|Name , symbol , atomic number||Potassium, K, 19|
|Element category||Alkali metals|
|Group , period , block||1 , 4 , p|
|Mass fraction of the earth's envelope||2.41%|
|Atomic mass||39.0983 (1) et al|
|Atomic radius (calculated)||220 (243) pm|
|Covalent radius||203 pm|
|Van der Waals radius||275 pm|
|Electron configuration||[ Ar ] 4 s 1|
|1. Ionization energy||4th.340 663 69 (9) eV ≈ 418.81 kJ / mol|
|2. Ionization energy||31.62500 (19) eV ≈ 3 051.35 kJ / mol|
|3. Ionization energy||45.8031 (17) eV ≈ 4 419.3 kJ / mol|
|4. Ionization energy||60.917 (19) eV ≈ 5 878 kJ / mol|
|5. Ionization energy||82.66 (16) eV ≈ 7 975 kJ / mol|
|Crystal structure||body-centered cubic|
|density||0.856 g / cm 3 (20 ° C )|
|magnetism||paramagnetic ( Χ m = 5.7 10 −6 )|
|Melting point||336.53 K (63.38 ° C)|
|boiling point||1047 K (774 ° C)|
|Molar volume||45.94 · 10 −6 m 3 · mol −1|
|Heat of evaporation||79.1 kJ / mol|
|Heat of fusion||2.334 kJ mol −1|
|Speed of sound||2000 m s −1 at 293.15 K.|
|Specific heat capacity||757.8 J kg −1 K −1|
|Work function||2.30 eV|
|Electric conductivity||14.3 · 10 6 A · V −1 · m −1|
|Thermal conductivity||100 W m −1 K −1|
|Oxidation states||−1, +1|
|Normal potential||−2.931 V (K + + e - → K)|
|Electronegativity||0.82 ( Pauling scale )|
|For other isotopes see list of isotopes|
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .
Potassium ( Latin , from Arabic القلية, DMG al-qalya 'plant ash') is a chemical element with the element symbol K and the atomic number 19. In the periodic table it is in the first main group or the 1st IUPAC group and is one of the alkali metals .
On November 19, 1807, Humphry Davy reported to the Royal Society in London that he had succeeded in obtaining two different metals by electrolysis of weakly humidified caustic alkalis; One of the metals, first extracted on October 6, 1807, he called potassium (the English and French term for potassium that is still used today) because it can be extracted from potash , while the other, obtained a few days later, is sodium (which is still used today English and French name for sodium ), because it is contained in the various modifications of sodium carbonate (soda). In the German-speaking world, the Sodium Davy is since 1811, as suggested by Berzelius called sodium, while for the Potassium Davy to one of Klaproth introduced in 1796 express potassium (from Arab.القَلْيَة al-qalya = ash, obtainable from plant ash) took over.
In nature, potassium only occurs as a cation in potassium compounds. This is due to the fact that it only has one outer electron and that it gives it up very willingly in order to obtain a stable and low-energy outer electron shell . Covalent potassium compounds are therefore not known. The average concentration in seawater is 399.1 mg K + / kg = 408.4 mg K + / l.
Naturally occurring minerals containing potassium are:
- Sylvin - KCl
- Sylvinite - KCl · NaCl
- Carnallite - KCl · MgCl 2 · 6H 2 O
- Kainite - KCl · MgSO 4 · 3H 2 O
- Schönite - K 2 SO 4 · MgSO 4 · 6 H 2 O
- Polyhalite - K 2 SO 4 · MgSO 4 · MgSO 4 · 2 CaSO 4
- Orthoclase (potassium feldspar) - K [AlSi 3 O 8 ]
- Muscovite (potash mica ) - KAl 2 (OH, F) 2 [AlSi 3 O 10 ]
Potassium salts form evaporation deposits on former seabeds , which makes the extraction of potassium salts in these areas economical. Potash salt is mined in Canada, Russia, Belarus, Kazakhstan, Germany, Israel, the United States, Jordan and other locations around the world. The largest deposits ever found are 1,000 meters below the surface in the Canadian province of Saskatchewan. The deposits are located in the Elk Point Group, which formed in the central Devonian . In Saskatchewan , where several large mines had been in operation since the 1960s , the technique of freezing wet sand (the Blairmore Formation) was introduced to drive mine shafts through them. The main potash mining company in Saskatchewan is the Potash Corporation . The Dead Sea water is used as a source of potash by Israel and Jordan, while the concentration in normal oceans is too low for commercial production at current prices.
