{"id":409,"date":"2011-02-11T03:30:43","date_gmt":"2011-02-11T07:30:43","guid":{"rendered":"http:\/\/web.colby.edu\/colbyatsea\/?p=409"},"modified":"2011-02-11T10:02:51","modified_gmt":"2011-02-11T14:02:51","slug":"winkler-titrations-measuring-dissolved-oxygen","status":"publish","type":"post","link":"https:\/\/web.colby.edu\/colbyatsea\/2011\/02\/11\/winkler-titrations-measuring-dissolved-oxygen\/","title":{"rendered":"Winkler Titrations &#8211; Measuring Dissolved Oxygen"},"content":{"rendered":"<div id=\"attachment_410\" style=\"width: 253px\" class=\"wp-caption alignright\"><a href=\"http:\/\/web.colby.edu\/colbyatsea\/files\/2011\/02\/winkler-bottle.jpg\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-410\" class=\"size-medium wp-image-410\" src=\"http:\/\/web.colby.edu\/colbyatsea\/files\/2011\/02\/winkler-bottle-243x300.jpg\" alt=\"\" width=\"243\" height=\"300\" srcset=\"https:\/\/web.colby.edu\/colbyatsea\/files\/2011\/02\/winkler-bottle-243x300.jpg 243w, https:\/\/web.colby.edu\/colbyatsea\/files\/2011\/02\/winkler-bottle.jpg 264w\" sizes=\"(max-width: 243px) 100vw, 243px\" \/><\/a><p id=\"caption-attachment-410\" class=\"wp-caption-text\">Winkler Bottle<\/p><\/div>\n<p>We have been asked for another example of a chemical analysis being performed on the ship.\u00a0\u00a0 One of the most famous measurements is the determination of dissolved oxygen in seawater.\u00a0\u00a0 The CTD has an <a href=\"http:\/\/www.ysi.com\/media\/pdfs\/DO-Diagram-of-Clark-Electrode-Operation.pdf\">electrochemical sensor<\/a> that measures oxygen continuously as it is lowered into the ocean.\u00a0\u00a0 However, the electrochemical sensor drifts with time and all CTD measurements must be calibrated against lab measurements.\u00a0 Samples are taken from the Niskin bottles on the rosette for the calibration of the oxygen sensor.\u00a0\u00a0 The lab measurements are based on the classic Winkler titration of oxygen first developed by <a href=\"http:\/\/en.wikipedia.org\/wiki\/Lajos_Winkler\">Lajos Winkler<\/a> in 1888.<!--more--><\/p>\n<p>The Winkler analysis of dissolved oxygen has three steps designed to turn odorless, colorless dissolved oxygen into something that we can see.<\/p>\n<p>Samples are collected in Winkler, or BOD, bottles that are specifically designed with a conical top to help exclude bubbles.<\/p>\n<p>A basic solution of Mn(II), MnCl<sub>2<\/sub> (aq), is added to the sample bottle.\u00a0 Under alkaline conditions dissolved oxygen will oxidize manganese(II) ions to manganese(IV), MnO(OH)<sub>2<\/sub>.\u00a0\u00a0\u00a0 This reaction is fast and stoichiometric so that each mole of oxygen produces two moles of Mn(IV).<\/p>\n<p>2 Mn<sup>2+<\/sup> (aq) + <span style=\"color: #ff0000\">O<\/span><sub><span style=\"color: #ff0000\">2 <\/span><\/sub><span style=\"color: #ff0000\">(aq)<\/span> + 4OH<sup>&#8211; <\/sup>\u2192 2 MnO(OH)<sub>2<\/sub>(s) \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 (1)<\/p>\n<p>Excess Mn(II) and base are added in step one so that oxygen is the limiting reagent <a href=\"http:\/\/web.colby.edu\/colbyatsea\/2011\/02\/01\/life-is-controlled-by-the-limiting-nutrient\/\">(see the blog on limiting reagents<\/a>).\u00a0\u00a0 Sodium iodide is also added during step one.\u00a0 In base, iodide does not react, but it is added to be ready for the next step of the reaction.\u00a0 \u00a0You can get an idea of the amount of oxygen in the sample by observing the brown MnO(OH)<sub>2<\/sub> precipitate that forms in the bottle.\u00a0 In this step we are converting a dissolved gas to a solid. \u00a0The amount of solid is proportional to the amount of oxygen that was in the bottle.<\/p>\n\r\n<!