Acids and bases in seawater

The importance of the many add-base pairs in seawater in determining the acidity of the ocean depends on their concentrations and equilibrium constants. Evaluating the concentrations of an acid and its conjugate anion (base, Ba ) as a function of pH (pH = —log [H ]) requires knowledge of the equation describing the acid/base equilibrium (hydrogen ion exchange), the apparent equihbrium constant, K, and information about the total concentration, [Ba]x, of the acid in solution  

Combining Eqs. (4.2) and (4.3) gives expressions for the concentration of the acid, HBa, and its conjugate base, Ba, as functions of the apparent equilibrium constant, IC, and the hydrogen ion concentration, [H+j  

A plot of these logarithmic equations (Fig. 4.1) illustrates that the concentration of the acid dominates the solution concentration below pH = pK (on the acid side), and in the region where pH is greater than pK (the basic side), the conjugate base, Ba , dominates. At a pH equal to pK the concentrations of the acid and basic forms are equal, [HBa] = [Ba ]. 

The final constraint is that of charge balance, which in this simple solution involves the only two ions  

This equation constrains the system to a single location on the plot (where the lines for these two concentrations cross in Fig. 4.1), which uniquely fixes the pH and concentrations of acids and bases in the system. In this simple system the solution is acidic (pH = 4) because the concentration of the hydrogen ion and anion must be equal. 

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These simple equations and ideas provide the basis for describing the carbonate system in terms of the/coj, DIG, pH, and alkalinity of seawater. We will build up a plot similar to that in Fig. 4.1 for the important acids and bases in seawater. These are listed along with their concentrations and apparent equihbrium constants in Table 4.1. It will then be demonstrated how the constraint of charge balance (called alkalinity) determines the pH of seawater. 

Speciation calculations can be performed for the weak acids and bases in a feshion similar to that presented earlier for Fe(III). The results of these calculations as a function of pH are shown in Figure 5.19. At the pH of seawater, the dominant species are carbonate, bicarbonate, ammonium, hydrogen phosphate, dihydrogen phosphate, and boric and silicic acid. In waters with low O2 concentrations, significant concentrations of HS can be present. 

Ohe pH of aqueous solutions—blood plasma, seawater, detergents, sap, and reaction mixtures—is controlled by the transfer of protons between water molecules and other molecules and ions. In Chapter 10, we learned about the properties of acids and bases. In this chapter, we see how to use solutions of weak acids or bases and their salts to maintain a desired pH. We also see how to identify the stoichiometric points of titrations. Finally, we meet the solubility equilibria that are the basis of qualitative analysis, the identification of the ions present in a sample. 

This submarine vs. subaerial hypothesis for the origin of the two types of deposits (Kuroko deposits, epithermal vein-type deposits) can reasonably explain the difference in metals enriched into the deposits by HSAB (hard-soft acids and bases) principle proposed by Pearson (1963) (Shikazono and Shimizu, 1992). Relatively hard elements (base metal elements such as Cu, Pb, Zn, Mn, Fe) are extracted by chloride-rich fluids of seawater origin, while soft elements (Au, Ag, Hg, Tl, etc.) are not. Hard elements tend to form chloro complexes in the chloride-rich fluid, while soft elements form the complexes in H2S-rich and chloride-poor fluids. Cl in ore fluids is thought to have been derived from seawater trapped in the submarine volcanic and sedimentary rocks. 

Solutions of substances that are good conductors of electricity are called electrolytes. Sodium chloride, the major constituent of seawater, is a strong electrolyte. Most salts, as well as strong acids and bases, are strong electrolytes because they remain in solution primarily in ionic (charged) forms. Weak acids and bases are weak electrolytes because they tend to remain in nonionic forms. Pure water is a nonconductor of electricity. 

Most of the titratable charge in seawater is supplied by bicarbonate because its concentration is much greater than that of carbonate or any of the other weak bases in seawater, such as B(OH). A typical acid titration curve for a seawater sample is shown in Figure 15.7. If the titration is performed in an open container, initial addition of acid does not cause much of a drop in pH. During this phase of the titration, is readily consumed, first by carbonate (Eq. 5.57) and then by bicarbonate (Eq. 5.56). Most of the buffering is provided by bicarbonate because of its high concentration. Once most of the bicarbonate has been consumed, further addition of acid causes a rapid decline in pH. 

Depending upon the environment to which the copper and its alloys are exposed, various forms of corrosive attack occur. The environments of interest are (i) atmospheric (ii) fresh water and seawater (iii) soil and (iv) chemical solutions, including acids and bases. The forms of corrosion of copper and its alloys in different environments... 

Dickson A. G. and Riley J. P. (1979a) The estimation of acid dissociation constants in seawater from potentiometric titrations with strong base I. The ion product of water— K. Mar. Chem. 7, 89-99. 

