The Winkler method is a widely used analytical technique to measure the concentration of dissolved oxygen in water. This method involves several steps, including the addition of reagents that react with oxygen, leading to the formation of a colored compound that can be quantified using spectrophotometry. The Winkler method is sensitive to changes in temperature, and as such, it can be used to investigate the effects of temperature on oxygen concentration in water.
Global warming and increased sea temperatures have significant impacts on the environment, including changes in oxygen concentration in aquatic ecosystems. As the temperature of water increases, the solubility of oxygen in water decreases, leading to a decrease in the concentration of dissolved oxygen. This decrease in oxygen concentration can have severe consequences for aquatic organisms, including fish, invertebrates, and plants.
One of the primary impacts of decreasing oxygen concentration is the development of hypoxic or anoxic conditions, where there is insufficient oxygen to support the life of aquatic organisms. This can lead to the death of fish and other aquatic organisms, and it can also affect the growth and reproduction of plants and other organisms. In addition, low oxygen concentration can also affect the decomposition of organic matter, leading to the accumulation of nutrients and the development of harmful algal blooms.
Symbol equation for oxygen saturation in water:
O2(g) ⇌ O2(aq)
Symbol equation for the reaction of dissolved oxygen with manganese(II) sulfate:
2MnSO4(aq) + O2(aq) + 2NaOH(aq) → 2Mn(OH)2(s) + 2Na2SO4(aq)
Symbol equation for the reaction of manganese(II) hydroxide with iodine:
2Mn(OH)2(s) + 4I-(aq) → 2MnO(OH)(s) + I2(aq) + 2H2O(l)
The Winkler method involves several steps, including the addition of reagents that react with oxygen in water. In the first step, a water sample is collected and treated with manganese(II) sulfate and sodium hydroxide to react with dissolved oxygen, forming manganese(II) hydroxide. In the second step, iodine is added to the solution, reacting with manganese(II) hydroxide to form manganese(IV) oxide, which is a brown-colored solid. The amount of manganese(IV) oxide formed is proportional to the concentration of dissolved oxygen in the water sample and can be quantified using spectrophotometry.
Materials:
Water sample
Alkaline iodide-azide reagent
Sulfuric acid solution
Sodium thiosulfate solution
Starch solution
Potassium iodate (KIO3) standard solution
Potassium iodide (KI) solution
Equipment:
BOD bottle or similar glass bottle with stopper
Erlenmeyer flask
Burette
Pipette
Volumetric flask
Magnetic stirrer or stirring rod
pH meter
Graduated cylinder
Weighing scale
Method:
Prepare the alkaline iodide-azide reagent by dissolving 50 g of potassium hydroxide (KOH) and 10 g of sodium azide (NaN3) in 1 liter of distilled water. Then, add 100 g of potassium iodide (KI) and mix until dissolved. Store in a brown glass bottle.
Prepare the sulfuric acid solution by diluting concentrated sulfuric acid (H2SO4) to 1 mol dm-3 with distilled water. Store in a brown glass bottle.
Prepare the sodium thiosulfate solution by dissolving 25 g of sodium thiosulfate pentahydrate (Na2S2O3·5H2O) in 1 liter of distilled water. Store in a brown glass bottle.
Prepare the starch solution by dissolving 1 g of starch in 100 ml of distilled water. Store in a brown glass bottle.
Prepare the potassium iodate (KIO3) standard solution by dissolving 1.2042 g of KIO3 in 1 liter of distilled water. This solution should be standardized against the sodium thiosulfate solution before use.
Fill a BOD bottle or similar glass bottle with the water sample to be tested. Add 1 ml of alkaline iodide-azide reagent and mix thoroughly.
Allow the bottle to stand for at least 5 minutes to ensure complete reaction. The alkaline iodide-azide reagent reacts with dissolved oxygen in the water sample to produce iodide ions (I–) according to the following reaction:
O2 + 2I– + 2OH– → 2H2O + I2
Add 1 ml of sulfuric acid solution to the bottle and mix thoroughly. This will convert the iodide ions to molecular iodine (I2), which will react with the sodium thiosulfate solution in the titration step.
Titrate the iodine with the sodium thiosulfate solution until the yellow color of iodine disappears. Add a few drops of starch solution as an indicator to help identify the endpoint of the titration, which is the point where all iodine has been consumed.
Record the volume of sodium thiosulfate solution used in the titration. Repeat steps 6-9 at different water temperatures, ranging from 5°C to 50°C.
Standardize the sodium thiosulfate solution against the KIO3 standard solution before use, using the following equation:
KIO3 + 5KI + 6H2SO4 → 3I2 + 3K2SO4 + 6H2O
Calculate the molarity of the sodium thiosulfate solution using the following equation:
Molarity of Na2S2O3 = (Molarity of KIO3 x Volume of KIO3 x 6) / (Volume of Na2S2O3 x 5)
Calculate the concentration of dissolved oxygen in each water sample
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