MS20 Laboratory: Physical and Biological Factors Affecting Oxygen in Sea Water

Transcription

1 MS20 Laboratory: Physical and Biological Factors Affecting Oxygen in Sea Water Objectives Understand the relationship between oxygen concentration and temperature Understand the relationship between oxygen concentration and salinity Use The Winkler titration method to determine oxygen concentration in a water sample Investigate the relationship of latitude and longitude on oxygen concentration Investigate the effect of de[p[th on oxygen concentration Introduction The amount of a gas in seawater is a function of both biological and physical factors. In the case of a gas like oxygen, the biological processes of plant photosynthesis and plant and animal respiration strongly affect the oxygen concentration. This biological process is shown in Figure One. Figure One. Chemistry of photosynthesis and respiration In this process, carbon dioxide and water combine in the presence of light to produce additional plant material (glucose, cellulose) and oxygen. To accurately predict oxygen concentration, several biologically related factors must be considered, such as the number of organisms producing oxygen, the amount of respiration taking place, the amount of light available to the organisms, the nutrients available, and the turbidity of the water. In addition, there are also several physical factors that affect the amount of oxygen dissolved in seawater. These include salinity, temperature, and pressure, as well as mixing resulting from wind or currents. The concentration of oxygen can also indicate the "health" of the water, especially in regard to pollution. Procedure 1: Winkler Method for Determination of Oxygen Concentration in a Sea Water Sample General Chemical Reactions That Occur During Analysis for Oxygen in Seawater (The first three steps of this procedure have been carried out by your instructor) MS20 Lab Oxygen rev.3/27/2005 Page 1 of 16

2 1. To a carefully collected se water sample, add a manganous sulfate solution and a alkaline potassium hydroxide-iodide solution to the seawater. This forms manganous hydroxide-which is a precipitate. 2. The newly formed manganous hydroxide Mn(OH) 2 reacts with the dissolved oxygen in the seawater to form a hydrated tetravalent oxide of manganese. 3. Next add sulfuric acid to the solution in the presence of iodide to release free iodine. 4. The free iodine released from this reaction is equivalent to the dissolved oxygen present in the seawater. In the presence of a starch solution this free iodine has an intense blue color. 5. Using the titration method, determine the amount of I 2 present (and therefore the oxygen content) by adding a standardized sodium thiosulfate solution until the blue color disappears. This takes place when all the free iodine is gone. Sampling for Oxygen Analysis Because this experiment requires that the samples react with the chemical solution for a given period of time, all the samples will be collected and chemically fixed collections by the instructor before the lab period begins. The following samples have been collected (see Figure Two). The model lake samples are collected from the surface first, then sequentially down into the "lake" o / oo salinity, room temperature aquarium (standard) o / oo salinity, room temperature aquarium 3. 0 o / oo salinity, polluted room temperature aquarium 4. Model lake-surface 5. Model lake-75 cm down 6. Model lake-near bottom Figure Two. Water sampling procedure MS20 Lab Oxygen rev.3/27/2005 Page 2 of 16

3 The water samples have been collected and the oxygen fixed in the following manner: 1. Each BOD (biological oxygen demand) bottle was filled right to the brim with the water sample. BOD bottles (as shown in Figure Three, step 1) were filled with the rubber tubing resting on the bottom of the bottle so that "extra" oxygen was not added by splashing or agitation. 2. Using the automatic pipettes, 1 ml of prepared manganous sulfate solution (step 2) was added to each BOD bottle followed immediately by 1 ml of the alkaline potassium iodide solution (step 3). After adding the potassium iodide solution, the water in most of the BOD bottles turns a straw yellow. However, those samples with little or no oxygen (polluted environment and lake bottom) will have a very weak yellow color or even a pale gray. 3. The bottles were stoppered then shaken (step 4); the contents were allowed to settle for 15 minutes, and then the bottles were shaken again. 4. To each bottle 1 ml of concentrated sulfuric acid (step 5) was added. The bottles were restoppered and shaken until the precipitate dissolved. The solution turns slightly darker. 5. The sample has now been "fixed" and can be analyzed any time during the lab period. The "fixing" process means that free iodine has been released in exact proportion to the oxygen that was in the sample at the time of fixing. The addition of oxygen to the sample after fixing will not affect the amount of free iodine now in the sample and so will not alter the titration results. Figure Three. Oxygen analysis flow chart MS20 Lab Oxygen rev.3/27/2005 Page 3 of 16

