SODIUM

KNUT H. SCHRØDER, NORWEGIAN UNIVERSITY OF SCIENCE & TECHNOLOGY

Sodium is the seventh most abundant element in rocks and the fifth most abundant metal. It reacts with water and oxygen in the air, and in liquid ammonia it forms a blue solution described as solvated electrons. However, the sodium ion itself is quite inert, with high solubility for its salts and weak complexation abilities. For that reason, no classical analytical methods for the determination of sodium ions are available. Ion-selective electrodes can be used, but they are not very adaptable. Emission spectroscopy (for example, flame photometry) is very convenient, and online and automatic continuous surveillance can be worked out.

Sodium compounds are among the most frequently used materials for industrial and domestic use, and salt is needed for human life. For that reason, it has been of strategic importance in hot, humid tropical regions.

Because of the inertness of the sodium ion, regular chemical reactions with sodium ions are limited, but sodium ions are essential to life for many reasons. The extracellular concentration of sodium ions in the human body and in animals is about 10 times higher than what is found inside the cells. This is not expected when passive diffusion through the cell membranes is considered. To keep that high gradient across a membrane requires energy, that is, an active transport using ATP. This is a means of energy storage for sudden use and for forming electrical potential gradients in nerve cells. A similar mechanism does not occur in plants, and this is one of the most important differentiating characteristics between animals and plants.

SALTY Lake Natron, in Tanzania, is highly alkaline, containing soda (sodium carbonate), salt, and magnesite deposits. The drying soda forms white beds as water evaporates. BERNHARD EDMAIER/SCIENCE PHOTO LIBRARY

Pure NaCl is NaCl, and for that reason all brands will be identical. However, some manufacturers claim their salt is saltier than "regular table salt," and less salt should be required to obtain the proper "salty" taste of food. Obviously, in solution all brands of pure salts are identical, as the same amount of salt reaches the papillae on the tongue. However, if the salt is not dissolved when it passes the papillae, the sensing of the taste may be different because a quite concentrated solution is achieved on that spot. The physical shape of NaCl crystals differs from one producer to another, and this is important for dissolution in saliva.

Soluble sodium salts are found in salt ores in several places on Earth; among the most important salts are sodium chloride, carbonates, and trona [(Na₃H(CO₃)₂2H₂O]. Of special interest are the salts in the ocean and in inland lakes such as the Great Salt Lake (Utah), the Dead Sea, and the salt lakes in East Africa. The salts originate from the corrosion of silicates. The composition of the different lakes, as well as the pH value of the waters, differs substantially from that of the ocean, with a weakly buffered pH value in the range of 8–8.3 to a relatively stable pH of 10.5 for the carbonate-rich lakes in East Africa. The latter is valid for all such lakes because such aqueous systems consist of both HCO₃² and CO₃²², and this corresponds to a buffering maximum of the carbonate equilibrium system. In textbooks, the pK₂ for carbonate is reported to be about 10.3, but this value differs from one textbook to another and is valid only for diluted systems. Surprisingly, some of these lakes contain living organisms and even fish.

The solid salts on the shores of the East African lakes are mainly sodium chloride and trona beside each other, and the shapes of the two crystals are quite different. Interestingly, one side of these lakes is always richer in one of the salts than the shore on the opposite side. One explanation for this phenomenon can be found in the solubility of the two salts. Sodium chloride has solubility almost independent of the temperature, and evaporation caused by wind and high temperature causes the salt to precipitate. Trona, however, has a quite temperature-dependent solubility. Daily temperature changes and wind differ substantially from one shore to the opposite one, and this is a possible explanation. This difference in solubility behavior is exploited to separate the salt and the trona in many East African local saltworks, such as in Lake Katwe in Uganda.

Knut H. Schrøder is a professor in analytical chemistry at NTNU (Norwegian University of Science & Technology) in Trondheim. His main research interest is in developing electroanalytical methods for online monitoring of environmental and industrial processes.

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