-- <script src=\"https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/jquery.tools.min.js\"><\/script> -->\r\n<script type=\"text\/javascript\" src=\"https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/flowplayer-3.2.2.js\"><\/script>\r\n<style>\r\n\/* player container *\/\r\ndiv.player {\r\n\theight:272px;\r\n\twidth:480px;\r\n\tcursor:pointer;\r\n\tfloat:left;\r\n\ttext-align:center;\r\n\tmargin-right:15px;\r\n}\r\n\/* play button height *\/\r\ndiv.player img {\r\n\tmargin-top:95px !important;\r\n}\r\n<\/style>\r\n\r\n<div class=\"player\"  \r\n    href=\"mp4:king\/colbyatsea\/OxygenI\" \r\n    style=\"background-color:#000000;background-image:url(http:\/\/web.colby.edu\/colbyatsea\/files\/2011\/02\/oxy1.jpg)\"> \r\n \r\n    <!-- play button --> \r\n    <img decoding=\"async\" src=\"https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/play_large.png\" style=\"border:0px !important;\" alt=\"Play this video\" \/>      \r\n<\/div> \r\n<script type=\"text\/javascript\">\r\n$f(\"div.player\", \"https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/flowplayer-3.2.2.swf\", {\r\n\tplugins: {\r\n\t\trtmp: {\r\n\t\t\turl: 'https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/flowplayer.rtmp-3.2.1.swf',\r\n\t\t   \tnetConnectionUrl: 'rtmp:\/\/flash.colby.edu\/vod',\r\n\r\n\t\t   \t\/\/ make the rtmp plugin query the stream length from the server\r\n\t\t   \tdurationFunc: 'getStreamLength'\r\n\t\t},\r\n\t\tcontrols: {\r\n\t\t\tfullscreen: true,\r\n\t\t\theight: 30,\r\n\t\t\tautoHide: true\r\n\t\t}\r\n\t},\r\n   clip:{\r\n         autoBuffering:true,\r\n         autoPlay:true,\r\n         scaling:'fit',\r\n\t\t provider: 'rtmp'\r\n   }\r\n}); \r\n<\/script>\r\n\r\n\n<p>The sample bottle must remain tightly capped during the first step to prevent oxygen from the air from reacting with Mn(II).\u00a0 <a href=\"http:\/\/web.colby.edu\/colbyatsea\/2011\/01\/20\/sampling-at-sea-everyone-must-obey-the-bottle-cop\/\">Recall from the CTD video<\/a> that all the air bubbles were carefully removed from the bottle during sampling and the Bottle Cop makes certain that the samples for dissolved oxygen are taken first.\u00a0\u00a0 The capped bottles are allowed to sit for over 30 minutes to completely react with all the oxygen.<\/p>\n<p>Next, excess acid is added to the bottle and the MnO(OH)<sub>2<\/sub> (s) formed in step one reacts stoichiometrically with iodide (I<sup>&#8211;<\/sup>) to form a yellow, triiodide (I<sub>3<\/sub><sup>&#8211;<\/sup>) solution in two steps.\u00a0 The cool part of this reaction is that iodide reacts in acid, but oxygen does not.\u00a0 This means that the samples will no longer react with oxygen from the air. Why is this important?<\/p>\n<p><span style=\"color: #ff0000\">MnO(OH)<\/span><sub><span style=\"color: #ff0000\">2<\/span><\/sub><span style=\"color: #ff0000\">(s)<\/span> + 2 I<sup>&#8211;<\/sup>(aq) + 4H<sup>+<\/sup> \u2192 Mn<sup>2+<\/sup>(aq) + I<sub>2<\/sub>(aq) + 3H<sub>2<\/sub>O\u00a0\u00a0 (2)<\/p>\n<p>I<sup>&#8211;<\/sup> (aq) + <span style=\"color: #ff0000\">I<\/span><sub><span style=\"color: #ff0000\">2<\/span><\/sub><span style=\"color: #ff0000\"> (aq)<\/span> \u2192\u00a0I<sub>3<\/sub><sup>&#8211; <\/sup> (aq) \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0(3)<\/p>\n<p>Again, iodide and acid are added in excess.\u00a0\u00a0 The limiting reagent in each step is highlighted in red.\u00a0 Triiodide is yellow so we now have a solution that we can see.\u00a0 The more oxygen in the original sample the more yellow the solution.\u00a0 On the Melville, Melissa uses an automated titration system measure the triiodide by the stoichiometric reaction of triiodide with thiosulfate, S<sub>2<\/sub>O<sub>3<\/sub><sup>2-.<\/sup>.<\/p>\n<p><span style=\"color: #000000\">2 S<\/span><sub><span style=\"color: #000000\">2<\/span><\/sub><span style=\"color: #000000\">O<\/span><sub><span style=\"color: #000000\">3<\/span><\/sub><sup><span style=\"color: #000000\">2-<\/span><\/sup><span style=\"color: #000000\">(aq)<\/span> + <span style=\"color: #ff0000\">I<\/span><sub><span style=\"color: #ff0000\">3<\/span><\/sub><sup><span style=\"color: #ff0000\">&#8211;<\/span><\/sup><span style=\"color: #ff0000\"> <\/span><span style=\"color: #ff0000\">(aq)<\/span> \u2192 S<sub>4<\/sub>O<sub>6<\/sub><sup>2-<\/sup>(aq) + 3 I<sup>&#8211; <\/sup>(aq)\u00a0\u00a0\u00a0 (4)<\/p>\n<p>When the solution returns to a constant color all of the triiodide has been converted back to I<sup>&#8211;<\/sup>. \u00a0In the video the black box on the right monitors the color of the reaction. \u00a0The tube entering the top of the bottle adds thiosulfate. \u00a0 It may seem easier to simply measure the intensity of the yellow solution.