DISSOLVED OXYGEN IN WATER

Transcription

1 DISSOLVED OXYGEN IN WATER LAB DO 1.CALC INTRODUCTION Oxygen is an essential element to life, both terrestrial and aquatic. Oxygen gas dissolves in water through several different avenues. Simple diffusion from the air and aeration from water movement affect the dissolved oxygen (DO) content. For example, water in a shallow, fast moving stream is more likely to have DO than a stagnant pool of water. Biological factors that affect DO content are photosynthesis by aquatic plants, the amount of decaying organic material, and even human activities that occur at a body of water. During the day, photosynthesis by aquatic plants increases the DO content of water. At night, plants and animals continue to consume the DO, resulting in a decrease in DO concentration. Decaying organic material is broken down by aerobic bacteria (bacteria that require oxygen), and this process depletes the DO concentration of water as well. Physical factors like temperature, altitude, and atmospheric pressure also affect the DO content of water. The oxygen content of water is often an important indicator of water quality and directly influences the organisms supported by a body of water. In general, the higher the DO concentration, the more diverse the biological population will be. Table 1 shows the concentrations of DO necessary to support certain species. Catfish and carp require significantly less DO than trout and bass. This explains why catfish and carp (big goldfish) commonly live in ponds where the water is not well aerated and decomposing material decreases the DO concentration. Mosquito larvae require the least DO of all. A small puddle of water is enough to support their growth! This is the reason it is important to be sure that after a heavy rain, there are no standing pools of water in your yard. Especially in western Pennsylvania, where West Nile virus is a concern, it is important to eliminate the type of environment where mosquitoes can breed. The use of laundry detergents that contain phosphorus is good example of how human activity can affect water quality and DO concentrations. All organisms rely on the presence of phosphorus to survive, but in general, levels of this nutrient are low. Up until the 1970 s, most laundry detergents in the US contained phosphates as a primary ingredient. These phosphates were sent with household water to sewage plants, and then would be released into local water supplies. The increase in phosphorus in the water resulted in a process called eutrophication. Eutrophication is the name for nutrient enrichment of an ecosystem that is naturally low in nutrients. The increase in dissolved DO1.CALC-1

2 phosphorus in the water resulted in algal blooms, or accelerated growth of algae. Increased growth resulted in increased amounts of decaying organic material in the water. The bacteria that aid in the decomposition of the algae are aerobic species. During the decomposition process, therefore, the bacteria deplete the water supply of DO, resulting in the death of many fish and other organisms that live in the water. Since the relationship between the presence of high concentrations of phosphates in water and the decrease in water quality has been recognized, the use of phosphates in detergents has been terminated. This is only one example of policy changes made to prevent the continuing decline in water quality. Table 1. Minimum DO RequirementsError! ORGANISM MINIMUM DO (mg/l) Trout/Smallmouth bass 6.5 Caddisfly & mayfly larvae 4.0 Catfish 2.5 Carp 2.0 Mosquito larvae 1.0 PURPOSE The purpose of this experiment is to determine the level of dissolved oxygen in various water samples and to determine the percent saturation of oxygen in these samples. EQUIPMENT/MATERIALS LabPro Small plastic or paper cup (optional) Graphing Calculator with DataMate Tissues or paper towels Vernier Dissolved Oxygen Probe Wash bottle with distilled water 250-mL beaker Sodium Sulfite Calibration Solution Disposable pipet DO Electrode Filling Solution 100% calibration bottle Barometer SAFETY Always wear an apron and goggles in the lab. DO1.CALC-2

3 PROCEDURE ADVANCED PREPARATION 1. Prepare the Dissolved Oxygen Probe for use. a. Unscrew the membrane cap from the tip of the probe. b. Using a disposable pipet, fill the membrane cap with 1 ml of DO Electrode Filling Solution. c. Carefully thread the membrane cap back onto the electrode. d. Place the probe into a container of distilled water. Remove membrane cap Add electrode filling solution Replace membrane cap 2. Plug the Dissolved Oxygen Probe into Channel 1 of the LabPro. Use the link cable to connect the TI Graphing Calculator to the interface. Firmly press in the cable ends. 3. Turn on the calculator, press APPS and start the DATAMATE program by entering the number that appears next to Datamate. 4. With the probe still in the water, wait 10 minutes while the probe warms up. The probe must stay connected to the interface at all times to keep it warmed up. If disconnected for a period longer than a few minutes, it will be necessary to warm it up again. 5. Select 1:SETUP from the main screen. 6. Set up the calibration for the Dissolved Oxygen Probe. If your instructor directs you to manually enter the calibration values, select 2:CALIBRATE, then 3:MANUAL ENTRY. Enter the slope and intercept values, select 1:OK, then 1:OK then proceed to Collection and Storage of Samples. If your instructor directs you to perform a new calibration, follow this procedure. Zero-Oxygen Calibration Point a. Select 2:CALIBRATE, then 2:CALIBRATE NOW. Insert probe at an angle Submerge probe tip 1-2 cm DO1.CALC-3