4 Standardizing the Thiosulfate Solution If we know the oxygen content of a given water sample, we can use this as our standard and calibrate the thiosulfate solution to it. This assumes, of course, that we know the exact condition of our water sample, its salinity, temperature, and degree of oxygen saturation. If we use aerated fresh water at room temperature, we can closely approximate the actual oxygen content of the water by consulting the proper oceanographic tables. Table One shows the theoretical saturations of oxygen at 0 o / oo salinity and at the temperature ranges typically encountered in nature. Table One. Theoretical Oxygen Saturation at Various Temperatures TEMPERATURE 0 2 TEMPERATURE 0 2 TEMPERATURE 0 2 (ºC) ml/l (ºC) ml/l (ºC) ml/l Sample 1, from the open aquarium at 0 o / oo salinity, will represent your standard. In order to calibrate your thiosulfate solution, you will need to know how many milliliters of thiosulfate are needed to titrate the standard. Procedure 1. Transfer exactly 50 mlof the known sample into a 125 mlerlenmeyer flask, using a 50 ml volumetric flask for the transfer (Figure Three, steps 6 and 7). 2. Fill a 50 ml buret with thiosulfate solution (step 8). Record the buret level in Table Two below. 3. Add approximately 5 ml of the starch solution (step 9) to your sample in the flask. The solution in the flask becomes dark blue. Slowly titrate (add thiosulfate from the buret) until the solution becomes colorless (step 10). You might hold a sheet of white paper behind the flask to help determine when the solution becomes colorless. At this point, you may still observe a few blue particles- don't worry about them. You have reached the end point! Record the final amount in the buret in Table Two. After calculating the difference between the starting and ending points on the buret, you will know the number of milliliters of thiosulfate required to render the solution colorless. MS20 Lab Oxygen rev.3/27/2005 Page 4 of 16

5 4. Calibrate the thiosulfate solution by noting the number of milliliters of solution required to titrate a standard volume (50 ml) of fresh water ( 0 o / oo ), room temperature. Run the standard twice, taking the second 50 ml aliquots from the same BOD bottle as you took the first. Average your results from the two runs. Find the theoretical oxygen content from Table One. For the remaining samples (the unknowns), complete steps 6 through 10 on Figure Three for each analysis. Titrate each unknown once, unless you miss the end point. To calculate the concentration of each unknown, you will use the following formula unknown O 2 concentration = known O 2 concentration x ml thiosulfate used for unknown / ml thiosulfate for known where the known O 2 concentration is the one you determined from the standard. Record your results on your answer sheet. Procedure 2. Dissolved Oxygen (DO) in the Sea: Oceanographic Data Tables The following tables contain actual hydrographic data taken by oceanographic vessels throughout the world s oceans. Hydrocasts always include the temperature, salinity and depth. Dissolved oxygen and phosphate data are often included. The exact location of the hydrocast is given by the latitude and longitude. 1. Mark on Map A all the stations given in the data tables. 2. Graph dissolved oxygen versus depth. Answer the following questions. A. How do surface values compare from place to place? B. How does dissolved oxygen vary with depth per locale? From locale to locale? C. What factors affect oxygen concentration? D. Are there any subsurface maximums? Minimums? If so, explain what causes them. MS20 Lab Oxygen rev.3/27/2005 Page 5 of 16

6 Charles H. Gilbert; August 21; 21º11 N, 158º18 W;wind 090º,14kt; weather, cloudy ) O 2 (ml/l) PO 4 -P (µg-at/l) σ t (g/ml) Charles H. Gilbert; August 20; 21º09 N, 158º20 W; wind 030º,11kt; weather, cloudy ) O 2 (ml/l) PO 4 -P (µg-at/l) σ t (g/ml) Hugh M. Smith; September 17; 21º14 N, 158º19 W; wind 050º, 8kt; weather, partly cloudy ) O 2 (ml/l) PO 4 -P (µg-at/l) σ t (g/ml) MS20 Lab Oxygen rev.3/27/2005 Page 6 of 16

7 Crawford; Station 441; 24º 16'S 07º 43'W; Date: 9/19 ) O 2 (ml/l) PO 4 -P (µg-at/l) Total P (µg-at/l) MS20 Lab Oxygen rev.3/27/2005 Page 7 of 16

8 Crawford; Station 440; 24º 16'S 09º 11'W; Date: 9/18 ) O 2 (ml/l) PO 4 -P (µg-at/l) Total P (µg-at/l) ? Thor; August 10, 41º32 N 29º24 E ) σ t (g/ml) O 2 (ml/l) H 2 S (ml/l) MS20 Lab Oxygen rev.3/27/2005 Page 8 of 16

9 Horizon; November 23; 24º57 S 145º01 W; wind 180º, force 2; weather, missing; sea, slight. ) O 2 (ml/l) PO 4 -P SiO 3 -Si ph (µg-at/l) (µg-at/l) σ t u MS20 Lab Oxygen rev.3/27/2005 Page 9 of 16

