Reaction equation of Winkler method (Summery)

Reprinted below is a description of the Winkler method written in the student experiment.

Principle of Winkler method

Add manganese chloride and sodium hydroxide solution to a certain amount of samples to precipitate manganese hydroxide.

Mn²⁺ + 2OH^‐  → Mn(OH)₂↓    (colloidal white precipitation)                         (1)

At this time, the oxygen dissolved in the water oxidizes some of the manganese hydroxide (divalent Mn) to trivalent Mn(OH)₃. (or to manganese hydroxide MnO(OH)₂. Mn(OH)₃ is used in the above equation because it is easier to perform thermodynamic calculations.)

2Mn(OH)₂ + 1/2O₂ + H₂O → 2Mn(OH)₃↓    (brown precipitation)                         (2)

Add potassium iodide and hydrochloric acid to it, then the oxidized manganese ions are reduced by potassium iodide in acidic conditions, and iodine liberates.

2Mn(OH)₃ + 2I ^̶  + 6H⁺  →  2Mn²⁺ + I₂ +6H₂O                         (3)

Titrate liberated-iodine with sodium thiosulfate solution whose concentration is known and indirectly determines the amount of oxygen.

I₂ + 2S₂O₃^{2 ̶}  → 2I ^̶  + S₄O₆^{2 ̶}                          (4)

After all, four molecules of sodium thiosulfate (or four moles of thiosulfate ions) are equivalent to one molecule of oxygen (O₂) (or one mole of O₂ molecules).

adding hydrochloric acid to the mixed precipitate of Mn(OH)₂ and Mn(OH)₃, the precipitate of Mn(OH)₂ will also dissolve and become Mn²⁺. Because the Mn oxidation number of Mn(OH)₂ remains unchanged at +2, it is not in the redox reaction (without I₂ generation).

　To do this in practice, add (1) a manganese chloride solution (known as fixing solution I) and (2) a potassium iodide-sodium hydroxide mixture (known as fixing solution II) sequentially to sample water collected at the observation site. Dissolved oxygen is fixed as Mn(OH)₃ (called oxygen fixation) and later acidified with hydrochloric acid to liberate iodine.  Titrate liberate-iodine with sodium thiosulfate solution.

[Reagent preparation]

① Manganese chloride solution (Fixing solution Ⅰ)

Dissolve 200 g of manganese chloride (MnCl₂-4H₂O) in 500 mL of deionized water and add 2 mL of pure concentrated hydrochloric acid.

② Potassium iodide-sodium hydroxide mixture (Fixing solution Ⅱ)

Dissolve 180 g sodium hydroxide in 500 mL deionized water and 200 g pure potassium iodide*. Store in a plastic container (because this is highly alkaline and will cause the glass lid to stick). After use, the dispensing vessel should be washed with deionized water and acid.

* Use new reagents within 1 year after opening the package, as old or exposed to strong light one will release I2 and cause color change, which may lead to errors.

③ Hydrochloric acid (6 mol/L) (200ml)

Dilute pure concentrated hydrochloric acid (12 mol/L) 2 times. You can measure it with a measuring cylinder.

Add 100 mL of water in a beaker (500 mL). Measure 100 mL of concentrated hydrochloric acid in a draft with a measuring cylinder. Gradually add the concentrated hydrochloric acid to the beaker in the draft. Put the hydrochloric acid solution into the reagent bottle in the draft. Immediately after adjustment, the reagent is warm, so do not put the lid on. When cooled to room temperature, put the lid on.

Warning： Handle concentrated hydrochloric acid in a draft chamber wearing protective glasses because hydrochloric acid emits vapor. Add concentrated hydrochloric acid to water gradually. Wipe up spilled hydrochloric acid with a wet tissue and wash the tissue with tap water.

④ Starch solution

Knead about 1 g of starch with a small amount of water to form an even, itchy consistency and add to 100 mL of hot water (deionized water). Continue to boil while heating slowly until clear. When cooled, put the mixture into reagent bottles. 

[note]   For long-term storage, add benzoic acid at a rate of 0.1 g per 100 mL of starch solution or acetic acid at a rate of 5 mL to prevent spoilage. If the bottle is too old, or if the bottom of the bottle becomes stagnant, the blue coloring of the iodine solution will weaken. Then replace it with a new one. For long-term stable use, a glycerin solution of starch is better. While warming glycerin, dissolve 10-20% soluble starch in it.

⑤ Standard stock solution of potassium iodate (0.016669 mol/L)

Dissolve 1.7835 g potassium iodate (KIO₃) in deionized water to make the total volume exactly 500 mL. Mix well by tipping up and down 20 times. Put this undiluted solution (10 times the concentration of the actual solution used in the experiment) into a brown bottle and keep it in a cool, dark place as much as possible. Before use, dilute the solution to 1/10 of its original concentration.