4 b. Remove the probe from the water and place the tip of the probe into the Sodium Sulfite Calibration Solution. Important: No air bubbles can be trapped below the tip of the probe or the probe will sense an inaccurate dissolved oxygen level. If the voltage does not rapidly decrease, tap the side of the bottle with the probe to dislodge the bubble. The readings should be in the 0.2- to 0.5-V range. c. When the voltage stabilizes (~1 minute), press ENTER. d. Enter 0 as the known value in mg/l. Saturated DO Calibration Point e. Rinse the probe with distilled water and gently blot dry. f. Unscrew the lid of the calibration bottle provided with the probe. Slide the lid and the grommet about 1/2 inch onto the probe body. g. Add water to the bottle to a depth of about 1/4 inch and screw the bottle into the cap, as shown. Important: Do not touch the membrane or get it wet during this step. h. Keep the probe in this position for about a minute. The readings should be above 2.0 V. When the voltage stabilizes, press ENTER. 1/4 water i. Enter the correct saturated dissolved-oxygen value (in mg/l) from Table 1 (for example, 8.66 ) using the current barometric pressure (see Table 2 for conversions from in Hg to mm Hg) and air temperature values. j. Record the slope and intercept from your calibration on the Data Sheet. k. Select 1:OK then 1:OK to return to the setup screen. COLLECTION AND STORAGE OF SAMPLES 1. It is important to sample as far away from the shore as is safe and under the surface of the water. In slow-moving water, it is necessary to take samples below the water s surface at various depths. 2. When collecting a sample with a cup or container, prevent mixing of the water sample and air by collecting your sample from below the water surface. 3. If you are going to take readings after returning to the laboratory, make sure that there are no air bubbles in the water-sample container and that the container is tightly stoppered. The sample should be stored in an ice chest or refrigerator until measurements are to be made. Storing water samples for later testing decreases DO1.CALC-4

5 sample accuracy and is only recommended in cases where measuring at the site is not possible. 4. When taking readings in cold (0 10 C) or warm (25 35 C) water, allow more time for the dissolved oxygen readings to stabilize. Automatic temperature compensation in the Dissolved Oxygen Probe is not instantaneous and readings may take up to 2 minutes to stabilize depending on the temperature. DATA COLLECTION 1. Rinse the tip of the probe with distilled water. 2. Place the tip of the probe into the stream at Site 1, or into a cup with sample water from the stream. Submerge the probe tip to a depth of 4-6 cm. 3. Gently stir the probe in the water sample. Note: It is important to keep stirring until you have collected your DO value. 4. When the readings stabilize (stable to the nearest 0.1 mg/l) record this value on the Data Sheet (round to the nearest 0.1 mg/l). 5. Repeat Steps 1-4 to test a second sample or a different site. PROBE STORAGE AND CARE Follow these steps when storing the dissolved oxygen probe: Long-term storage (more than 24 hours): Remove the membrane cap and rinse the inside and outside of the cap with distilled water. Shake the membrane cap dry. Also rinse and dry the exposed anode and cathode inner elements (blot dry with a lab wipe). Reinstall the membrane cap loosely onto the electrode body for storage. Do not screw it on tightly. Short-term storage (less than 24 hours): Store the Dissolved Oxygen Probe with the membrane end submerged in about 1 inch of distilled water. References Johnson, R. L.; Holman, S.; Holmquist, D.D. Water Quality with Calculators; Vernier Sovtware & Technology: Beaverton, OR, 2000; pp Johnson, R. L.; Holman, S.; Holmquist, D. D. Water Quality with Calculators; Vernier Software & Technology: Beaverton, OR, 2000; pp Starr, Cecie. Biology: Concepts and Applications. Ecosystems. Brooks/Cole-Thomson Learning, Inc. Fifth Edition p DO1.CALC-5

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7 DATA SHEET DISSOLVED OXYGEN IN WATER Name Name Period Class Date Stream: Date: Site name: Time: Site number: Date test completed: Comments: (e.g. weather, geography, vegetation along stream) Calibration date: Calibration values: slope intercept Data and Calculations Column Procedure: A. Record the dissolved oxygen reading from sensor. B. Record the water temperature from a Temperature Probe or thermometer (Test 1). C. Record the atmospheric pressure from a barometer. D. From Table 2, record the 100% dissolved oxygen value using measured temperature and atmospheric pressure. E. Percent saturation = A / D X 100 DO1.CALC-7

8 STREAM WATER (McCLURE S RUN) Column A B C D E Reading Dissolved oxygen (mg/l) Water temperature ( C) Atmospheric pressure (mmhg) 100% dissolved oxygen (mg/l) Percent saturation (%) Example 8.2 mg/l 18.4 C 760 mmhg 9.5 mg/l 86 % Average % POND WATER ( LAKE BRITTAIN) Column A B C D E Reading Dissolved oxygen (mg/l) Water temperature ( C) Atmospheric pressure (mmhg) 100% dissolved oxygen (mg/l) Percent saturation (%) Example 8.2 mg/l 18.4 C 760 mmhg 9.5 mg/l 86 % Average % OBSERVATIONS & QUESTIONS 1. Why is it important to know the temperature of the water and the barometric pressure when you determine the DO content of your water? DO1.CALC-8

9 2. Do the stream water and pond water have different DO concentrations? Which DO concentration (% saturation) is higher? 3. Consider the difference between a stream and a pond. What factors (biological or physical) do you think affected the DO concentration in these two samples? 4. Which water could support more organisms, the pond or the stream? Why? DO1.CALC-9

10 Table 2: 100% Dissolved Oxygen Capacity (mg/l) 770 mm 760 mm 750 mm 740 mm 730 mm 720 mm 710 mm 700 mm 690 mm 680 mm 670 mm 660 mm 0 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C Table 3 Barometric Pressure Conversion in Hg mm Hg in Hg mm Hg DO1.CALC-10