Oxygen in sea-water

SOLUBILITY OF OXYGEN IN SEA-WATER FROM A FREE, DRY ATMOSPHERE AT 760 mm. 

The relative abundance of isotopes from different sources may vary a little. Atmospheric oxygen is slightly richer in oxygen-18 than the combined oxygen in sea water, and the relative abundance in water from different sources is not constant. Elementary sulphur in the Texas deposits has a different isotopic composition from that of the combined sulphur in the surrounding rocks. A range of 3.8% has been observed in the ratio of boron-10 and 11 from various sources (Briscoe and Robinson, 1925), but no... 

Seiwell, H.R., 1937. Consumption of oxygen in sea water under controlled laboratory conditions. Nature, 140 504—507. 

Figure 5 The vertical distributions of iron, nitrate, silicate, and oxygen in sea water. This figure shows how iron is depleted to picomolar levels in surface waters and has a profile that mimics other plant nutrients.

The method described here is a modification of the classical Winkler procedure which we are convinced still remains the most reliable and precise means of analysing for dissolved oxygen in sea water. If a very precise estimate of extremely low oxygen concentrations is required on a routine scale then an absorptiometric determination of the iodine liberated in the Winkler method may prove superior to a titration procedure (Oulman and Baumann, Sewage Ind. Wastes, 28 1461, 1956) but not unless oxygen concentrations are less than about 0.1 mg-at/liter. 

In a similar vein, mean seawater temperatures can be estimated from the ratio of 0 to 0 in limestone. The latter rock is composed of calcium carbonate, laid down from shells of countless small sea creatures as they die and fall to the bottom of the ocean. The ratio of the oxygen isotopes locked up as carbon dioxide varies with the temperature of sea water. Any organisms building shells will fix the ratio in the calcium carbonate of their shells. As the limestone deposits form, the layers represent a chronological description of the mean sea temperature. To assess mean sea temperatures from thousands or millions of years ago, it is necessary only to measure accurately the ratio and use a precalibrated graph that relates temperatures to isotope ratios in sea water. 

Pitting Environments. In ordinary sea water the dissolved oxygen in the water is sufficient to maintain passivity, whereas beneath a barnacle or other adhering substance, metal becomes active since the rate of oxygen replenishment is too slow to maintain passivity, activation and pitting result. 

In sea-water, the increase of pH adjacent to the surface of cathodes brought about by the reduction of oxygen leads to the deposition of films of calcium carbonate and magnesium hydroxide . Such film deposition often results in a gradual decrease in the rate of galvanic corrosion of the more negative members of couples immersed in sea-water. 

Dissolved oxygen is a very important factor in the corrosion of metals immersed in sea water. Because of its biological significance, a vast amount... 

The open-circuit potential of most metals in sea water is not a constant and varies with the oxygen content, water velocity, temperature and metallurgical and surface condition of the metal. 

At sufficiently high rates of flow in natural waters enough oxygen may reach the surface to cause partial passivity, in which case the corrosion rate may decrease. In sea-water, owing to the high concentration of chloride ions, the corrosion rate increases with velocity. In one series of tests, corrosion under static conditions was 0-125mm/y, 0-50mm/y at 5 ft/s and 0-83 mm/y at 15 ft/s. 

Nickel-iron alloys fully immersed in sea-water may suffer localised corrosion which can be severe under conditions where oxygen is constantly renewed at the surface and the formation of protective corrosion products is hindered, e.g. in fully-aerated flowing sea-water. In quieter, less oxygenated conditions, average corrosion rates of Fe-36Ni are low and well below those for mild steel, as exemplified in the data given in Table 3.33 . However the resistance to localised attack is not improved to the same extent. 

Palladium is considerably less resistant to anodic corrosion than platinum, though it may be used for evolution of oxygen from alkaline solutions. It is attacked rapidly when used as an anode in sea-water, and dissolves quantitatively in acid chloride solutions. 

Paints used for protecting the bottoms of ships encounter conditions not met by structural steelwork. The corrosion of steel immersed in sea-water with an ample supply of dissolved oxygen proceeds by an electrochemical mechanism whereby excess hydroxyl ions are formed at the cathodic areas. Consequently, paints for use on steel immersed in sea-water (pH 8-0-8-2) must resist alkaline conditions, i.e. media such as linseed oil which are readily saponified must not be used. In addition, the paint films should have a high electrical resistance to impede the flow of corrosion currents between the metal and the water. Paints used on structural steelwork ashore do not meet these requirements. It should be particularly noted that the well-known structural steel priming paint, i.e. red lead in linseed oil, is not suitable for use on ships bottoms. Conventional protective paints are based on phenolic media, pitches and bitumens, but in recent years high performance paints based on the newer types of non-saponifiable resins such as epoxies. 

Oxygen, dissolved in sea-water in equilibrium with a normal . atmosphiere 76C mm) of iair saiurated with water vapour... 

Note the table gives the quantity of oxygen dissolved in sea-water at dirferent temperatures and chlorinities when in equilibrium with a normal atmosphere saturated with water vapour, li thus represents the condition approached by the surface water when biological activity is not excessive. 

Table 21.22 Saturated solubilities of atmospheric gases in sea-water at various temperatures Concentrations of oxygen, nitrogen and carbon dioxide in equilibrium with 1 atm (lOI 325 N m ) of designated gas...

Magnesium (eighth most abundant element) is found principally as Mg+2 ion in salt deposits, particularly as the slightly soluble carbonate, MgC03, and also in sea water. The element is oxidized by atmospheric oxygen and is not found in an uncombined state in nature. 

It follows that, depending on the temperature, whenever limestone precipitates, it includes the oxygen isotope in which the water is rich in sea waters, for example, the limestone is enriched in oxygen-18 ... 

Calcium carbonate is also the main constituent of the shells of sea animals, which make their shells from elements acquired from the surrounding waters. Now, the degree of fractionation of the oxygen isotopes as well as the formation of mineral carbonates and of animal shells in sea waters are determined on the basis of the temperature-dependent fractionation of the isotopes of oxygen the oxygen isotope composition of these materials reflects, therefore, the temperature at the time of their formation. Thus determining the isotope ratio between the stable isotopes of oxygen... 

The chemical method for the determination of the chemical oxygen demand of non-saline waters involves oxidation of the organic matter with an excess of standard acidic potassium dichromate in the presence of silver sulfate catalyst followed by estimation of unused dichromate by titration with ferrous ammonium sulfate. Unfortunately, in this method, the high concentrations of sodium chloride present in sea water react with potassium dichromate producing chlorine ... 

Longinelli A, Craig H (1967) Oxygen-18 variations in sulfate ions in sea-water and saline lakes. Science 156 56-59... 

---