Topic : Determination of the amount of dissolved oxygen in a water sample by iodometry-the winkler’s method.
Objective: To determine the amount of dissolved oxygen in a water sample by iodometry- the winkler’s method.
Apparatus: volumetric pipette, 3 conical flask, burette, burette clamp, Pasteur pipette, reagent bottle, conical flask stopper, retord stand, white tile Materials: 2 ml manganese sulphate solution, 2 ml alkaline-iodine solution, 0.025M sodium thiosulphate solution, 2ml concentration sulphuric acid, starch solution
1. When sampling water, care must be taken to ensure that a good representative sample of the water to be analyzed is obtained. For most purposes, this includes attention to dissolved gases. Therefore, the water sample should be taken in a clean bottle which must be filled to overflowing and tightly sealed with stopper without introduction of air. If the water is sampled from a tap, it must be allowed to run for at least 5 minutes prior to sampling. For this purpose, you may collect the water into a 1 L reagent bottle fitted with a stopper. 2. The stopper is removed carefully from the conical flask and 2 mL of the manganese sulfate solution is added, discharging the reagent from the tip of a pipette put well below the water surface. Stopper is replaced.
3. Similarly, 2 mL of the alkaline-iodide solution is introduced. 4. The stopper is placed in the bottle, be sure that no air becomes entrapped. Some overflow may occur. The content is mixed thoroughly by inversion and rotation. Manganese hydroxide is precipitated and will settled on standing. 5. When the precipitate has settled, 2 mL of concentrated sulfuric acid is introduced with the tip of the pipette well below the surface of the solution. 6. The stopper is replaced and mixed until the precipitate dissolves completely. The dissolved oxygen now liberates free iodine from the potassium iodide present. 7. Pipette 200 mL of the acidified sample into a 500 mL conical flask and titrate with 0.025 M sodium thiosulphate until the solution becomes pale yellow. 8. Add a few drops of starch indicator and continue the titration to the disappearance of blue color. (If the blue color doesn’t appear after adding starch, repeat the titration and adding starch before start of titration)
9. Repeat the titration twice.
Treatment of data:
Titration number 1 2 3 Final volume (cm3) 18.0025.5018.50
Initial volume (cm3) 10.0018.0025.50
Volume used (cm3) 8.007.507.00
Average volume of titrant required for titration = 7.50 cm3 Calculation
Concentration (ppm) of dissolved oxygen in the water sample. Mol of S2O3 2- used=0.0001875mol
Mol of S2O3 2- =0.000046875mol
Mass of O2=[0.000046875×2(15.999)]=1.4999mg
1000ml of tap water=7.5mg O2
Concentration of DO in water sample=7.3mg/L
The Winkler test is used to determine the concentration of dissolved oxygen in water samples. Dissolved oxygen (D.O.) is widely used in water quality studies and routine operation of water reclamation facilities. An excess of manganese (II) salt, iodide (I–) and hydroxide (OH–) ions is added to a water sample causing a white precipitate of Mn(OH)2 to form. This precipitate is then oxidized by the dissolved oxygen in the water sample into a brown manganese precipitate. In the next step, a strong acid is added to acidify the solution. The brown precipitate then converts the iodide ion (I–) to iodine. The amount of dissolved oxygen is directly proportional to the titration of iodine with a thiosulfate solution. Iodemetry reacts directly, fast and quantitively with many organic and inorganic substances.
Because of its relatively low, pH independent redox potential and reversibility of the iodide/iodine reaction, iodometry can be used both to determine amount of reducing agents and of oxidizing agent. In my first titration, initial reading is 10.00 and final reading is 18.00. The titrants used is 8.00. In my second titration, initial reading is18.00 and final reading is 25.50. The titrants used is 7.50. In my third titration, my initial reading is25.50 and my final reading is 18.50. The titrant used is7.00. Adequate dissolved oxygen is necessary for good water quality.
Oxygen is a necessary element to all forms of life. Natural stream purification processes require adequate oxygen level in order to provide for aerobic life form. As dissolved oxygen levels in water drop below 5.0mg/L, aquatic life is put under stress. The lower the concentration, the greater the stress. Oxygen levels that remain below 1-2mg/L for a few hours can result in large fish kills. Total dissolved gas concentration in water should not exceed 110 percent. Concentration above this levels can be harmful to aquatic life. Conclusion: The concentration of DO in tap water is 7.5ppm.
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