Extraction and presentation
Metallic potassium can be extracted using several methods. Electrolytic processes such as the Degussa process or a thermal process from IG Farben , Griesheim plant were used. Most of potassium, however, with the in the 1950s by Mine Safety Appliances made introduced MSA method in which at 870 ° C under a protective gas atmosphere with metallic sodium is a reduction of potassium chloride is carried out. The resulting gaseous potassium is deposited in a cold trap by condensation.
Commercially available Na-K alloys are also accessible by varying the distillation parameters.
or the thermal decomposition of potassium azide :
as well as a subsequent distillation under high vacuum are available.
Similar to the other alkali metals , potassium often reacts very violently with many other elements and compounds , especially with non-metals , and occurs only chemically bound in the natural environment . Potassium has a higher reactivity than sodium and reacts violently with water to form potassium hydroxide and release of hydrogen . High-speed recordings of the reaction of alkali metals with water suggest a Coulomb explosion .
- Reaction of potassium with water
Due to the strongly exothermic reaction , the hydrogen ignites when air enters. This can lead to deflagrations and explosions . In dry oxygen , the metal burns with an intense violet flame to form potassium hyperoxide KO 2 and potassium peroxide K 2 O 2 .
- Reaction of potassium with oxygen
- Reaction of potassium with water and carbon dioxide
- Reaction of potassium with ethanol
Like all alkali metals , potassium is readily soluble in liquid ammonia , forming a blue-violet solution . With the halogens bromine and iodine in liquid or solid form, potassium is explosively converted to the corresponding halides .
To dispose of potassium, it is usually recommended to carefully introduce small pieces of the metal into a large excess of tert-butyl alcohol , with which it reacts to form the alcoholate and hydrogen . Since this reaction proceeds very slowly, it can happen that small potassium residues remain unnoticed, covered with a crust of potassium tert- butanolate . Therefore, care must be taken to ensure that the reaction is complete. Alternatively, 1-butanol can also be used for small amounts of potassium , which reacts with the potassium more quickly but is still controllable. Under no circumstances should "lower" alcohols ( propanols , ethanol or methanol ) be used, as these react too violently with potassium and are also more flammable .
In the air , the shiny silver-white cut surface of the fresh metal is covered with a bluish shimmering layer of oxide and hydroxide and a little carbonate within seconds . Left to stand in the air, like all alkali metals, it slowly reacts completely to form carbonate . Metallic potassium is therefore used in anhydrous organic liquids , such as. B. stored paraffin oil. In contrast to sodium , potassium can still form crusts of oxides , peroxides and hydroxides after prolonged storage , which coat the metal in the form of reddish-yellow layers and which can explode if touched or pressed . Safe disposal is then no longer possible; the only option here is to burn off the entire container under controlled conditions. Potassium welded into polyethene packaging can also become coated with an explosive crust, so such packaging is not suitable for storage .
Liquid mixtures are formed with sodium in a wide range of concentrations at room temperature , see: NaK (alloy) . The phase diagram shows a compound Na 2 K which melts incongruously at 7 ° C and a eutectic at −12.6 ° C with a sodium content of 23% w .
Potassium has been used as a cooling fluid in some fast nuclear reactors in the form of a eutectic Na-K alloy . Otherwise, metallic potassium is of little technical importance because it can be replaced by the cheaper sodium .
