Acidification of seawater

Magnesium hydroxide is commonly produced from seawater, which is rich in Mg2+ ion. The average concentration of Mg2+ in seawater is about 1,300 mg/L. The first step of the process involves removal of interfering substances from seawater, the most notable being the water-soluble calcium bicarbonate. Bicarbonate removal is crucial, as it can form insoluble calcium carbonate, a side product that cannot be separated from magnesium hydroxide readily. Acidification of seawater converts bicarbonate into carbon dioxide, which is degassed by heating. Alternatively, seawater is treated with bme to convert calcium bicarbonate to carbonate ... 

The acidification of seawater samples is required for the storage of seawater in order to prevent the precipitation, adsorption of trace metals on the container walls or volatilization loss from solution. Generally, nitric acid is added to seawater to bring the pH of the solutions to below 1.5. The addition of acids immediately after collection on board ship may be preferable for sample storage rather than addition before analysis in the laboratory. 

An alternative pretreatment for seawater is acidification of the bicarbonate followed by degasification to remove the carbon dioxide generated. The precipitation step for the seawater process is given by (76) ... 

Yamamoto et al. [6] studied preservation of arsenic- and antimony-bearing samples of seawater. One-half of the sample (201) was acidified to pH 1 with hydrochloric acid immediately after sampling, and the remaining half was kept without acidification. In order to clarify the effect of acidification on storage, measurements were made over a period of a month after sampling. Results are given in Table 1.1. In this study, a standard addition method and calibration curve method were used for comparison and it was proven that the two gave the same results for the analyses of seawater. 

Of particular concern are the impacts of seawater acidification on biocalcification and the burial rates of sedimentary carbon. Carbonate ion concentrations in the surface waters have already declined by 16%. Thus, it is not surprising that the abundance of tropical/subtropic planktonic foraminiferan species appears to have declined since the 1960s. This information was obtained by studying the rapidly accumulating sediments of the Santa Barbara Basins off the coast of California. 

Although sample preservation is to be avoided, sometimes there are no other options. For chromium sampling, the water sample was acidified at pH 2 and no Cr(III) was lost to the wall of the precleaned polyethylene (PE) or the polypropylene (PP) bottle for more than one month.48-49 While it is clear that Cr(III) is stable for a long time in an acidified sample, it was reported that acidification of coastal seawater resulted in the rapid reduction of Cr(VI) to Cr(III)48 On the other hand, a stable Cr(VI) concentration could be ensured at a nearly neutral pH, especially under a C02 blanket.48-50... 

A large discrepancy between the two concentration techniques was found for the copper results. The average difference was 72 27 ngl-1. Bruland and Frank analysed the Chelex column effluent by the solvent extraction technique, and found 63 and 135ngl-1 as the copper content for the samples at 25 m and 2500 m, respectively. These values are almost equal to the difference between the Chelex and solvent extraction results. Therefore, they concluded that about 60% of the copper in seawater (unfiltered and unacidified) is not removed by the Chelex technique. As Riley et al. suggested, copper in seawater is not liberated by the Chelex resin because of association with colloids and fine particulates [62]. In order to avoid this error, acidification and heating of seawater is necessary prior to the Chelex treatment. According to the results of Bruland and Franks, acidification and storage followed by solvent extraction appears to be superior to the Chelex resin concentration for the quantitative determination of copper in seawater [15]. Similar problems have been pointed out by Eisner and Mark, Jr. [63] and Florence and Batley [64]. 

More simultaneous measurements of NH3 in the ocean and in the atmosphere are needed to reduce the considerable uncertainties of the ocean/atmosphere flux estimates. The ongoing acidification of the ocean will shift the NH3/NH4 equilibrium to NH. On the one hand this might have implication for the atmospheric distribution of NH3, since the uptake capacity of the ocean will be increased with unknown consequences for chemistry of the atmosphere (e.g. the aerosol formation) over the ocean. On the other hand this might have severe implications for the nitrification rates in seawater because they are influenced by the pH. When the pH drops from 8 to 7, nitrification rates can be reduced by 50% (Huesemann et al., 2002). (One explanation for this is that the ammonia monooxygenase enzyme uses rather NH3 than NH4 as substrate.) Most recently it was suggested that atmospheric NH3 serves as a foraging cue for seabirds such as the blue petrel (Nevitt ei a/., 2006) is an excretion product of... 

Wright R. E., Norton S. A., Brakke D. E., andFrognerT. (1988) Acidification of stream water by whole-catchment experimental addition of dilute seawater. Nature 334, 422-424. 

I filtered the mixture. The filtrate, now free of cells and debris, was nearly dark, but it regained its luminescence upon neutralization with a small amount of sodium bicarbonate. Indeed, the experiment showed that the luminescence substance of the jellyfish was extracted into the solution at pH 4. But my real surprise came next. When I added a small amount of sea water to the solution, its luminescence became explosively strong. Because the composition of seawater is known, I quickly discovered that the activator is Ca. The discovery of Ca. as the activator in turn suggested that EDTA should serve as a better inhibitor of luminescence than acidification. Based on this information, we devised a method of extracting the light-emitting principle. We collected and extracted about 10,000 jellyfish in that summer. 

Free fatty acids, hydroxy acids and their esters may be extracted from seawater after filtration with various organic solvents in high yields after acidification to pH 2—3 or extraction at pH 8 and subsequent acidification to pH 2—3 (A. Saliot, pers. comm., 1979). Chloroform is the most commonly employed extracting solvent, three repeated extractions being sufficient to quantitatively extract 1—2 1 of seawater (Treguer et al., 1972). 

Now, the average pH at the surface layer of seawater is 8.1 [6]. Acidification of the ocean is progressing by an increase in the concentration of carbon dioxide in the air. It may be necessary to revise the pH of a solvent used in the elution test if the pH continues to fall in the future. 

Solutions that contain high concentrations (10 M or more) of a weak conjugate acid-base pair and that resist drastic changes in pH when small amounts of strong acid or strong base are added to them are called buffered solutions (or merely buffers). Human blood, for example, is a complex buffered solution that maintains the blood pH at about 7.4. (Section 17.2, Blood as a Buffered Solution ) Much of the chemical behavior of seawater is determined by its pH, buffered at about 8.1 to 8.3 near the surface. (Section 17.5, Ocean Acidification ) Buffers find many important applications in the laboratory and in medicine ( Figure 17.1). Many biological reactions occur at the optimal rates only when properly buffered. If you ever work in a biochemistry lab, you will very likely to have prepare specific buffers in which to run your biochemical reactions. 

The specifics of sampling and storage for water are dependent upon the sample. Clearly, different procedures are required for groundwater, seawater, waste water, and atmospheric precipitation. Samples should be collected at a reproducible location and depth below the surface. In general, the contamination control procedures discussed above should be followed. Although it may be necessary to remove bacteria and other suspended solids from samples using membrane filters, it is necessary to investigate these methods for possible contamination or influence upon the types of species present. Acidification of water samples may also induce chemical transformations of some elements. 

Abollino et al. [690] compared cathodic stripping voltammetry and graphite furnace AAS in determination of cadmium, copper, iron, manganese, nickel, and zinc in seawater. The effects of UV irradiation, acidification, and online sample preconcentration were studied. 

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