What does chlorinity mean?
Chlorinity is a technical measure used in oceanography to express the amount of chlorine present in seawater in the form of dissolved chloride ions. It is conventionally defined as the mass of chloride, plus small amounts of bromide and iodide counted as chloride equivalents, per kilogram of seawater, expressed in parts per thousand. Open-ocean seawater has a chlorinity of roughly 19.4‰. Historically, chlorinity was enormously important because chemists found that the relative proportions of the major dissolved salts in seawater are nearly constant worldwide: by titrating a sample for chloride alone, researchers could reliably calculate its total salinity using the classic Knudsen formula. Although modern salinity is determined electronically from conductivity, chlorinity remains relevant in historical records, calibration work, and studies of estuarine or brackish waters. The term belongs firmly to scientific register and appears mainly in marine chemistry literature rather than general usage.
nounThe total amount of chlorine, in the form of chloride ions, contained in seawater, conventionally expressed in grams per kilogram of seawater (parts per thousand). It is a standard chemical measure used in oceanography, historically as the basis for calculating salinity.
- The concentration of chloride ions in seawater, expressed in grams per kilogram (parts per thousand), used especially in oceanography and traditionally as the basis for determining salinity via the Knudsen relation.
- More broadly, the measure of chlorine (as chloride) content of any brine or saline solution.
"Oceanographers measured the chlorinity of the sample to estimate its overall salinity."
"The standard chlorinity of open-ocean seawater is close to 19.4 parts per thousand."
"By measuring chlorinity with silver nitrate titration, early hydrographers could compute salinity without analyzing every constituent salt."
Rarely used in the plural; the term normally appears as an uncountable mass noun referring to a measured property. A plural may occasionally occur when comparing values across different samples or water bodies.
"The chlorinities of the two samples differed by less than one part per thousand."
Before modern instruments, oceanographers couldn't weigh every salt in the sea — so they simply titrated for chloride and calculated salinity from chlorinity instead.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of chlorinity
The word chlorinity combines 'chlorine' — coined in the early 19th century from Greek 'chloros', meaning pale green, a reference to the gas's characteristic color — with the suffix '-ity', forming nouns denoting a quality or degree. It emerged within nineteenth-century scientific English as chemists and hydrographers sought precise terminology for the composition of seawater. It thus shares its root with other chlorine-derived terms such as chloride, chlorophyll, and chloroform, all ultimately traceable to that same Greek adjective for greenish-yellow.
How chlorinity is actually used
A technical term confined to oceanography, marine chemistry, and related sciences; it would not appear in everyday conversation. It is typically reported in parts per thousand (‰) or grams per kilogram, and in modern practice it has largely been superseded by conductivity-based salinity measurements, though it remains important in historical datasets and calibration standards.
Easily confused with chlorinity
Chlorine is the chemical element itself, whereas chlorinity is a measure of how much dissolved chloride (chlorine in ionic form) is present in water such as seawater.
Salinity measures the total dissolved salts in water, while chlorinity measures only the chloride portion, from which salinity is traditionally derived.