Winkler test for dissolved oxygen
The Winkler test is used to determine the concentration of dissolved oxygen in water samples. Dissolved oxygen (D.O.) is widely used in water quality studies and routine operation of water reclamation facilities. An excess of manganese(II) salt, iodide (I–) and hydroxide (OH–) ions is added to a water sample causing a white precipitate of Mn(OH)2 to form. This precipitate is then oxidized by the dissolved oxygen in the water sample into a brown manganese precipitate. In the next step, a strong acid (either hydrochloric acid or sulfuric acid) is added to acidify the solution. The brown precipitate then converts the iodide ion (I–) to iodine. The amount of dissolved oxygen is directly proportional to the titration of iodine with athiosulfate solution.[1] Today, the method is effectively used as its colorimetric modification, where the trivalent manganese produced on acidifying the brown suspension is directly reacted with EDTA to give a pink color.[2] As manganese is the only common metal giving a color reaction with EDTA, it has the added effect of masking other metals as colorless complexes.

History
The test was originally developed by Ludwig Wilhelm Winkler, in later literature referred to as Lajos Winkler, while working at Budapest University on his doctoral dissertation in 1888.[3] The amount of dissolved oxygen is a measure of the biological activity of the water masses. Phytoplankton and macroalgae present in the water mass-produce oxygen by way of photosynthesis. Bacteria and eukaryotic organisms (zooplankton, algae, fish) consume this oxygen through cellular respiration. The result of these two mechanisms determines the concentration of dissolved oxygen, which in turn indicates the production of biomass. The difference between the physical concentration of oxygen in the water (or the theoretical concentration if there were no living organisms) and the actual concentration of oxygen is called the biochemical demand in oxygen. The Winkler test is often controversial as it is not 100% accurate and the oxygen levels may fluctuate from test to test despite using the same constant sample.

Sample method
In the first step, manganese(II) sulfate (at 48% of the total volume) is added to an environmental water sample. Next, potassium iodide (15% in potassium hydroxide 70%) is added to create a pinkish-brown precipitate. In the alkaline solution, dissolved oxygen will oxidize manganese(II) ions to the tetravalent state. 2 MnSO4(s) + O2(aq) → 2 MnO(OH)2(s)
MnO(OH)2 appears as a brown precipitate. There is some confusion about whether the oxidised manganese is tetravalent or trivalent. Some sources claim that Mn(OH)3 is the brown precipitate, but hydrated MnO2 may also give the brown colour. 4 Mn(OH)2(s) + O2(aq) + 2 H2O → 4 Mn(OH)3(s)
The second part of the Winkler test reduces (acidifies) the solution. The precipitate will dissolve back into solution. The acid facilitates the conversion by the brown, Manganese-containing precipitate of the Iodide ion into elemental Iodine.
The Mn(SO4)2 formed by the acid converts the iodide ions into iodine, itself being reduced back to manganese(II) ions in an acidic medium. Mn(SO4)2 + 2 I–(aq) → Mn2+(aq) + I2(aq) + 2 SO42–(aq)
Thiosulfate is used, with a starch indicator, to titrate the iodine. 2 S2O32–(aq) + I2 → S4O62–(aq) + 2 I–(aq)

Analysis
From the above stoichiometric equations, we can find that: 1 mole of O2 → 2 moles of MnO(OH)2 → 2 mole of I2 → 4 mole of S2O32–
Therefore, after determining the number of moles of iodine produced, we can work out the number of moles of oxygen molecules present in the original water sample. The oxygen content is usually presented as mg/dm3.

Limitations
The success of this method is critically dependent upon the manner in which the sample is manipulated. At all stages, steps must be taken to ensure that oxygen is neither introduced to nor lost from the sample. Furthermore, the water sample must be free of any solutes that will oxidize or reduce iodine.
Instrumental methods for measurement of dissolved oxygen have widely supplanted the routine use of the Winkler test, although the test is still used to check instrument calibration.

BOD
To determine five-day biochemical oxygen demand (BOD5), several dilutions of a sample are analyzed for dissolved oxygen before and after a five-day incubation period at 20 °C in the dark. In some cases, bacteria are used to provide a source of oxygen to the sample; these bacteria are known as "seed". The difference in DO and the dilution factor are used to calculated BOD5. The resulting number (usually reported in parts per million or milligrams per liter) is useful in determining the relative organic strength of sewage or other polluted waters.
The BOD5 test is an example of analysis that determines classes of materials in a sample.

Winkler bottle
A Winkler bottle is a piece of laboratory glassware specifically made for carrying out the Winkler test. These bottles have conical tops and a close fitting stopper to aid in the exclusion of air bubbles when the top is sealed. This is important because oxygen in trapped air would be included in the measurement and would affect the accuracy of the test.[

Iodometry, also known as iodometric titration, is a method of volumetric chemical analysis, a redox titration where the appearance or disappearance of elementary iodineindicates the end point.
Note that iodometry involves indirect titration of iodine liberated by reaction with the analyte, whereas iodimetry involves direct titration using iodine as the titrant.
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Basic principles
Iodometry is commonly used to analyse the concentration of oxidizing agents in water samples, such as oxygen saturation in ecological studies or active chlorine in swimming pool water analysis. To a known volume of sample, an excess but known amount of iodide is added, which the oxidizing agents oxidizes iodide to iodine. Iodine dissolves in the iodide-containing solution to give triiodide ions, which have a dark brown color.
The triiodide ion solution is then titrated against standard thiosulphate solution to give iodide again using starch indicator: I3- + 2 e- ⇌ 3 I- (Eo = + 0.5355 V)
Together with reduction potential of thiosulfate:[1] S4O62- + 2 e- ⇌ 2 S2O32- (Eo = + 0.08 V)
The overall reaction is thus: I3- + 2 S2O32- → S4O62- + 3 I- (Eo = + 0.4555 V)
For simplicity, the equations will usually be written in terms of aqueous molecular iodine rather than the triiodide ion, as the iodide ion did not participate in the reaction in terms of mole ratio analysis.
The disappearance of deep blue color due to the decomposition of the iodine-starch clathrate marks the end point
The reducing agent used does not necessarily need to be thiosulfate, stannous chloride, sulphites, sulfides, arsenic(III), and antimony(III) are commonly used alternatives.[2] At higher pH (> 8)) At low pH would also react with the thiosulfate:: :S2O32− + 2 H+ → SO2 + S + H2O
Some reactions involving certain reductants are reversible at certain pH, thus the pH of the sample solution should be carefully adjusted before the performing the analysis. For example, the reaction: H3AsO3 + I2 + H2O → H3AsO4 + 2 H+ + 2 I- is reversible at pH < 4.
The volatility of iodine is also a source of error for the titration, this can be effectively prevented by ensuring an excess iodide is present and cooling the titration mixture. Strong light, nitrite and copper ions catalyzes the conversion of iodide to iodine, so these should be removed prior to the addition of iodide to the sample.
For prolonged titrations, it is advised to add dry ice to the titration mixture to displace air from the erlenmeyer flask so as to prevent the aerial oxidation of iodide to iodine. Standard iodine solution is prepared from potassium iodate and potassium iodide, which are both primary standards): IO3- + 8 I- + 6 H+ → 3 I3- + 3 H2O