January, 2002, Chemistry Experiment:

Effect of Sulfite Ions
on Dissolved Oxygen:
A Pollution Investigation

- Purpose
- Background Information
- Equipment
- Software Set-up
- Experimental Procedure
- Data Analysis
- Conclusions and Extensions

Dissolved Oxygen Sensor (PS-2108)

Purpose:

Students will measure the concentration of dissolved oxygen in an aqueous environment, both before and after the addition of sodium sulfite, a chemical commonly found in untreated industrial effluent. Students will analyze how sulfites and related chemical compounds affect the availability of oxygen in waterways exposed to typical pollutants.

Background Information:

Sodium sulfite is commonly used industrially, for example in processes such as paper making, dyeing, bleaching, photographic development, and engraving. If runoff from an industrial factory or plant is untreated and effluent is released to the environment, local waterways can be adversely affected. Sulfites and other chemical pollutants, including sulfates, nitrates, ammonia, and heavy metals, produce direct chemical demands on oxygen in the water due to the oxidation-reduction reactions that result. Dissolved oxygen levels lower than 3 parts per million are stressful to most aquatic organisms, and dramatic events like fish kills can result when there is excessive demand on dissolved oxygen in an ecosystem. The overall health of an ecosystem is influenced as well by such factors as pH, temperature, carbonate buffering, water movement, and various populations of organisms all competing for shared resources. Since different aquatic organisms thrive under different environmental conditions, any small change in the complex chemistry of the environment can have far-reaching effects.

Hypothesize: What kinds of redox reactions might be expected when sulfite ions encounter dissolved oxygen? Predict how the concentration of dissolved oxygen will be affected by the addition of sodium sulfite to a sample of aerated water.

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Equipment:

For each lab group:

Additional equipment:

  • 2-M sodium sulfite solution (25.2 g Na2SO3 / 100 mL)
  • large bottle or aquarium pump to aerate water

Note: To saturate deionized water with air, fill a clean container one-third full with deionized water, seal it, and shake vigorously for 10 seconds. Alternatively, bubble air through the deionized water for 15 minutes using an aquarium pump.

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Software Setup:

  1. Click on one of the links below to download a pre-configured DataStudio file for this dissolved oxygen experiment, and then open the file.

    PASPORT users: Windows (.zip file) or Macintosh (.sit file)

    ScienceWorkshop 500 users: Windows (.zip file) or Macintosh (.sit file)

    When the file is opened, you should see a graph display of Oxygen Concentration versus Time, as well as a digits display of oxygen concentration.

  2. Connect the Dissolved Oxygen Sensor to an Xplorer or USB link (PASPORT users), or plug the sensor into the 500 interface (ScienceWorkshop 500 users). If you are using the ScienceWorkshop 500 interface, be sure the sensor is associated correctly in the Experiment Setup window when you open the DataStudio file.

  3. Resize and arrange the displays as needed so that you can see them all.

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Experimental Procedure:

Sensor calibration:

  1. PASPORT users: Refer to the "Setup and Calibration" instructions and the Calibration Table on the Quick Start card for calibration procedures.

  2. ScienceWorkshop 500 interface users: Refer to the Dissolved Oxygen Sensor Instruction Manual and Experiment Guide, including Table 1 on page 28, for calibration procedures.
Data Recording:
  1. Measure 400 mL of room-temperature aerated deionized water into the 600-mL beaker.

  2. Stir gently but continuously with the Dissolved Oxygen Sensor, or use a stir plate / hot plate and clamp the probe above a magnetic stir bar. Click the Start button (  ) to begin collecting data.

  3. Monitor the dissolved oxygen concentration of the water for the meter reading to stabilize. Once the reading stabilizes record data for 30 more seconds.

  4. After 30 seconds, begin dropping 1 mL of the 2-M Na2SO3 solution into the water.

  5. Continue stirring and record data until the reaction stops, then click the Stop (  ) button.

Additional data runs: As time permits, additional data runs can be recorded using oxygenated water that is slightly colder or warmer than room temperature. Also, students can test a larger or smaller volume of Na2SO3 solution, or can vary the speed with which they add the solution to the water.

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Data Analysis:

  1. Examine the graph display to view your data, using the Scale to Fit button (  ) in the Graph toolbar to resize the axes.

  2. Determine the minimum and maximum values for the concentration of dissolved oxygen: click the Statistics button (  ) in the Graph toolbar and look for the minimum and maximum values to appear in the graph legend.

  3. Use the Slope tool (  ) to determine when during the data run the concentration of dissolved oxygen changed most quickly.

  4. If additional data runs were recorded, compare the Oxygen Concentration graphs over time for each run.

Conclusions and Extensions:

  1. What effect did the addition of the sodium sulfite solution have on the amount of dissolved oxygen in the water? Describe the changes that were observed over time.

  2. Sodium sulfite (Na2SO3) reacts with oxygen (O2) in aqueous solution to form sodium sulfate (Na2SO4). In this redox reaction, sulfite is oxidized to sulfate and oxygen is reduced. Write a complete balanced reaction for this process.

  3. Describe the possible effects of an industrial accident that results in a large release of sodium sulfite into a nearby stream. What physical evidence might suggest that these oxidation-reduction reactions took place?

  4. Because dissolved oxygen is corrosive to metals, in many industries the use of oxygen scavengers is common to prevent rusting or pitting of metals. However, when sodium sulfite is the scavenger of choice, the sodium sulfate produced forms a soft sludge that can foul heat transfer surfaces or otherwise damage equipment. At higher temperatures such as in steam generators, sodium sulfite can decompose to form sodium hydroxide. For oxygen scavenging, what alternatives to sodium sulfite are available? Research other options used industrially and report on their benefits and drawbacks.