Materials Included In Kit

Additional Materials Required

Prelab Preparation

Sodium Thiosulfate, 0.0025 M: Dissolve 0.62 g of reagent sodium thiosulfate pentahydrate (Na₂S₂O₃•5H₂O) in approximately 500 mL of distilled or deionized water in a 1-L volumetric flask. Dilute to the 1-L mark with additional water. Prepare fresh within one week of use. The solution may be standardized by titration with potassium iodide/potassium iodate.

Starch Indicator Solution, 5%: Combine 5 grams of soluble starch with a few milliliters of distilled or deionized water and mix to a uniform paste. Add boiling water up to 100 mL. Cool to room temperature and refrigerate until ready to use. For best results, prepare fresh within one week of use.

Temperature baths: Use hot water and ice to set up various temperature baths for the students.

Safety Precautions

Sulfuric acid is extremely corrosive to eyes, skin and other tissue. Winkler solution #2 contains sodium hydroxide and potassium iodide—it is a concentrated base solution and is caustic and severely corrosive. Concentrated sodium hydroxide solutions are especially dangerous to the eyes. Keep sodium carbonate and citric acid on hand to clean up acid and base spills, respectively. Wear goggles or safety glasses whenever working with chemicals, heat or glassware in the laboratory. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Tested samples and any unused sodium thiosulfate or starch indicator solution may be rinsed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b. Winkler’s solution #1 contains manganese sulfate and should be disposed of as heavy metal waste according to Flinn Suggested Disposal Method #27f. Winkler’s solution #2 contains concentrated sodium hydroxide solution and may be neutralized for disposal according to Flinn Suggested Disposal Method #10.

Lab Hints

• In general, one 50-minute lab period should be scheduled for preliminary discussion and baseline analysis of distilled water at room temperature (to familiarize students with the Winkler titration method). Students can then use the time before the next scheduled lab period to plan the independent project, the effect of temperature on the amount of dissolved oxygen in water. Preparation and analysis of test solutions can then be completed in a subsequent 50-minute lab period.
• The solubility of oxygen in water decreases as the temperature increases (this is true for all gases). The following table gives literature values for the maximum amount of oxygen in water as a function of temperature at standard atmospheric pressure. (The reference values are for water containing <0.1 g l of sodium chloride).
  {11942_Hints_Table_1}
• The concentration of dissolved oxygen depends on pressure—the solubility of a gas increases as the partial pressure of the gas above the liquid increases. For most areas of the country, the pressure correction for the dissolved oxygen concentration will be small (±3%). To find the 100% saturation level at a pressure other than standard atmospheric pressure (1 atm or 760 mm Hg), multiply the literature value by the ratio of the barometric pressure to standard atmospheric pressure. Example: The solubility of oxygen in water at 20 °C and 740 mm Hg pressure is 8.9 ppm (9.1 ppm x 740/760).
• The small-scale titration procedure recommended in this activity gives accurate results (±3%) and is faster and easier to perform than a traditional titration. (In this activity, the water sample analyzed is 20 mL and the sodium thiosulfate concentration is 0.0025 M.) for larger water volumes (e.g., for field studies), multiply both the water volume and the sodium thiosulfate concentration by the same factor to keep the milliliters sodium thiosulfate added equal to parts per million of dissolved oxygen. Example: Use 0.025 M sodium thiosulfate solution to analyze 200 ml of water.
• For a sample at 15 °c or colder, have students take only a 10-ml aliquot in step 9. they will then multiply the mL of sodium thiosulfate titrant by two to get the ppm of dissolved oxygen.

Teacher Tips

• The manganese–oxygen complex produced in step 1 of the Winkler procedure is variously described as MnO(OH)₂, MnO₂ or even Mn(OH)₃. The stoichiometry of the reaction requires that manganese is oxidized to the +4 oxidation state.

Answers to Prelab Questions

1. What is the general relationship between dissolved oxygen levels and water quality?
   In general, the amount of dissolved oxygen in water is highest when the water is clean and unpolluted. As the water quality declines, the DO level also decreases.
2. Consider the graph in Figure 1: What would be the range of dissolved oxygen values in ppm for water that is 90–110 percent saturated at 15 °C?
   The dissolved oxygen concentration will vary between 9.3 ppm (90% saturated) and 11.4 ppm (110% saturated) at 15 °C (59 °F).
3. Determine the overall mole ratio for the reaction of oxygen with sodium thiosulfate in the Winkler titration: How many moles of sodium thiosulfate must be added in the titration for every mole of oxygen in the water sample?
   
   {11942_Answers_Equation_4}
4. Describe the criteria for a “fair test” procedure to study the effect of a single variable (e.g., nitrate levels), on the amount of dissolved oxygen in water.
   In a fair test, all of the variables or conditions in the experiment, except for the dependent variable, should be held constant (controlled). At least two different values or levels of the dependent variable should be investigated, in addition to a control or baseline experiment. To test the effect of nitrate levels on dissolved oxygen, for example, at least two concentrations, such as 10 and 30 ppm nitrates, would be tested in addition to distilled water.

