MS20 Laboratory Seawater Salinity and Density

Transcription

1 MS20 Laboratory Seawater Salinity and Density Introduction As you perform these experiments, pay particular attention to the results different methods produce different levels of precision and accuracy. Care in your technique should produce repeatable results, but do not expect that each technique will produce exactly the same numbers. For each method you will be measuring the salinity of: one known sample, whose salinity is 35º/ ºº and two unknown samples, A and B. In this lab, you will: determine salinity by conductivity evaporation hydrometer (and temperature and TDS diagram) refractometer and determine density by: hydrometer (and temperature and TDS diagram) refractometer Procedures A. Determining salinity by conductivity (demonstration) Figure 1. Light intensity increases as salinity increases. Therefore it is possible to make a meter to measure electrical conductivity. However, in our experience, the meters are expensive, difficult to maintain and give erroneous results. B. Determining salinity by evaporation 1. Fill each beaker (labeled 35 º/ ºº A & B ) to about 200 ml from the dispensers on the side of the room, making sure that you have the correct sample in each beaker. 2. Weigh each labeled Petri dishes (with the same labels as the sample beakers) to the nearest 0.01 gram. Record the masses of the dishes in Table B (page 4) on the answer sheet. 10/8/2012 Page 1 of 11

2 3. Using a pipette, transfer about 10 ml of each of the three salt solutions to the corresponding labeled Petri dishes. Weigh each Petri dish with the water to the nearest 0.01 gram and record the masses on Table B. Determine the mass of the water samples by subtracting the weight of the dish only, and record the masses. 4. Carefully take the Petri dishes to the back of the room, where your instructor will place them in the drying oven. Leave them in the oven until dry this will take the majority of the lab period. You will be doing exercises C through F while waiting for the samples dry. 5. Once the samples are dry allow them to cool for a few minutes, then weigh each Petri dish and record the results on Table B (dish + salt). Subtract the masses of the dishes to determine the mass of each of the salt samples and record the results on the answer sheet. 6. Determine the salinity of each sample using the equation below. Record your answers on the data sheet (Table B, page 4). C. Determining salinity by measuring density (using a hydrometer) and temperature (using a thermometer). Figure 2. As the density of the fluid increases, the hydrometer displaces less water. Since the upper tube is very narrow this difference in volume corresponds to a significant change in depth. The density of the fluid is read from the graduations on the stem. NOTE: The entire hydrometer is sealed in thin glass, and most of the mass is located in the metallic base. This device is carefully calibrated, very fragile, and very expensive. Handle your hydrometer carefully, as the tiniest crack in the glass makes it worthless for measuring density. 10/8/2012 Page 2 of 11

3 1. Pour approximately ml of water from the known (35 o / oo ) salinity sample into a 250 ml graduated cylinder. 2. Measure and record the water temperature and density in Table C. 3. Determine the salinity using the TDS diagram and record in Table C. 4. Repeat steps 1-3 above for the A and B unknown samples. D. Determining salinity using a refractometer Figure 3: The refractometer. Before you begin, look through the eyepiece and focus by turning the eyepicece. The boundary of the shaded region should cross the scale at the zero line. If it does not, inform your instructor so that she/he can adjust it or assign you a different refractometer.. 1. Place a drop of the water sample on the refractometer and lower the flipper into place. Look through the eyepiece and aim toward a the light. The boundary between the upper blueshaded region of the scale and the lower white region crosses the calibration scale at a value that represents the salinity of the sample. 2. Rinse the prism and cover plate in deionized water and dry with a cloth towel. 3. Repeat steps 1-3 for the A and B unknowns and record the salinities in Table D. 10/8/2012 Page 3 of 11

4 Data Sheets MS20 Laboratory: Record all data in the appropriate metric units (centimeters, grams, etc.). Remember to use significant figure rules and to indicate appropriate units (if the scale reads 13.4 g, your answer is not 13.4, but 13.4 g (or 13.4 grams). A. Demonstration (No data to record) B. Determining salinity by evaporation TABLE B: SAMPLE MASS OF EMPTY DISH MASS OF DISH + WATER MASS OF WATER MASS OF DISH + SALT MASS OF SALT SALINITY OF SAMPLE 35 o / oo A B C. Determining salinity from a TDS diagram by measuring density (using a hydrometer) and temperature (using a thermometer). TABLE C: SAMPLE TEMPERATURE ( C) DENSITY (GRAMS/ML) SALINITY ( O / OO ) 35 o / oo A B 10/8/2012 Page 4 of 11

5 Figure 3: TDS Diagram. 10/8/2012 Page 5 of 11

6 D. Determining salinity using a refractometer TABLE D: SAMPLE SALINITY ( O / OO ) 35 o / oo A B Combined Salinity Data from Tables B, C and D Record the given salinities for each exercise. (B) EVAPORATION 35 o / oo A B (C) HYDROMETER (D) REFRACTION Note: Save your graduate of 35 o / oo salinity water for the next procedure! E. Determining density as temperature changes 1. Using the 35 o / oo salinity water in the graduate from the previous exercise, add a bit more 35 o / oo until the graduate is as full as possible without overflowing when the hydrometer is in it. Take the graduate and a glass thermometer to the warm water bath at the side of the room. 2. Stir the water in the graduate constantly but gently (the thermometers are glass!) to insure uniform heating of the water in the graduate. Warm the graduate in the water bath provided until the temperature is 30 C. Take the graduate back to your table and measure the density with a hydrometer and record both the density and temperature in Table E2. 3. Fill a 2000 ml beaker (stainless steel) about ¾ full of cold tap water. 4. Place the 250 ml graduate containing 35 o / oo into the beaker. Stir the water in the graduate constantly with the thermometer except when you remove the thermometer and put in the hydrometer to measure the density. Record densities and temperatures in Table E2. Be sure to remove the hydrometer and thoroughly stir the water in the 250 ml graduate between density readings. NOTE: The amount of time between readings is not as important as making sure that you have a density recorded for every drop in temperature of about 2 degrees. 10/8/2012 Page 6 of 11

