Winkler titration

<h2 id="history">History</h2>
<p>The test was originally developed by Ludwig Wilhelm Winkler, in later literature referred to as <a href="/facts/Lajos_Winkler/xRcppxkk">Lajos Winkler</a>, while working at Budapest University on his doctoral dissertation in 1888.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> The amount of dissolved oxygen is a measure of the biological activity of the water masses. <a href="/facts/Phytoplankton/tK5MUIkA">Phytoplankton</a> and macroalgae present in the water mass-produce oxygen by way of <a href="/facts/Photosynthesis/TMV7w693">photosynthesis</a>.  <a href="/facts/Bacteria/wC8xSCsU">Bacteria</a> and eukaryotic organisms (zooplankton, fish) consume this oxygen through <a href="/facts/Cellular_respiration/kb6gpsvL">cellular respiration</a>. 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.
</p>
<h2 id="chemical-processes">Chemical processes</h2>
<p>In the first step, <a href="/facts/Manganese(II)_sulphate/RVBG26R6">manganese(II) sulphate</a> (at 48% of the total volume) is added to an environmental water sample.  Next, <a href="/facts/Potassium_iodide/6023uFnx">potassium iodide</a> (15% in <a href="/facts/Potassium_hydroxide/NBhPBuXc">potassium hydroxide</a> 70%) is added to create a pinkish-brown precipitate.  In the alkaline solution, dissolved oxygen will oxidize manganese(II) ions to the <a href="/facts/Tetravalent/ck7cHByj">tetravalent</a> state.
</p>
2 Mn2+(aq) + O2(aq) + 4 OH−(aq)  → 2 MnO(OH)2(s)
<p>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 <a href="/facts/Electron_shell/QxI30eh9">trivalent</a>. Some sources claim that Mn(OH)3 is the brown precipitate, but hydrated MnO2 may also give the brown colour.
</p>
4 Mn(OH)2(s) + O2(aq) + 2 H2O → 4 Mn(OH)3(s)
<p>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.
</p>
MnO(OH)2(s) + 4 H+(aq) → Mn4+(aq) + 3 H2O(l)
<p>The acid facilitates the conversion by the brown, Manganese-containing precipitate of the Iodide ion into elemental Iodine.
</p><p>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.
</p>
Mn(SO4)2 + 2 I−(aq) →  Mn2+(aq) + I2(aq) + 2 SO2−4(aq)
<p><a href="/facts/Thiosulfate/LYu0KhDu">Thiosulfate</a> is used, with a starch indicator, to <a href="/facts/Titrate/gGUfvQcW">titrate</a> the iodine.
</p>
2 S2O2−3(aq) + I2 → S4O2−6(aq) + 2 I−(aq)
<h2 id="analysis">Analysis</h2>
<p>From the above <a href="/facts/Stoichiometric/I5p9cJzV">stoichiometric</a> equations, we can find that:
</p>
1 mole of O2 → 2 moles of MnO(OH)2 → 2 mole of I2 → 4 mole of S2O2−3
<p>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).
</p>
<h2 id="limitations">Limitations</h2>
<p>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 <a href="/facts/Solutes/MGchhMQV">solutes</a> that will <a href="/facts/Oxidize/Pyd5RVcj">oxidize</a> or <a href="/facts/Redox/Pyd5RVcj">reduce</a> iodine.
</p><p>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.
</p>
<h2 id="bod5">BOD5</h2>
<p>To determine five-day <a href="/facts/Biochemical_oxygen_demand/8vDcugIo">biochemical oxygen demand</a> (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.
</p><p>The BOD5 test is an example of analysis that determines classes of materials in a sample.
</p>
<h2 id="winkler-bottle">Winkler bottle</h2>
<p>A Winkler bottle is a piece of <a href="/facts/Laboratory_glassware/M4CMuHRh">laboratory glassware</a> 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.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>
</p>

<h2 id="further-reading">Further reading</h2>

The Wikibook <i>School Science</i> has a page on the topic of: <i>Winkler test for dissolved oxygen</i>

<ul><li>Moran, Joseph M.; Morgan, Michael D., & Wiersma, James H. (1980).  <i>Introduction to Environmental Science</i> (2nd ed.). W.H. Freeman and Company, New York, NY <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-7167-1020-X</li>
<li>Eaton, Andrew D.; Greenberg, Arnold E.; Rice, Eugene W.; Clesceri, Lenore S.; Franson, Mary Ann H., eds. (2005). <i>Standard Methods For the Examination of Water and Wastewater</i> (21 ed.). American Public Health Association. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-87553-047-5. Also available on CD-ROM and <a href="http://www.standardmethods.org/">online</a> by subscription.</li>
<li><a href="https://fortress.wa.gov/ecy/publications/SummaryPages/1703202.html">"Standard Operating Procedure EAP0 23, Version 2.5. Collection and Analysis of Dissolved Oxygen (Winkler Method)"</a>. Washington State Department of Ecology. July 2017.</li>
<li>Y.C. Wong & C.T. Wong. <i>New Way Chemistry for Hong Kong A-Level Volume 4</i>, p. 248. <i><a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 962-342-535-X</i></li>
<li>Manganese (III) consistently claimed <a href="http://ocw.mit.edu/NR/rdonlyres/Earth--Atmospheric--and-Planetary-Sciences/12-097January--IAP--2006/35FAFFC3-135B-4266-93B4-2144F7487004/0/dissolved_oxygen.pdf">[1]</a> (NB: Gives unbalanced equation for formation of MnO(OH)2). Claims manganese (III) gives manganese (IV) consistently. <a href="http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg7.htm">[2]</a> <a href="http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg3.htm">[3]</a> <a href="http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg11.htm">[4]</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Chiya Numako and Izumi Nakai (1995). "XAFS studies of some precipitation and coloration reaction used in analytical chemistry". Physica B: Condensed Matter. 208–209 (1–4): 387–388. Bibcode:1995PhyB..208..387N. doi:10.1016/0921-4526(94)00706-2. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>A. H. de Carvalho, J. G. Calado and M. L. Moura, Rev. Port. Quim., 1963, 5, 15 <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Lajos Winkler (1888). "Die Bestimmung des in Wasser Gelösten Sauerstoffes". Berichte der Deutschen Chemischen Gesellschaft. 21 (2): 2843–2855. doi:10.1002/cber.188802102122. <a href="/wiki/Lajos_Winkler" target="_blank">/wiki/Lajos_Winkler</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Whitney King, "Winkler titrations – measuring dissolved oxygen", Colby at Sea, 11 February 2011, retrieved and archived 11 July 2012. <a href="http://web.colby.edu/colbyatsea/2011/02/11/winkler-titrations-measuring-dissolved-oxygen/" target="_blank">http://web.colby.edu/colbyatsea/2011/02/11/winkler-titrations-measuring-dissolved-oxygen/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
</ol>

Winkler titration open-in-new

Winkler titration