Alkali metals are so reactive that they never occur in uncombined form in nature. Many metals can be isolated from their minerals by reduction with carbon or hydrogen, but alkali metal ions are extremely difficult to reduce. A more powerful technology for metal reduction was needed before alkali metals could be isolated in pure elemental form.
Potassium was discovered in 1807 by Sir Humphry Davy , who derived it from caustic potash (K OH ). Potassium was the first metal that was isolated by electrolysis .
Potassium was not known in Roman times, and its names are not Classical Latin but rather neo-Latin .
• The name kalium was taken from the word " alkali ", which came from Arabic al qaliy = "the calcined ashes".
• The name potassium was made from the word " potash ", which is English, and originally meant an alkali extracted in a pot from the ash of burnt wood or tree leaves.
Sir Humphry Davy believed that chemical union was electrical in nature- and so, a strong electric current might be able to overcome the forces binding compounds together, and lead to the isolation of new elements. In 1807 he tested this hypothesis on caustic potash ( potassium hydroxide , KOH) and caustic soda ( sodium hydroxide , NaOH), which had previously been suspected to be oxides of unknown metals.
Davy fired a small piece of KOH in a furnace and placed it on a platinum plate. He connected the plate to the negative terminal of an enormous battery made of 250 stacked cells made of 6" x 4" copper and zinc plates. The positive terminal was connected to a platinum wire and touched to the top of the KOH. The results were spectacular. Davy wrote:
"The potash began to fuse at both its points of electrization. There was a violent effervescence at the upper surface; at the lower, or negative surface, there was no liberation of elastic fluid (gas), but small globules having a high metallic lustre, and being precisely similar in visible characters to quicksilver, appeared, some of which burnt with explosion and bright flame, as soon as they were formed, and others remained, and were merely tarnished, and finally covered with a white film which formed at their surfaces."
According to Davy's brother, when Davy saw the globules of potassium metal break through the potash and burst into flame, "he could not contain his joy- he actually bounded about the room in ecstatic delight". A few days later, the experiment was repeated with sodium hydroxide, and sodium metal was discovered.
OCCURRENCE
Potassium makes up about 2.4% of the weight of the Earth 's crust and is the seventh most abundant element in it. As it is very electropositive , potassium metal is difficult to obtain from its minerals . It is never found free in nature. Potassium salts such as carnallite , langbeinite , polyhalite , and sylvite are found in ancient lake and sea beds. These minerals form extensive deposits in these environments, making extracting potassium and its salts more economical. The principal source of potassium, potash , is mined in Saskatchewan , California , Germany , New Mexico , Utah , and in other places around the world. 3000 feet below the surface of Saskatchewan are large deposits of potash which are important sources of this element and its salts, with several large mines in operation since the 1960s. Saskatchewan pioneered the use of freezing of wet sands (the Blairmore formation) in order to drive mine shafts through them. The oceans are another source of potassium, but the quantity present in a given volume of seawater is relatively low compared with sodium .
Potassium can be isolated through electrolysis of its hydroxide in a process that has changed little since Davy . Thermal methods also are employed in potassium production, using potassium chloride .
ISOTOPES
There are 24 known isotopes of potassium. Three isotopes occur naturally: 39 K (93.3%), 40 K (0.012%) and 41 K (6.7%). Naturally occurring 40 K decays to stable 40Ar (11.2%) by electron capture and by positron emission , and decays to stable 40Ca (88.8%) by beta decay ; 40 K has a half-life of 1.250×10 9 years.
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