Potassium

Typical wastewaters

• potash mining and brine discharge K⁺ with chloride; supplies in Saskatchewan, the largest potash producing area, reach 51 mg/L
• potassium chloride softener regenerant and softened water K⁺ released at 14 mg per 17 mg/L of hardness removed; softened water 82 to 411 mg/L for hardness of 100 to 500 mg/L as CaCO₃
• potash mine effluent and runoff (German Werra and Weser catchments) K⁺ in potash mine effluents and runoff; receiving rivers and streams reached 72 g/L total salt (about 149 mS/cm) K⁺ found the most toxic ion to freshwater organisms at equal concentration, calcium ameliorating
• palm oil mill effluent dissolved K⁺ 9.5 to 29.1 mg/L with nitrogen 7.6 to 20.7 and phosphorus 5.3 to 8.7 mg/L eight smallholder mills, Nigeria
• distillery spent wash (sugarcane vinasse) dissolved potassium with high COD, nitrogen, phosphorus and sulfur 42.5 percent K removed by microalgal cultivation
• coal mine water (open cast and underground, Dhanbad) K⁺ and Al the highest of 18 metals measured in mine water, with TDS and COD above drinking water standards

1 · Identity

Symbol, number

K, 19

Oxidation states in water

+1 only, as the hydrated K⁺ ion; no hydrolysis, no complexes of consequence, no precipitates in treatment. Potassium-40 makes it the largest natural source of beta activity in water.

Note

The element entry already says K⁺ passes straight through most treatment plants and counts toward salinity rather than toxicity. This short chapter adds the two treatment reagents that carry it and the one exposure route that matters.

2 · Occurrence in water

Natural sources

Weathering of feldspar and mica and dissolution of evaporites; present in all natural waters at far lower levels than sodium (WHO fact sheet: occurs widely in the environment, including all natural waters).

Anthropogenic sources

Potassium permanganate dosing (up to about 10 mg/L potassium added, normally less), potassium chloride softener regeneration (tens to hundreds of mg/L in softened water), potash mining and brine discharge (Saskatchewan supplies reach 51 mg/L), fertiliser runoff and sewage.

3 · Speciation

Potassium is K⁺ at every pH and redox condition water can reach. It forms no hydroxide, carbonate or sulfate precipitate at the concentrations of treatment, only weak ion pairs, and it is taken up and released by clays and ion exchange resins in competition with sodium, calcium and magnesium. Its chemistry in a plant is entirely that of its partner ions: permanganate, chloride, hydroxide.

Solubility

Potassium chloride, sulfate, nitrate, carbonate and hydroxide are among the most soluble inorganic salts (the element entry); no solid controls potassium in water.

Hydrolysis

None; K⁺ does not hydrolyse and potassium hydroxide is a strong base used for pH correction where sodium must be kept low.

Complexation

Weak ion pairs only; no complexation of treatment significance.

Precipitates

None in water treatment; potassium leaves evaporation ponds as sylvite and carnallite (the element entry).

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Potassium is stable in a sample; the only pitfall is contamination from potassium bearing glassware and reagents at low levels. Report whether the sample is upstream or downstream of a permanganate dose or a softener.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Essential; daily requirement above 3000 mg; no evidence that potassium in municipally treated water, even with permanganate, poses a risk (WHO). Hyperkalaemia is possible in susceptible individuals (kidney dysfunction, heart disease, hypertension, diabetes, adrenal insufficiency, pre-existing hyperkalaemia, interfering medications, the elderly and infants) drinking potassium chloride softened water.

Bioaccumulation

Not applicable; potassium is homeostatically regulated.

Ecotoxicity

No US EPA aquatic life criterion; potassium contributes to salinity toxicity in brine discharges but no figure was read.

Flags

• The softened water potassium figures are Health Canada calculations for a fully potassium regenerated softener, not measurements.
• The seawater figure is a single PubChem abundance figure.
• The 0.25 mg potassium per mg permanganate is arithmetic on the formula weights, not a source figure.
• The permanganate equations are as printed by the EPA manual; the doses come from a 1986 compilation quoted there.
• The potassium-40 remark is from the element entry; the WHO radiological screening treatment of potassium-40 was not re-read.
• Standard Methods 3500-K is cited from the table of contents; the method text was not opened.

