# What Is Forchhammer’s Principle?

> Seawater tastes saltier in some places than others, yet its major dissolved salts usually keep nearly the same proportions. If chloride rises in a sample from the open ocean, sodium and the other major ions tend to rise with it in predictable...

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Published: 2026-09-01T14:24:04+00:00
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Seawater tastes saltier in some places than others, yet its major dissolved salts usually keep nearly the same proportions. If chloride rises in a sample from the open ocean, sodium and the other major ions tend to rise with it in predictable ratios. This regularity is known as Forchhammer's principle or the principle of constant proportions.

The rule gave oceanographers a practical shortcut. Measuring every dissolved salt in every bottle would be slow and difficult, especially aboard a nineteenth-century ship. A reliable relationship between one measurable component and total salt content allowed researchers to estimate salinity from a smaller set of observations.

[NOAA's account](https://oceanservice.noaa.gov/facts/forchhammers-principle.html) credits Danish geologist Johan Georg Forchhammer with analyzing samples from the North Atlantic and Arctic Ocean in the 1860s. His work showed that total saltiness varied largely because water was added or removed, while the mixture of major salts remained broadly consistent.

The principle is easier to picture with dilution. If a bucket of seawater loses pure water to evaporation, every major salt becomes more concentrated. Their ratios remain nearly unchanged because none of the salts left with the water vapor. Adding rain produces the reverse result. Ocean circulation blends these altered waters across long distances, helping maintain a recognizable chemical recipe throughout most open-ocean regions.

## A baseline with measurable limits

Its name can obscure an important qualification. Constant means approximately stable at the scale used for routine salinity work, rather than mathematically identical in every sample. Modern instruments can detect regional departures that nineteenth-century analyses could not resolve. Those differences carry information about rivers, ice, biology, seafloor reactions and the movement of water masses. Repeated sampling shows whether a departure is local, seasonal, or part of a persistent water-mass signature.

Oceanographers still need the baseline because salinity influences density and density helps determine whether a water parcel sinks or remains above another layer. Consistent measurements reveal boundaries between water masses and allow observations from distant cruises to be compared. The principle links a chemical pattern first recognized in bottles with the physical circulation mapped by modern sensors.

## Forchhammer compared seawater from different regions

**Johan Georg Forchhammer** worked before research vessels carried electronic sensors. He and his collaborators collected bottles from different locations, then used chemical reactions to determine their contents. The analyses included chlorine compounds and several other major components known at the time.

The samples did not all contain the same mass of salt. Rain, river input, ice melt and evaporation alter how much freshwater is mixed with seawater. Once Forchhammer compared the salts as proportions of the total, the differences became much smaller.

William Dittmar later analyzed 77 samples gathered during the HMS Challenger expedition of 1873 to 1876. The [Journal of Marine Research](https://elischolar.library.yale.edu/journal_of_marine_research/566/) history by John Lyman and Richard Fleming describes Dittmar's results as a strong confirmation of the constant-ratio idea. Values derived from that work influenced ocean chemistry for decades.

## The principle describes major ions

**Sea salt composition** is more complex than pure sodium chloride. The major dissolved ions include chloride and sodium, with substantial magnesium and sulfate. Calcium and potassium contribute smaller shares. Their charges balance across the solution and their combined concentration helps determine the physical properties of seawater.

Oceanographers call many of these major ions **conservative constituents**. Their residence times in the ocean are long compared with the time required for currents to mix ocean basins. Inputs and removals happen slowly enough that circulation spreads them widely before their relative abundances change much.

The principle concerns **major-ion ratios**, not identical concentrations. Evaporation removes water and leaves salts behind, raising salinity while preserving most major-ion ratios. Rain and melting ice add freshwater, lowering all of those concentrations together. Rivers can produce larger local departures near coasts.

Minor substances behave differently. Nutrients such as nitrate and phosphate are consumed by organisms near the surface and released again as organic matter decomposes. Dissolved oxygen changes through photosynthesis, respiration and contact with the atmosphere. Their distributions provide information that a simple salinity measurement cannot replace.

