Oxygen Winkler method

M. Novic, B. Pihlar, and M. Dular. Use of flow-injection analysis based on iodometry for automation of dissolved-oxygen (Winkler method) and chemical oxygen demand (dichromate method) determinations. Fresen. Z. Anal. Chem., 332 750-755, 1988. 

One standard method for determining the dissolved O2 content of natural waters and wastewaters is the Winkler method. A sample of water is collected in a fashion that prevents its exposure to the atmosphere (which might change the level of dissolved O2). The sample is then treated with a solution of MnS04, and then with a solution of NaOH and KI. Under these alkaline conditions Mn + is oxidized to Mn02 by the dissolved oxygen. 

Implementation Confirmation of the in situ monitors results is obtained when river water samples are brought to the lab and tested for dissolved oxygen using a lab dissolved oxygen probe (a polarographic electrode-based measurement) and the classic Winkler oxygen titration method. 

Perhaps the most important application of redox chemicals in the modern laboratory is in oxidation or reduction reactions that are required as part of a preparation scheme. Such preoxidation or prereduction is also frequently required for certain instrumental procedures for which a specific oxidation state is essential in order to measure whatever property is measured by the instrument. An example in this textbook can be found in Experiment 19 (the hydroxylamine hydrochloride keeps the iron in the +2 state). Also in wastewater treatment plants, it is important to measure dissolved oxygen (DO). In this procedure, Mn(OH)2 reacts with the oxygen in basic solution to form Mn(OH)3. When acidified and in the presence of KI, iodine is liberated and titrated. This method is called the Winkler method. 

The Winkler method for dissolved oxygen in water is based on the rapid oxidation of solid Mn(OH)2 to Mn(OH)3 in alkaline medium. When acidified, the Mn(III) readily releases iodine from iodide. A 150-mL water sample, in a stoppered vessel, was treated with 1.00 mL of a concentrated solution of Nal and NaOH and 1.00 mL of a man-ganese(II) solution. Oxidation of the Mn(OH)2 was complete in about 1 min. The precipitates were then dissolved by addition of 2.00 mL of concentrated H2SO4, whereupon an amount of iodine equivalent to the Mn(OH)3 (and hence to the dissolved O2) was liberated. A 25.0-mL aliquot (of the 254 mL) was titrated with 13.67 mL of 0.00942 M thiosulfate. Calculate the mass in milligrams O2 per milliliter sample. (Assume that the concentrated reagents are O2 free, and take their dilutions of the sample into account.)... 

All vertical profiles of the dissolved gases were obtained at the deepest site of the western basin, at the point with coordinates 45°06 N, 58°23 E ( Station A2 ). The samples were taken with Molchanov bottles from the standard depth levels. The dissolved oxygen content was obtained through the Winkler method, and the content of hydrogen sulfide through titration. 

Chemical reaction Fixes a particular constituent Samples to be analysed for dissolved oxygen using the Winkler method... 

One way to do this is to determine the AG° values for each half-reaction and then add them to determine the value of AG° for the complete redox reaction. Let us examine one of the steps of the Winkler method for determining dissolved oxygen (DO) as an example of this technique. The last step of the Winkler method is the titration of iodine (Igoq)) to iodide (I ) using thiosulfate, 8203 , which is itself oxidized to tetrathionate, 8406 . The reaction takes place in acid solution. The two half-reactions are... 

This procedure is called Winkler method and it is often used to determine dissolved oxygen in natural waters (33). 

Measure oxygen on site electrometrically or chemically according to Winkler method... 

Dissolved oxygen Winkler s method - Specific electrode method. 

The main requirement in the use of membranes in the construction of oxygen sensors is that the membrane must be permeable to oxygen, while restricting the passage of water and electrolyte. On this basis, the use of a single point calibration method based on the use of air, with the assumed linearity of response for other measurement, is feasible. Another calibration method involves the use of solutions previously verified by the Winkler oxygen titrimetric method. 

Modifications of the original Winkler method (e.g., Strickland and Parsons, 1960 Carpenter, 1965 Carrit and Carpenter, 1966) have mainly improved the technical details of the analytical procedure. It is noteworthy that the precision of historical Winkler determinations of oxygen is close to that of today s routine analyses. 

The Winkler method is an iodometric titration. As dissolved oxygen in seawater does not directly oxidize the iodide ion to iodine, a multi-step oxidation is performed using manganese as a transfer medium. 

Modern versions of the Winkler method improve the sensitivity and accuracy of the method by computer control of the titration procedure and the endpoint detection. Instead of visual observation of the decolouration of the blue starch-iodine complex, either the starch-iodine complex colour or the iodine colour itself is measured photometrically in the visible to ultraviolet (UV) spectral range. The spectral absorbance of an I3- solution (oxygen sample before titration) is depicted in Fig. 4-1. Grasshoff (1981) described a dead-stop titration of iodine with thiosulphate using amperometric endpoint detection. Bradburg and Hambly (1952) have compared various endpoint detections for iodine-thiosulphate titrations in low concentration ranges and stated relative sensitivities for visual-starch, colouri-metric-starch, amperometric, UV absorption as 1 0.2 0.002 0.0015. 

One of the most accurate methods for measuring dissolved oxygen concentrations in water is the Winkler method. Under alkaline conditions manganese(ii) ions are rapidly oxidized to manganese(iii) by dissolved oxygen, producing a pale brown precipitate of manganese(iii) hydroxide, Mn(OH)3 ... 

Applying the Winkler method to calculate biological oxygen demand... 

Q13 Which of the following chemicals is not involved in the Winkler method for determining the concentration of dissolved oxygen in water ... 

The method described here is a modification of the classical Winkler procedure which we are convinced still remains the most reliable and precise means of analysing for dissolved oxygen in sea water. If a very precise estimate of extremely low oxygen concentrations is required on a routine scale then an absorptiometric determination of the iodine liberated in the Winkler method may prove superior to a titration procedure (Oulman and Baumann, Sewage Ind. Wastes, 28 1461, 1956) but not unless oxygen concentrations are less than about 0.1 mg-at/liter. 

The dissolved oxygen concentrations are determined immediately and after five days. The method for dissolved measurement involves either a modified Winkler titration or a membrane-covered oxygen electrode. The difference between initial and final dissolved oxygen multiphed by the dilution factor is the BOD value. 

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