Virtually all inorganic reactions involved in geological and biological systems are ionic in nature, and ions and their reactions are essential to plant and animal life. Ions and ion exchange determine in a large measure the fertility of soils. Yet these same ions can seriously interfere with the beneficial use of water by man.
In 1884, the Swedish chemist Arrhenius studied the fact that certain substances which dissolve in distilled water make it a good conductor of electricity, while other substances do not increase its conductivity.
To explain the mechanism by which these substances conduct electrical current, he proposed his theory of ionization in 1887. He suggested that when certain substances (later called electrolytes) dissolve in water, they form electrically charged particles called "ions." Further, because the solution as a whole is electrically neutral, he assumed the existence of two types of ions, one charged positively and the other charged negatively.
An ion can be defined as an electrically charged atom or group of atoms in solution. Positively charged ions are called "cations" because they migrate to the cathode, or negative electrode when a current of electricity is passed through the solution. Negatively charged ions are called "anions" because they migrate to the anode or positive electrode. The word "ion," derived from the Greek, means "to go" or "to wander."
When sodium chloride (table salt) is dissolved in water, each molecule that is ionized produces two ions, a sodium ion with a single positive charge and a chloride ion with a single negative charge.
| Na+ | + | CI- | = | NaCI |
| Sodium Ion | + | Chloride Ion | = | Sodium Chloride Molecule |
Ions exist independently in solution and possess specific properties, which may differ greatly from those of their atoms or molecules. For example, metallic sodium reacts violently with water, producing hydrogen gas and caustic soda, but sodium ions exist calmly in solution.
Chlorine gas is poisonous, but chloride ions are not. In fact, both sodium and chloride ions are essential to life.
Many impruities exist as ions in natural waters. The most common of these are:
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These electrically charged dissolved particles make ordinary natural water a good conductor of electricity. Conversely, pure water has a high electrical resistance, and resistance is frequently used as a measure of its purity.
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