From AMS Glossary
(Symbol Cl.) A measure of the chloride content, by mass, of seawater (grams per kilogram of seawater, or per mille).
Originally chlorinity was defined as the weight of chlorine in grams per kilogram of seawater after the bromides and iodides had been replaced by chlorides. To make the definition independent of atomic weights, chlorinity is now defined as 0.3285233 times the weight of silver equivalent to all the halides. The chlorinity of seawater is generally determined in order to permit the calculation of salinity, although other methods of determining salinity can be used. By using normal water as a comparison standard, Knudsen burettes and pipettes for the analysis, and Knudsen's tables to compute the results, determinations as accurate as those of a time-consuming gravimetric analysis can be made with a rapid titration of the seawater against silver nitrate solution, employing potassium chromate or other suitable indicator for the end point. It is customary to express chemical analyses of seawater in terms of chlorinity or of chlorosity.