Dissolved Oxygen (DO)

Reference:

Standard Methods; 4500-O A, 4500-O B, 4500-O C, 4500-O G

Scope and Application

The concentration of Dissolved Oxygen (DO) in natural water and wastewater is a function of the temperature of the air and water, the degree of hardness of the water, and the demand for oxygen in the body of water. The solubility of oxygen increases with decreasing water temperature (oxygen solubility in water is inversely proportional to temperature). DO is an important parameter in wastewater treatment processes, most notably when dealing with an activated sludge system.

There are two methods for measuring DO: an electrometric method using a Dissolved Oxygen meter and an iodometric method commonly known as the Winkler titration. The most common method used employs the Dissolved Oxygen meter which compensates for pressure and temperature variations in water, and is the preferred method for measuring DO in polluted and highly colored waters.

The use and calibration of a dissolved oxygen electrode will vary from manufacturer to manufacturer. Refer to the manufacturer’s instruction manual for the specific procedures to use. The rest of this section on the measurement of dissolved oxygen will focus on the analysis by titration.

The Winkler titration (we will discuss the sodium azide modification of this method in this manual) is useful in a number of DO measurements;

  1. To determine the initial or final DO in the Biochemical Oxygen Demand (BOD5) test;
  2. to calibrate or verify readings obtained with a DO meter; and
  3. to make a direct determination of DO on a water sample.

Apparatus

 

Reagents

The first two solutions can be prepared in house. The laboratory practices committee recommends that buffer solutions be purchased due to ease of use and being relatively inexpensive compared to the time, effort, and chemicals used to prepare them in house. One supplier also provides prepackaged reagents that can be added directly to the sample to be analyzed.

  1. Manganous sulfate – Dissolve 480 g MnSO4•4H2O, 400 g MnSO4•2H2O, or 364 g MnSO4•H2O in distilled or deionized water. Filter the solution and dilute to 1 liter.
  2. Alkali-iodide-azide – Place 500 g sodium hydroxide (NaOH) in 500 mL distilled water; in a separate container dissolve 150 g potassium iodide (KI) in 200 mL distilled water. Allow both solutions to cool. Carefully combine these two chemicals in a Pyrex® or other heat resistant container. Add the two slowly, stirring constantly (this can be best accomplished using a magnetic stirbar and stirplate). Heat will be created. Avoid breathing the fumes or body contact with the solution. Allow the solution to cool to room temperature. To this add 10 g sodium azide (NaN3) that has been dissolved in 40 mL distilled water. The sodium azide is poisonous, handle it carefully. Dilute to 1 liter. This solution will decompose in time and should be replaced after three months.
  3. Sulfuric acid – concentrated
  4. Starch solution – Dissolve 2 g of soluble starch in 100 mL of hot distilled water. To preserve the solution, add 0.2 g salicylic acid.
  5. Sodium thiosulfate (0.025 M) – Place 6.205 g sodium thiosulfate (Na2S2O3•5H2O) to distilled water. Add 0.4 g NaOH (solid). Dilute to 1 liter. The laboratory practices committee recommends that this solution be purchased. This will alleviate the need to purchase or prepare the potassium bi-iodate and potassium iodide listed below and remove the requirement to standardize the sodium thiosulfate.
  6. Potassium bi-iodate (0.0021 M) – Dissolve 812.4 mg potassium bi-iodate, KH(IO3)2 in distilled water. Dilute to 1 liter. The potassium bi-iodate should have been dried in an oven at 103 –105°C and then cooled in a desiccator before being weighed to prepare the solution.
  7. Potassium iodide – free from iodate

Storage / Preservation

When used for the direct determination of DO, the samples measured must be analyzed immediately. There is no method available for preservation of DO samples. Therefore, all samples are grab samples. The DO level of the sample can be fixed (by performing the "Preparation of Sample" step) to allow time to transport to a laboratory setting for completion of the analysis. The sample should be kept cool and protected from sunlight until the analysis can be completed.

 

Procedure

Standardization of Sodium Thiosulfate

  1. In a 500 mL wide-mouth Erlenmeyer flask place 2 g potassium iodide in 100-150 mL distilled water. Swirl to dissolve.
  2. Add 2 drops of concentrated sulfuric acid and 20.0 mL of standard potassium bi-iodate solution.
  3. Record the initial volume of sodium thiosulfate in the burette.
  4. Titrate the potassium iodide solution with sodium thiosulfate to a straw yellow color.
  5. Add several drops of starch, a blue / purple color will develop. Titrate dropwise until the blue / purple color disappears (solution will become clear).
  6. Record the final volume of sodium thiosulfate in the burette.
  7. Calculate the volume of sodium thiosulfate used.

Note: The amount of sodium thiosulfate used should be equal to the amount of potassium bi-iodate solution titrated (20 mL). This verifies that the sodium thiosulfate is 0.025 M (± 0.001).

Preparation of Sample

  1. Fill a 300 mL BOD bottle with the sample to be analyzed.
  2. With a pipette, add 1 mL of manganous sulfate.
  3. With a pipette, add 1 mL of alkali-iodide-azide.
  4. Stopper and invert the bottle several times to mix.
  5. Allow the floc in the solution to settle to about half the volume of the bottle.
  6. Invert the bottle several times to mix the floc back into the solution.
  7. Allow the floc in the solution to settle to about half the volume of the bottle.
  8. After settling, add 1 mL concentrated sulfuric acid, stopper and gently invert several times. Continue until the precipitate has dissolved back into solution.
  9. If a brown color develops, there is dissolved oxygen in the sample. If no color develops or it is very faint, it may be appropriate to obtain another sample at this time to ensure that there has not been an error in the sample preparation.

Titration

  1. Pour 200 mL of the sample to be titrated in a 500 mL wide-mouth flask.
  2. Record the initial volume of sodium thiosulfate in the burette.
  3. Titrate with standardized sodium thiosulfate solution to a pale yellow endpoint (swirl the flask gently while titrating or use a magnetic stirrer).
  4. Add several drops of starch solution and continue titrating dropwise until the disappearance of the blue / purple coloration.
  5. Record the final volume of sodium thiosulfate in the burette.
  6. Calculate the volume of sodium thiosulfate used.

Calculations

When a 200 mL sample is used, 1 mL of sodium thiosulfate solution (0.025 M) is equivalent to 1 mg/L Dissolved Oxygen in the sample. (If 8.7 mL of sodium thiosulfate was used, then the DO of the sample is 8.7 mg/L).

 

Goals for a New Analyst Performing Dissolved Oxygen Analysis

How to prepare samples for analysis

pH / ISE meter

Dissolved oxygen electrode

How to calculate calibration value

How to calibrate

General use and care of the electrode

BOD bottles

Graduated cylinders

Burette

Reagents necessary to perform analyses

Pipettes and tips

 

Dissolved Oxygen Analysis; the Tips for Techs version

Electrometric Analysis

 

Analysis by Titration