Table of Contents
How to test dissolved oxygen in water (Principle)
Dissolved oxygen determination measures the amount of dissolved or free oxygen present in water or waste water. Aerobic bacteria and aquatic organisms such as fish need dissolved oxygen to survive. If the amount of free oxygen present in the waste water process is too low, the aerobic bacteria will die. The treatment of sewage done by aerobic bacteria will be hampered.
DO is determined by the titrimetric method which was developed by Winkler. Dissolved oxygen in water or waste water is not capable of reacting with KI. That is why an oxygen carrier such as Mn(OH)2 is needed. The reactions are given below:
- 2NaOH + MnSO4 = Mn(OH)2 + Na2SO4
- 2Mn(OH)2 +O2 = 2MnO(OH)2 [Brown precipitate]
- MnO(OH)2 + H2SO4 = MnSO4 + 2H2O + [O]
- 2KI + H2SO4 + [O] = K2SO4 + H2O + I2
- I2 + I– = I3–
- 2S2O32- + I3– = 3I– + S4O62-
Reagents required
- Manganous sulphate solution (MnSO4.4H2O): 100g MnSO4.4H2O was dissolved in 200 cm3 distilled water.
- Alkaline Iodide-azide solution: 100g NaOH was dissolved in 100 cm3 distilled water. Then allowed to stand for some days. Then the clear liquid form the above was taken and mixed with 30g KI and 2g sodium azide and made up to 200 cm3 with distilled water.
- Sodium thiosulphate (0.0125 mol/L): 125cm3 0.1 mol Na2S2O3 to 1L distilled water.
- Potassium Iodide (0.025 mol/L)
- Starch solution
- Concentrated H2SO4
Equipments required
- Digital weight balance
- Measuring cylinder
- Pipette
- Dropper
- Conical flask and stopper
- Burette
- Funnel
- Water sample
Procedure
- First of all, sample water was collected in a 100mL measuring cylinder.
- Then the sample was poured into a conical flask and covered the flask with the stopper.
- After that, 1mL of MnSO4 solution was added to the sample by removing the stopper.
- Then 1mL of Alkaline iodide azide solution was added and placed the stopper again to avoid inclusion of any air bubbles.
- The solution was mixed gently.
- Then 1mL concentrated H2SO4 was added.
- Solution was mixed gently until the precipitation completely dissolved.
- Titrated the solution against standard sodium thiosulphate (Na2S2O3).
- When pale yellow color appeared, titration was stopped and 3-4 drops of starch was added (dark blue color appeared).
- Then titrated again until the dark blue color completely disappeared. The point at which the blue color disappeared was the final end point. That makes the procedure of How to test dissolved oxygen in water.
Data of collected sample
| Source of water sample | Buriganga water (Sadarghat VIP terminal) |
| Water temperature | 230C |
| Humidity | 70% |
| Time of sample collection | 10:05 am |
| Weather condition | Haze |
| Date | 01-02-23 |
Titration table
Titration of sample water with 0.0125M Na2S2O3 solution
| Observation no. | Vol of sample water (mL) | Initial Burette reading of Na2S2O3 (mL) | Final Burette reading of Na2S2O3 (mL) | Total vol of Na2S2O3 used (mL) | Mean volume (mL) |
| 1 | 100.00 | 00.00 | 11.30 | 11.30 | 10.75 |
| 2 | 100.00 | 11.50 | 21.70 | 10.20 |
Calculation
10mL 0.0125M Na2S2O3 is equivalent to 1mg of O2 dissolved in water
So, 10.75mL 0.0125M Na2S2O3 is equivalent to (1X10.75)/10 = 1.075mg of O2 dissolved in water
Here,
The volume of Na2S2O3 used = 10.75 mL
Volume of water sample = 100 mL
That means, 100mL H2O contained = 1.075 mg of O2
So, 1000mL H2O contained = (1.075X1000)/100 = 10.75 mg of O2
Result
The DO of collected sample was 10.75 mg/L
Discussion
DO refers to the level of free O2 present in water or other liquids. It is an important parameter for the determination of water quality because of its influence on the organisms living within water. In limnology (the study of lakes), dissolved oxygen is a crucial factor second only to water itself.