7 5. When your 35 o / oo water has reached a temp of 25º add a little ice to the water bath to cool it down. Continue to measure temperatures and densities every 2º or so. Stop when your salt water reaches a temperature of about 10º Table E2. Temperature and density of a 35 o / oo water sample Temperature ( C) Density (g/cm 3 ) Add ice here stop 6. Plot Density versus Temperature from Table E2 in Figure 4 below. Figure 4. Density versus Temperature for a 35 o / oo water sample Figure 4. Density versus Temperature for a 35 o / oo water sample 10/8/2012 Page 7 of 11

8 F: Constructing a Swallow float An aquarium has been set up to represent a model ocean with two water masses of different densities. These water masses have been dyed with food coloring to help show the boundary between the different density layers. Your job is to create a float that remains at the interface of the two water masses. 1. Samples from each of the water layers have been drawn and are in hydrometers next to the aquarium. The blue layer is denser cold salty water (bottom) and the yellow layer is lighter warm fresh water (top). Measure the density of each layer using the hydrometers provided. Record the densities below. surface (yellow) layer: deep (blue) layer: 2. Weigh a 0.5 dram glass vial (plus cap) to the nearest 0.01 g using a 200 g balance. mass of the empty vial and cap: 3. Fill the vial about 1/3 full with lead shot and measure its volume to the nearest 0.1 cm 3 by immersing the vial in fresh water in a 25 ml graduate. Read the volume of water in the graduate precisely before you add the vial. Tilt the graduate slightly and slide the vial into the graduate slowly so that you don t splash any water out or trap any air bubbles! Then read the volume of water again. Subtract the first volume from the second to arrive at the volume of the vial. Remember Archimedes principle! volume of the vial: ml 4. Calculate the density needed to just sink through the upper surface and float on the lower layer. In other words, calculate the density halfway between the upper and lower layers. This is easily done by adding the density of the upper and lower layers and dividing by 2. (density of blue layer + density of yellow layer) / 2 = g/cm 3 5. We now know the volume of the vial and the density it should be. We need to find the mass. Recall that density = mass / volume so, by rearranging the equation mass = density X volume 6. Calculate the desired mass of your float by multiplying the desired density (step 4 above) by the volume (step 3 above) 10/8/2012 Page 8 of 11

9 Calculated mass of float: g 7. Now adjust the mass of our vile to equal the calculated float mass. Place the empty vial and its cap on the balance and slowly add lead shot until the mass equals the calculated value. When you have done this, you are ready to test your float. Tell your professor when you are ready to launch the Swallow Float! 10/8/2012 Page 9 of 11

10 Names Lab day and time Questions 1. Write a description of what you observed in the electrical conductivity demonstration. 2. Consider the various methods of determining salinity. Which is the most accurate, which the least, and why? (Define accuracy as being close to the "true" value). Think of real-world conditions where you may be using this equipment. 3. Assume there was "dirt" (fine silt or clay particles) in the sample you evaporated. How would this have affected the salinity as you measured it? 4. Could you have evaporated the salt out of the seawater at room temperature? If you could, would this have affected the salinity measurement? Explain. 5. Why might the Persian Gulf be significantly more saline than other ocean basins (hint: it is the same reason that the Mediterranean Sea is more saline than the rest of the Atlantic, and the same reason that the Dead Sea is the saltiest body of water on Earth)? (hint #2: The answer is related to air circulation remember Horse Latitudes?) 6. A fully loaded ship moving from a fresh water port heads over to the Persian Gulf, an area known for high salinities. Will the ship ride higher or lower in the Gulf compared with the port it left? Explain. 10/8/2012 Page 10 of 11

11 7. Does a ten degree change in temperature have a greater or lesser effect on density than a 10 o / oo change in salinity? (Use TDS diagram) 8. Of the methods for determining salinity you have examined so far in this laboratory, which takes the most time? Which method might the most difficult to do aboard ship? 9. In Table E1, are your measured densities different from the theoretical values? How can you explain these differences? (Hint: What are the possible the sources of error?) 10. Is the change in density with temperature a "linear" change over your experimental temperature range? (Figure 4). 11. Assume you were heating up a liter of 20 o / oo water with a sunlamp. The temperature increases from 10ºC to 30 ºC. At the same time some of the water evaporated and the salinity went from 20 to 40 o / oo. What is the initial and what is the final density? Which is the more important factor (temperature or salinity) affecting the density? (Hint see TDS diagram) 12. Would it be possible to make a Swallow float for a fresh water lake? Explain your answer. 10/8/2012 Page 11 of 11