Sample Data

Answers to Questions

1. Graph or analyze the class results for the effect of different variables, such as temperature or sodium chloride concentration, on the amount of dissolved oxygen in water.
   The following graph shows literature data for the dissolved oxygen concentration as a function of temperature
   
   {11942_Answers_Figure_2}
2. Propose a simple explanation based on physical principles for why the solubility of oxygen and other gases in water decreases as the temperature is raised.
   Note: Accept all reasonable explanations—this is not an easy question! Most textbook explanations for the effect of temperature on the solubility of gases focus on the enthalpy of solution for oxygen in water and Le Chatelier’s principle. Dissolving oxygen in water is an exothermic process. According to Le Chatelier’s principle, the solubility equilibrium for an exothermic reaction shifts back to reactants when the temperature increases. From a thermodynamic viewpoint, however, it makes sense to consider both the enthalpy and entropy contributions to free energy (and hence the equilibrium constant). The entropy change for oxygen dissolving in water is negative. As the temperature increases, this process becomes less favored.
   
   ΔG= – RTlnK         ΔG = ΔH – TΔS
   
   As T increases, –TΔS and ΔG both become more positive, so the value of the equilibrium constant K decreases.
3. Explain why the amount of dissolved oxygen in a lake or stream increases during the day and decreases at night.
   Oxygen is produced during the daylight hours as a result of photosynthesis by blue-green algae. At night, photosynthesis stops, but the respiration (metabolism) of all organisms continues. Respiration requires oxygen and thus depletes the oxygen dissolved in water until it can be replenished. 
4. How does the amount of oxygen dissolved in water (9–0 mg/L) compare with the amount of oxygen dissolved in air? Given that air contains 21 mole percent oxygen, calculate the concentration of oxygen (milligrams of oxygen per liter of air) at STP, assuming the ideal gas volume.
   At STP, one mole of air occupies an ideal gas volume of 22.4 L. There are 0.21 moles of oxygen, therefore, per 22.4 L of air.
   
   {11942_Answers_Equation_5}

References

This kit was adapted from Flinn ChemTopic™ Labs, Chemistry in the Environment, Volume 22; Cesa, I., Editor; Flinn Scientific, Inc.: Batavia, IL (2006).

Introduction

Without a critical supply of oxygen gas dissolved in water, fish and other aquatic organisms would drown. The amount of dissolved oxygen in water is one of the most important indicators of water quality and environmental health. How does the environmental factor of temperature affect the level of dissolved oxygen?

Concepts

• Dissolved oxygen
• Winkler titration
• Biological oxygen demand
• Water quality

Background

Dissolved oxygen (DO) is the amount of gaseous oxygen, O₂, dissolved in a body of water. The most common units for measuring DO levels are milligrams of oxygen per liter of water (mg/L), or parts per million (ppm) O₂, where 1 ppm = 1 mg/L. Oxygen enters into the water by aeration, diffusion from air and as a byproduct of photosynthesis. In general, high flow rates or water turbulence will increase oxygen levels in water due to aeration. Slow moving or stagnant water usually has very low oxygen levels. Oxygen levels also change throughout the day as a result of photosynthesis, usually peaking in late afternoon. The amount of oxygen that will dissolve in water depends on temperature and pressure and is very sensitive to environmental conditions.

Dissolved oxygen is inversely related to temperature—as the water temperature increases, the amount of oxygen that can dissolve decreases. In the summer, extremely warm water temperatures may result in very low dissolved oxygen. Typical environmental factors that affect DO levels include the amount of organic matter or waste from decaying vegetation, the presence of nitrates, phosphates and other nutrients, and the concentration of electrolytes, such as Na⁺, Ca²⁺, Mg , Cl^– and HCO₃^– ions. Dissolved oxygen is one of the most important indicators of the overall health of a body of water. When water contains a large amount of oxygen, the quality of the water is generally very good. Water with consistently low dissolved oxygen levels (<3–4 ppm) is extremely stressful to aquatic organisms and may harbor only a few species adapted to such conditions. DO levels less than 2 ppm will not support fish life.

The relationship between the amount of dissolved oxygen in water and water quality is usually expressed in terms of “percent saturation.” Percent saturation refers to how close the water is to holding its maximum amount at a given temperature. Rivers that have oxygen saturation levels between 90% and 110% are considered healthy. Water less than 90% saturated may contain large amounts of oxygen-demanding organic material. Water with over 110% saturation can result from excessive turbulence. The graph shown in Figure 1 is used to determine percent oxygen saturation based on the concentration of dissolved oxygen in ppm at a specific temperature. Example: Water containing 9.0 ppm DO at a temperature of 12 °C is about 80% saturated.

