1. Why does temperature decrease when a sample is brought to the surface?

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1 Salinity and Density Lab Apparatus ml volumetric flasks 6 10 ml volumetric flasks 1 Drying oven set at Plastic beakers (20 ml) 5 Hydrometer and tables 1 Refractometer 1 Salinometer (20 liters) Distilled water (20 liters) ml beakers 6 Hot plates 6 Magnetic stirrers and stirring bars Food coloring Silver Nitrate, 500 ml (37 g/liter) Indicator, 100 ml (3.5 K 2 CrO 4 g/liter) 6 Burettes ml Erlenmeyer flasks 6 10 ml Pipettes 2 Triple beam balances 2 Thermometers Procedures Temperature Oceanographers use two terms in describing temperature, one is the in situ temperature while the other is the potential temperature, usually signified by the Greek letter theta (). The in situ (Latin for in place) temperature is that measured in the water. The potential temperature is what the temperature would be if raised to the surface adiabatically, that is without exchange of heat or water with the surrounding water. Oceanographers are interested in the potential temperature since deep water usually acquires its temperature characteristics while at the surface. Thus potential temperature may give an indication of what the temperature was at the time the water mass was formed. 1. Why does temperature decrease when a sample is brought to the surface? 2. A water sample has an in situ temperature of -1ºC at 4,000m. Use Table II-6 to determine what the potential temperature () is. = What is the percentage change in the temperature?

2 Density The density of seawater is dependent upon its pressure, temperature and salinity. Ignoring pressure effects, almost all of the ocean has a density in the range of and g/cm 3. The greatest effect on density is from the compressibility of water. A parcel of water with a density of at the surface would have a density of at 5,000m. Since the range of densities in the ocean is fairly narrow, oceanographers have adopted a convention to make the numbers easier to use, that is to use only the decimal portion. The following definitions apply = ( S,T,p - 1) x 10 3 = in situ density t = ( S,T,0-1) x 10 3 = density at the surface = ( S,,0-1) x 10 3 = density at the surface Where is the density, S is salinity, T is Temperature, and p is pressure.

3 3. Determine the t of a sample which was measured to be g cm -3. What is the t of a sample collected at 14.33ºC and (use Table II-I) 4. Use a 100 ml volumetric flask and determine the weight of both fresh and seawater. Then determine the density of both. Fresh Wt. of Flask Flask + Water Wt. of Water Density How did you calculate density? What are the units? Do the units come out right? 5. Weigh two plastic beakers. Using a 10 ml volumetric flask, very carefully measure 10 ml of distilled water in a weighing dish, and do the same with seawater. Put the dishes in a drying oven at C. When dry, determine the weight of salt residue. Distilled Beaker Wt. Wt. with water Wt. of Water Wt. dried Wt. of Solids What was the density? What are the units? Distilled Compare the density to that found in #4. Are there differences? Why? How does the presence of salt affect the density of water? What is the purpose of drying a sample of distilled water.

4 What are your conclusions from this? What is the salt content of each sample in parts per hundred (%)? i.e. What is the weight of salt in 100 g of water? Fresh What is the salt content in parts per thousand ( )? Fresh 6. An alternative method of measuring density uses a hydrometer. Often these measure the specific gravity of a fluid. This is defined as M Specific Gravity = M x w Where M x is the mass of some liquid or solid M w is the mass of an equal volume of water (usually at 4ºC). Specific gravity is the density of a substance divided by the density of water. Since water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is the same number as density but without any units. Use a hydrometer to determine the specific gravity of the seawater. What else is necessary to determine density accurately with the hydrometer?

5 Follow the directions in 210.B Hydrometric Method below Sample Temp Sp.Gr. Density Salinity Salt Fresh * make sure you include units in the table 7. In the following table collect the density (g/kg) data from the previous experiments. Sample Fresh Wt. Dry Wt. Hydrometer Distilled Salt Water How do the three methods (the fresh wt., dry wt. and the hydrometer) compare in terms of determining density? Why? Salinity Salinity may be defined as the content of dissolved salts in sea water usually expressed as S. This is a convention which approximates the weight in grams, in vacuo, of the solids obtained from 1 kg of sea water (weighed in vacuo). The definition states that this weight is after the solids have been dried to a constant weight at 480 C, the organic matter completely oxidized, the bromide and iodide replaced by an equivalent amount of chloride, and carbonates converted to oxides. Ocean water contains slightly more salts (halides, carbonate, and bicarbonate) than is expressed by its salinity value. The median salinity of the ocean is g/kg of seawater or 34.69, where stands for parts per thousand. About 75% of the ocean falls into the range of Therefore, measurements of salinity have to be made with considerable precision because the range is so narrow. For some processes, changes of only a few hundreths of a part per million are important. For example, there is a slow decrease in salinity in the deep Pacific from about at 30ºS to at 40ºN, which is evidence for a northward movement of this large volume of water. The ions in seawater remain in the same relative ratios, no matter what the salinity. In practice, because of this Principle of Constant Proportionality, the salinity may be defined in terms of chlorinity by the Knudsen equation: S = x Cl