In the research laboratory potassium is occasionally for drying of solvents used, especially when the boiling point of the solvent above the melting point of potassium, but below the melting point of sodium is located. Then the potassium is melted in the boiling solvent and its surface does not become encrusted. This means that significantly less alkali metal is required and it can be used up almost entirely in the drying process, so that only very small residues have to be disposed of.
When left standing for a long time, potassium develops crusts of peroxo compounds (K 2 O 2 and KO 2 ), even under protective liquid ( petroleum ), which react very explosively . Even with low pressure , especially when lifting pieces of potassium out with pliers or trying to cut with a knife , these peroxo compounds can trigger explosions . As countermeasures, either melting in glass ampoules or storage under protective liquid and inert gas are recommended . Furthermore, due to the risk of explosion , halogenated solvents must not be dried with potassium. Reactions of potassium in the interplay of solvents, which can easily give off oxygen , are also dangerous .
- Doping of tungsten wires for the production of filament lamps
- Na-K heat alloy (see sodium )
- In the form of potassium hyperoxide KO 2 as potassium air filter (see Kali cartridges ) inter alia submarines for respiratory air regeneration in use
Importance as a fertilizer
The industrial agriculture uses fertilizers to the in the harvest to replace lost potassium. Most agricultural fertilizers contain potassium chloride , while potassium sulfate is used for chloride sensitive crops or crops with higher sulfur content . Few fertilizers contain potassium nitrate . Other fertilizers containing potassium are Kornkali with magnesium oxide , Patentkali, Flory, Nitrophoska and potassium phosphate .
The essential macronutrient potassium has a wide range of effects in plants . In the xylem , it serves as an osmotic which is crucial for building up root pressure. Potassium in the leaf cells increases turgor , which leads to cell elongation and leaf surface growth . By increasing the turgor, it also provides for an opening of the stomata , which promotes carbon dioxide uptake and thus has a direct influence on photosynthesis . If potassium is present in sufficient quantities, it promotes the formation of C3 sugars in a first step , which are processed into starch , cellulose , lignin and proteins in other metabolic processes influenced by potassium . Plants that suffer from a lack of potassium show symptoms mainly on the older leaves . Potassium is retranslocated from them and transported into young leaves via the phloem . Typical symptoms of a potassium deficiency are point, intercostal and leaf margin chlorosis and leaf margin necrosis . There is also a compressed habit and possibly wilting . Strong sunlight can cause photo-oxidation in the leaves. A larger excess causes root burns and a calcium or magnesium deficiency .
Significance for the human body
- Bioelectricity of cell membranes , d. H. normal neuromuscular irritability, stimulation and conduction of the heart
- Regulation of cell growth
- Influence of protective (protective) endothelial vascular functions
- Maintaining normal blood pressure
- Regulation of the acid-base balance by influencing the renal net acid excretion
- Affecting the release of hormones (e.g. insulin from the beta cells )
- Carbohydrate utilization and protein synthesis
Recommended and Actual Potassium Intakes
To maintain all vital physiological processes, it is advised to consume at least 2 g of potassium daily. The Food and Nutrition Board (FNB) of the USA and Canada, however, considers an intake of 4.7 g / day (120 mmol / day) to be appropriate for all adults from a preventive perspective. Based on new findings, this amount of potassium (from food) is necessary to prevent, reduce or delay chronic diseases such as increased blood pressure , salt sensitivity , kidney stones , loss of bone mass or strokes .
According to the National Nutrition Survey II (NVS II), the potassium intake is located in Germany in the median at 3.1 g / day (females) or 3.6 g / day (men). The intake of 4.7 g potassium per day recommended by the FNB is not achieved by 75% of men and 90% of women.