Gaps

• No source read gives potassium in groundwater, surface water, municipal or industrial wastewater as ranges; potash and brine effluents belong to the ledger's mining chapter.
• No cation chromatography standard or flame photometry detection limit was read.
• Reverse osmosis and ion exchange removal figures for potassium are not printed in the sources read; the textbook chapters are cited from memory.
• The potassium permanganate reactions with sulfide and with arsenite are not written here because the reagent row already carries the iron and manganese equations the EPA manual prints; they belong to the sulfur and arsenic chapters.

Sources

Identity

Name and symbol

Potassium, K

Atomic number

19 protons

Position

group 1 · period 4 · s-block · alkali metal

CAS number

7440-09-7

Atomic structure

Atomic mass

39.0983 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
[Ar] 4s¹

Electrons per shell

2, 8, 8, 1

Valence electrons

1 outer shell

Physical properties

State at room temperature

Solid

Melting point

336.53 K (63.38 °C)

Boiling point

1,032 K (758.85 °C)

Density

0.89 g/cm3

Appearance

silvery white, faint bluish-purple hue when exposed to air

Thermal conductivity

102.5 W/(m·K)

Electrical resistivity

72 nΩ·m at 20 °C

Electrical conductivity

13.89 MS/m

Crystal structure

body-centered cubic

Molar heat capacity

29.6 J/(mol·K)

Chemical properties

Oxidation states

+1

Electronegativity

0.82 (Pauling Scale)

Ionisation energy

4.341 eV
1st 418.8, 2nd 3,052, 3rd 4,420 kJ/mol

Electron affinity

0.501 eV

Atomic radius

empirical 203, covalent 203, van der Waals 275 pm

Ionic radius

K⁺ 138 pm

Reactivity

An alkali metal with a low first ionisation energy (419 kJ/mol) and one 4s electron; one of the most reactive and electropositive metals, more reactive than sodium, and stored under kerosene.

with water

Reacts violently with cold water, giving potassium hydroxide and hydrogen with enough heat to ignite the hydrogen, which burns with a lilac flame: $\ce{2 K + 2 H2O -> 2 KOH + H2}$

with oxygen, air

Oxidises within seconds in air to flaky white peroxide; burned in air or oxygen it gives mainly the orange superoxide KO2 with some peroxide $\ce{K2O2: K + O2 -> KO2}$

with acids

Reacts violently with dilute acids to the potassium salt and hydrogen.

with halogens

Reacts vigorously with all the halogens to the ionic potassium halides KF, KCl, KBr and $\ce{KI: 2 K + Cl2 -> 2 KCl}$

Typical compounds

• KCl potassium chloride sylvite, muriate of potash; the fertiliser salt
• KOH potassium hydroxide caustic potash; liquid soaps and detergents
• KNO₃ potassium nitrate saltpeter, niter; fertiliser and gunpowder
• K₂CO₃ potassium carbonate potash; glass making
• KO₂ potassium superoxide regenerates oxygen from carbon dioxide in submarines
• K₂Cr₂O₇ potassium dichromate oxidiser for quantitative analysis

Occurrence, production and use

Crustal abundance

2.09×104 milligrams per kilogram

Oceanic abundance

3.99×102 milligrams per liter

Occurrence and sources

The metal is the seventh most abundant and makes up about 2.4% by weight of the earth's crust. Most potassium minerals are insoluble and the metal is obtained from them only with great difficulty.

Certain minerals, however, such as sylvite, carnallite, langbeinite, and polyhalite are found in ancient lake and sea beds and form rather extensive deposits from which potassium and its salts can readily be obtained. Potash is mined in Germany, New Mexico, California, Utah, and elsewhere. Large deposits of potash, found at a depth of some 3000 ft in Saskatchewan, promise to be important in coming years.

Potassium is also found in the ocean, but is present only in relatively small amounts, compared to sodium.

• sylvite (KCl), sylvinite (KCl and NaCl), carnallite (KCl with MgCl2) evaporite deposits formed by dried seas and lakes; deposits of billions of tonnes of potassium chloride worldwide
• langbeinite (K2Mg2(SO4)3) and polyhalite (K2Ca2Mg(SO4)4 with 2 H2O) ancient lake and sea beds; potash mined in Germany, New Mexico, Utah and the Saskatchewan basin in Canada
• dissolved K+ the ocean, in much smaller amounts than sodium; the Dead Sea holds nearly 2 billion tonnes of potassium chloride
• feldspars and micas igneous rocks, the bulk of the 2.4 percent crustal share, sparingly soluble and not a practical source

Extraction, production

Potash mining and beneficiation

Sylvinite is mined underground or by solution mining and the potassium chloride is separated from sodium chloride by flotation, dissolution and recrystallisation, heavy media separation or solar evaporation; products are muriate of potash (KCl, 95 percent or purer), sulphate of potash and langbeinite. World mine production about 48 million tonnes of K2O equivalent in 2024 (estimate); capacity 65.2 million tonnes. Physical separation, so no reaction is written.

