These sieves are of quite a singular construction. They are immense organic molecules possessing a number of interesting properties. First, like many other plastics, they are insoluble in water and organic solvents. Secondly, they consist of what is known as ionogenic groups, i.e., groups capable of producing different kinds of ions in a solvent (water, in particular). These compounds may therefore be classed as electrolytes.

The hydrogen ion in them can be replaced by some metal, a process known as ion exchange. Accordingly, these unique compounds are known as ion exchangers. Those capable of interacting with cations (positively charged ions) are called cation exchangers, and those which interact with negatively charged ions are called anion exchangers. The first organic ion exchangers were synthesized in the middle thirties of our century and immediately gained wide recognition. This is not surprising, because with the aid of ion exchangers hard water can be made soft and salt water fresh.

Imagine two columns, one filled with a cation exchanger and the other with an anion exchanger. Suppose we want to purify water containing common salt. We first pass the water through the cation exchanger. In it all the sodium ions are replaced by hydrogen ions, so that instead of sodium chloride our water will now contain hydrochloric acid. Then we pass the water through the anion exchanger. If it is in hydroxyl form (i.e., the anions in it capable of exchanging are hydroxyl ions) all the chloride ions in the solution will be replaced by hydroxyl ions. And the hydroxyl ions immediately form water molecules with the free hydrogen ions. Water which originally contained sodium chloride can be completely demineralized by passing it through ion-exchange columns. The product is not inferior in properties to the best grades of distilled water.

But the demineralization of water is not the only thing that made ion exchangers widely known. It was found that ion exchangers retain different ions at different strengths. Lithium ions are held more fastly than hydrogen ions, potassium ions, more fastly than those of sodium, rubidium ions, more fastly than those of potassium, etc. Ion exchangers offered an easy way of separating metals. At present ion exchangers play an important part in various branches of industry. For example, for a long time photographic laboratories had no suitable method of collecting precious silver from their wastes. This important problem was solved with the aid of ion-exchange filters.

Well, and will man ever be able to use ion exchangers for extracting valuable metals from sea water? The answer is yes. And though sea water contains a large amount of different salts, the recovery of the noble metals from it is evidently a thing of the near future.

Just now the difficulty is that when sea water is passed through the cation exchanger the salts contained in it virtually prevent the small amount of valuable metals from depositing on the cation exchanger. However, recently resins of a new type known as electron-exchange resins were synthesized. These not only exchange their ions for the metal ions in the solution, but also reduce the metal by donating electrons to it. Recent tests have shown that if a solution containing silver is passed through such resins, metallic silver, rather than silver ion, soon begins to deposit on the resin, and the properties of the latter persist for a considerable length of time. Thus, if a mixture of salts is passed through the electron exchanger the ions which are reduced the most readily can be converted into atoms of a pure metal.

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