Chlorinity

Chlorinity is the mass in grams of chloride-equivalent halides (chloride, bromide, iodide expressed as chloride) per kilogram of seawater, measured historically by silver-nitrate titration. It was the practical basis for salinity before conductivity methods, tied to it by the UNESCO 1967 relation S = 1.80655 x Cl. That coefficient was chosen so the new scale matched the older Knudsen formula at a salinity of 35. Chlorinity works because the major-ion ratios of seawater are nearly constant, the principle of constant proportions, so one ion concentration fixes total salt content. Practical Salinity and Absolute Salinity under TEOS-10 have since superseded it.