Potassium metal by electrolysis or thermal reduction

The metal is made by electrolysis of the molten hydroxide, essentially Davy's method, or by heating potassium compounds with sodium, carbon, silicon or calcium carbide. The full product set is not stated, so no equation. Demand for the metal is tiny next to the salts.

Potassium superoxide by burning the metal in dry air

$\ce{K + O2 -> KO2}$

Minor; balanced from the stated reactants and product. In breathing equipment it regenerates oxygen: $\ce{2 KO2 + H2O + 2 CO2 -> 2 KHCO3 + O2}$ (as stated by the source).

Uses

Potassium forms an alloy with sodium (NaK) that is used as a heat transfer medium in some types of nuclear reactors.

Potassium forms many important compounds. Potassium chloride (KCl) is the most common potassium compound. It is used in fertilizers, as a salt substitute and to produce other chemicals. Potassium hydroxide (KOH) is used to make soaps, detergents and drain cleaners. Potassium carbonate (KHCO3), also known as pearl ash, is used to make some types of glass and soaps and is obtained commercially as a byproduct of the production of ammonia. Potassium superoxide (KO2) can create oxygen from water vapor (H2O) and carbon dioxide (CO2) through the following reaction: 2KO2 + H2O + 2CO2 => 2KHCO3 + O2. It is used in respiratory equipment and is produced by burning potassium metal in dry air. Potassium nitrate (KNO3), also known as saltpeter or nitre, is used in fertilizers, match heads and pyrotechnics.

The greatest demand for potash has been in its use for fertilizers. Potassium is an essential constituent for plant growth and is found in most soils.

An alloy of sodium and potassium (NaK) is used as a heat-transfer medium. Many potassium salts are of utmost importance, including the hydroxide, nitrate, carbonate, chloride, chlorate, bromide, iodide, cyanide, sulfate, chromate, and dichromate.

• Agriculture (fertilisers): muriate of potash (KCl), the bulk potassic fertiliser; sulphate of potash and potassium magnesium sulphate for chloride sensitive crops; potassium nitrate about 85 percent of United States potash sales went to fertiliser in 2024; potassium has no substitute as a plant nutrient (USGS, US figure)
• Chemicals (potassium salts and alkali): potassium hydroxide for liquid soap and detergent; potassium carbonate for glass; potassium nitrate, chlorate, bromide, iodide, sulphate, chromate and dichromate as reagents
• Chemicals (caustic potash by chlor-alkali electrolysis): potassium hydroxide made by electrolysing potassium chloride brine in the chlor-alkali cell; the CAK BREF describes purifying sylvinite derived KCl by crystallisation or flotation first
• Chemicals (cyanides and silicones): potassium cyanide, made alongside sodium cyanide in the speciality inorganic chemicals sector for electroplating, metal treatment and gold leaching; potassium hydroxide as the catalyst of the equilibrium polymerisation step in silicone manufacture
• Food and beverage: potassium hydroxide as a cleaning in place agent, listed by the FDM BREF among the alkalis discharged from cleaning and disinfection
• Pharmaceuticals: potassium chloride in pharmaceuticals and saline drips
• Mining: potash is one of the industrial minerals covered by the MWEI BREF
• Nuclear: sodium potassium alloy (NaK) as a reactor heat transfer fluid

Safety, toxicity

GHS classification, signal word Warning

• H302 Harmful if swallowed Acute toxicity, oral
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H411 Toxic to aquatic life with long lasting effects to the aquatic environment, long-term hazard

Discovery and name

Discovered by

Humphry Davy

Discovered

1807

First isolated

not in sources

Named by

not in sources

Origin of the name

after potash, from which it was first isolated

It is one of the most reactive and electropositive of metals. Except for lithium, it is the lightest known metal. It is soft, easily cut with a knife, and is silvery in appearance immediately after a fresh surface is exposed. It rapidly oxidizes in air and must be preserved in a mineral oil such as kerosene.

As with other metals of the alkali group, it decomposes in water with the evolution of hydrogen. It catches fire spontaneously on water. Potassium and its salts impart a violet color to flames.