Just like animals and human beings living on land need oxygen, fish and other aquatic organisms also require O2 to survive. A very high or very low DO level can harm aquatic life and affect water quality. Oxygen level depends on whether water is flowing or not, whether there are rocks or other obstacles for water to flow over, the number of plants growing in the water and the temperature of the water. The factors affecting DO level are divided into biotic and abiotic factors.
Factors affecting DO
Abiotic factors include temperature, salinity and atmospheric pressure. The level of DO will be greater in cooler waters than in warmer waters. Because higher temperature results in increased molecular vibrations, which essentially reduce the space available between water molecules. So, the capacity of water to hold DO decreases with increased temperature.
Increased salinity also causes low level of DO in water. Because salt competes more efficiently for intramolecular spaces due to their ionic charges.
Altitude also affects DO level in water because of different densities of oxygen available for dissolution. At higher altitude, atmospheric O2 is less dense and DO concentration will be lower. At sea level where atmospheric O2 is more dense, the DO will be higher.
Biotic factors include photosynthesis. Aquatic plants and algae contribute dissolved oxygen to water bodies during daylight hours through photosynthesis. Photosynthesis splits two H2O molecules into two H2 and one O2, where O2 is released into the water for underwater photosynthetic organisms. Aerobic respiration consumes oxygen for energy needed for sustaining life therefore reduces the O2. So, at night when photosynthesis not actively occurs, the level of DO in the water decreases and reach the lowest concentration just before the sun rises the next morning. On the other hand, DO is the highest when photosynthesis rates are the greatest from mid to afternoon.
Use of Iodometric method
The determination of DO was done via iodometric method. The iodometric method is one of the most precise and reliable procedures for DO analysis. This is a titration based method based on the reaction of DO with Mn2+ ions. The use of iodometric method has some advantages and disadvantages too.
The advantages are:
- Very accurate and precise
- Relatively inexpensive
- Available in kits from various manufacturers
The disadvantages are:
- Cannot monitor DO continuously
- More time consuming than membrane electrode method
- Nitrite, iron (Fe2+/Fe3+) and color in the sample can interfere with accurate measurements
Reactions described
In this experiment, according to iodometric method Mn2+ in a known amount was added which formed an insoluble precipitate of Mn(OH)2. The pH was increased by this base.
2NaOH + MnSO4 = Mn(OH)2 + Na2SO4
The obtained Mn(OH)2 reacts with dissolved oxygen to form a brown precipitate of manganic oxide MnO(OH)2.
2Mn(OH)2 +O2 = 2MnO(OH)2 [Brown precipitate]
Manganic oxide than reacts with concentrated H2SO4 to liberate nascent oxygen.
MnO(OH)2 + H2SO4 = MnSO4 + 2H2O + [O]
Nascent oxygen plays role in the oxidation of KI to I2.
2KI + H2SO4 + [O] = K2SO4 + H2O + I2
The liberated I2 is then titrated against sodium thiosulphate using starch as an indicator. Thiosulphate reduces iodine to iodide ions. It itself gets oxidized to tetrathionate ion.
I2 + I– = I3–
2S2O32- + I3– = 3I– + S4O62-
The solution was blue as long as iodine was present. When all of the iodine had been removed from solution by Na2S2O3, the color disappeared. The data was collected and calculated the value of DO.
The calculated value for DO in 1000mL was 10.75 mg. Which means the sample water contained 10.75 mg of O2 per liter. Healthy water should have a dissolved oxygen concentration above 6.5-8.0 mg/L. Different fishes and invertebrate species require different level of DO for the maintenance of health and reproduction. Long term exposure to low DO level may not directly kill an organism but significantly increases it’s susceptibility to stress and diseases. If DO level decreases to about 3-4 mg/L, even the strongest fish may suffocate. And when it is less than 2mg/L for one to four days, it may become lethal. Higher dissolved oxygen level is also harmful. High DO level causes oxidative stress. A concentration of 5mg/L is recommended for optimum fish health.
Precautions
- Temperature of the water should be recorded at the time of sample collection.
- Data should be recorded carefully
- During titration, end point should be observed carefully.
- Dissolved oxygen is measured in mg/L unit.
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