From AMS Glossary
(Also called Copenhagen water, standard seawater.) A seawater preparation, the chlorinity of which lies between 19.30 and 19.50 grams per kilogram (or per mille) and has been determined to within ±0.001 per mille.
Normal water is used as a convenient comparison standard for chlorinity measurements of seawater samples by titration. It is prepared by the Hydrographical Laboratories, Copenhagen, Denmark.