This experiment is mainly used to determine dissolved oxygen in water.
Operation method
iodometry
Principle
Manganese sulfate and alkaline potassium iodide are added to the water sample to produce manganese hydroxide (Mn2+) precipitate, at this time, the nature of manganese hydroxide is extremely unstable, rapidly oxidized by the dissolved oxygen in the water into high-valent manganese (Mn4+) to produce a brown precipitate of tetravalent manganese hydroxide. After adding acid, the precipitate is dissolved, and the tetravalent manganese can oxidize iodine ions and release free iodine equivalent to the amount of dissolved oxygen. The amount of dissolved oxygen can be calculated by titrating the released iodine with sodium thiosulfate using starch as indicator. The reaction formula is as follows: MnSO4 + 2NaOH = Na2SO4 + Mn(OH)2↓2Mn(OH)2 + O2 = 2 MnO(OH)2↓ (brown precipitate) MnO(OH)2 + 2H2SO4 = MnSO4 + 3 H2OMn(SO4)2 + 2 K Ⅰ = Mn(SO4)2 + K2SO4 + I22Na2S2O3 + I2 = Na2S4O6+2NaⅠ
Materials and Instruments
Water samples Move I. Reagents Caveat 1. When collecting water samples, care should be taken not to aerate the water samples or have air bubbles remaining in the sampling bottles. Available water samples can be rinsed dissolved oxygen bottle, along the wall of the bottle directly pouring water samples or siphon method will be inserted into the bottom of the dissolved oxygen bottle, injection of water samples to the overflow of the volume of 1/3 ~ 1/2. 2. After the water sample is collected, fixative should be added to the sample immediately and stored in a dark place, while recording the water temperature and atmospheric pressure. 3. if the water sample is strongly acidic or alkaline, it can be adjusted to neutral with sodium hydroxide or sulfuric acid solution and then measured. 4. in the absence of interference, this method is applicable to a variety of water samples with dissolved oxygen concentrations greater than 0.2 mg/L and less than twice the saturation level of oxygen (about 20 mg/L). When the water contains nitrite, iron ions, free chlorine, may interfere with the determination, then the iodine correction method should be used. The specific method is to add manganese sulfate and alkaline potassium iodide solution to fix the water sample, add NaN3solution, or alkaline potassium iodide - sodium azide solution added to the water sample, Fe3+When Fe 3+ is high, add KF complexing mask. 5. Sodium azide is a highly toxic, explosive reagent, alkaline potassium iodide-sodium azide solution can not be directly acidified, otherwise toxic azide acid mist may be produced. 6. When titrating, the indicator should not be added too early, otherwise it will cause experimental error and make the result low. Common Problems I. Introduction to dissolved oxygen The molecular oxygen dissolved in water is called Dissolved oxygen (DO). The content of dissolved oxygen in water is related to factors such as atmospheric pressure, water temperature and salinity. Decrease in atmospheric pressure, increase in water temperature, and increase in salinity will all lead to a decrease in DO content. Clean surface water is near saturation with dissolved oxygen. When there is a large number of algal blooms, dissolved oxygen may be oversaturated; when the water body is contaminated by organic matter, inorganic reducing substances, it will reduce the dissolved oxygen content, or even tend to zero, when the anaerobic bacterial reproduction is active, water quality deterioration. For more product details, please visit Aladdin Scientific website.
Manganese sulfate Potassium iodide Sodium hydroxide Sulfuric acid Starch Potassium dichromate Sodium thiosulfate Sodium carbonate
Iodine measuring flask Burette Pipette
1. Manganese sulphate solution: weigh 480 g of manganese sulphate ( MnSO4-4H2O ) or 364 g of MnSO4-H2O and dissolve it in water, then add it to acidified potassium iodide solution, and it should not produce blue color in the presence of starch.
2. alkaline potassium iodide solution: weigh 500 g of sodium hydroxide dissolved in 300 ~ 400 mL of water, another 150 g of potassium iodide or 135 g Na Ⅰ dissolved in 200 mL of water, until the sodium hydroxide solution cooled down, the two solutions will be combined, mixing, and water to 1,000 mL of water. if the precipitation is left overnight, pour out the supernatant night, stored in brown bottles, tightly corked with a rubber stopper, and stored away from light. Store in a brown bottle, cork tightly with rubber stopper and keep away from light. After acidification, this solution should not turn blue when encountering starch.
3. (1+5) Sulfuric acid solution.
4. Concentrated sulfuric acid.
5. 1% starch solution: weigh 1 g of soluble starch, make a paste with a small amount of water, and then dilute it with freshly boiled water to 100 mL. After cooling, add 0.1 g of salicylic acid or 0.4 g of zinc chloride for antiseptic purposes.
6. Potassium dichromate standard solution C ( 1/6K2Cr2O7 ) = 0.0250 mol/L: weigh 1.2258 g of potassium dichromate dried at 105~110°C for 2 h and cooled, dissolve in water, and condense to 1 000 mL.
7. Sodium thiosulfate solution (about 0.01 mol/L): weigh 3.2 g of sodium thiosulfate ( NaS2SO3-5H2O ) dissolved in boiled and cooled water, add 0.2 g of anhydrous sodium carbonate, dilute with water to 1,000 mL and store in a brown bottle. Before use, calibrate with 0.0250 mol/l potassium dichromate standard solution (reagent 6). Calibration of sodium thiosulfate standard solution: In 250 mL iodine measuring flask, add 100 mL of water and 1 g of potassium iodide, add 10.00 mL of 0.0250 mol/l potassium dichromate standard solution (reagent 6), 5 mL of sulfuric acid solution, tightly stoppered, shaken, placed in a dark place for 5 min, and then titrate the solution with sodium thiosulfate standard solution (reagent 7) until the solution is light yellow. Add 1 mL of starch solution (reagent 5), continue titration until the blue color just faded, record the amount and calculate the concentration of sodium thiosulfate solution according to the following formula.
C( NaS2O3 ) = 10.00 x 0.0250/VWhere: V - volume of sodium thiosulfate solution consumed during titration, mL;8 - molar mass of oxygen (1/2 O2 ), g/mol;C - concentration of sodium thiosulfate solution, mol/l.
II. Experimental steps
1. Fixation of dissolved oxygen
Insert a pipette under the liquid level of the dissolved oxygen bottle, add 1 mL of manganese sulfate solution (reagent 1), 2 mL of alkaline potassium iodide solution (reagent 2), cap the bottle, mix it upside down several times, let it stand, and wait until the brown precipitate drops to half of the bottle, then mix it upside down once more, and wait for the precipitate to drop to the bottom of the bottle. It is usually fixed at the sampling site.
2. Sediment dissolution
Gently open the stopper of the bottle, immediately insert a pipette into the dissolved oxygen bottle under the liquid level and add 2.0 mL of concentrated sulfuric acid, carefully cover the stopper, mix upside down and shake well until all the precipitates are dissolved, and place in a dark place for 5 min.
3. Titration
Pipette the above solution into a 250 mL conical flask, titrate with calibrated sodium thiosulfate solution until the solution turns light yellow, add 1 mL of starch solution (reagent 5), continue titration until the blue color just fades, and record the amount of sodium thiosulfate solution.
III. Calculation of results
Dissolved oxygen ( O2, mg/l) = C x V x 8 x 1 000/100Formula:C - Concentration of sodium thiosulfate standard solution, mol/l;8 - molar mass of oxygen (1/2 O2), g/mol;V - volume of sodium thiosulfate solution consumed in the titration, mL.