3. 2. Determination of dissolved oxygen in water using the Winkler method
The Winkler method, determines the dissolved oxygen concentration through a series of oxidation – reduction reduction reactions. First, Mn2+ (as MnSO4) is added to a 250- or 300-mL sample. Next, the alkali – – iodide iodide reagent (KI in NaOH) is added. Under these caustic conditions, if oxygen is present in the water sample, the Mn2+ will be oxidized to Mn4+, which precipitates as a brown hydrated oxide. This reaction is relatively slow and the solution must be shaken several times to complete the reaction. This reaction can be represented by the following equations:
The titration is complete when all of the I2 has been converted to I-. The endpoint of this titration can be determined through colorimetric indicators. The most common indicator is starch, which turns from deep blue to clear. The DO concentration can be determined using the following equation, which also reflects the series of redox reactions in the equations given above:
Several modifications of the Winkler method have been developed to overcome interferences. The azide modification, the most common modification, effectively removes interference from nitrite, which is commonly present in water samples from biologically treated wastewater effluents and incubated biochemical oxygen demand samples. Nitrite interferes by converting I- to I2, thus
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overestimating the dissolved oxygen in the sample. This is illustrated in the following equations:
Note that N2O2 is oxidized by oxygen, which enters the sample during the titration procedure and is converted to NO2- again, establishing a cyclic reaction that can lead to erroneously high results. This final result yields oxygen concentrations that are far in excess of the amounts that would normally be expected. Nitrite interference can easily be overcome through the addition of sodium azide (NaN3). Azide is generally added with the alkali – KI reagent, and when sulfuric acid is added, the following reactions result in the removal of NO2- :
Other methods can also be used to remove ferrous iron (the permanganate modification), ferric iron (the potassium fluoride modification), and suspended solids (the alum flocculation modification). We will be using only the azide modification in this laboratory practical. You will be given one water sample for titration using the Winkler method. For this laboratory exercise you do not have to be concerned with preservation of the sample or sample-handling practices, but in the real world many precautions need to be taken. Most important is the preservation of field samples that need to be analyzed in the laboratory. Two approaches are used to preserve samples for later DO determination. First, you can ‘‘fix’’ your samples using the procedures described below and then perform the titration when the samples are brought to the laboratory. Samples should be stored in the dark and on ice until titration. This preservation technique will allow you to delay the titration for up to 6 hours. However, this procedure may give low results for samples with a high iodine demand. In this case it is advisable to use the second option, which is to add 0.7 mL of concentrated sulfuric acid and 0.02 g of sodium azide. When this approach is used, it is necessary to add 3 mL of alkali – iodide reagent (below) rather than the usual 2 mL. In addition, avoid any sample treatment or handling that will alter the concentration of DO, including increases in temperature and the presence of atmospheric headspace in your sample container. You will titrate your samples using the procedures described below. Conduct at least three careful titrations. Safety Pr ecaution s : Avoid skin and eye contact with caustic and acidic solutions. If contact
occurs, rinse your hands and/or flush your eyes for several minutes. Seek immediate medical advice for eye contact. Use concentrated acids in the fume hood and avoid breathing their vapors. Sodium azide is a toxin and should be treated as such.
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Chemicals and Solutions Manganese sulphate: Dissolve 48 g of MnSO4.4H2O, 40 g of MnSO42H2O, or 36.4 g of MnSO4 H2O in about 80 mL of deionized water. Filter the solution and dilute to 100 mL. Alkali – iodide – azide reagent . Dissolve 50 g of NaOH (or 70 g of KOH) and 13.5 g of NaI (or 15 g of KI) in deionized water and dilute to 100 mL. Add 1 g of NaN3 dissolved in 4 mL of deionized water. Concentrated H 2SO4. (1.0 mL of this solution is equivalent to approximately 3 mL of alkali – iodide – azide solution.) Starch solution . Dissolve 2 g of laboratory-grade soluble starch and 0.2 g of salicylic acid (as a preservative) in 100 mL of hot distilled water. Allow to cool before use. Refrigerate. Sodium thiosulphate titrant, 0.0250 M: . Dissolve 6.205 g of Na2S2O35H2O in deionized water. Add 1.5 mL of 6 M NaOH or 0.4 g of solid NaOH and dilute to 1.0 L. Standardize with biiodate solution. Standard potassium biiodate solution, 0.00210 M. Dissolve 0.8124 g of KH(IO3)2 in deionized water and dilute to 1.000 L. Glassware For each student group: three Erlenmeyer flasks, 25-mL buret, 20.00-mL pipet, Pasteur pipets, three 1.00-mL pipets (at least one of these should be a wide-bore pipet for the viscous azide reagent) Procedure Standar dization of Sodium Th iosulphatee Ti tr ant
Note: The thiosulphate titrant may already have been standardized by your demonstrator. If so, skip to step 5. 1. Dissolve approximately 2 g of KI (free of iodate) in an Erlenmeyer flask containing 100 to 150 mL of deionized water. 2. Add 1 mL of 6 M H2SO4 or a few drops of concentrated H2SO4 and pipet 20.00 mL of standard biiodate solution into the flask. Recall from the reactions given in the theory section that I2 will be formed from the reaction when any DO is present. 3. Titrate the liberated I2 with thiosulphate titrant until a pale straw (yellow) color is reached. Add a few drops of starch indicator, which will result in a blue color, and continue the titration to the endpoint, which is clear. 4. If all solutions were made properly, 20.00 mL of the biiodate solution will require 20.00 mL of thiosulfate titrant. If this result is not achieved, calculate the exact molar concentration of your titrant.
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Ti tr ation of Water Samples
5. To a 250- or 300-mL sample bottle, add 1 mL of MnSO4 solution, followed by 1 mL of alkali – iodide – azide reagent. If your pipets are dipped into the sample (as they should be), rinse them before returning them to the reagent bottles. If the solution turns white, no DO is present. 6. Stopper the sample bottles in a manner to exclude air bubbles and mix by inverting the bottle rapidly a few times. When the precipitate has settled to half the bottle volume, repeat the mixing and allow the precipitate to resettle. 7. Add 1.0 mL of concentrated H2SO4. 8. Restopper and mix by inverting the bottle rapidly and dissolve the precipitate. You may open the bottle and pour the sample at this point since the DO and reagents have been ‘‘fixed’’ and will not react further. 9. Titrate 200 mL of the sample with sodium thiosulphate solution (0.025 M), stirring the contents of the flask until the yellow-brown colour fades to a pale straw colour. Add a few drops of starch solution and a blue colour will develop. Continue titrating a drop at a time until the blue colour disappears 10. Repeat the titration for two more samples and determine the mean value, standard deviations and relative standard deviations of the dissolved oxygen level for the water sample. Calculation For titration of 200 mL of sample with 0.025 M sodium thiosulphate: 1 ml Na2S2O3 solution=1 mg L-1 dissolved oxygen
If sodium thiosulphate is used at a strength other than 0.025 M, and if the sample volume titrated is other than 200 ml), the dissolved oxygen in the sample may be calculated from the following formula:
___________________________________________________________________________References: American Public Health Association, APHA (1998). Standard Methods for the Examination of Water and Wastewater. 20th ed. 1220pp. American Public Health Association, Washington DC, USA.APHA (1999) Csuros M. (1997) .Environmental Sampling and Analysis, CRC press
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