Dissolved Oxygen (DO) is a fundamental measure of aquatic health, despite oxygen gas having a naturally low solubility in water. This limited resource is rapidly consumed by natural processes that break down and oxidize organic pollutants and reduced inorganic materials.
When contamination is present, this process depletes the available DO. Therefore, low dissolved oxygen levels serve as a critical red flag, directly indicating the presence of oxygen demanding waste and signifying inferior water quality that can harm aquatic ecosystems.
Principle
When manganese sulfate solution is added to sample followed by strong alkali, DO rapidly oxidizes an equivalent amount of the dispersed divalent manganous ions, which precipitate to hydroxides of higher valency state.
In the presence of iodide ion in an acidic solution the oxidized manganese reverts back to divalent state with the liberation of iodine equivalent to the original DO contents. The iodine is than titrated with thiosulfate.
Reagents
1) Manganese sulfate: Dissolve 480 g MnSO4, 4H2O or 400 g MnSO4, 2H2O or 364.0 g MnSO4, H2O in distilled water, filter and dilute to 1L.
The solution should liberate not more than a trace of iodine when added to an acidified solution of iodide.
2) Alkali-azide-iodide reagent: Dissolved 500 g NaOH (or 700 g KOH) and 135 g Nal (or 150 g KI) in the distilled water. Add 10 gm of solution azide (NaN3) in 40 ml distilled water.
Make the volume to one liter. The reagent should not give colour with starch solution when diluted and acidified.
3) Concentrated H2SO4 (approx. 36 N): One ml acid is equivalent to about 3ml of iodide reagent.
4) Standard sodium thiosulfate (0.025N): Dissolve 6.205 g Na2S2O3, 5H2O in distilled water, boil and make the volume to 1L.
Standardize with standard K2Cr2O7 solution.
5) Starch indicator: Grind 5 g starch and 0.1 g Hgl with 30 ml of cold distilled water, and slowly pour it in a 1L boiling water with constant stirring.
Boil for 3 minutes and cool to room temperature. After cooling decant off the clear solution and use it as an indicator.
Procedure
Collect the sample in a BOD bottle taking care to see that no air bubble is entrapped therein. Add to it 2ml of MnSO4 followed by 2ml of azide solution well below the surface of the liquid.
Stopper with care to exclude air bubbles if any. Shake the bottle thoroughly by inverting it several times.
Allow it to stand still so as to settle the precipitate at the bottom. Carefully remove the stopper and add 1ml of concentrated H2SO4, stopper it again and shake well so as to dissolve the precipitate in H2SO4, without loosing much time titrate 200 ml of solution with standard Na2S2O3 to a pale straw color.
Add 1 or 2 ml of starch solution and continue titration up to the first disappearance of the blue color.
Note: DO determination is conducted on site. In absence of the facilities, sample should be fixed with the addition of MnSO4 and alkali azide solution before it’s transport to any other place.
Calculation
Mg/L DO =
Vol. of 0.025 N Thio x 1000 x 0.2
200
Reactions
MnSO4 + 2KOH - Mn (OH)2 (white ppt) +
K2SO4
2Mn(OH)2 + O2 - 2MnO (OH)2(brown ppt )
MnO(OH)2 + 2H2SO4 -Mn(SO)4 +3H2O
Mn(SO4)2 +2KI - MnSO4 + K2SO4 + I2 Since I2 = O2
2Na2S2O3 + I2 - 2Nal + Na2S4O6
NO2 interference
2NH3 +3O2 - NO2 for ming bacteria - NO2 +2H + 2H2O
2NO2 + O2 +H2 -NO2 for ming bacteria - 2NO - + 2H+
Removal of NO2 interference by azide:
NaN3 +H+ - HN3 +Na+
HN3 + NO2 + H+ - N2 + N2O + H2O