Determination of dissolved oxygen

The reaction velocity is comparatively slow, but increases with increasing concentration of acid. The addition of three drops of a neutral 20 per cent ammonium molybdate solution renders the reaction almost instantaneous, but as it also accelerates the atmospheric oxidation of the hydriodic acid, the titration is best conducted in an inert atmosphere (nitrogen or carbon dioxide). 

The iodometric method has the advantage over the permanganate method (Section 10.95) that it is less affected by stabilisers which are sometimes added to commercial hydrogen peroxide solutions. These preservatives are often boric acid, salicylic acid, and glycerol, and render the results obtained by the permanganate procedure less accurate. 

Better results are obtained by transferring 25.0 mL of the diluted hydrogen peroxide solution to a conical flask, and adding 100 mL 1M(1 20) sulphuric acid. Pass a slow stream of carbon dioxide or nitrogen through the flask, add 10 mL of 10 per cent potassium iodide solution, followed by three drops of 3 per cent ammonium molybdate solution. Titrate the liberated iodine immediately with standard 0.1M sodium thiosulphate in the usual way. 

The above method may also be used for all per-salts. 

Discussion. One of the most useful titrations involving iodine is that originally developed by Winkler18 to determine the amount of oxygen in samples of water. The dissolved oxygen content is not only important with respect to the species of aquatic life which can survive in the water, but is also a measure of its ability to oxidise organic impurities in the water (see also Section 10.103). Despite the advent of the oxygen-selective electrode (Section 16.36) direct titrations on water samples are still used extensively.19 

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Another important example of redox titrimetry that finds applications in both public health and environmental analyses is the determination of dissolved oxygen. In natural waters the level of dissolved O2 is important for two reasons it is the most readily available oxidant for the biological oxidation of inorganic and organic pollutants and it is necessary for the support of aquatic life. In wastewater treatment plants, the control of dissolved O2 is essential for the aerobic oxidation of waste materials. If the level of dissolved O2 falls below a critical value, aerobic bacteria are replaced by anaerobic bacteria, and the oxidation of organic waste produces undesirable gases such as CH4 and H2S. 

Ross, F. F., Water Waste Treatment, 1964, 9, 528 private comm., 1966 One of the reagents required for the determination of dissolved oxygen in polluted water is a solution of sodium azide in 50% sulfuric acid. It is important that the diluted acid should be quite cold before adding the azide, since hydrogen azide boils at 36°C and is explosive in the condensed liquid state. 

The reactions have been utilized for the determination of dissolved oxygen or hydrogen peroxide. Quantitative Analysis... 

J.-M. Zen, C.-W. Wang, Determination of Dissolved Oxygen by Catalytic Reduction on a Nafion /Ruthenium Oxide Pyrochlore Chemically Modified Electrode, J. Electroanal. Chem. 368 (1994) 251—256. 

International Standard Organization. 1983. Water quality. Determination of dissolved oxygen. Iodometric method. ISO 5813. International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland. 

The best-known example of a membrane-covered electrode is the Clark electrode for determination of dissolved oxygen (Fig. 14.49,10). A membrane (usually PTFE) with pores of a size that lets only oxygen diffuse through is placed over a thin film of electrolyte on top of a platinum or gold electrode, the potential of this being controlled so as to reduce oxygen. The anode is usually a silver disc that acts simultaneously as reference electrode. It can be used for oxygen determination in gas or liquid phases. 

Karchmer, J.H. Polarographic Determination of Dissolved Oxygen. Study... 

Johll, M. E. Poister, D. Ferguson, J. Statistical Comparison of Multiple Methods for the Determination of Dissolved Oxygen Levels in Natural Waters, Chem. Educ. 2002, 7, 1146-1148. 

McCormick, P. G. The Determination of Dissolved Oxygen by the Winker Method— A Student Laboratory Experiment, /. Chem. Educ. 1972, 49, 839-841. 

Oxygen wave At the dropping mercury electrode, oxygen produces two waves, the first due to formation of peroxide, the second due to further reduction to water this can be an interference in the determination of other species but is used in the determination of dissolved oxygen. 

Standard Method 4500-0 describes two methods for determination of dissolved oxygen (DO) in water Winkler s iodometric method and the elecrometric method (Standard Methods, 1998). The iodometric method is very accurate and precise, but the electrometric method is far more convenient for field use (e.g. in wastewater treatment system monitoring and control) and produces an electronic output that can easily be converted to digital form for microprocessor monitoring or control of wastewater treatment systems. Also, electrometric methods are not subject to certain interferences (i.e. oxidation or reduction of the iodine indicator). In addition, the iodometric method end point may be obscured by the presence of turbidity or color in textile wastewater samples. The electrochemical method is almost exclusively used in testing of textile wastewater. 

Sakai, T., Piao, S., Teshima, N. et al. (2004) Flow injection system with in-line Winkler s procedure using 16-way valve and spectrofluorimetric determination of dissolved oxygen in environmental waters. Talanta, 63 (4), 893-898. 

Winkler s method [30] for the determination of dissolved oxygen is based on the reaction of O2 with Mn(OH)2 (MnSO and KOH are added to the... 

Potassium iodide is also used in the production of photographic film and in a number of chemical tests, such as the determination of dissolved oxygen in water and the presence... 

The Winkler determination of dissolved oxygen involves the formation of MnOas> by the oxidation of Mn with oxygen. The MnOas) is then reacted with 1 to form Isoq) Determine the E value of a solution that contained 8 mg DO/liter at a point when half of the MnOa,s) formed by reaction with DO has been reduced to Mn by 1 (which itself is oxidized to laa ))- The concentration is approximately 1 M. The following reagent concentrations and quantities are used in the determination ... 

A conventional polarograph with a dropping mercury electrode may indeed be employed for the determination of dissolved oxygen. 

As a rule, subsampling for the determination of dissolved oxygen should be carried out as soon as possible after the sampler has been recovered. The bottles are unstoppered and after flushing the nozzle of the hydrocast sampler with sample water, the end of the nozzle tube is inserted into the sample bottle almost to the bottom. The nozzle should be transparent and sufficiently narrow to ensure that the sample stream carries with it any air bubbles and the air segment in the tube but wide enough to fill the bottle rapidly. 

The same instrumentation as used for the determination of dissolved oxygen is required for the determination of thiosulphate in seawater (see Chapter 4). The same type of sample bottles is used, but with a content of about 100 mL instead of SOmL. The piston burette should be calibrated carefully and be readable to 0.01 mL. 

Potassium iodate Exactly 356.7 mg of dried (180 "C) potassium iodate (a.g.) are dissolved and made up to 1000mL. The solution is 1.667 nunol/L. (The reagent is the same as used for the determination of dissolved oxygen see Chapter 4.)... 

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