Winkler test for dissolved oxygen
Encyclopedia
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 a thiosulfate
solution. 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. 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.
, while working at Budapest University on his doctoral dissertation in 1888. 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% the oxygen levels may fluctuate.
(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.
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.
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.
Thiosulfate is used, with a starch indicator, to titrate the iodine.
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.
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.
(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.
Gives manganese (IV) consistently
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
precipitate. In the next step, a strong acid (either hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
or sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
) is added to acidify the solution. The brown precipitate then converts the iodide ion (I–) to iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
. The amount of dissolved oxygen is directly proportional to the titration of iodine with a thiosulfate
Thiosulfate
Thiosulfate is an oxyanion of sulfur. The prefix thio indicates that thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. Thiosulfate occurs naturally and is produced by certain biochemical processes...
solution. 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
EDTA
Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...
to give a pink color. 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 WinklerLajos Winkler
Lajos Winkler was a Hungarian analytical chemist. He is best known today for his discovery of the Winkler method for the measurement of oxygen dissolved in water.- Life :...
, while working at Budapest University on his doctoral dissertation in 1888. The amount of dissolved oxygen is a measure of the biological activity of the water masses. Phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
and macroalgae present in the water mass produce oxygen by way of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and eukaryotic organisms (zooplankton, algae, fish) consume this oxygen through cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
. 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% the oxygen levels may fluctuate.
Sample method
In the first step, manganese(II) sulfateManganese(II) sulfate
Manganese sulfate usually refers to the inorganic compound with the formula MnSO4. This pale pink deliquescent solid is a commercially significant manganese salt. Approximately 260 thousand tonnes of manganese sulfate were produced worldwide in 2005. It is the precursor to manganese metal and...
(at 48% of the total volume) is added to an environmental water sample. Next, potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
(15% in potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
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
Electron shell
An electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" , followed by the "2 shell" , then the "3 shell" , and so on further and further from the nucleus. The shell letters K,L,M,.....
. 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
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
, 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 solutesSolution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
that will oxidize or reduce
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
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.
BOD5
To determine five-day biochemical oxygen demandBiochemical oxygen demand
Biochemical oxygen demand or B.O.D. is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. The term also refers to a chemical procedure for...
(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.
Further reading
- Moran, Joseph M.; Morgan, Michael D., & Wiersma, James H. (1980). Introduction to Environmental Science (2nd ed.). W.H. Freeman and Company, New York, NY ISBN 0-7167-1020-X
- Standard Methods for the Examination of Water and Wastewater – 20th Edition ISBN 0-87553-235-7. This is also available on CD-ROM and online by subscription
- http://www.ecy.wa.gov/programs/wq/plants/management/joysmanual/4oxygen.html
- Y.C. Wong & C.T. Wong. New Way Chemistry for Hong Kong A-Level Volume 4, p. 248. ISBN 962-342-535-X
- Manganese (III) consistently claimed http://ocw.mit.edu/NR/rdonlyres/Earth--Atmospheric--and-Planetary-Sciences/12-097January--IAP--2006/35FAFFC3-135B-4266-93B4-2144F7487004/0/dissolved_oxygen.pdf (NB: Gives unbalanced equation for formation of MnO(OH)2). Claims manganese (III) http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg7.htm http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg3.htm http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg11.htm
Gives manganese (IV) consistently