10 Atlantis Cruise 242; Station 5636; June 17; 19º01 N 39º21 E µg-at/l ) O 2 (ml/l) PO 4 -P Total P * * Atlantis Cruise 242; Station 5637; June 17; 19º05 N 39º28 E µg-at/l ) O 2 (ml/l) PO 4 -P Total P * ? * * 21.88? * * MS20 Lab Oxygen rev.3/27/2005 Page 10 of 16

11 Atlantis Cruise 242; Station 5608; May 16; 25º21.9 N 36º07.9 E ) O 2 (ml/l) PO 4 -P (µgat/l) Total P (µgat/l) * * * ? * * * * Atlantis Cruise 242; Station 5609; May28; 07º59.0 N 58º59.0 E µg-at/l ) O 2 (ml/l) PO 4 -P Total P * ? * ? * * * MS20 Lab Oxygen rev.3/27/2005 Page 11 of 16

12 R/V Anton Bruun; Station 80; 15º43 N 90º µg-at/l ) O 2 (ml/l) PO 4 -P NO 3 -N NO 2 -N SiO 3 -Si u u R/V Anton Bruun; Station 80; 15º43 N 90º µg-at/l ) O 2 (ml/l) PO 4 -P NO 3 -N NO 2 -N SiO 3 -Si u ? MS20 Lab Oxygen rev.3/27/2005 Page 12 of 16

13 Meteor; March 9; 56º37 N 44º54.5 W ) σ t O 2 (ml/l) O 2 (%) Meteor; March 30; 59º38 N 40º42.5 W ) σ t O 2 (ml/l) O 2 (%) MS20 Lab Oxygen rev.3/27/2005 Page 13 of 16

14 Name Lab Section Date ANSWER SHEET Oxygen Lab Table Two Calibration water temp theoretical oxygen starting point ending point mls of thiosulfate used 1 st run 2 nd run Average = Calculate your unknown concentrations using the average mls of thiosulfate from Table Two and the known oxygen concentration from Table One in this equation: unknown O 2 concentration = known O 2 concentration x ml thiosulfate used for unknown / ml thiosulfate for known 6. Determine the oxygen content of sample 2: 35 o / oo salinity water, room temperature. milliliters of thiosulfate solution calculate Determine the oxygen content of sample 3: polluted lake environment. milliliters of thiosulfate solution calculate Determine the oxygen content of sample 4: model lake surface sample. milliliters of thiosulfate solution calculate Determine the oxygen content of sample 5: model lake - 75 cm down. milliliters of thiosulfate solution calculate Determine the oxygen content of sample 6: model lake - bottom. milliliters of thiosulfate solution calculate 0 2 Answer the following questions: 1. Assume you misread your buret end point and added too much thiosulfate solution for your standard. How would this affect all of your other results? MS20 Lab Oxygen rev.3/27/2005 Page 14 of 16

15 2. What if someone mixed up the numbers of the water bottles in the laboratory "lake" so that the surface sample was the "polluted environment" sample? At what stage in the experiment could you first recognize the mix-up? Explain! 3. In winter, some fresh water lakes have the same temperature from top to bottom (isothermal). Would the oxygen values also be the same from top to bottom? Explain! 4. Consider the oxygen values listed in One. Also compare the measured oxygen values for the water at 0 o / oo and 35 o / oo salinity. In the open ocean, which factor, temperature or salinity, would have a more important effect on oxygen concentration? (hint: consider the range of temperatures and salinities that might occur going from the equator to the polar regions) 5. Graph the distribution of oxygen in the model lake that you observed. Would this be typical of an actual lake environment? Why or why not? MS20 Lab Oxygen rev.3/27/2005 Page 15 of 16

16 6. The percentage of saturation of a gas in seawater can be calculated from the physical properties of the water (temperature, salinity, and atmospheric pressure). Sometimes, however, in the case of oxygen we have super saturation occurring in the surface layers. Why? (hint: consider the coastal ocean and processes that add O 2 to the water) 7. Oxygen is a non-conservative gas- that is, it is affected by biological processes. Assume for the moment, however, that no biological processes are taking place. Sketch in the oxygen curve (below right) to indicate the theoretical oxygen concentrations with depth. Consider that the oxygen gets into the seawater through the air/sea interface only, and its concentration is controlled by the physical factors of temperature and salinity (ignore pressure effects and biological effects). (hint: consider what salinity and temperature are doing in each zone before plotting the oxygen curve!) MS20 Lab Oxygen rev.3/27/2005 Page 16 of 16