Alkalinity of seawater

Table 16-2 presents what might be termed the minimum set of constituents that must be considered in the case of cloud/rainwater. If we consider the amount of water, L, to be fixed by atmospheric physical processes, the minimum number of input components that can vary are SO2, NH3, CO2, and whatever solute is present from the CCN, often one or another sulfate compound between H2SO4 and (NH4)2S04. Occasionally, salt particles from the ocean surface may be sufficiently abundant to provide enough solute to influence the pH via the inherent alkalinity of seawater, and we will consider that as a second, somewhat more complicated possibility. 

In practice, the alkalinity of seawater is determined by titrating a sample until the pH drops to 3, well below the bicarbonate equivalence point (pH 4.5). Although most of the titratable negative charge is contributed by bicarbonate and carbonate, the other weak bases present in seawater do consume some acid above and below the bicarbonate... 

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. 

The low amount of liquid water associated with particles (volume fraction 10-10, compared to clouds, for which the volume fraction is on the order of 10-7) precludes significant aqueous-phase conversion of S02 in such droplets. These particles can contribute to sulfate formation only for very high relative humidities (90% or higher) and in areas close to emissions of NH3 or alkaline dust. Seasalt particles can also serve as the sites of limited sulfate production (Sievering et al. 1992), as they are buffered by the alkalinity of seawater. The rate of such a reaction as a result of the high pH of fresh seasalt particles is quite rapid, 60 pM min-1, corresponding to 8% h 1 for the remote oceans (S02 = 0.05 ppb). Despite this initial high rate of the reaction, the extent of such production may be quite limited. For a seasalt concentration of 100 nmol m 3, the alkalinity of seasalt... 

Obtaining maximum performance from a seawater distillation unit requires minimising the detrimental effects of scale formation. The term scale describes deposits of calcium carbonate, magnesium hydroxide, or calcium sulfate that can form ia the brine heater and the heat-recovery condensers. The carbonates and the hydroxide are conventionally called alkaline scales, and the sulfate, nonalkaline scale. The presence of bicarbonate, carbonate, and hydroxide ions, the total concentration of which is referred to as the alkalinity of the seawater, leads to the alkaline scale formation. In seawater, the bicarbonate ions decompose to carbonate and hydroxide ions, giving most of the alkalinity. 

Superior antimicrobial activity in alkaline pH (seawater is always above pH 8), in the presence of nitrogenous organic matter, and due to lower volatility has been documented for bromine antimicrobials3 4. The pKa acid dissociation constants for HOC1 and HOBr are 7.4 and 8.7, respectively the dissociated acids are less effective antimicrobials4,5. Undissociated hypohalous acids are more effective because they are far better halogenating agents compared to the dissociated anion (hypohalite). Table 1 shows the effect of acid dissociation on antimicrobial performance in well-controlled laboratory experiments. 

Until fairly recently, the analysis of seawater was limited to a number of major constituents such as chloride and alkalinity. 

Atienza et al. [657] reviewed the applications of flow injection analysis coupled to spectrophotometry in the analysis of seawater. The method is based on the differing reaction rates of the metal complexes with 1,2-diaminocycl-ohexane-N, N, N, A/Metra-acetate at 25 °C. A slight excess of EDTA is added to the sample solution, the pH is adjusted to ensure complete formation of the complexes, and a large excess of 0.3 mM to 6 mM-Pb2+ in 0.5 M sodium acetate is then added. The rate of appearance of the Pbn-EDTA complex is followed spectrophotometrically, 3 to 6 stopped-flow reactions being run in succession. Because each of the alkaline-earth-metal complexes reacts at a different rate, variations of the time-scan indicates which ions are present. 

As mentioned above, several other weak bases can consume acid during the alkalinity titration of seawater. In order of decreasing ability to react with H", they include ... 

THE EFFECT OF POC AND PIC FORMATION AND DEGRADATION ON THE pH, CARBONATE ALKALINITY, AND XCO2 OF SEAWATER... 

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