Salinity HYDROLOGY INVESTIGATION Oceanography Chapter 7 Salinity

HYDROLOGY INVESTIGATION 1 Salinity Salinity is the total amount of dissolved salts in water, expressed as grams of salts per kilogram of water (g/kg) or as parts per thousand (ppt). Seawater is a complex mixture of over 100 different ions. However, 11 of them (major constituents) make up more than 99.99% of all dissolved materials. Water’s colligative properties are: * The heat capacity of water decreases with increasing salinity *As salinity increases, freezing point decreases *As salinity increases, evaporation slows *Osmotic pressure increases as salinity increases

Salinity HYDROLOGY INVESTIGATION 2 Major Constituents of Seawater (% by weight) Chloride (Cl - )55.07% Sodium (Na + )30.62% Sulfate (SO 4 2- )7.72% Magnesium (Mg 2+ )3.68% Calcium (Ca 2+ )1.17% Potassium (K + )1.10% Bicarbonate (HCO 3 - )0.40% Bromide (Br - )0.19% Strontium (Sr 2+ )0.02% Boron (B 3+ )0.01% Fluoride (F - )0.01%

Salinity HYDROLOGY INVESTIGATION Fig. 7-3, p. 189 A representation of the most abundant components of a kilogram of seawater at 35‰ salinity. Note that the specific ions are represented in grams per kilogram, equivalent to parts per thousand (‰).

Salinity HYDROLOGY INVESTIGATION The Components of Ocean Salinity Came from, and Have Been Modified by, Earth’s Crust Processes that regulate the major constituents in seawater. Ions are added to seawater by rivers running off crustal rocks, volcanic activity, groundwater, hydrothermal vents and cold springs, and the decay of once-living organisms. Ions are removed from the ocean by chemical entrapment as water percolates through the mid-ocean ridge systems and seamounts, sea spray, uptake by living organisms, incorporation into sediments, and ultimately by subduction.

Salinity HYDROLOGY INVESTIGATION Fig. 7-4b, p. 190

Salinity HYDROLOGY INVESTIGATION The major constituents are very well mixed through the world’s oceans and are found in nearly constant proportions. This means that although the salinity may vary from place to place, the major constituents are present in the same relative proportions.

Salinity HYDROLOGY INVESTIGATION The Ratio of Dissolved Solids in the Ocean is Constant Forchhammer’s principle, also known as the principle of constant proportions states that although the salinity of various samples of seawater may vary, the ratio of major salts is constant. How do scientists determine the salinity of seawater? Salinity can be determined by measuring the chlorinity of the sample. Since the chlorinity is easy to measure, and the principle of constant proportions applies to all seawater, scientists can use the following formula to determine salinity: Salinity in ‰ =  Chlorinity in ‰

Salinity HYDROLOGY INVESTIGATION The Ocean Is in Chemical Equilibrium Is the ocean becoming progressively saltier with age? No, the ocean is in chemical equilibrium. The proportion and amounts of dissolved solids remain constant. This concept is known as the “steady state ocean.“ Ions are being added to and removed from the ocean at the same rate. Residence time: is the average length of time an element spends in the ocean. Residence time can be calculated by the equation: Residence Time = ___Amount of element in the ocean___ The rate at which the element is added to (or removed from) the ocean

Salinity HYDROLOGY INVESTIGATION Seawater’s Constituents May Be Conservative or Nonconservative Conservative constituents of seawater are those constituents that occur in constant proportions. Conservative elements have long residence times and are the most abundant dissolved material in the ocean. Nonconservative constituents have short residence times, and are usually associated with seasonal, biological or short geological cycles.

Salinity HYDROLOGY INVESTIGATION 3 Salinity Salinity determines the distribution of plants and animals that live in the ocean. Salinity also affects other properties of seawater, such as its density and the amount of dissolved oxygen. 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 ppt are called brackish. Waters with salinity greater than 40 ppt are called hypersaline.

Salinity HYDROLOGY INVESTIGATION

Salinity HYDROLOGY INVESTIGATION 4 Salinity Determination Methods Hydrometer method: The addition of salts results in an increase in the density of seawater as compared to that of pure water. The 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. 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) = x Chlorinity (ppt)

Salinity HYDROLOGY INVESTIGATION 5 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.

Salinity HYDROLOGY INVESTIGATION 6 Calibration - Titration Method Concentration of standard reflect composition of seawater 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.

Salinity HYDROLOGY INVESTIGATION 7 Hydrometer Reading Fill the 500-mL graduated cylinder with sample water until the water level is 2 to 3 cm from the top of the cylinder. Determine the temperature of your sample following the Hydrology Temperature Protocol. Place the hydrometer in the cylinder and allow it to settle. Follow the manufacturer’s instructions with the hydrometer (the hydrometer should not touch the cylinder walls, and should be read from the bottom of the meniscus). Read the specific gravity from the hydrometer scale.

Salinity HYDROLOGY INVESTIGATION 8 Element Reporting (Hydrometer Method) Make measurements weekly, at the same time of day. Measure the temperature and specific gravity for each group, calculate the average, and record all values on the Hydrology Investigation Data Work Sheet. Using the temperature and specific gravity values for each group, determine the salinity values from Table HY-P-2. Calculate the average salinity. If all salinity values are within 2 ppt of the average, submit to the GLOBE Student Data Server the temperature, specific gravity and salinity from the student(s) whose salinity is closest to the calculated average. If only two measurements were used to calculate the average, report the temperature, specific gravity, and salinity from either group.

Salinity HYDROLOGY INVESTIGATION 9 Element Reporting (Titration Method) Make measurements weekly, at the same time of day. Record the salinity in ppt on the Hydrology Investigation Data Work Sheet. Take the average salinity values measured by the student groups. If the recorded values are all within 0.4 ppt of the average, submit the average to the GLOBE Student Data Server.

Salinity HYDROLOGY INVESTIGATION

Salinity HYDROLOGY INVESTIGATION Specific gravity (spg): how many times as dense a mineral is compared to water Archimedes principle: mineral submerged, loss in weight/weight of displaced water Specific gravity= weight of sample in air weight of equal volume in air (loss of weight in H 2 0)

Salinity HYDROLOGY INVESTIGATION HISTORICAL Specific gravity is one of the first fundamental principles of hydrostatics, and its discovery is ascribed to Archimedes (287 B. C.). The incentive to his discovery and the method of his reasoning is described in the following story which carries with it the flavor of the East: King Hiero ordered his silversmith to make for him a crown of pure gold. When the crown was brought before him he suspected the honesty of the silversmith, and desired to know if Archimedes could devise a way of testing the question without injuring the crown. The philosopher pondered the problem for a long time without succeeding, but one day as he stepped into a bath his attention was attracted by the overflow of water. A new train of ideas was started in his ever receptive brain. Wild with enthusiasm he sprang from the bath, and forgetting his robe,

Salinity HYDROLOGY INVESTIGATION Table 7-3, p. 192

Salinity HYDROLOGY INVESTIGATION Table 7-1, p. 188

Salinity HYDROLOGY INVESTIGATION Table 7-2, p. 189

Salinity HYDROLOGY INVESTIGATION Fig. 7-4a, p. 190