Potassium - K
Potassium (K) is a soft, silvery alkali metal in group 1. In natural waters and foods it occurs mainly in compounds and as potassium ions (K+), rather than as the highly reactive metal.
Properties and occurrence · Uses · Health and safety · Environment and water
| Atomic number | 19 |
|---|---|
| Standard atomic weight | 39.0983(1) |
| Electronegativity (Pauling scale) | 0.82 |
| Density at 20 °C | Approximately 0.86 g cm−3 |
| Melting point | 63.5 °C |
| Boiling point | 759 °C |
| Non-bonded atomic radius | 0.275 nm |
| Common oxidation state | +1 |
| Stable isotopes | 39K and 41K |
| Electron configuration | [Ar] 4s1 |
| First ionisation energy | 418.81 kJ mol−1 |
| Isolation of metal | Humphry Davy, 1807 |
Property sources: Royal Society of Chemistry, CIAAW and CNESST (density). Standard atomic weight is dimensionless; the corresponding molar mass is approximately 39.0983 g mol−1. Parentheses express uncertainty in the final digit.
Properties and occurrence of potassium
The symbol K derives from kalium. Humphry Davy first isolated potassium metal in 1807 by electrolysis of potassium hydroxide. Its common oxidation state in compounds is +1.
Natural potassium contains the stable isotopes 39K and 41K and a small proportion of radioactive 40K. This naturally occurring isotope is part of normal environmental radioactivity; it should not be described as a third stable isotope.
Potassium occurs in minerals including feldspars, micas, sylvite and carnallite, and in dissolved salts. Weathering releases potassium from rocks and soils. Some released potassium is taken up by organisms, held on exchange sites or fixed in clay minerals; its mobility depends on the soil and water conditions.
Uses of potassium compounds
A major use of potassium compounds is in fertilisers, supplying an essential plant nutrient. Other applications include potassium carbonate in glass manufacture and potassium hydroxide in liquid soaps and detergents. The properties and hazards of a particular compound depend on its full composition, not only its potassium content.
In water treatment, potassium permanganate is used as an oxidant, and potassium chloride can be used to regenerate certain household softeners. These are applications of potassium compounds; potassium metal is not a water-purification chemical.
Health effects and safe handling
Potassium as a nutrient
Potassium supports normal fluid balance, nerve signalling and muscle function. Foods including vegetables, fruit, beans, dairy products and nuts contribute to intake. Dietary needs vary with age and life stage; nutritional guidance is distinct from a drinking-water treatment specification.
Healthy kidneys help regulate potassium, but impaired kidney function and some medicines can reduce potassium excretion. Excess potassium in the blood, called hyperkalaemia, can cause dangerous heart-rhythm disturbances. People advised to restrict potassium should discuss supplements, potassium-based salt substitutes and potassium-softened drinking water with their healthcare professional.
Potassium metal
Potassium metal reacts violently with water and moisture. It releases heat, flammable hydrogen and corrosive potassium hydroxide, creating fire, explosion and severe-burn hazards. These hazards must be distinguished from exposure to potassium ions in ordinary food or water.
The reaction with water can be represented as:
2 K + 2 H2O → 2 KOH + H2
Handling, storage and emergency response require trained personnel and the material's safety data sheet. Water and carbon dioxide are unsuitable extinguishing agents for a potassium-metal fire. The safety of a potassium-containing chemical must be assessed for that specific substance and task.
Potassium in the environment and water
Related page: Potassium in water (K + H₂O) — occurrence, reactions and water-treatment considerations.
Potassium is an essential plant nutrient. Its availability depends on mineral weathering, soil exchange processes, fertilisation and plant uptake. Natural abundance does not mean unlimited inputs are harmless: excess salt loading or an unsuitable nutrient balance can impair soil and crop performance.
Drinking-water quality
WHO has not established a health-based drinking-water guideline value for potassium because usual concentrations are well below levels of health concern. This does not define one safe concentration for every person, particularly when treatment adds potassium and a consumer has been advised to limit intake.
Potassium-chloride softeners
A softener regenerated with potassium chloride exchanges calcium and magnesium for potassium. It reduces hardness while adding potassium to the treated water. Potassium itself is not a principal hardness ion like calcium or magnesium.
WHO advises susceptible individuals to obtain medical advice about consuming water treated with potassium-chloride softeners. Where a clinician advises limiting potassium from drinking water, an unsoftened drinking-water supply or a suitable bypass can avoid this added source, provided the supplied water otherwise meets drinking-water requirements.
Reducing potassium in water
Reverse osmosis can reduce dissolved potassium; actual performance depends on the system, feed composition and operating conditions. Ordinary sediment filters do not remove dissolved potassium ions, and a hardness softener should not be selected as a potassium-removal process.
For process water, establish the required potassium and total-salt limits from the application, then select and verify a suitable demineralisation process. Consider the full analysis and the management of concentrate, spent media or regeneration waste rather than assessing potassium alone.
Read more about potassium in water and dietary reference information.
Potassium page last reviewed: by Lenntech.
Back to the periodic table of elements.
Sources and further reading
- RSC: potassium properties, uses and history.
- CIAAW: potassium atomic weight and isotopes.
- CNESST: potassium metal physical properties and safety (French).
- NIH: potassium nutrition, medicines and hyperkalaemia.
- ILO/WHO: International Chemical Safety Card 0716 — potassium metal.
- WHO: potassium in drinking water and softener considerations.
- Health Canada: potassium from water softeners (PDF).
- CDC: water-treatment processes and their limitations.
- USDA NRCS: soil nutrients and potassium cycling (PDF).