The amount of dissolved oxygen in water can be determined using a standard wet-chemical technique called the Winkler method, or by means of special dissolved oxygen sensors or meters. (The Winkler method is commonly used to calibrate or standardize dissolved oxygen sensors.) There are three basic steps in the Winkler procedure (Equations 1–3):

1. Manganese sulfate and a basic potassium iodide solution are added to convert the dissolved oxygen to an insoluble manganese–oxygen complex. This step “fixes” the dissolved oxygen and prevents the oxygen from being consumed by or reacting with other substances. Both the manganese sulfate and iodide solutions are added in excess to ensure that all of the oxygen has been sequestered. These solutions should be added as soon as possible (but no later than 24 hours) after a water sample has been collected in the field.
2. Concentrated sulfuric acid is added to dissolve the manganese–oxygen complex, which then reacts with iodide ions to generate iodine.
3. The iodine released in this reaction is titrated using a standard sodium thiosulfate solution with starch indicator (to make the endpoint more visible).

When 20 mL of the “fixed” water is titrated with 0.0025 M sodium thiosulfate solution, the volume in mL of sodium thiosulfate added is exactly equal to the amount of dissolved oxygen in ppm.

Experiment Overview

The purpose of this cooperative class project is to investigate the effect of temperature on the amount of dissolved oxygen in water. Dissolved oxygen levels will be measured in the laboratory using the Winkler method. Different student groups will analyze water at different temperatures and results will be compared.

Materials

Prelab Questions

1. What is the general relationship between dissolved oxygen levels and water quality?
2. Consider the graph in Figure 1: What would be the range of dissolved oxygen values in ppm for water that is 90–110 percent saturated at 15 °C?
3. Determine the overall mole ratio for the reaction of oxygen with sodium thiosulfate in the Winkler titration. How many moles of sodium thiosulfate must be added in the titration for every mole of oxygen in the water sample?

Safety Precautions

Sulfuric acid is extremely corrosive to eyes, skin and other tissue. Winkler solution #2 contains sodium hydroxide and potassium iodide—it is a concentrated base solution and is caustic and severely corrosive. Concentrated sodium hydroxide solutions are especially dangerous to the eyes. Notify the teacher immediately in case of an acid or base spill. Wear chemical splash goggles, chemical-resistant apron and chemical-resistant gloves. Avoid contact of all chemicals with eyes and skin and wash hands thoroughly with soap and water before leaving the lab. Please follow all laboratory safety guidelines.

Procedure

1. Add about 100 mL of distilled water to a 250-mL Erlenmeyer flask or plastic bottle. Stopper the flask and vigorously shake the flask 10–15 times to saturate the water with dissolved oxygen. Measure and record the temperature of the water.
2. Pour the water from the flask into a large (20 x 150 mm) test tube. Fill the test tube completely, all the way to the top, until the water just begins to overflow, and then immediately stopper the test tube. (Some water will be displaced. There should be no air bubbles trapped in the test tube.)
3. Remove the stopper and quickly add 6 drops of Winkler solution #1 (manganese sulfate solution) directly to the water in the test tube. Use a Beral-type pipet or a dropper bottle to add the solution, and hold the pipet or dropper as close to the water surface as possible.
4. Wearing gloves, use a clean Beral-type pipet to carefully add 6 drops of Winkler solution #2 in the same fashion. Caution: Winkler solution #2 is a concentrated base solution and is caustic and corrosive.
5. Stopper the test tube (some liquid will overflow), and carefully invert the stoppered test tube several times to mix the contents—a brown precipitate will quickly form. (This step fixes, or sequesters, the dissolved oxygen in the water. Fixed samples can be held up to 48 hours before they are analyzed by titration.)
6. Allow the brown precipitate formed in step 5 to settle to at least one-half the volume of the test tube (approximately 10–15 minutes).
7. Wearing gloves, carefully add 6–7 drops of concentrated sulfuric acid using a disposable glass pipet. Caution: Concentrated sulfuric acid is extremely corrosive. Clean up all spills immediately and notify the teacher.
8. Replace the stopper in the test tube and invert several times to mix. The acid should cause the precipitate to dissolve, giving a clear amber (yellow–gold) solution.
9. Using a 25-mL graduated cylinder, measure and pour (decant) 20.0 mL of treated water from the test tube into a 125-mL Erlenmeyer flask. (Use the top solution only—do not transfer any of the precipitate with the water.)
10. Fill the 10-mL syringe with 0.0025 M sodium thiosulfate solution and measure and record the initial (starting) volume.
11. Slowly add sodium thiosulfate solution from the syringe to the treated water in the flask until the water fades to a pale straw color.
12. Add 6 drops of starch solution to the treated water in the flask and swirl to mix. The sample will turn dark blue.
13. Continue adding the sodium thiosulfate solution dropwise to the water until the blue color fades completely. (This is the colorless endpoint.) Measure and record the final volume of sodium thiosulfate in the syringe.
14. Calculate the volume of sodium thiosulfate added and record the dissolved oxygen concentration in the data table. Note: For 20.0 mL of water, the milliliters (mL) of sodium thiosulfate added equals the dissolved oxygen concentration in parts per million (ppm).
15. Using the same general procedure, analyze the amount of dissolved oxygen in water at a second temperature (e.g., 30 °C). Different student groups should choose different temperatures between 5 and 45 °C to get a nice range for comparison. Note: For samples at temperatures of 15 °C or less, use only 10.0 mL of treated water in step 9.

Student Worksheet PDF

11942_Student1.pdf