The Winkler test is used to determine the concentration of

dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can ...

in water samples. Dissolved oxygen (D.O.) is widely used in

water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...

studies and routine operation of water reclamation facilities to analyze its level of

oxygen saturation Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is Dissolution (chemistry), dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the g ...

. In the test, an excess of manganese(II) salt, iodide (I⁻) and hydroxide (OH⁻) ions are added to a water sample causing a white

precipitate In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...

of Mn(OH)₂ to form. This precipitate is then oxidized by the

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

that is present in the water sample into a brown

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

-containing precipitate with manganese in a more highly oxidized state (either Mn(III) or Mn(IV)). In the next step, a strong acid (either

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

) is added to acidify the solution. The brown precipitate then converts the iodide ion (I⁻) to

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

. The amount of dissolved oxygen is directly proportional to the titration of iodine with a

thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...

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

ethylenediaminetetraacetic acid Ethylenediaminetetraacetic acid (EDTA), also called EDTA acid, is an aminopolycarboxylic acid with the formula . This white, slightly water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-solubl ...

to give a pink color. As manganese is the only common metal giving a

color reaction In chemistry, a color reaction or colour reaction is a chemical reaction that is used to transform colorless chemical compounds into colored derivatives which can be detected visually or with the aid of a colorimeter. The concentration of a colo ...

with ethylenediaminetetraacetic acid, 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. The amount of dissolved oxygen is a measure of the biological activity of the water masses.

Phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...

and macroalgae present in the water mass-produce oxygen by way of

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

Bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and eukaryotic organisms (

zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

, fish) consume this oxygen through

cellular respiration Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cell ...

. 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.

Chemical processes

In the first step,

manganese(II) sulphate Manganese(II) sulfate usually refers to the inorganic compound with the formula MnSO4·H2O. This pale pink deliquescent solid is a commercially significant manganese(II) salt. Approximately 260,000 tonnes of manganese(II) sulfate were produced wor ...

(at 48% of the total volume) is added to an environmental water sample. Next,

potassium iodide Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are u ...

(15% in

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which utili ...

70%) is added to create a pinkish-brown precipitate. In the alkaline solution, dissolved oxygen will oxidize manganese(II) ions to the

tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an atom is a measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Valence is generally understood to be the number of chemica ...

state. : 2 Mn²⁺ + O₂ + 4 OH⁻ → 2 MnO(OH)₂ Mn has been oxidised to 4+, and MnO(OH)₂ appears as a brown precipitate. There is some uncertainty about whether the oxidised manganese is tetravalent or

trivalent In chemistry, the valence (US spelling) or valency (British spelling) of an atom is a measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Valence is generally understood to be the number of chemica ...

. Some sources claim that Mn(OH)₃ is the brown precipitate, but hydrated MnO₂ may also give the brown colour. : 4 Mn(OH)₂ + O₂ + 2 H₂O → 4 Mn(OH)₃ The second part of the Winkler test reduces (acidifies) the solution. The precipitate will dissolve back into solution as the H⁺ reacts with the O²⁻ and OH⁻ to form water. : MnO(OH)₂ + 4 H⁺ → Mn⁴⁺ + 3 H₂O 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⁻ → Mn²⁺ + I₂ + 2

Thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...

is used, with a starch indicator, to

titrate Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of Quantitative research, quantitative Analytical chemistry, chemical analysis to determine the concentration of an identified analyte (a substance to be ...

the iodine. : 2 + I₂ → + 2 I⁻

Analysis

From the above

stoichiometric Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...

equations, we can find that: :1 mole of O₂ → 2 moles of MnO(OH)₂ → 2 mole of I₂ → 4 mole of 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 in milligrams per liter (mg/L).

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 In chemistry, a solution is defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are ...

that will

oxidize Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

reduce Reduction, reduced, or reduce may refer to: Science and technology Chemistry * Reduction (chemistry), part of a reduction-oxidation (redox) reaction in which atoms have their oxidation state changed. ** Organic redox reaction, a redox reacti ...

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 demand Biochemical oxygen demand (also known as BOD or biological oxygen demand) is an analytical parameter representing the amount of dissolved oxygen (DO) consumed by aerobic bacteria growing on the organic material present in a water sample at a s ...

(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 standardized community to uptake oxygen while consuming the organic matter in 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 Laboratory glassware is a variety of equipment used in science, scientific work, traditionally made of glass. Glass may be blown, bent, cut, molded, or formed into many sizes and shapes. It is commonly used in chemistry, biology, and analytical ...

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.Whitney King
"Winkler titrations – measuring dissolved oxygen"
''Colby at Sea'', 11 February 2011, retrieved an
archived
11 July 2012.

History

Chemical processes

Analysis

Limitations

BOD₅

Winkler bottle

References

Further reading

History

Chemical processes

Analysis

Limitations

BOD5

Winkler bottle

References

Further reading

BOD₅