Modified Winkler Method for Dissolved Oxygen

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Modified Winkler Method for Dissolved Oxygen Determination

Tiffany T. Lee

Department of Biological Sciences, College of Science
University of Santo Tomas, Manila, Philippines 1008

Date of Submission: February 17, 2011


In this experiment, the Winkler Method was used to measure the dissolved oxygen (d.o.) in a water sample from the pond in the Arch of the Centuries which had the owl statues. Then, the amount of oxygen is determined through a series of reaction. Usually, Winkler bottles or dissolved oxygen bottles are used in this method; but in this laboratory analysis, 50mL syringes were used instead. The significance of a Winkler Method is that it can be used to determine the health or cleanliness of a lake or stream, to know the amount and type of biomass a freshwater system can support, and to measure the amount of decomposition occurring in the lake or stream. Also, if there is sufficient dissolved oxygen, a body of water can sustain life; but if there is depletion in dissolved oxygen, this may cause major shifts in the kinds of aquatic organisms found in water bodies.


The amount of dissolved oxygen present in water or wastewater is crucial for most forms of life. Dissolved oxygen is a good indicator of water quality. Oxygen dissolves into water by means of the atmosphere and plants. The primary source of oxygen for a body of water is from microscopic algae or submerged plants. The concentration of oxygen is greatest during the daylight hours due to photosynthesis.

Also, Dissolved Oxygen is affected by temperature. As the temperature of the water goes up, the lower the concentration of dissolved oxygen gas. Simply stated, the water temperature helps determine the maximum amount of oxygen gas that water can dissolve. This dissolved oxygen concentration, in return, helps determine water's ability to support oxygen consuming creatures. Cooling a system down by 10 degrees slows down the rates of such reactions by a similar factor. During warm summer months, competition among water inhabitants for dissolved oxygen can become quite severe with rising water temperatures, bacteria and fish require.

Aerobic substances and aquatic life such as fish must have dissolved oxygen to survive. According to Hitchman, Aerobic waste water treatment processes use aerobic and facultative bacteria to break down the organic compounds found in wastewater into more stable products that will not harm the receiving waters. Wastewater treatment facilities such as lagoons or ponds, trickling filters and activated sludge plants depend on these aerobic bacteria to treat sewage. If the amount of DO present in the wastewater process becomes too low, the aerobic bacteria that normally treat the sewage will die. The process will not operate efficiently and septic conditions will occur. Also, anaerobic substances develop causing rivers and lakes to turn dark black and smell foul. The DO test is used to monitor the process to guarantee that there is enough dissolved oxygen present to keep the process from becoming diseased. The objective of this activity is to determine the primary standard used for the titrant and indicator used in each case.

Experimental Section
A. Preparing the Buret
The buret was rinsed with liquid detergent and water. It was noted that water should not form beads inside the buret. The buret was rinsed with distilled water. Then, it was rinsed with the titrant. B. Preparation of 0.1 M Na2S2O3 titrant

250mL of distilled water was boiled for at least 5 minutes. The water was then cooled and approximately 6-7 grams of Na2S2O3•5H2O was added. After which the solid was dissolved by stirring. A pinch of Na2CO3 was added. The solution was stored in an amber bottle reagent. C. Preparation of the Primary Standard

Approximately 1-2 grams of KIO3 was dried at 160ºC for an hour. This was done by placing the KIO3 in a glass which was weighed and heated in an oven without any cover....
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