Determination of Dissolved Oxygen In a Water Essay

INTRODUCTIONIn an alkaline antecedent, dissolved oxygen pull up stakes oxidize manganese(II) to the trivalent state. 8OH-(aq) + 4Mn2+(aq) + 2H2O(l) 4Mn(OH)3(s)The analysis is completed by titrating the iodine produced from jet iodide by manganese(III) hydroxide. 2Mn(OH)3(s) + 2I-(aq) + 6 H+(aq) 2Mn2+(aq) + I2(aq) + 6H2O(l) Sodium thiosulphate is utilise as the titrant. succeeder of the method is critically dependent upon the manner in which the sample is manipulated. At all stages, every method must be made to fasten that oxygen is neither introduced to nor lost from the sample. Furthermore, the sample must be unaffectionate of any solutes that will oxidize iodide or reduce iodine.Chemicals Manganese(II) sulphate dissolver disposed(p) by dissolving 48 g of MnSO4.4H2O in water system to v 100 cm3 solution alkaline potassium iodide solutionprepared by dissolving 15 g of KI in astir(predicate) 25 cm3 of water, adding 66 cm3 of 50% NaOH, and diluting to 100 cm3 concentrated s ulphuriv(VI) savage 0.0125 M sodium thiosulphate solution starch solution (freshly prepared).Apparatus 250 cm3 volumetric flaskful, 250 cm3 cone-shaped flask, measuring cylinders, titration apparatus, magnetic stirrerProcedure1.Use a 250 cm3 volumetric flask to collect water sample. Fill the flask completely with water without lodging any air bubbles. 2.Add 1 cm3 of manganese(II) sulphate solution to the sample utilise a pipette. Discharge the solution well below the surface (some run out will occur). 3.Similarity introduce 1 cm3 of alkaline potassium iodide solution. Be true that no air becomes entrapped. Invert the bottle to distribute the go down uniformly. Hazard precedent Care should be taken to avoid exposure to any overflow, as the solution is quite alkaline. 4.When the precipitate has settled at least 3 cm below the stopper, introduce 1 cm3 of concentrated sulphuric acid well below the surface. Replace the stopper and carefully mix until the precipitate disappear s. A magnetic stirrer is helpful here. 5.Allow the mixture to stand for 5 minutes and then pull 100cm3 of the acidified sample into a 250 cm3 conical flask. 6.Titrate with 0.0125M sodium thiosulphate until the iodine colour becomes faint. Then add 1 cm3 of starch solution and continue adding the thiosulphate solution until the blue colour disappears. 7.Record the volume of thiosulphate solution used and calculate the dissolved oxygen content in the sample in mg dm-3. Remarks1.If the water sample has a low DO value, it is recommended to withdraw 200 cm3 of the acidified sample into a 500 cm3 flask for the titration describe in step 5. 2.This experiment can be further developed into a project to study the extent of water pollution. (a)The water sample chthonic investigation is divided into two portions. One portion of the sample is immediately analysed for dissolving oxygen using the Winkler method. The other portion is stored in the blue(a) for five days. (b)Repeat the analysis w ith the water sample that has been stored in the dark for five days. The difference between the two measurements is the five-day biochemical oxygen demand (BOD5), calculated in mg dm-3.