Dissolved oxygen is the number of mg / l of oxygen gas dissolved in water. dissolved oxygen in water can be derived from the process of photosynthesis by phytoplankton and aquatic plants or on the diffusion of air. Dissolved oxygen levels can be determined by titration and electronic measuring instrument DO meter. Winkler way based on two oxidation – reduction is widely used and is the standard way in the determination of dissolved oxygen. This method is based on the fact that the sodium oxide reacts with manganese sulfate, yielding a white precipitate and manganese hydroxides.
MnSO4 + 2NaOH -> Mn (OH) 2 + Na2SO4
The presence of oxygen in the solution is very alkaline, manganese hydroxide is oxidized into manganese oksihidrat white (brown). So the number oksigan that there can be roughly estimated from the intensity of the brown color of the precipitate. In very acidic media, manganese ions released and reacts with ions of potassium iodide free yod yod free form.
Free yod amount equivalent to the amount of oxygen present in the sample. Yod amount can be determined through titration with sodium thiosulphate.
(1) Manganese Sulfate Solution (MnSO4.4H2O) dissolve 48 grams or 40 grams of MnSO4. 2H2O in distilled water. Make to 100 ml of distilled water. Manganese chloride can be used in addition to manganese sulphate manganese chloride solution can be prepared by dissolving 100 grams of pure crystal of manganese chloride tetrahydrate in 200 ml of distilled water.
(2) Alkali iodide (Winkler reagent). Dissolve 50 grams of NaOH and 13.5 grams of 15 grams of NaI or KI in 100 of distilled water.
(3) Concentrated Sulfuric Acid.
(4) Raw solution Sodium Thiosulfate (Na2S2O3.5H2O) 0.1 N. Dissolve 24.83 grams of sodium thiosulfate in distilled water, put in a 1 liter flask and add distilled water to mark boundaries. Add into a solution of 5 ml of chloroform thiosulfate to prevent damage to the solution
(5) titers (Na2S2O3) 1/80 N (0.0125 N). Dilute the main liquor thiosulfate standard solution of 12.5 ml to 1 liter with distilled water.
(6) Solution Kanji. Dilute 30 ml of 20% KOH solution to 400 ml with distilled water. Add 2 grams of starch in it. Stir until the solution becomes almost clear. Let stand for 1 hour solution.
Gradually add hydrochloric acid. Check the pH often as possible until the solution becomes neutral. Add 1 ml of glacial acetic acid.
Dissolved oxygen measurement procedure with the titration method (Winkler)
Figure 32. Measurement of dissolved oxygen by titration
(1) Bottle Winkler
(2) Pipette drops
(3) The titration
(4) Pipette volume
(1) Water samples
(2) alkali iodide (perekasi Winkler)
(3) concentrated H2SO4
(4) A solution Manganese sulfate / MnSO4 48%
(5) 0.025 N sodium thiosulfate
(6) Indicator amylum 1%
How it Works:
(1) Added thereto 1 mL and 1 mL reagent MnSO4 Winkler, then shaken and wait until a precipitate is formed.
(2) Added 2 mL of concentrated H2SO4, shaken until the sediment dissolves.
(3) Taken 50.0 mL sample of the solution is titrated with 0.025 N sodium thiosulfate until pale yellow.
(4) Added inikator starch (blue).
(5) Titrated back with a solution of sodium thiosulphate, from blue to become clear.
(6) Noting how mL of sodium thiosulfate used.
O2 levels (mg / l) = 8000 x mL Na2S2O3 x N Na2S2O3 mL sample