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Seawater - on average, seawater in the worlds oceans has a salinity of about 3.5%, or 35 parts per thousand (also expressed as 35 ppt).  This means that every 1 Kg of seawater has approximately 35 grams of dissolved salts.

When culturing saltwater organism, less than a 10% variation in salinity (± 5% or ± 2ppt) is often required.  The measuring methods cover a wide range of cost, convenience and accuracy.  Salinity is the sum of all dissolved ions in the water (not just sodium and chloride), expressed as parts per thousand (grams per kilogram).

One measurement method is to titrate chlorinity (chloride) and calculate the salinity based on the assumption that the composition is the same as seawater.  Usually titrations are based on volumes, which can introduce a 2.5% error if not performed correctly.  It is very time consuming compared to other preferred methods.

Electrical conductivity is another method of determining salinity.  Conductivity measurements are best at 25°C and any difference from this requires compensation.  Some units give readings in millisiemens (mS) and require a salinity conversion table - others read directly in salinity.  For more information on conductivity meters please click HERE.

Specific Gravity
Density is the basis of the hydrometer measuring method.  Salinity can then be determined from tables based on density (specific gravity) and temperature.  Density measurements are affected by solids other than salts, so errors may be introduced for samples containing solid materials (such as sand etc.)

Refractometers measure total dissolved solids, expressed as density (SG).  They provide fast, convenient and accurate (± 1ppt) salinity measurements if operated under ideal test conditions and are less expensive than other alternative methods.  In most cases the instrument should also incorporate automatic temperature compensation.

For more information on hand held refractometers which can provide measurements for root vegetables please see the E-line optical refractometer, OPTi digital refractometer, DR portable refractometer, and Eclipse hand held refractometer web pages.

Every effort has been made to ensure the accuracy of the contents of this FAQ.  However, Xylem Inc. can assume no responsibility for errors contained in this FAQ.
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