Potassium
Because potassium and sodium are chemically very similar, their salts were not at first differentiated. The existence of multiple elements in their salts was suspected from 1702, and this was proven in 1807 when potassium and sodium were individually isolated from different salts by electrolysis. Potassium in nature occurs only in ionic salts. As such, it is found dissolved in seawater (which is 0.04% potassium by weight), and is part of many minerals.
Most industrial chemical applications of potassium employ the relatively high solubility in water of potassium compounds, such as potassium soaps. Potassium metal has only a few special applications, being replaced in most chemical reactions with sodium metal.
Potassium ions are necessary for the function of all living cells. Potassium ion diffusion is a key mechanism in nerve transmission, and potassium depletion in animals, including humans, results in various cardiac dysfunctions. Potassium accumulates in plant cells, and thus fresh fruits and vegetables are a good dietary source of it. Conversely, most plants except specialist halophytes are intolerant of salt, and sodium is present in them only in low concentration. This resulted in potassium first being isolated from potash, the ashes of plants, giving the element its name. For the same reason, heavy crop production rapidly depletes soils of potassium, and agricultural fertilizers consume 95% of global potassium chemical production.
Creation and occurrence
Potassium is formed in the universe by nucleosynthesis from lighter atoms. The stable form of potassium is created in supernovas via the explosive oxygen-burning process.
Elemental potassium does not occur in nature because it reacts violently with water (see section Precautions below). As various compounds, potassium makes up about 2.6% of the weight of the Earth's crust and is the seventh most abundant element, similar in abundance to sodium at approximately 1.8% of the crust. In seawater, potassium at 0.39 g/L (0.039 wt/v%) is far less abundant than sodium at 10.8 g/L (1.08 wt/v%).
Orthoclase (potassium feldspar) is a common rock-forming mineral. Granite for example contains 5% potassium, which is well above the average in the Earth's crust. Sylvite (KCl), carnallite (KCl·MgCl2·6(H2O)), kainite (MgSO4·KCl·3H2O) and langbeinite (MgSO4·K2SO4)) are the minerals found in large evaporite deposits worldwide. The deposits often show layers starting with the least soluble at the bottom and the most soluble on top. Deposits of niter (potassium nitrate) are formed by decomposition of organic material in contact with atmosphere, mostly in caves; because of the good water solubility of niter the formation of larger deposits requires special environmental conditions.
| Symbol | K | |
| Atomic Number | 19 | |
| Atomic Weight | 39.0983 | |
| Oxidation States | +1 | |
| Electronegativity, Pauling | 0.82 | |
| State at RT | Solid, Metal | |
| Melting Point, K | 336.8 | |
| Boiling Point, K | 1047 |
Interesting Facts about Potassium
- Potassium and its close periodic table neighbor sodium are solids at room temperature. Their alloys however are not. NaK alloys containing 40 to 90 percent of potassium by weight are liquids at room temperature. The commercially available 78% K, 22% Na alloy stays liquid at temperatures as low as -12.6 oC (9.3 oF).
- All living cells need potassium to maintain fluid balance, therefore we and all other forms of life on Earth need potassium minerals to survive. Potassium is available in all meats, plants and dairy products. Fruit and vegetables are the best sources of potassium.
- Several neurotoxins work by disrupting our cells’ biological use of potassium. This can result in severe pain, or even death. These neurotoxins include agitoxin, charybdotoxin and margatoxin (scorpion stings), apamin (bee stings), and dendrotoxin (mamba snake bites).
- Most of the universe’s potassium atoms were made in the final moments of giant stars as they exploded in supernovae. Potassium is made in the oxygen burning shell of stars when they explode. This is not normal burning, of course; it is nuclear fusion. Potassium is made, along with several other elements including sulfur, and silicon, during explosive oxygen burning in supernovae.
- All plants need potassium to survive; over 90% of all human use of potassium compounds is in the manufacture of plant fertilizers.
- People whose diets are low in potassium can suffer from hypokalemia. Severe hypokalemia can be life threatening. Symptoms include irregular heartbeat, fatigue, muscle cramps and constipation. It is unusual for people to be deficient in potassium solely as a result of getting too little in their diets. Usually hypokalemia is caused by other issues such as diarrhea and/or vomiting, use of antibiotics, and kidney disease.
- Most people are familiar with carbon dating, which uses the decay of the radioactive carbon-14 isotope to find the ages of once-living things such as animal and plant matter. The radioactive isotope potassium-40 gives us a way of dating rocks. Potassium-40 decays to argon-40 and calcium-40 with a half-life of 1.25 billion years. The ratio of potassium-40 to argon-40 trapped in rock is used to determine how long it is since the rock has solidified.
Appearance and Characteristics
Harmful effects:
In healthy people with normal kidney function, a potassium intake from foods does not seem to pose potential for increased risk, because excess potassium is readily excreted in the urine. In people whose urinary excretion of potassium is impaired, a potassium intake below 4.7 g (120 mmol)/day is appropriate because of adverse cardiac effects. If the digestive system is bypassed and potassium salts are injected into a vein, the heart can be stopped.
Due to its highly reactive nature, elemental potassium must be handled with extreme care.
Characteristics:
- Potassium is silvery-white, low melting, metal soft enough to be easily cut with a knife. It tarnishes rapidly in air, forming a dull oxide coating.
- Potassium burns with a lilac colored flame. It is extremely reactive, reacting violently with water, for example, to produce hydrogen gas and potassium hydroxide.
- Potassium is a very light metal (the second least dense metal after lithium) and would float on water if it were not so reactive.
Uses of Potassium
- Potassium is vital for plant growth. Plants use it, for example, to make proteins, hence the greatest demand for potassium compounds is in fertilizers.
- Potassium hydroxide is a strong alkali and an important industrial chemical. It is used in the manufacture of soft soaps and as an electrolyte in alkaline batteries.
- Potassium chloride is used as a healthier alternative to table salt.
- Toughened glass can be made by immersing glass in molten potassium nitrate.
- Potassium nitrate is the main explosive ingredient in gunpowder.
Sodium and then potassium reacting with water.
Will potassium metal react with ice? The still image you see above might not be the whole story.
