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Winkler's Iodometric Method

Water, of all the natural substances, comes closest to being the universal chemical solvent. This remarkable property makes it one of the most important components of all living organisms. Oxygen forms one of the abiotic factors governing the physiological and ecological operations of both plants and animals. It is the aerobic respiration that has been preferred by the living organisms as against anaerobic perhaps to oxidize the calorific compounds and mobilize energy. Thus oxygen becomes an indispensable factor for bodily metabolisms. Oxygen gets diffused in aquatic bodies from the atmosphere while the strength of oxygen in the system is enriched by the photosynthetic activities of phytoplanktons and regular hydrophytes. A minimum of 4 ppm [4 parts per million] of oxygen is required in the dissolved state to maintain and keep balance of the aquatic population. However, the amount of oxygen varies in water bodies according to its geographical location and usage. To know the normal fluctuation of oxygen in the water bodies, its quantitative estimation using Winkler's iodometric method at regular intervals is found to be significant.

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Glassware - Stoppered sample bottles [300ml], Pipettes, Burette, Conical flask, Measuring jars.
Reagents -
1. 0.025N Sodium thio sulphate [Na2S2O3. 5H2O]
{Also known as hypo by photographers, is colorless and odorless. 24.82 g of Na2S2O3. 5H2O is dissolved in distilled water and made up to a volume of 1l. This solution is stabilized by the addition of a pellet of Sodium hydroxide [NaOH] or 0.4 g Borax. This forms the stock solution of 0.1N. This solution is diluted four times to obtain 0.025N solution and has to be stored in a brown glass bottle.}
2. Alkaline potassium Iodide [KI] [Potassium iodide is a deliquescent white solid prone to oxidation on exposure to air.]
{100 g of potassium Hydroxide [KOH] and 50 g Potassium Iodide [KI] dissolved in 200 ml distilled water.}
3. Manganese sulfate [MnSO4]
{It is a pale pink/ red powder, which is slightly efflorescent and soluble in cold water. 100 g MnSO4.4 H2O is dissolved in 200 ml distilled water and heated to dissolve maximum salt. The solution is filtered after cooling.}
4. Starch solution {Amylodextrin is a white, odorless and tasteless powder. 1 g of starch is dissolved in 100 ml distilled water and warmed for complete solubility.}
5. Concentrated Sulphuric acid [H2SO4] [Is a clear, colorless oily liquid with high affinity for water.]
Fill a given water sample in a 300 ml stoppered bottle taking care to avoid any kind of bubbling or trappings of air bubbles. Add 2 ml each of MnSO4 and alkaline KI well below the surface of water along the sides of the wall of the bottle. A brown precipitate is formed indicating the presence of oxygen. Shake the contents well and kept aside for a few minutes for the precipitate to settle down. Once the precipitate settles, add 2ml of concentrated H2SO4 and shake well to dissolve the precipitate. Take 50 ml of this solution in a conical flask and titrate against Na2S2O3 using a few drops of starch as indicator. At the endpoint, the initial dark blue changes to colorless [as plain water].
Frequently asked questions:
1. What type of reaction forms Mn(OH)3 ?
The process is a heterogeneous reaction. It involves the combination of a gas with a solid colloid.
2. What is the function of MnSO4 and KI?
They fix the dissolved oxygen in the sample so that external contamination is avoided during titration.