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Salinity

Salinity. Salinity is the total amount of dissolved salts in water; grams of salts per kilogram of water (g/kg) or as parts per thousand (ppt). Seawater has 11 major constituents that make up more than 99.99% of all dissolved materials.

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Salinity

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  1. Salinity • Salinity is the total amount of dissolved salts in water; grams of salts per kilogram of water (g/kg) or as parts per thousand (ppt). • Seawater has 11 major constituents that make up more than 99.99% of all dissolved materials. • Although salinity may vary, the major constituents are well mixed and present in the same relative proportions.

  2. Major Constituents of Seawater (% by weight) • Chloride (Cl-) 55.07% • Sodium (Na+) 30.62% • Sulfate (SO42-) 7.72% • Magnesium (Mg 2+) 3.68% • Calcium (Ca 2+) 1.17% • Potassium (K+) 1.10% • Bicarbonate (HCO3 -) 0.40% • Bromide (Br -) 0.19% • Strontium (Sr 2+) 0.02% • Boron (B 3+) 0.01% • Fluoride (F-) 0.01%

  3. Why Study Salinity… • Determines the distribution of plants and animals that live in the ocean. • Affects other properties of seawater, such as its density and the amount of dissolved oxygen.

  4. Significant Values • The average salinity of the world’s oceans is 35 ppt. • Freshwater has a salinity of <1 ppt. • Inshore waters with salinity values between 1 - 25 ppt are called brackish. • Waters with salinity greater than 40 ppt are called hypersaline.

  5. Salinity Hydrometer Method • Addition of salts to pure water causes an increase in density. Salinity can be calculated by measuring the specific gravity of a water sample using a hydrometer, correcting for the effect of temperature and converting the readings to salinity by using conversion tables. Specific Gravity = density of sample Density of pure water

  6. Hydrometer Reading • Be sure hydrometer is clean • Fill 500 mL graduated cylinder with sample water • Determine the temperature of your sample • Place the hydrometer in cylinder and let settle. It should not touch the cylinder walls, and should be read from the bottomof the meniscus. • Read the specific gravity from the hydrometer scale • Using the specific gravity and temperature values, determine salinity from salinity table in Teacher’s Guide • Read three times. The values should be within 2 ppt of the average. Discard outliers.

  7. Calibration - Hydrometer Method • 35 ppt standard: • Measure out 17.5 g NaCl (table salt) and pour into a 500-mL graduated cylinder. • Fill the cylinder to the line with distilled water and carefully swirl the solution to mix the standard, until all salt crystals have dissolved. • Pour the solution into a 1-quart plastic bottle and label. • Prepare a blank using 500 ml of distilled water. • Follow the directions for a water sample. • Check technique every six months.

  8. Salinity titration method • The amount of halogens (chloride, bromide, iodine, and fluoride) in the water sample (chlorinity) is determined using a silver nitrate titration method. The salinity of the sample can then be calculated using the following formula: Salinity (ppt) = 1.80655 x Chlorinity (ppt) • Use a test kit that meet specifications described in the Tool Kit in the GLOBE Teacher’s Guide • Follow the instructions in the test kit to take three measurements. Values should be within the precision stated in the test kit specifications. Discard outliers and retake measurements.

  9. Calibration - Titration Method • Concentration of standard reflect composition of seawater. • 38.6 ppt sea water titration standard: • Measure out 17.5 g NaCl (table salt) and pour this into a 500-mL graduated cylinder. • Fill the cylinder to the line with distilled water and carefully swirl the solution to mix the standard, until all salt crystals have dissolved. • Pour the solution into a 1-quart plastic bottle and label. • Follow directions for a water sample. • Calibrate every six months to check technique.

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