The exact concentration of the standard solution of potassium iodate can be calculated by recording the exact weight weighed on an electronic balance. Therefore, it is not necessary to adjust it to exactly 3.567 g. Consider performing the experimental manipulation in a dexterous manner. Wash reagent bottles and other containers together to maintain the adjusted concentration.

⑥ Sodium thiosulfate solution (0.02 mol/L)

Dissolve about 3.2 g of anhydrous sodium thiosulfate (Na2S2O3) in deionized water to make the total volume 1 L. Mix well by tipping up and down 20 times. Store in a reagent bottle.

Since we are titrating with this sodium thiosulfate solution, we need to know the exact concentration. Sodium thiosulfate easily absorbs water to form hydrates. Even if you weigh an exact amount of anhydrous sodium thiosulfate, you will not be able to adjust the exact concentration of the solution. Therefore, the exact concentration should be determined using potassium iodate standard solution. The method of calibration is explained in the next chapter.

(7) Capacity test of oxygen bottles (wash and dry oxygen bottles on the second day of the experiment, and measure the dry weight on the third day)

To test the capacity of the oxygen bottles, measure the weight of the water in the bottles. First,  wash oxygen bottles with water, and drain the water inside. They will be placed in a dryer, and once completely dry,  measure their tare weight. On the second day of the experiment, we will fill beakers with deionized water (about 200 mL), wrap them and leave them overnight to acclimate to room temperature; on the third day, we will measure the temperature of the deionized water and the room temperature. Fill the oxygen bottle with this deionized water, cover it, and wipe off any water droplets on the outside with tissue paper. Measure the weight of the oxygen bottle with water to determine the weight of the water and convert it to volume. Complete this volume test before beginning the measurement of dissolved oxygen concentration in seawater.

Perform experimental manipulations (1) through (6). If there is extra time, do additional experiments.

① Evaluation of sodium thiosulfate concentration

Since sodium thiosulfate powder reagent can absorb water, it is difficult to adjust an accurate concentration. Titrate a standard solution of iodic acid with sodium thiosulfate to determine its concentration. Do it three times and take the average. If there is a clear failure, start over.

(1) Put about 0.02 mol/L sodium thiosulfate solution in a burette. Thiosulfate is corrosive, so be careful not to get it in your eyes. Remove the burette from the stand and hold it in your hand. Set the funnel on top of the burette, and slowly pour the solution into it. After pouring, remove the funnel.

(2) Dilute the potassium iodate standard stock solution (stored in a reagent bottle) to 1/10 of its original concentration (using a whole pipette and volumetric flask for accuracy). Measure  10 mL of the diluted solution with a whole pipette accurately and transfer it to a conical beaker. Add 0.2-0.3 g of small crystals of potassium iodide and 1 mL of hydrochloric acid (6 mol /L) to liberate the iodine. Immediately add deionized water to make the total volume approximately 100 mL.

(3)Titrate the solution in the conical beaker with sodium thiosulfate.  When the yellow color of iodine fades, add about 1 mL of starch solution as an indicator. Set the endpoint as the first moment when the resulting blue color disappears, and determine the exact concentration from the volume of sodium thiosulfate solution required until then. If the concentration of the sodium thiosulfate solution deviates significantly from the target concentration of 0.02 mol/L, it is likely that a serious error has been made in the experimental operation.

[Explanation of the reaction equation]

Iodide ion (I^-) is oxidized to I₂ under hydrochloric acidic conditions. At this time, iodic acid (IO^3-) is reduced to I₂.

Potassium iodide crystals are over-fed, so all IO₃^- in the conical beaker is turned to I₂. I₂ and I^- combine to form I₃^-.

I³⁻ displays yellow in color.

Reduce this I₂ with thiosulfate.

When I₂ is completely reduced, I₃^- is also completely reduced at the same time. The endpoint is the point where the color disappears.

Note that I₂ is black solid, but slightly dissolves to become I₂ (aq).

Additional experiment (if you have extra time)

In experimental manipulation (2), place 10 mL of the stock solution of the iodic acid standard in a conical beaker (10 times the amount of iodic acid added in the initial experiment). Add only 0.2-0.3 g of small crystals of potassium iodide, 1 mL of hydrochloric acid, and about 50 mL of deionized water in it. Then, the IO₃^- in the conical beaker will react to all of the added I^-. In other words, the equilibrium in the reaction equation (2) shifts to the left, and I₂ (aq) is generated because there is no I^- in the conical beaker.  Since I₂ is hardly soluble, I₂(s) precipitates by the reaction equation (4). Let us check this. How can we dissolve this black solid? Just add potassium iodide.