\u00a0 This would probably work for really clear water samples, but it doesn\u2019t work for water samples that are murky or have some other source of color.<\/p>\n\r\n<!-- <script src=\"https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/jquery.tools.min.js\"><\/script> -->\r\n<script type=\"text\/javascript\" src=\"https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/flowplayer-3.2.2.js\"><\/script>\r\n<style>\r\n\/* player container *\/\r\ndiv.player {\r\n\theight:272px;\r\n\twidth:480px;\r\n\tcursor:pointer;\r\n\tfloat:left;\r\n\ttext-align:center;\r\n\tmargin-right:15px;\r\n}\r\n\/* play button height *\/\r\ndiv.player img {\r\n\tmargin-top:95px !important;\r\n}\r\n<\/style>\r\n\r\n<div class=\"player\"  \r\n    href=\"mp4:king\/colbyatsea\/OxygenII\" \r\n    style=\"background-color:#000000;background-image:url(http:\/\/web.colby.edu\/colbyatsea\/files\/2011\/02\/oxy2.jpg)\"> \r\n \r\n    <!-- play button --> \r\n    <img decoding=\"async\" src=\"https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/play_large.png\" style=\"border:0px !important;\" alt=\"Play this video\" \/>      \r\n<\/div> \r\n<script type=\"text\/javascript\">\r\n$f(\"div.player\", \"https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/flowplayer-3.2.2.swf\", {\r\n\tplugins: {\r\n\t\trtmp: {\r\n\t\t\turl: 'https:\/\/web.colby.edu\/colbyatsea\/wp-content\/plugins\/fp_rtmp\/flowplayer.rtmp-3.2.1.swf',\r\n\t\t   \tnetConnectionUrl: 'rtmp:\/\/flash.colby.edu\/vod',\r\n\r\n\t\t   \t\/\/ make the rtmp plugin query the stream length from the server\r\n\t\t   \tdurationFunc: 'getStreamLength'\r\n\t\t},\r\n\t\tcontrols: {\r\n\t\t\tfullscreen: true,\r\n\t\t\theight: 30,\r\n\t\t\tautoHide: true\r\n\t\t}\r\n\t},\r\n   clip:{\r\n         autoBuffering:true,\r\n         autoPlay:true,\r\n         scaling:'fit',\r\n\t\t provider: 'rtmp'\r\n   }\r\n}); \r\n<\/script>\r\n\r\n\n<p>Notice that the samples can be handled in the air because step four occurs in acid.\u00a0 Melissa can process twelve oxygen samples in about an hour.\u00a0\u00a0\u00a0 She has processed over 600 samples during the cruise.\u00a0 That\u2019s a lot of titrations!<\/p>\n<p>&#8211; Whitney<\/p>\n","protected":false},"excerpt":{"rendered":"<p>We have been asked for another example of a chemical analysis being performed on the ship.\u00a0\u00a0 One of the most famous measurements is the determination of dissolved oxygen in seawater.\u00a0\u00a0 The CTD has an electrochemical sensor that measures oxygen continuously &hellip; <a href=\"https:\/\/web.colby.edu\/colbyatsea\/2011\/02\/11\/winkler-titrations-measuring-dissolved-oxygen\/\">Continue reading <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":184,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"ngg_post_thumbnail":0,"footnotes":""},"categories":[17554,17553,1178,1],"tags":[],"_links":{"self":[{"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/posts\/409"}],"collection":[{"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/users\/184"}],"replies":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/comments?post=409"}],"version-history":[{"count":10,"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/posts\/409\/revisions"}],"predecessor-version":[{"id":424,"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/posts\/409\/revisions\/424"}],"wp:attachment":[{"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/media?parent=409"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/categories?post=409"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/web.colby.edu\/colbyatsea\/wp-json\/wp\/v2\/tags?post=409"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}