Gases, dissolved in seawater

The main gases dissolved in seawater are nitrogen, oxygen, and carbon dioxide. In well-aerated waters, the concentrations of O2 and CO2 are about 5 x 10 " and 10 mol/L, respectively. 

The content of solute oxygen depends mainly on temperature and salt content. The gases dissolved in seawater do not reach saturation levels by a long shot, not even in areas close to the surface. Table 4 lists the balanced saturation values for oxygen, nitrogen and carbon dioxide at a gas pressure of 101 kPa and at different temperatures and salt contents [9]. 

Solutions Seawater is a solution that contains nearly every element found on Earth. Most elements are present in tiny quantities. Sodium and chloride ions are the most common ions in seawater. Several gases, including oxygen, nitrogen, and carbon dioxide, also are dissolved in seawater. 

The primary source of noncondensable gases usually is air dissolved in the condenser water. Figure 11-126 shows the dissolved-gas content of fresh water and seawater, calculated as equivalent air. The... 

Chapters 2 and 3 discuss the determination of anions. Direct application of many of the classical procedures for anions fail for seawater owing to interfering effects of the simple matrix. Suitable modifications are discussed that are amenable to seawater. Dissolved gases in seawater are of interest in certain contexts and their determination is discussed in Chap. 4. 

Spencer and Brewer [ 1 ] have reviewed methods for the determination of dissolved gases in seawater (O2, CO2, N2, H2S, inert gases). 

Fully hydrated gas molecules are solutes. Some gases, such as the noble gases, are conservative, whereas others, such as CO2, O2, and CH4, are nonconservative. The chemistry of CO2 is further complicated by its spontaneous reaction with water. In some cases, gases in seawater and the sediments can collect into bubbles. In this form, the gases are not truly dissolved. 

One of the most notable features of seawater is its high degree of saltiness. In previous chapters, we have discussed various sources of this salt, these being rivers, volcanic gases, and hydrothermal fluids. These elements have ended up in one of four places (1) as dissolved ions in seawater, (2) as sedimentary minerals, (3) as hydrothermal minerals, and (4) as volatiles that reside in the atmosphere. The minerals are recycled via geologic uplift and subduction. Upon return to Earth s surface, these minerals are chemically weathered via acid attack by the atmospheric volatiles remobilizing the salts for return to the ocean in river runoff. 

Fundamental physical processes such as gas diffusivities in seawater and air-sea gas exchange as well as details of measurement techniques for dissolved gases in seawater are not discussed in the context of this chapter. 

In these equations P is the headspace pressure in atmospheres, a is the gas-to-seawater ratio (Vg/V ), and is the total volume of the flask (here 1 L). By this procedure the initial concentration of a dissolved gas (C f) in seawater can be determined. Normally single extractions are done, but subsequent extractions can be used to check the amount of gas remaining in the seawater. The uncertainties associated with the headspace method can be evaluated by referring to Equation 2. In addition to the headspace gas concentration, Cg, the uncertainty in C f is dependent on the uncertainties in H and the ratio a. By using a propagation of error procedure on Equation 2, the uncertainties associated with the vacuum extraction flask can be estimated for different gases (2). The results show that for a gas that is not very soluble (H > 3), the total uncertainty is 5 %. For a more soluble gas H < 0.5) whose Henry s constant is not accurately known in seawater, the uncertainty is 30 %. 

The tests demonstrate the ability of the purge-cryogenic trap method to recover quantitatively the dissolved gases in seawater, which can then be... 

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