Winkler oxygen titration

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. 

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. 

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. 

Fig. 7 Vertical distribution versus density (agy kg m 3) of temperature (T), salinity (S), transmission (Xmiss), dissolved oxygen measured with YSI oxygen sensor (02SB), dissolved oxygen measured by Winkler titration (02), hydrogen sulfide (H2S), phosphate (PO4), silicate (Si), nitrite (N02), ammonia (NH4), dissolved manganese (Mn diss), bivalent iron (Fe(II)), and trivalent iron (Fe(III)) at a station near the Bosporus (Cast 16, RV Knorr 172-05 cruise, April 04,2003). Concentrations of chemical parameters are in xM...

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. 

DO can be measured by a fairly tricky wet chemical procedure known as the Winkler titration. The DO is first trapped, or "fixed", as an orange-colored oxide of manganese. This is then dissolved with sulfuric acid in the presence of iodide ion, which is converted to iodine by the oxidized manganese. The iodine is titrated using standard sodium thiosulfate. The original dissolved oxygen concentration is calculated from the volume of thiosulfate solution needed. 

Historic data on Secchi disk depth in the northern Adriatic Sea in 1911 through the present, with few interruptions of data collection, provide a measure of water transparency that could be interpreted to depict surface water productivity. These data coupled with surface and bottom water dissolved oxygen content determined by Winkler titrations and nutrient loads outline the sequence of eutrophication in the northern Adriatic Sea. Similar historical data from other coastal areas around the world demonstrate a decrease in water clarity due to phytoplankton production in response to increased nutrient loads that are paralleled by declines in water column oxygen levels. 

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... 

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. 

Manganese(II)chloride 40g of MnCl2 5H20 are dissolved and made up to 100 mL with distilled water. This solution is the same reagent as is used for the determination of oxygen (Winkler titration Chapter 4). 

Data pairs of sensor signals (voltage or equivalent) and oxygen saturations determined by Winkler-titration are collected for various temperatures (commonly 5-30 °C in steps of 5 K). 

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. 

---