Neutral molecules have activity coefficients essentially equal to unity in solutions of less than 10 mM ionic strength. At higher salt concentrations, most neutral molecules are increasingly salted out of water that is, the activity coefficient > 1, so that a, /c, < 1 for molecules in higher ionic strength solutions. In our discussion of dilute aqueous acids and bases, we will assume ideal behavior of the neutral species. The importance of salting out of dissolved CO2 will be reflected in considering dissolved carbonic species in seawater (Chapter 4). 

In this chapter we describe the distribution of CO2, H2CO3, HCOf, and C03 in natural waters, examine the exchange of CO2 between atmosphere and waters, evaluate the buffering mechanisms of fresh waters and seawater, and define their capacities for acid and base neutralization. 

One of the most important components of the chemical perspective of oceanography is the carbonate system, primarily because it controls the acidity of seawater and acts as a governor for the carbon cycle. Within the mix of adds and bases in the Earth-surface environment, the carbonate system is the primary buffer for the aridity of water, which determines the reactivity of most chemical compoimds and solids. The carbonate system of the ocean plays a key role in controlling the pressure of carbon dioxide in the atmosphere, which helps to regulate the temperature of the planet. The formation rate of the most prevalent authigenic mineral in the environment, CaCOs, is also the major sink for dissolved carbon in the long-term global carbon balance. 

Concentrations of the species of the acid-base pairs of carbonate, borate and water in seawater as a function of pH. (Salinity, S = 35, temperature, T-20"C and... 

The reaction of amino acids with sugars to form Schiff s base-type compounds has been often suggested to be a feasible process in the formation of coloured condensed material in seawater, particular in the presence of clay minerals (Hedges, 1978). It is also worthy to note that natural free sugars and amino acids when heated in seawater form fluorescent compounds whose excitation and emission maxima correspond with those formed on reaction of amines with aldehydes (Honda et al., 1974). These experiments were performed at high temperatures (G. Liebezeit, R. Dawson and K. Mopper, unpublished results) and there is some suggestion that the apparent disappearance of amino acids on standing in seawater at room temperature, may be connected in part to this abiotic process. 

Blank values. Ihe overall blank values of the analytical method, from weighing of the specimen to the prepeuation of the sample, are mainly caused by the added reagents, Le., the buffer solution, acids and bases, the dithiodicarbamate and, last but not least, the ultra-pure water if used instead of seawater for dilution or for the determination of the blank values. If no water of sufficient purity (ultra-pure water) is available, seawater extracted with Na-DBDTC is an accetable substitute. The use of reference seawater is another option (c.g., NASS and CASS reference material see Section 12.1.6). The blanks are derived firom the difference between the certified and the measmed values and should result in the same (similar) blank values from different reference materials. Typical blank values for Fe have been found to be between 100 and 150ng/L, for Zn between 60 and 90 ng/L, and for Cd and Pb below 10 ng/L. Variations depend on the stocks of chemicals employed. In particular the... 

Various approaches to the analysis of dissolved silicon have been tried. Most of them are based on the formation of /J-molybdosilic acid [ 199-203 ]. Dissolved silicon exists in seawater almost entirely as undissociated orthosilicic acid. This form and its dimer, termed reactive silicate , combine with molybdosilicic acid to form a- and /I-molybdosilicic acid [180]. The molybdosilicic acid can be reduced to molybdenum blue, which is determined photometrically [206]. The photometric determination of silicate as molybdenum blue is sufficiently sensitive for most seawater samples. It is amenable to automated analysis by segmented continuous flow analysers [206-208]. Most recent analyses of silicate in seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably the only silicon species in seawater that can be used by siliceous organisms [204]. 

Fukishi and Hiiro [222] determined sulfide in seawater by this technique. The method is based on the generation of hydrogen sulfide by the addition of sulfuric acid to the water sample. The gas permeated through a microporous polytetrafluoroethylene (PTFE) tube, and was collected in a sodium hydroxide solution. The carbon dioxide in the permeate was removed by adding a barium cation-exchange resin to the sodium hydroxide solution. Injection into the... 

Other methods for the determination of chlorine in seawater or saline waters are based on the use of barbituric acid [13] and on the use of residual chlorine electrodes [ 14] or amperometric membrane probes [15,16]. In the barbituric acid method [12], chlorine reacts rapidly in the presence of bromide and has completely disappeared after 1 minute. This result, which was verified in the range pH 7.5-9.4, proves the absence of free chlorine in seawater. A study of the colorimetric deterioration of free halogens by the diethylparaphenylene-diamine technique shows that the titration curve of the compound obtained is more like the bromine curve than that of chlorine. The author suggests... 

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. 

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. 

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