12.097 Environmental Chemistry of Boston Harbor – IAP 2006
Lab 1: DETERMINATION OF DISSOLVED OXYGEN BY WINKLER TITRATION 1. Background Knowledge of the dissolved oxygen (O2) concentration in seawater is often necessary in environmental and marine science. It may be used by physical oceanographers to study water masses in the ocean. It provides the marine biologist with a means of measuring primary production - particularly in laboratory cultures. For the marine chemist, it provides a measure of the redox potential of the water column. The concentration of dissolved oxygen can be readily, and accurately, measured by the method originally developed by Winkler in 1888 (Ber. Deutsch Chem. Gos., 21, 2843). Dissolved oxygen can also be determined with precision using oxygen sensitive electrodes; such electrodes require frequent standardization with waters containing known concentrations of oxygen. They are particularly useful in polluted waters where oxygen concentrations may be quite high. In addition, their sensitivity can be exploited in environments with rapidly-changing oxygen concentrations. However, electrodes are less reliable when oxygen concentrations are very low. For these reasons, the Winkler titration is often employed for accurate determination of oxygen concentrations in aqueous samples. 2. Scope and field of application This procedure describes a method for the determination of dissolved oxygen in aqueous samples, expressed as mL O2 (L water) -1. The method is suitable for the assay of oceanic levels of oxygen in uncontaminated seawater and is based on the Carpenter (1965) modification of the traditional Winkler titration. 3. Definition The dissolved oxygen concentration of seawater is defined as the number of milliliters of dioxygen gas (O2 ) per liter of seawater (mL L -1 ). 4. Principle of Analysis The chemical determination of oxygen concentrations in seawater is based on the method first proposed by Winkler (1888) and modified by Strickland and Parsons (1968). Oxygen in the water sample oxidizes iodide ion (I-) to iodine (I2) quantitatively. The amount of iodine generated is then determined by titration with a standard thiosulfate (S2O3-2) solution. The endpoint is determined by using starch as a visual indicator. The amount of oxygen can then be computed from the titer: one mole of O2 reacts with four moles of thiosulfate.
12.097 Environmental Chemistry of Boston Harbor – IAP 2006 At the time of sampling, dissolved oxygen is fixed by the addition of Mn(II) under basic conditions, resulting in a brown precipitate, manganic hydroxide (MnO(OH)2). Prior to analysis, the sample is acidified to pH 1.0-2.5. This causes the precipitated hydroxides to dissolve, liberating Mn(III) ions. Mn(III) ions oxidize previously added iodide ions to iodine. Iodine forms a complex (I3-) with surplus iodide ions. Iodine and the complex exist in equilibrium; thus, I3- serves as a reservoir of I2. The iodine is then titrated with thiosulfate; iodine is reduced to iodide and the thiosulfate is oxidized to tetrathionate. The stoichiometric equations for the reactions described above are: Mn +2 + 2OH − → Mn (OH )2 1 oxidation of Mn(II) to Mn(III) 2 Mn(OH )2 + O2 + H 2 O → 2 MnO (OH )2 2 2 Mn(OH )3 + 2 I − + 6 H + → 2 Mn +2 + I 2 + 6 H 2 O oxidation of I- to I2
I 2 + I − ↔ I 3− I 3− + 2 S 2 O3−2 → 3I − + S 4 O6−2
oxidation of S2O3-2 to S4O6-2; reduction of I3- to I-
The thiosulfate solution is not stable and therefore must be standardized with a primary standard, typically potassium iodate (KIO3). Standardization is based on the co-proportionation reaction of iodide with iodate, thereby forming iodine. As described above, the iodine binds with excess iodide, and the complex is titrated with thiosulfate. One mole of iodate produces three moles iodine, which are consumed by six moles of thiosulfate. IO3− + 8 I − + 6 H + → 3I 3− + 3H 2 O I 3 + 2 S 2 O3 − 2−
→ 3 I − + S 4 O6
A note to the student: The entire...
References: Carpenter, J.H. (1965). The Chesapeake Bay Institute. Technique for the Winkler oxygen method. Limnol. Oceanogr., 10, 141–143. Grasshoff, K. Ehrhardt, M, and K. Kremling (1983). Methods of Seawater Analysis. Grasshoff, Ehrhardt and Kremling, eds. Verlag Chemie GmbH. 419 pp. Murray J.N., Riley, J.P. and Wilson, T.R.S. (1968). The solubility of oxygen in Winkler reagents used for the determination of dissolved oxygen. Deep-Sea Res., 15, 237–238. Strickland, J.D.H., and Parsons, T.R. (1968). Determination of dissolved oxygen. in A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada,Bulletin, 167, 71–75. Williams, P.J.leB., and Jenkinson, N.W. (1982). A transportable microprocessorcontrolled precise Winkler titration suitable for field station and shipboard use. Limnol. Oceanogr., 27 (3), 576–584. Winkler, L.W. (1888). Die Bestimmung des in Wasser gelösten Sauerstoffen. Berichte der Deutschen Chemischen Gesellschaft, 21: 2843–2855.
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