Importance of the sodium / potassium ratio
Potassium plays an important role in regulating membrane potential in the body . The intracellular potassium concentration is around 150 mmol / l, extracellularly 4 mmol / l. The intracellular sodium concentration is around 10 mmol / l, extracellular around 140 mmol / l. These differences in concentration are maintained by the Na / K-ATPase and are vital for the function of the cell. A permanent shift in these cellular concentrations can lead to cardiac arrest in systole with increased K + concentration ( hyperkalaemia ), i.e. K + >> 4.5 mmol / l, and to hypokalaemia (K + <3.5 mmol / l) Decrease in the ability of muscles to contract, increased excitement, disruption of the conduction of excitation, extrasystoles of the heart. K + is (besides Na + ) decisive for the osmotic pressure of the cells, i.e. for the water content of the cell. Too low a K + concentration in the blood leads to cardiac arrest in diastole . The normal range in serum is 3.6–4.5 mmol / l, in urine 26–123 mmol / l. That is, K + is constantly being lost, which needs to be replaced with food.
This is particularly a problem today, as the western diet is characterized by the consumption of animal foods. However, the intake of plant-based foods rich in potassium is significantly reduced compared to before. Before the development of agriculture, the potassium intake was 10.5 g / day, compared to the average 3.4 g / day according to NVS II. On the other hand, the sodium intake, which was shifted into the opposite, was previously only 0.8 g / day and has increased to an average of 3.1 g / day (women) and 4.3 g / day (men) due to our high-salt diet. This affects the natural potassium to sodium ratio in the human body.
Potassium is the natural antagonist of sodium , and a balanced ratio of the two minerals is particularly important for the regulation of physiological processes. Excessive sodium intake can lead to potassium depletion. Conversely, potassium has a natriuretic effect. In this respect, the Na / K ratio in the food is more decisive than the concentration of the individual cations on their own. The WHO recommends a molar ratio of the two minerals of 1: 1. The WHO recommendations of less than 2 g sodium per day and at least 3.5 g potassium per day meet this ratio.
In addition to sodium, potassium is of great importance for regulating blood pressure . Epidemiological studies have shown that increased potassium intake is associated with lowering blood pressure and reduces the risk of stroke. The antihypertensive effect of potassium has also been shown in supplementation attempts.
The so-called DASH (Dietary Approaches to Stop Hypertension) diet (rich in whole grain cereal products, fruit, vegetables, poultry, fish and nuts) could also lower the blood pressure . In contrast to the usual diet, this diet contains less table salt and saturated fats, a relatively large amount of potassium, but also more other nutrients such as magnesium and calcium, which are also held responsible for lowering blood pressure. For this reason, a diet rich in fruits and vegetables (rich in potassium) in combination with a moderate reduction in sodium intake should be recommended, as a sodium to potassium ratio of 1 or less has a beneficial effect on blood pressure. It does not make sense to adjust the potassium intake to a high sodium intake. The European Food Safety Authority (EFSA) also confirms the blood pressure-increasing effect of sodium.
Potassium also has a positive influence on bone metabolism, as a higher potassium intake prevents the increased excretion of calcium, which is caused by a high salt intake. Potassium thus promotes renal calcium retention in the kidneys and prevents calcium breakdown from the bones . In this context, the influence of the accompanying anion, the composition of the food and the age on the acid-base status must also be taken into account. Clinical studies have shown that potassium citrate counteracts calcium loss via the kidneys and calcium breakdown from the bones. A prospective controlled intervention study in 161 postmenopausal women with osteopenia showed that the partial neutralization of diet-induced acid exposure (using 30 mmol potassium citrate per day, equivalent to 1.173 g potassium) over a period of twelve months significantly increases bone density and significantly improves bone structure. Potassium citrate was just as effective as raloxifene , an estrogen receptor modulator used in the treatment and prevention of osteoporosis in postmenopausal women.
Potassium is an important electrolyte . Potassium ions are mainly found inside the cell ( intracellular space ), where they are involved in maintaining the resting potential . Therefore, high potassium losses, such as through heavy sweating , can lead to cramps and states of exhaustion.
Foods rich in potassium have a diuretic / dehydrating effect . In dialysis -dependent patients with impaired renal function , it is important that they avoid strong potassium-containing foods, as this can cause hyperkalemia in life-threatening conditions.
