Winkler Method for Dissolved Oxygen Measurement

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Winkler Method for Dissolved Oxygen Measurement

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Ideal gas law, Boyle-Mariotte law, Gay-Lussac law, Avogadro's volume law, Winkler method, dissolved oxygen measurement, sodium thiosulfate titration, iodometry techniques, gas constant R, oxygen concentration calculation, stoichiometric calculations, manganese oxidation, iodine oxidation, hydrochloric acid reaction, SI units, International System units, J/(mol·K), kelvins, K^-1.mol^-1, pascals, Pa, cubic meters, m³, moles, mol, thiosulfate concentration, manganese chloride MnCl2, potassium iodide KI, sulfuric acid, starch iodide indicator, oxygen dosage, water analysis, titration reaction, stoichiometric study, gas behavior, ideal gases, pressure volume relationship, temperature effect on gases, molar volume, amount of substance, reaction kinetics, oxidation reaction, precipitation reaction, iodometry, analytical chemistry techniques, water quality measurement, environmental analysis, chemical reaction balancing, equation stoichiometry, laboratory procedures, experimental methods, measurement precision, reproducibility of results, Lajos Winkler, doctoral thesis, historical scientific methods, chemical education, educational content, scientific research methods, laboratory testing, water testing, dissolved oxygen levels, aquatic chemistry, environmental monitoring, chemical analysis techniques, reaction mechanisms, manganese hydroxide Mn(OH)2, manganese(III) hydroxide Mn(OH)3, diiodide formation, thiosulfate titration reaction

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Abstract

The Winkler method is an indirect technique for determining dissolved oxygen in water, involving iodometry and titration with sodium thiosulfate.

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[...] Stoichiometry I. Study of a reaction producing a gas: action of hydrochloric acid on a metal Balance equation of the reaction equation: Measurement 1 Measurement 2 Measurement 3 Mass of metal m 0.0354g 0.0340g 0.0352g Volume of gas V 36.1 mL=36.1*10-6m3 33.8 mL=33.8*10-6m3 35.5 mL=35.5*10-6m3 Temperature T 21°C=294.15 °K 21°C=294.15°K 21°C=294.15°K Atmospheric pressure " 745.9mmHg =99445.169Pa 746.3mmHg =99498.498Pa 755.5mmHg =100725.06Pa Saturation vapor pressure of water at T 2486 Pa 2486 Pa Partial pressure of dihydrogen 99445,169-2486 =96959.169Pa 99498,498-2486 =97012.498 Pa" 100725,06-2486 =98239.06 Pa Quantity of magnesium material that has reacted n =m/M=0.0354/24.305 =1.456*10^-3mol n =m/M=0.0340/24.305 =1.399*10^-3mol n =m/M=0.0352/24.305 =1.448*10^-3mol Value of R constant of perfect gases (J.K^-1.mol^-1) R=PV/nT =8.17 J.K^-1.mol^-1 R=PV/nT =7.97 J.K^-1.mol^-1 R=PV/nT =8.18 J.K^-1.mol^-1 Value retained for R (average of 3 values) 8.11 K^-1.mol^-1 The details of the reasoning are given in the table. [...]

[...] So 4 moles of thiosulfate for one mole of oxygen. n(O2)=n(S2O3-2)/4=CVT/4=0,0125*0,00887/4=2,77*10-5 mol [O2]=n(O2)/V=2,77*10-5 /0,1=2,77 *10-4 mol/L M(O2)=2*M(O)=2*16=32 g/mol Cm=[O2]*M(O2)=8,86*10-3 g/L Rappels: The law of perfect gases", is also known as the Boyle-Mariotte law, Gay-Lussac law or Avogadro's volume law, describes the behavior of ideal gases. It states that, at a constant temperature, the volume of a gas is inversely proportional to its pressure and directly proportional to its amount of matter Mathematically, the law of perfect gases is expressed as follows: PV=nRT where : - P is the pressure of the gas (in pascals, - V is the volume occupied by the gas (in cubic meters, m³), - n is the amount of substance of the gas (in moles, mol), - R is the gas constant, which depends on the units used for pressure and volume. [...]

[...] The manganese precipitates then return to their ionic forms Mn2+ and Mn3+. Next, we introduce potassium iodide in excess, whose iodide ions are oxidized exclusively by the Mn3+ ions. Thus, the amount of oxygen is entirely found in the amount of Mn3+ that reacts completely with iodine, oxidizing it to diiodide. Finally, the diiodide formed is dosed using sodium thiosulfate in the presence of a colored indicator, starch iodide, allowing the determination of the amount of dissolved oxygen in the water. [...]

[...] It begins with the dissolution of manganese chloride MnCl2 in the water to be analyzed in a basic medium, while ensuring that no air is included in the sample to measure only the oxygen already dissolved. This step induces the formation of a precipitate of manganese hydroxide, Mn (OH)2. Initially present in the form of Mn2+, manganese undergoes oxidation by oxygen to become Mn3+, forming the brown precipitate Mn (OH)3. This oxidation reaction of manganese by oxygen can take some time, even with sustained agitation, usually around 30 minutes. [...]

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