Guide · The Potassium Edition
Four passes through the same topic: where the name comes from, what explains the levels found in foods, which instruments are used to measure it, and what wording regulators allow. Four review questions close the guide.
A section on the history of science. It contains no statements about body functions.
Before chemistry knew the element, craftsmen knew its salt. Soap makers, glassworks and textile dyers leached wood ash with hot water and boiled the clear solution down in large iron pots. What remained was a white, strongly alkaline powder: potassium carbonate, named in everyday speech after the vessel — potash.
A second naming line ran alongside it. The Arabic term al-qalya also meant plant ash and gave German the word Kali, later Kalium. Anyone holding a bag of fertilizer labeled “potash” today is holding a linguistic fossil from the era before element chemistry.
In the fall of 1807, at the Royal Institution in London, Humphry Davy applied a current to molten caustic potash. Tiny, shiny silver beads gathered at the cathode, tarnished instantly in air and skittered across water. Davy had isolated a metal no one had seen before; a few days later he did the same with soda.
The discovery was only possible because the voltaic pile had recently made a strong enough source of direct current available. Chemically, it ended the idea that potash was an indivisible basic substance: it turned out to be a compound of a metal that could be obtained in no other way.
Davy called his find potassium, derived from potash. In German-speaking countries, however, the kali line won out, and when Jöns Jacob Berzelius introduced the modern element symbols he assigned it a K. Since P was already taken by phosphorus, it stayed that way — to this day every periodic table in the world shows a K, while the English name points to an entirely different word root.
Values from physical chemistry reference works; they concern the element as a substance, not its role in nutrition.
| Property | Value |
|---|---|
| Atomic number | 19 |
| Symbol | K |
| Standard atomic weight | 39.10 u |
| Melting point | 63.5 °C (146.3 °F) |
| Density | 0.86 g/cm³ |
| First isolated | 1807, London |
Information on food composition. No health claims are derived from it.
Potassium salts dissolve extremely well. That single property explains much of what you see in content tables. Because the figures refer to a hundred grams of the ready-to-eat product, the number rises whenever water has been removed: dried apricots reach more than four times the value of the fresh fruit, even though the fruit itself has gained nothing.
The reverse is also true. Products with a high water content — melon, cucumber, buttermilk — end up far down the lists without their cells being any poorer than those of other plants. Comparing tables means, first of all, comparing dry matter.
Composition data from food analysis compendia, rounded to the nearest ten. Individual batches vary up and down.
| Product | Potassium per 100 g |
|---|---|
| Cocoa powder, low-fat | 1900 mg |
| Soybeans, dried | 1800 mg |
| Apricots, dried | 1370 mg |
| White beans, dried | 1310 mg |
| Tomato paste | 1000 mg |
| Hazelnuts | 640 mg |
| Spinach, raw | 550 mg |
| Avocado | 485 mg |
| Potatoes, raw | 410 mg |
| White mushrooms | 390 mg |
| Salmon | 380 mg |
| Banana | 360 mg |
| Whole milk | 155 mg |
Good solubility has a practical downside: what is in the raw food does not necessarily reach the plate. When potatoes are diced, soaked and then cooked in plenty of water, a considerable share of the potassium moves into the cooking water — in test series, up to half of the starting value, depending on the size of the cubes and the amount of water.
Steaming, oven baking and cooking with the skin on, by contrast, keep most of it in the food. For reading tables this means: the listed value refers to one particular state of the food, and cooking shifts it.
A description of analytical methods. It is not a guide to interpreting your own results.
Hold a potassium salt in a burner flame and it turns violet. The light appears because excited electrons fall back to their ground state and emit radiation in a characteristic wavelength range. Since even traces of sodium drown out this color with their strong yellow, the flame is viewed through blue cobalt glass, which filters out the yellow.
This observation became a quantitative method: flame photometry measures the light intensity at around 767 nanometers and converts it into a concentration using a calibration series. For decades it was the standard method in food and soil analysis.
