Determination of Dissolved Oxygen in Water

INTRODUCTION

The major inputs of Dissolved Oxygen (DO) to natural water are from the atmosphere and photosynthetic reactions. Where the algae and phytoplankton production is high, the saturation of oxygen can occur during the daytime. The solubility of oxygen in water depends on temperature and other factors. The presence of oxygen is essential for the survival of aquatic life in water. A rapid fall of DO in river water is one of the first indications of pollution.

Dissolved oxygen (DO) levels in environmental water depend on the physiochemical and biochemical activities in the water body and it is important and useful in pollution and waste treatment process control. Oxygen reacts with Mangnous hydroxide to form higher hydroxides, which, when acidified, liberate iodine equivalent to the amount of oxygen fixed. This iodine is titrated with a standardized Sodium thiosulfate solution using starch as an indicator.

Two methods are commonly used to determine to DO concentration: (1) The iodometric method which is a titration-based method and depends on the oxidizing property of DO and (2) The membrane electrode procedure, which works based on the rate of diffusion of molecular oxygen across a membrane.

Significance :

  • It is necessary to know the DO level to assess quality of raw water and to keep a check on stream pollution.
  • A minimum DO of 4 to 5 mg/l is desirable for the survival of aquatic life.
  • Higher values of DO may cause corrosion of Iron and steel.
  • DO test is used to evaluate the pollution strength of domestic and industrial wastes.
  • Winkler’s Modified Method :

    The Winkler’s Method is the technique used to measure DO in freshwater structures. It is used as an indicator of the health of a water body, where higher DO concentrations are associated with high production and little contamination. This test is performed on-site, as delays between sample collections and testing may result in a variation in oxygen content.

    In this method of analysis the oxygen present in the water sample oxidizes the divalent manganous ion to its higher valency, which precipitates as a brown hydrated oxide after addition of sodium hydroxide (NaOH) and potassium iodide (KI). Upon acidification, manganese reverts to divalent state and liberates iodine from KI equivalent to DO content in the sample. The liberated iodine is titrated against sodium thiosulphate (0.025N), using freshly prepared 2% starch solution as indicator. If the oxygen is absent in the sample, the MnSO4 react with the alkali to form white precipitate Mn(OH)2.

    When MnSO4 and alkali-iodide reagent (NaOH + KI) are added to a sample in the absence of oxygen, a pure white precipitate is formed.

    Mn2+ + 2OH- → Mn (OH)2 (white precipitate)

    Mn2+ is oxidized to Mn4+ and precipitates brown hydrated oxide in the presence of oxygen.

    Mn2+ + 2OH- + 0.5O2 → MnO2 ( brown hydrated precipitate) + H2O

    The oxidation of Mn2+ to MnO2 is called fixation of the oxygen, and occurs slowly at low temperature.

    Mn(OH)2 + 0.5O2 → MnO2 + H2O

    After the floc has settled, sulfuric acid is added. MnO2 is oxidized to produce I2 under low pH conditions. I2 is insoluble in water and forms complexes when there is an excess of iodide ions in solution, preventing iodine ions from escaping.

    MnO2 + 2I- + 4H+ → Mn2+ + I2 + 2H2O

    I2 + I- ↔ I3

    The sample is now ready to be titrated with thiosulfate solution.

    Relevant Indian Standard for Dissolved Oxygen Test :

    1. IS 3025 (Part 38)-1989: Method of Sampling and Test (Physical and Chemical) for Water and Wastewater, Dissolved Oxygen (DO), First Revision.