Determination of dissolved oxygen in waters and waste water

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Introduction

Dissolved oxygen (DO) levels in natural and waste waters depend on the physical, chemical and biological activities in the water body and its determination is a key test in water pollution and wastewater treatment process control. Before selecting a method for the determination of DO in water samples, the effect of interference, especially oxidizing and reducing materials that may be present in the sample are considered. Certain oxidizing agents liberate iodine from iodides (positive interference) and some reducing agents reduce iodine to iodide (negative interference). Most organic matter is oxidized partially when the oxidized manganese precipitate is acidified, thus causing negative errors.

There are basically two methods of DO determination; the Winkler or iodometric method and the electrometric method, which uses electrodes. The iodometric method is a titration procedure based on the oxidizing property of DO while the membrane electrode procedure is based on the rate of diffusion of molecular oxygen across a membrane. The choice of procedure depends on the interference present, the accuracy desired and in some cases convenience and experience. Several modifications have been made on the iodometric method to minimize the effect of interference, among which is the Azide modification, the permanganate modification, the alum flocculation modification and the cupper sullfate-sulframic acid flocculation modification. The Azide modification of the Winkler technique will be employed in this practical. This method is appropriate for most wastewater and effluents, (which constitute our samples) and is the most precise and reliable titrimetric procedure for DO analyses. It effectively removes interferences caused by nitrite, which is the most common interference in biologically treated effluents and incubated BOD samples.

Apparatus

General laboratory glassware
DO bottles
Titration set up

Reagents

a. Manganous sulphate solution: Dissolve 480g MnSO4.4H2O, 400g MnSO4.2H2O, or 364g. MnSO4.H2O in distilled water, filter if dissolution is incomplete, and dilute to 1L. The MnSO4 solution should not give a color with starch when added to an acidified potassium iodide (KI) solution.
b. Alkali-iodide-azide reagent: Dissolve 500g NaOH (or 700g KOH) and 135g NaI (or 150g KI) in distilled water and dilute to 1L. Add 10g NaN3 dissolved in 40ml distilled water. This reagent should not give a colour with starch solution when diluted and acidified.
c. Sulphuric acid H2SO4 conc:
d. Starch: Use either an aqueous solution or soluble starch powder mixture: To prepare an aqueous solution, dissolve 2g laboratory grade soluble starch and 0.2g salicylic acid as a preservative in 100 ml hot distilled water
e. Standard sodium thiosulfate titrant (Na2S2O3): Dissolve 6.205g Na2S2O3.5H2O in distilled water. Add 1.5ml 6N NaOH or 0.4 g solid NaOH and dilute to 1000ml. Standardize with bi-iodate solution.
f. Standard bi-iodate solution, 0.0021M: dissolve 812.4mg KH (IO3)2 in distilled water and dilute to 1000ml
g. Potassium fluoride solution: dissolve 40g KF.2H2O in distilled water and dilute to 100 ml.

Procedure

Standardization: Dissolve approximately 2g KI free from iodate in 100 - 150 ml distilled water in an Erlenmeyer flask. Add 1 ml 6N H2SO4 or a few drops of conc H2SO4 and 20ml standard bi-iodate solution. Dilute to 200ml and titrate liberated iodine with thiosulfate titrant, adding a few drops of starch towards the end of titration, when a pale straw color is reached. Titrate to the first disappearance of the blue color.


1. Rinse a 300 ml BOD bottle with sample and pour the sample into the BOD.
2. Add 1 ml MnSO4 solution followed by 1 ml alkali-iodide azide reagent. If pipes are dipped into sample, rinse them before returning them to reagent bottles. Alternatively, hold pipette tips just above liquid surface when adding reagents.
3. Stopper carefully to exclude air bubbles and mix by inverting bottle a few times
4. When precipitate has settled sufficiently (to approximately half the volume of the bottle) to leave a clear supernatant above the manganese hydroxide floc, add 1.0 ml conc H2SO4
5. Re stopper and mix by inverting several times until dissolution is complete.
6. Titrate a volume corresponding to 200 ml original sample after correction for sample loss by displacement with reagent (for a total of 2 ml i.e. 1ml MnSO4 and 1 ml alkali-iodide-azide reagents in a 300 ml bottle, titrate 200 x 300/300-2) = 201ml.
7. Titrate with 0.025M Na2S2O3 solution to a pale straw color.
8. Add a few drops of starch solution and continue titration to first disappearance of blue color.

Calculation

For titration of 200 ml sample, 1 ml 0.025M Na2S2O3 should be equal to 1mg DO/l.
If the molarity of the titrant solution is not 0.025M the calculate DO as follows

DO = Vol of Na2S2O3 used x N/0.025

NB: you can also read more online.

Thank you, see you on Monday