Determination of the amount of dissolved oxygen Essay

Subject: Determination of the sum of dissolved O in a H2O sample by iodometry-the winkler’s method.

Aim: To find the sum of dissolved O in a H2O sample by iodometry- the winkler’s method.

Apparatus: volumetric pipette. 3 conelike flask. burette. burette clinch. Pasteur pipette. reagent bottle. conelike flask stopper. retord base. white tile Materials: 2 milliliter manganese sulphate solution. 2 ml alkaline-iodine solution. 0. 025M Na thiosulphate solution. 2ml concentration sulfuric acid. amylum solution

Procedure

1. When trying H2O. attention must be taken to guarantee that a good representative sample of the H2O to be analyzed is obtained. For most intents. this includes attending to dissolved gases. Therefore. the H2O sample should be taken in a clean bottle which must be filled to overruning and tightly sealed with stopper without debut of air. If the H2O is sampled from a pat. it must be allowed to run for at least 5 proceedingss prior to trying. For this intent. you may roll up the H2O into a 1 L reagent bottle fitted with a stopper. 2. The stopper is removed carefully from the conelike flask and 2 milliliter of the manganese sulfate solution is added. dispatching the reagent from the tip of a pipette put good below the H2O surface. Stopper is replaced.

3. Similarly. 2 milliliter of the alkaline-iodide solution is introduced. 4. The stopper is placed in the bottle. be certain that no air becomes entrapped. Some flood may happen. The content is assorted exhaustively by inversion and rotary motion. Manganese hydrated oxide is precipitated and will settled on standing. 5. When the precipitate has settled. 2 milliliter of concentrated sulphuric acid is introduced with the tip of the pipette good below the surface of the solution. 6. The stopper is replaced and assorted until the precipitate dissolves wholly. The dissolved O now liberates free I from the K iodide nowadays. 7. Pipette 200 milliliter of the acidified sample into a 500 milliliter conelike flask and titrate with 0. 025 M Na thiosulphate until the solution becomes pale xanthous. 8. Add a few beads of amylum index and go on the titration to the disappearing of bluish colour. ( If the blue colour doesn’t appear after adding amylum. reiterate the titration and adding amylum before start of titration )

9. Repeat the titration twice.
Treatment of informations:
Titration figure 1 2 3 Concluding 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 O in the H2O 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 pat water=7. 5mg O2
Concentration of DO in H2O sample=7. 3mg/L

Discussion:
The Winkler trial is used to find the concentration of dissolved O in H2O samples. Dissolved O ( D. O. ) is widely used in H2O quality surveies and everyday operation of H2O renewal installations. An surplus of Mn ( II ) salt. iodide ( I– ) and hydrated oxide ( OH– ) ions is added to a H2O sample doing a white precipitate of Mn ( OH ) 2 to organize. This precipitate is so oxidized by the dissolved O in the H2O sample into a brown manganese precipitate. In the following measure. a strong acid is added to sour the solution. The brown precipitate so converts the iodide ion ( I– ) to iodine. The sum of dissolved O is straight relative to the titration of I with a thiosulfate solution. Iodemetry reacts straight. fast and quantitively with many organic and inorganic substances.

Because of its comparatively low. pH independent redox potency and reversibility of the iodide/iodine reaction. iodometry can be used both to find sum of cut downing agents and of oxidising agent. In my first titration. initial reading is 10. 00 and concluding reading is 18. 00. The titrants used is 8. 00. In my 2nd titration. initial reading is18. 00 and concluding reading is 25. 50. The titrants used is 7. 50. In my 3rd titration. my initial reading is25. 50 and my concluding reading is 18. 50. The titrant used is7. 00. Adequate dissolved O is necessary for good H2O quality.

Oxygen is a necessary component to all signifiers of life. Natural watercourse purification processes require equal O degree in order to supply for aerophilic life signifier. As dissolved O degrees in H2O bead below 5. 0mg/L. aquatic life is put under emphasis. The lower the concentration. the greater the emphasis. Oxygen degrees that remain below 1-2mg/L for a few hours can ensue in big fish putting to deaths. Total dissolved gas concentration in H2O should non transcend 110 per centum. Concentration above this degrees can be harmful to aquatic life. Decision: The concentration of DO in pat H2O is 7. 5ppm.