Determination of Dissolved Oxygen (DO)

Objective

This experiment gives us the measure of dissolved oxygen in water or wastewater sample by Azide Modification of Winkler’s Test.

Related Theory

Dissolved Oxygen

“Dissolved oxygen is a relative measure of the amount of oxygen that is dissolved or carried in a given medium.” Dissolved oxygen is measured in mg/L.

Importance of Dissolved Oxygen (DO)

Aerobic bacteria and aquatic life such as fish must have DO up to some extent in water to survive. Wastewater treatment facilities such as lagoons or ponds, trickling filters and activated sludge plants depend on these aerobic bacteria to treat sewage.

The same type of aerobic wastewater treatment process occurs naturally in streams and ponds if organic matter is present, turning these bodies of water into “aerobic wastewater treatment plants.” If sufficient oxygen is not naturally supplied through wind and turbulence to replace the depleted oxygen, the body of water will develop a low DO and become anaerobic (or septic).  The results of septic water bodies include fish kills and anaerobic odors.

If the amount of free or DO present in the wastewater process becomes too low, the aerobic bacteria that normally treat the sewage will die. The process will not operate efficiently and septic conditions will occur.  The DO test is used to monitor the process to ensure that there is enough dissolved oxygen present to keep the process from becoming septic. A condition in which “free” or DO is present in an aquatic environment is called aerobic condition whereas in anaerobic conditions DO is not present in aquatic environment.

DO levels and water quality

0-2 mg/L: not enough oxygen to support life

2 -4 mg/L: only a few fish and aquatic insects can survive

4-7 mg/L: good for many aquatic animals, low for cold water fish

7-11 mg/L: very good for most stream fish

Sources of Oxygen

Dissolved oxygen consumption and production are influenced by plant and algal biomass, light intensity and water temperature (because they influence photosynthesis). Oxygen is introduced or removed from water by three basic methods

1. From atmosphere because of partial pressure.  
2. Plant produced oxygen due to photosynthesis

There is limited availability of oxygen in pipes, no aquatic life is present there & oxygen present in water is the one which is coming from source.

Anaerobic conditions are poisonous for aquatic life. If conditions changed from anaerobic to aerobic conditions then fish may survive and start living in that environment. But at bulk level this is highly costly to change these conditions from anaerobic to aerobic.  

Solubility of Oxygen in Water

According to Henry’s law, “the solubility of a gas in liquid is directly proportional to the pressure of the gas above the liquid at a particular temperature.”

The solubility of oxygen in water depends upon the temperature of the water and the value of solubility decreases with the increase in temperature. The solubility of atmospheric oxygen in fresh water ranges from 14.6mg/L at 0°C to about 7mg/L at 35°C at 1atm. At hot summer month, it is about 8mg/L. Amount of DO increases with decreasing salinity (freshwater holds more oxygen than saltwater does). Amount of DO in water decreases with decreasing atmospheric pressure.

The chart below shows the solubility of DO in mg/l in water at various temperatures.

 

Measurement of Dissolved Oxygen 

The dissolved oxygen can be measured by the following methods

1. The Winkler method 
2.  Azide Modification of Winkler Method 
3.  DO meter

 Azide Modification of Winkler Method Test

The sample is treated with Manganese Sulfate, Alkaline‑Iodide‑Azide reagent and finally Sulfuric Acid.  The first two chemicals combine with dissolved oxygen to form a compound which, when acid is added, releases free Iodine (from the potassium iodide).  Because the amount of Iodine released is equal to the amount of oxygen present, the sample can be titrated with Sodium Thiosulfate to determine the amount of dissolved oxygen present.  Under specific conditions, the amount of Sodium Thiosulfate used is equivalent to the amount of dissolved oxygen present in the sample.

Reagents

• Manganese Sulfate solution (MnSO₄) 
•  Alkaline Iodide Azide solution (mixture of  NaI +NaN₃) 
•  Sulfuric acid (H₂SO₄), concentrated 
•  Starch indicator solution 
•  Sodium Thiosulfate (Na₂S₂O₃ 5H₂O), 0.025 N

Apparatus

• Burette, graduated to 0.1 ml 
•  Burette stand 
•  300 ml glass BOD bottles with stoppers 
•  500 ml wide‑ mouthed Erlenmeyer flasks 
•  Pipettes with elongated tips and minimum volume of 1.0 ml (+/‑ 0.1 ml) 
•  Pipette bulb 
•  250 ml graduated cylinders 
•  Distilled water rinse bottle

Procedure

1. Take three BOD bottles. BOD bottles have the capacity of 300ml when lid is closed.
2. Fill the BOD bottle completely with sample till it overflow, but take special care to avoid adding air to the liquid being collected.
3. Place cap over it, volume more than 300ml will overflow & just 300ml will be left. 
4.  Add 1ml of Manganese Sulfate solution MnSO₄ (indicator of presence of oxygen) in each bottle.
5. Add 1 ml of Alkali Azide in each bottle and place lids over the bottles again.
6. If sample contain oxygen then there will brownish yellow precipitate formation and if there is no oxygen present then there will be white precipitate. White precipitates indicate sample as wastewater sample.
7. If brownish yellow precipitates appears then place the lid again. Now 2ml of sample is discard and volume become 300ml again.
8. Shake BOD bottle for 1 to 2 minutes 
9.  Leave the bottle for 10 – 15 minutes so that precipitates settle at the bottom of the bottle. After settlement of particles the sample becomes clear at top portion of bottle.
10. Add 1 ml of concentrated H₂SO₄ in all sample bottles. This conc. acid dissolves all the precipitates. Cover the bottle with lid and discard 1ml solution. Shake the bottles till all precipitates get dissolved 
11.  Solution colour will be reddish brown which is indication of Iodine production. The formation of brownish yellow colour is indication of Manganese Hydroxide.

Titration

1. Take 100ml of sample solution in titration flask and titrate it against 0.025N Sodium Thiosufhate (Na₂S₂O₃). 
2.  Addition must be drop wise. 
3.  During titration, solution colour turns reddish brown to brown, brown to orange, orange to yellow and from yellow to pale yellow. 
4.  As soon as, solution turned pale yellow during titration, add 1-2 ml indicator (Starch solution). Due to indicator, dark blue colour appears.
5. This solution is further titrated to get colourless solution which is the endpoint as well.
6. Note total volume consumed for titration. Calculate DO for each sample.

Environmental Significance

• Water with high DO level is good drinking purposes/community water supply.
•   In water or wastewater MO are present for degradation of organic matter and the amount of DO present tells what amount of organic matter was present.
• DO maintains aerobic conditions in water and wastewater as well. Anaerobic conditions cause smell.
• The domestic drains, have anaerobic conditions, product of anaerobic bacterial decomposition would be H₂S, NH₃, and CH₄ which would result in odour problems. CH₄ will catch fire if flammable conditions appear. 
• Higher the DO levels means higher speed of corrosion in water pipes. For the industrial purposes, water with the least possible amount of DO is preferred.
• DO test is used as a mean of controlling corrosion of iron & steel, particularly in water distribution system & in steam boilers.Determination of DO serve as the basis for BOD test. 

 

Observations and Calculations

Sr. No.	Sample Description	Sample Volume (ml)	Volume of Na2S2O3 Used (ml)	Dissolved Oxygen (mg/L)	Mean DO (mg/L)
1
2
3

Formula used
D.O =
Where,
N= Normality of Sodium Thiosulfate = 0.025
Equivalent weight of Oxygen = 8
F = Fraction of solution wasted = 0.99
Volume of Sample = 300 ml
F =
F = 300 – 3 / 300
F = 0.99