For more information on the physical effects of potassium
Potassium levels in foods
Foods rich in potassium include mushrooms, bananas, dates, raisins, beans, chilli, cheese, spinach and potatoes, which contain between 0.2 and 1.0 g of potassium per 100 g of food.
Potassium levels in foods rich in potassium in mg per 100 g; from the USDA National Nutrient Database (2011)
|Food 100 g||potassium|
|Soybean (dried)||1,800 mg|
|Apricots (dried)||1,370 mg|
|Wheat bran||1,350 mg|
|Tomato paste||1,014 mg|
|Beetroot -leaves (cooked)||909 mg|
|Orange juice concentrate||674 mg|
|Date (Deglet Nour)||656 mg|
|soy milk||638 mg|
|Sweet chestnuts (roasted)||592 mg|
|Buckwheat flour (whole grain)||577 mg|
|French fries (vegetable oil)||550 mg|
|Potatoes (unpeeled, baked)||535 mg|
|Soybeans (cooked)||515 mg|
|Grapefruit juice (white)||484 mg|
|Spinach (cooked)||466 mg|
|Plantains (cooked)||464 mg|
|White beans||454 mg|
|Tomato puree||439 mg|
|Kidney beans (cooked)||402 mg|
Since the potassium salts contained in food are very soluble in water, the potassium content of food can be noticeably reduced by so-called soaking ( soaking in water for about three to five hours). This is especially important for people with kidney and metabolic disorders.
Natural potassium consists of 0.0117 percent of the radioactive isotope 40 K and therefore has a specific activity of 31.6 Becquerel per gram. With 0.17 mSv per year, almost 10 percent of the natural radiation dose in Germany (an average of 2.1 mSv per year) is due to the body's own potassium.
40 K decays through β-decay : with a probability of 89% through β - -decomposition into stable 40 calcium and with 11% probability through β + -decay or electron capture (EC, K-capture) into stable 40 argon . The half-life is 1.248 billion years.
The decay from 40 K to 40 Ar is an essential source of argon in the earth's atmosphere and can also be used to determine the age by means of potassium-argon dating .
The metal potassium may according to a computer simulation under extreme pressure simultaneously fixed and liquid to be because, according to the simulation then though most of atoms in solid bars , however, to arrange some atoms for liquid to be. Under extreme pressure , potassium is a solid block from which liquid potassium leaks out at the same time.
In addition to the spectroscopic detection based on the color of the flame , potassium ions dissolved in water can be detected and determined potentiometrically with an ion-selective electrode specializing in K + . Most potassium electrodes use the specific complexation of potassium by valinomycin C 54 H 90 N 6 O 18 , which is embedded in a plastic membrane in a concentration of around 0.7%.
A qualitative proof of potassium is possible using perchloric acid . Potassium perchlorate (KClO 4 ) , which is sparingly soluble in water in the cold, forms as a white precipitate . However, other form cations such as rubidium , cesium and ammonium white, heavy in the cold -soluble precipitates .
Potassium can be determined quantitatively with the help of gravimetry . Here, potassium is as potassium tetraphenylborate (K [B (C 6 H 5 ) 4 ]) by adding to the solution with Kalignost precipitated and the resulting precipitate balanced.
In routine analysis ( clinical chemistry ( blood ), environmental chemistry , water chemistry ), potassium is determined quantitatively down to the trace level using flame photometry . The limit of quantification mentioned here is 100 µg / l. In atomic absorption spectrometry , 1 µg / l can still be detected with the flame technique , and 0.004 µg / l with the graphite furnace technique.
In its compounds, potassium occurs only as a monovalent cation with the oxidation state 1.
Potassium oxide is a very reactive white solid and has an antifluorite crystal structure in the cubic crystal system in the space group Fm 3 m (space group no. 225) . There the positions of the anions and cations are reversed relative to their positions in fluorite , with potassium ions bound to 4 oxide ions and oxide ions bound to 8 potassium ions.
