Tech Talk 28
When culturing saltwater organisms, less than a 10 percent variation in salinity (± 5% or ± 2 ppt) is often required. The measuring methods cover a wide range of cost, convenience and accuracy. Basically, 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 percent error if not done correctly. It is much more time consuming than other methods and used as a means of checking other techniques.
Density is the basis of the hydrometer measuring method. Salinity can then be determined from tables based on density (specific gravity) and temperature.
Refractometers are also density-based. They provide fast, accurate (± 1 ppt) salinity measurements. Generally, temperature-compensated models are suggested. Cost and care are the biggest drawbacks. Calibration using certified seawater is best; however, fresh water is a reasonable alternative for general aquaculture uses.
Electrical conductivity is another method of determining salinity. Conductivity measurements are best at 25°C and any difference from this requires compensation. If there is a question about temperature compensation, simply take a reading, change the temperature of the water by 5 or 10 degrees and measure again. It should give the same salinity. Some units give readings in millisiemens (mS) and require a salinity conversion table. Others read directly in salinity (called a salinity meter). Regular calibration with a standard solution is recommended.
The information in this Tech Talk was provided by Thomas Frakes, Technical Consultant, Aquarium Systems, Inc.