Today most automated lab analyzers work with a membrane electrode. The membrane contains valinomycin, a ring-shaped molecule from microbiology whose cavity matches the ionic radius of potassium exactly and largely rejects other alkali metals. A voltage builds up between the sample and a reference solution, and it can be converted into a concentration.
The electrode measures only what is dissolved in the submitted sample. What a single result means is decided not by the instrument but by the professional who ordered it.
Natural potassium is not a uniform substance but a fixed mixture of three isotopes. One of them, potassium-40, is unstable and decays with a half-life of about 1.25 billion years — partly to calcium-40, partly to argon-40. The second path is why volcanic rock can be dated with the potassium-argon method.
What makes it scientifically interesting is how constant the share is: it is the same roughly 0.012 percent everywhere. This makes it possible to calculate a body's total potassium from the gamma radiation it emits. Whole-body counters in shielded chambers use exactly this relationship to determine body composition without taking a sample.
Isotope shares per standard physics references. A nuclear physics figure with no connection to any body function.
| Isotope | Share of natural potassium | Property |
|---|---|---|
| Potassium-39 | 93.26 % | stable |
| Potassium-40 | 0.012 % | Half-life 1.25 billion years |
| Potassium-41 | 6.73 % | stable |
A reproduction of regulatory texts. It is not an interpretation for any individual case.
Anyone reading a Nutrition Facts panel often sees a percentage next to the amount. In the United States that percentage is based on the FDA Daily Value for potassium, 4,700 milligrams per day; the European Union uses 2,000 milligrams. These values make packages comparable and do not describe what any particular person needs.
Separately, scientific bodies publish estimates of adequate intake. Label values and intake estimates answer different questions and cannot be traded off against each other.
A side-by-side view of figures from different frameworks. It does not replace individual nutrition advice.
| Figure | Value | Source and purpose |
|---|---|---|
| Label reference value (US) | 4,700 mg/day | FDA Daily Value — Nutrition Facts label |
| Label reference value (EU) | 2,000 mg/day | Annex XIII, Regulation (EU) No 1169/2011 — label |
| Adequate intake estimate | 2,600–4,000 mg/day | Derived by scientific bodies; varies by sex and age |
In the European Union, health claims are only permitted if they appear on a positive list. The list is based on opinions from the European Food Safety Authority and took effect as a regulation in 2012. The wording is fixed: it may be quoted but not rephrased. For the link between potassium and nerve tissue, the list contains this single entry:
“Potassium contributes to normal functioning of the nervous system”
Per Regulation (EU) No 432/2012
The statement may only be used for products that reach the minimum content set by labeling rules. A food with a low level may not carry it, even if potassium can be detected in it.
The wording names an organ system as a whole. It describes neither a specific function nor a time course, and it is addressed to the general population, not to any group with a particular condition. Statements about diseases, their prevention or their course are not covered by the regulation, and so they do not appear on this page either.
Four points that remained open while putting the sections together.
Because two naming lines existed side by side. English follows potash and led to potassium; German follows the Arabic word for plant ash and led through Kali to Kalium. The symbol K comes from the second line and is used internationally.
About 0.012 percent of naturally occurring potassium is the isotope potassium-40, which decays very slowly. This share is the same in every rock, every food and every organism. Radiation safety assessments are made by the responsible authorities; this text makes none.
Labeling rules differ by market: in the United States potassium has been a required line on the Nutrition Facts panel since the 2016 label update, while in the European Union it is optional and may only be declared when the product contains a significant amount. If the line is missing on an imported product, you cannot conclude that the content is zero.
Mathematically, yes, by scaling the table values of the ingredients to the amounts used. The result is still an estimate: variety, origin and preparation shift the starting values, and tables do not account for cooking water. A personal assessment belongs in expert hands.
The unabridged edition takes the four parts of this page further: with larger content tables, the full procedure for both measurement methods, the references for the regulations cited and a glossary of technical terms.
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