Potassium peroxide is a very reactive fire-promoting solid that is not itself flammable , but reacts violently with flammable substances . It decomposes violently on contact with water . It is used as an oxidizer and bleach, as well as to purify air .
Potassium chloride forms colorless, odorless, easily water-soluble crystals . Potassium chloride occurs in nature as sylvine . Other widely used minerals and rocks containing potassium and chloride are carnallite (KCl · MgCl 2 · 6 H 2 O), kainite (KCl · MgSO 4 · 3 H 2 O) and sylvinite (KCl · NaCl). It is used in food technology as a firming agent and flavor enhancer . It is used for the production of potash fertilizers and almost all technically used potassium compounds such as potassium carbonate and potassium hydroxide .
Potassium fluoride is a white, hygroscopic powder that usually occurs as a dihydrate . It is used for the production of glazes , as a cement additive , as an oxide-dissolving component of aluminum welding powders, as a wood preservative and for glass etching .
Potassium iodide is used in the laboratory for the production of iodine potassium iodide solution (Lugol's solution). It is also used in the production of silver iodide and in the production of pharmaceuticals and in analytical chemistry for the detection of certain compounds .
Potassium hydroxide is a white, hygroscopic, odorless solid. In water , it dissolves to form the strong base potassium hydroxide with great heat development . In microsystem technology , it is used for the selective anisotropic etching of monocrystalline silicon . In microbiology it is used to differentiate between gram-positive and gram-negative bacteria in the rapid test method. It is used as an electrolyte in galvanic oxygen sensors. It fulfills the same purpose in the widespread alkali-manganese cells and the historically important nickel-cadmium accumulators . It is used as an acidity regulator in the food industry.
Potassium carbonate is a white, hygroscopic powder. It has a monoclinic crystal structure in the space group P 2 1 / c (space group no. 14) . It is used in the manufacture of glass , paint and photographic developers .
Potassium chlorate is a white stable salt that makes a colorless solution in water . The connection has a strong oxidizing and will among other things for the production of matches , primer caps and other pyrotechnic products used.
In its pure form, potassium perchlorate forms rhombic prisms that can be several centimeters in size. The crystal system is orthorhombic in the space group Pnma (space group no. 62) . It is often used in pyrotechnics because of its strong fire-promoting effect and its good shelf life , for example as an oxidizing agent in lightning bolts . It is now rarely used in rocket engines because it has been replaced by ammonium perchlorate .
Potassium dichromate is an orange salt. It has a triclinic crystal structure in the space group P 1 (space group no. 2) . In the laboratory it is used because of its good shelf life and as an oxidizing agent , reagent for the detection of hydrogen peroxide and basic titre substance . In engineering it is used in tannery , electroplating , matches and for the production of chromosulfuric acid .
Potassium hexacyanoferrate (II) forms yellow, monoclinic crystals . By oxidation with hydrogen peroxide or chlorine can be Kaliumhexacyanidoferrat (III) produced. In the food industry it is used as a release agent and stabilizer . However, it may only be used in small quantities in food , as hydrogen cyanide can form when heated or when exposed to acids .
Potassium cyanide (cyanide) is the potassium salt of hydrogen cyanide and is also highly toxic. It is used in industrial manufacturing processes , where it is mainly used for gold extraction ( cyanide leaching ) and in galvanic baths, but also in organic synthetic chemistry for the production of nitriles .
Potassium hydrogen tartrate (tartar) is a salt of tartaric acid . It is produced during the storage of wine or grape juice . It is used to produce tartaric acid and, together with sodium hydrogen carbonate, as a raising agent .
Soaps are sodium or potassium salts of fatty acids . For production , fats are boiled with caustic soda or potassium hydroxide . This process is called soap boiling , the chemical reaction saponification . The fats are broken down into glycerine and the alkali salts of the fatty acids (the actual soaps). Alternatively, soaps can be made directly from free fatty acids by reacting them with alkalis to form their salts . Suitable fatty acids are, for example, lauric acid , myristic acid , palmitic acid , stearic acid , oleic acid and ricinoleic acid .
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