Introduction
In an alkalic solution. dissolved O will oxidise Mns ( II ) to the trivalent province. 8OH- ( aq ) + 4Mn2+ ( aq ) + 2H2O ( cubic decimeter ) –> 4Mn ( OH ) 3 ( s )
The analysis is completed by titrating the I produced from K iodide by manganese ( III ) hydrated oxide. 2Mn ( OH ) 3 ( s ) + 2I- ( aq ) + 6 H+ ( aq ) –> 2Mn2+ ( aq ) + I2 ( aq ) + 6H2O ( cubic decimeter ) Sodium thiosulphate is used as the titrant.
Success of the method is critically dependent upon the mode in which the sample is manipulated. At all phases. every method must be made to guarantee that O is neither introduced to nor lost from the sample. Furthermore. the sample must be free of any solutes that will oxidise iodide or cut down I.
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Chemicals: Manganese ( II ) sulphate solution – prepared by fade outing 48 g of MnSO4. 4H2O in H2O to five 100 cm3 solution ; alkalic K iodide solution—prepared by fade outing 15 g of KI in approximately 25 cm3 of H2O. adding 66 cm3 of 50 % NaOH. and thining to 100 cm3 ; concentrated sulphuriv ( VI ) acid ; 0. 0125 M Na thiosulphate solution ; starch solution ( newly prepared ) .
Apparatus: 250 cm3 volumetric flask. 250 cm3 conelike flask. mensurating cylinders. titration setup. magnetic scaremonger
Procedure:
1. Use a 250 cm3 volumetric flask to roll up H2O sample. Fill the flask wholly with H2O without pin downing any air bubbles. 2. Add 1 cm3 of Mn ( II ) sulphate solution to the sample utilizing a pipette. Dispatch the solution good below the surface ( some flood will happen ) . 3. Similarity introduce 1 cm3 of alkalic K iodide solution. Be certain that no air becomes entrapped. Invert the bottle to administer the precipitate uniformly. [ Hazard Warning: Care should be taken to avoid exposure to any overflow. as the solution is rather alkaline. ] 4. When the precipitate has settled at least 3 centimeter below the stopper. present 1 cm3 of concentrated sulfuric acid good below the surface. Replace the stopper and carefully blend until the precipitate disappears. A magnetic scaremonger is helpful here. 5. Let the mixture to stand for 5 proceedingss and so retreat 100cm3 of the acidified sample into a 250 cm3 conelike flask. 6. Titrate with 0. 0125M Na thiosulphate until the iodine coloring material becomes faint. Then add 1 cm3 of starch solution and go on adding the thiosulphate solution until the bluish coloring material disappears. 7. Record the volume of thiosulphate solution used and cipher the dissolved O content in the sample in mg dm-3. Remarks
1. If the H2O sample has a low DO value. it is recommended to retreat 200 cm3 of the acidified sample into a 500 cm3 flask for the titration describe in measure 5. 2. This experiment can be farther developed into a undertaking to analyze the extent of H2O pollution. ( a ) The H2O sample under probe is divided into two parts. One part of the sample is instantly analysed for fade outing O utilizing the Winkler method. The other part is stored in the dark for five yearss. ( B ) Repeat the analysis with the H2O sample that has been stored in the dark for five yearss. The difference between the two measurings is the five-day biochemical O demand ( BOD5 ) . measured in mg dm-3.