## Constant proportions made salinity measurable

Early oceanographers often measured **chlorinity**, a quantity based on the halides in seawater. Since chlorinity tracked the other major salts, they could convert it to an estimate of total salinity. The historical relationship rested on the same near-constant composition described by Forchhammer's principle.

Modern instruments usually measure **electrical conductivity**. Salt ions carry electric charge through water, so conductivity rises with salinity. Temperature strongly affects conductivity and pressure also has an influence, requiring all three variables for accurate calculations. A CTD package combines conductivity, temperature and depth sensors in one instrument.

The [Nature Scitable overview](https://www.nature.com/scitable/knowledge/library/key-physical-variables-in-the-ocean-temperature-102805293/) explains that Practical Salinity is calculated from a conductivity ratio under the Practical Salinity Scale of 1978. It has no ordinary mass unit. The calculation provides a consistent way to compare ocean observations collected by different programs.

Current thermodynamic work also uses **Absolute Salinity**, which represents the mass fraction of dissolved material more directly and accounts for small composition differences. The [TEOS-10 standard](https://www.teos-10.org/) supplies equations for seawater properties, including density and heat content. Practical Salinity remains common in observations, while TEOS-10 offers refinements needed for high-precision calculations.

Salinity helps researchers identify water masses because water formed in a particular region often carries a recognizable combination of temperature and salt content. Tracking that signature shows how currents transport water through the ocean. Density calculated from temperature and salinity also reveals where water can sink or remain layered.

## The ocean contains important exceptions

**Constant proportions** are an excellent approximation for major ions in much of the open ocean. Coastal zones receive river water containing a different chemical mixture. Restricted basins can experience intense evaporation, while hydrothermal vents add and remove substances through reactions with hot rock.

Biology and carbonate chemistry affect calcium and carbon-related compounds. Organisms build shells from calcium carbonate and those shells may dissolve at depth. Sediments exchange chemicals with bottom water. Small deviations become significant when scientists need very precise density or chemical estimates.

A global study archived by [NOAA's library](https://repository.library.noaa.gov/view/noaa/29075/noaa_29075_DS1.pdf) found meaningful regional variability in modern magnesium-to-calcium and strontium-to-calcium ratios. The largest variation appeared in settings influenced by rivers, upwelling, polar conditions, or shelves. Such results refine the textbook principle rather than erasing its usefulness.

Researchers studying ancient oceans must be especially cautious. Fossil shells can preserve elemental ratios used as temperature or chemistry proxies, yet local ecology and seawater composition may influence the signal. Assuming perfectly constant ratios can make a regional change look global.

## A nineteenth-century shortcut still supports ocean science

Forchhammer's principle survived because it captures the dominant pattern of seawater chemistry. Ocean mixing is fast relative to the turnover of most major ions, keeping their ratios broadly uniform. Freshwater processes change total salinity much more readily than they change the recipe of sea salt.

The principle now sits behind instruments far more precise than Forchhammer could use. Autonomous floats, research vessels and moorings measure conductivity across the globe. Those readings help map currents and water masses, relying on standardized relationships between electrical behavior and salt content.

Scientists no longer treat the ocean as chemically identical everywhere. Trace elements, nutrients, gases and even some major-ion ratios reveal local processes. High-quality measurements can resolve departures once hidden within analytical uncertainty.

The enduring lesson is a matter of scale. At basin scale, major seawater salts follow a remarkably stable recipe. Near a river mouth, vent field, sea-ice zone, or sediment boundary, details become important. Forchhammer's principle supplies the broad baseline against which those revealing exceptions can be recognized.

**Related reading:** [the ocean's average salinity](https://www.argo.net/what-is-the-average-salinity-of-the-ocean/) and [how temperature and salinity affect seawater density](https://www.argo.net/how-temperature-and-salinity-affect-seawater-density/).

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