Chemistry of water -Part 18-

4- Measurement of dissolved oxygen:

The first approach relies on the volumetric titration of dissolved oxygen in accordance with its oxidizing characteristics.

[Photo taken by me, in the water treatment plant laboratory ُSEOR'. Mostaganem. Algeria. (2018)]

Reagents used:

• Potassium fluoride solution with a concentration of 400 g/L.
• Concentrated sulfuric acid.
• An alkaline solution of potassium iodine and sodium azide, NaN3. The solution is prepared in 80 grams of distilled water, and 15 grams of potassium iodine and 35 grams of sodium hydroxide are added to it. Then we add to the previous solution 1 gram of sodium azide dissolved in 10 milliliters of distilled water.
  Since nitrite is involved in oxidation and reduction reactions, sodium azide is added to the reagents, to remove nitrite from the aqueous sample.
• Solution of sodium thiosulfate.
• Solution of manganese sulfate.

Immediately after obtaining the aqueous sample, 1 ml of manganese sulfate solution is added, in addition to 1 ml of alkaline solution, meanwhile, atmospheric oxygen must not be allowed to enter the bottle containing the water sample to be calibrated. After 5 minutes, a white precipitate will be formed, and to dissolve this precipitate, 2 ml of concentrated sulfuric acid is added, then the sample is calibrated with sodium thiosulfate solution in the presence of the indicator to know the transformation point. The following relationship allows finding the amount of dissolved oxygen:

O2= (8000. V2. t. V1) / (V0 (V2- 2))

O2: Amount of dissolved oxygen (mg/L).
V1: Volume of the sodium thiosulfate solution (ml).
V2: Volume of the aqueous sample (ml).
t: Titer of thiosulfate solution.
V0: The volume of the sample titrated with sodium thiosulfate.

References:

• [General and inorganic chemistry book- M. Shkhashirou- H. Birqdad- Y. Qodsi- University publications. Algeria]

• [AQUAPROX- Livre: Traitement des eaux de refroidissement. Imprimé en France par EMD S.A.S- 53110 Lassay-les-Chateaux. N° d'imprimeur: 15566- Dépot légal: juin 2006. N° 842- Cyclus print 90°]

• [GÉOCHIMIE DES EAUX CONTINENTALES- Nathalie Gassama. ISBN 9782340-047402 ©Ellipses Édition Marketing S.A., 2021. 8/10 rue la Quintinie 75015 Paris]

• [Introduction to Water Chemistry (Pollution- Treatment- Analysis). Dr. Nasser Al-Hayek. Publication of the Higher Institute for Applied Sciences and Technology (HIAST). Syrian Arab Republic, 2017.]

• "Chapter A2. Selection of Equipment for Water Sampling" (PDF). National Field Manual for the Collection of Water-Quality Data (Report). Techniques of Water-Resources Investigations, Book 9. Reston, VA: U.S. Geological Survey. March 2003.

• Folco Laverdière, Anja Holstein, Laurent Thiebaut, Robert Mallee, Guillaume Gravejat, Benjamin Des clozeaux: Les principales methodes d’analyse,1999, p5.