seawater, which further complicates the problem. One way in which this last problem has been attacked is to make up buffered artificial seawater solutions and very carefully determine the relation between measurements and actual hydrogen ion activities or concentrations. The most widely accepted approach is based on the work of Hansson (1973). pH values measured in seawater on his scale are generally close to 0.15 pH units lower than those based on NBS standards. These two different pH scales also demand their own sets of apparent constants. It is now clear that for very precise work in seawater the Hansson approach is best. 

In this chapter, we introduced the reader to some basic principles of solution chemistry with emphasis on the C02-carbonate acid system. An array of equations necessary for making calculations in this system was developed, which emphasized the relationships between concentrations and activity and the bridging concept of activity coefficients. Because most carbonate sediments and rocks are initially deposited in the marine environment and are bathed by seawater or modified seawater solutions for some or much of their history, the carbonic acid system in seawater was discussed in more detail. An example calculation for seawater saturation state was provided to illustrate how such calculations are made, and to prepare the reader, in particular, for material in Chapter 4. We now investigate the relationships between solutions and sedimentary carbonate minerals in Chapters 2 and 3. 

In the previous chapter, the fact that stoichiometric and apparent constants have been widely used in seawater systems was discussed. Berner (1976) reviewed the problems of measuring calcite solubility in seawater, and it is these problems, in part, that have led to the use of apparent constants for calcite and aragonite. The most difficult problem is that while the solubility of pure calcite is sought in experimental seawater solutions, extensive magnesium coprecipitation can occur producing a magnesian calcite. The magnesian calcite should have a solubility different from that of pure calcite. Thus, it is not possible to measure pure calcite solubility directly in seawater. 

Aragonite is the only one of the four carbonate minerals examined that does not have a calcite-type rhombohedral crystal structure. For all the minerals examined, with the exception of aragonite, the two solution saturation states studied represent supersaturated conditions, because at a saturation state of 1.2 with respect to calcite, the seawater solution is undersaturated (0.8) with respect to aragonite. 

At the higher saturation state, the seawater solution is more than 5 times supersaturated with respect to aragonite so that aragonite would be expected to precipitate on the aragonite seed crystal. Results indicated that Mg2+ is adsorbed between 25 to 40 times less on aragonite than on calcite from solutions supersaturated with respect to both minerals. 

The Mg to Ca surface ratios for dolomite in both seawater solutions were statistically identical. A Mg to Ca surface ratio of 3 is predicted from a simple site-specific adsorption model, and is in reasonable agreement with observed values. 

The Mg to Ca surface ratios for calcite in both supersaturated seawater solutions were nearly identical. The lower Mg to Ca surface ratio obtained in the less supersaturated solution may be the result of incomplete coverage of the pure calcite crystal by the magnesian calcite overgrowth. The Mg to Ca surface ratio on calcite exposed to both saturation state solutions is in close agreement with the value of 1 obtained in a solution with a Mg2+ to Ca2+ ratio of 5 by Moller and his associates. 

Zhong S. and Mucci A. (1989) Calcite and aragonite precipitation from seawater solutions of various salinities Precipitation rates and overgrowth compositions. Chem. Geol. (in press). 

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