6 This equation is solely a definition and has no true meaning in any practical chemical sense. Recent developments have made measurement of salinity by means of conductivity the practical rule. Salinity has thus also been redefined in terms of conductivity, and these measurements are often noted as psu, practical salinity units. However, measurements based on silver nitrate titrations are still the accepted reference procedure. In addition, other methods are in use depending on the ease of use and accuracy desired. Physical oceanographers may want to know salinity to 4 decimals, while some biologists may be satisfied with ± 1. Some of these other methods include the refractometer which is based in the change of the index of refraction with changing salinity, and T-S-D diagram estimates based on temperature and density measurements. 8. One of the original methods, which is still used today, is the titration with silver nitrate. This technique has a precision of about ± Measure 15 ml of seawater into an erlenmeyer flask, add 15 ml of indicator solution (K 2 CrO 4 ). Titrate with the silver nitrate (AgNO 3 ) solution using the burette. (WARNING: silver nitrate stains your skin, so is an indicator of how good your lab technique is.) When you begin the titration, the solution starts off as a clear pale yellow liquid. Upon titration a white precipitate starts forming and as the end point is approached a pink/red precipitate forms. The pale yellow color of the solution turns to a full yellow then a definite pale red as the endpoint is exceeded. This titration determines the chlorosity (Cl/liter) in the sea water. Note that it is not in terms of cubic meters. To get the salinity value from the chlorosity, use Table II in the appendix of Strickland and Parsons. Chlorosity at 20C is defined below. Cl/liter (20) = V + C b + C s + C t V is the volume of titrant to three decimal places C b is a burette correction (usually within ±0.05 ml) C s is a standardization correction C t is a temperature correction (For our purposes ignore the correction factors) Chlorosity = V = Salinity = Find the density using the Table II, from Strickland and Parsons, provided by the instructor or use the relationship S = x Cl/liter. Salinity = Does this make sense given where you took the samples?

7 9. Evaporating seawater to determine salt content is tedious, impractical most of the time, and error prone. Salinity titrations are also relatively slow. Two other techniques which are faster rely on the refractive index (using a refractometer) and the now-preferred conductivity method (using a salinometer). Fresh Temp Salinity Temp Salinity The salinometer is based measuring electrical conductivity. Conductivity is dependent on temperature and salinity. These can be measured with great precision in the lab. Good instruments have an accuracy of ± The refractometer is a commonly used optical instrument. The refractive index of solutions changes depending on the amount of dissolved solids in the solution. The technique is generally accurate to about ± 1.0. Measure and record the seawater samples again using these two instruments. Record the results in the table below. Refractometer Salinometer Titration Hydrometer 10. Are there differences in the salinity results from the four different methods (hydrometer, titration, refractometer, salinometer)? What might that mean?

8 Inter-relationships and Conversions

9 11. There is a precise relationship among salinity, temperature and density. If any two of these are known then the third can be determined using this relationship. Graphically, this is done with a temperature-salinity-density (T-S-D) diagram. Determine the density and t of the seawater solution from the information you collected in #9 above and the T-S diagram. Density t 12. Tables are also available to determine the density from measurements. Ours tables are limited to salinities of with temperatures of 12-30ºC or salinities of30-40 and temperatures of -2.0 to 30ºC. Use your measured temperature and salinity (from #9) to determine the t of the seawater sample using Table II-1 provided by your instructor. Temperature Salinity t How does this compare to the graphical method used in #11. Which do you think is more accurate? Why? Practical Implications 13. Fill a large beaker about 1/2 full with seawater. Using a pipette bulb, carfefully suction hot seawater into the pipette. Without disturbing the water in the beaker too much, place the pipette tip near the bottom of the beaker. Hold the pipette steady (use a ringstand with clamp if necessary). Then slowly release the hot seawater from the pipette. Describe what happens. Why does this occur?

10 14. Using the same procedure as in #8 above, place dyed, room temperature freshwater into the pipette. Describe what happens. Why does this occur? 15. What happens to Saco River water as it flows into the ocean? Why? 16. The water flowing out of the Mediterranean Sea has been subjected to lots of evaporation. What does it do when it reaches the Atlantic Ocean? Why? 17. near the poles is colder than that near the equator. What happens to polar water? Why? 18. Deep ocean water percolates through cracks in the earth's crust near the mid-oceanic ridges and becomes heated by the magma. What happens to this water when it resurfaces above the ocean floor? Why?