Hach Co.: 8229:  Dissolved Oxygen - Winkler Method

  • Summary
  • Analytes
  • Revision
  • Data and Sites
Official Method Name
 Dissolved Oxygen (DO) - Azide Modification of Winkler Method Buret Titration
Current Revision
Revised 1997
Media
 WATER
Instrumentation
 Titration
Method Subcategory
 Inorganic
Method Source
  Hach Co.
Citation
  1997 Edition of the Hach Water Analysis Handbook
Brief Method Summary
 The Azide Modification of the Winkler Method is the standard test for dissolved oxygen. It uses a buret and 0.025 N sodium thiosulfate. The standard APHA reagents in solution form also are available. In the analysis, manganous ion reacts with the dissolved oxygen present in the alkaline solution to form a manganese (IV) oxide hydroxide flocculent. Azide is added at this time to suppress interference from any nitrite present which would react with the iodide. The solution is then acidified and the manganese (IV) floc is reduced by iodide to produce free iodine as I3- in proportion to the oxygen concentration. The liberated iodine is then titrated to the starch-iodide end point. NOTE: This method also is available using a digital titrator in lieu of the buret; the digital titrator version is NOT USEPA accepted, however.
Scope and Application
 Applicable to water, wastewater and seawater.
Applicable Concentration Range
 1-10 mg/L
Interferences
 Nitrite interference is eliminated by the azide in the reagents. Other reducing or oxidizing substances may interfere. If these are present, use an alternate method, such as the High Range Dissolved Oxygen Method (colorimetric) or a dissolved oxygen electrode.
Quality Control Requirements
 Analysis of standards for accuracy check. Also, the Hach Water Handbook contains a general discussion of QC practices.
Sample Handling

Sampling and sample handling are important considerations in obtaining meaningful results. The dissolved oxygen content of the water being tested can be expected to change with depth, turbulence, temperature, sludge deposits, light, microbial action, mixing, travel time and other factors. A single dissolved oxygen test rarely reflects the accurate overall condition of a body of water. Several samples taken at different times, locations, and depths are recommended for most reliable results.
Collect samples in a clean BOD bottle as described in Step 1. If storage is necessary, run Steps 1-4 of the procedure, and store in the dark at the temperature of the water source or water seal at 10oC to 20oC. (Sealing with water is done by pouring a small amount of water into the flared lip area of a stoppered bottle. Snap a BOD bottle cap over the flared lip.) Samples preserved in this manner can be held four to eight hours. Begin with Step 6 when analyzing.
Maximum Holding Time
 Samples should be analyzed immediately. If immediate analysis is not possible, follow the sampling handling instructions in the procedure.
Relative Cost
 Less than $50
Sample Preparation Methods
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