Standard seawater (SSW) has been employed by oceanographers as a reference material in the determination<br/>of salinity for over a century. In all that time, this is the first study to determine the uncertainty<br/>of the SSW manufacturing process. SSW is calibrated in reference to carefully prepared solutions of<br/>potassium chloride (KCl). All uncertainties in the preparation and measurement of KCl solutions and of<br/>new SSW are calculated. The expanded uncertainty of the SSW conductivity ratio is found to be 1 105,<br/>based on a coverage factor of 2, at the time of manufacture. There is no discernible “within batch”<br/>variability. No significant variability of quality within or between batches of KCl is found. Measurement of<br/>SSW “offsets” from the label conductivity ratio as long as 5 yr after the SSW batch manufacture are<br/>reported, and no significant change in label conductivity ratio for SSW batches P130 through P144 outside<br/>the expanded uncertainty of 1 105 is found. This last result is in contrast to some other studies, and<br/>herein are suggestions as to why this may be the case
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