Determination of Dissolved Oxygen (D.0)

Determination of Dissolved Oxygen (D.0)

DISCUSSION:

All living organisms are dependent upon oxygen in one form or the other to maintain the metabolic processes that produce energy for growth and reproduction. Aerobic processes are of great interest, which need free oxygen for wastewater treatment. Dissolved Oxygen (DO) is also important in precipitation and dissolution of inorganic substances in water. DO levels in natural waters and wastewaters depend on physical, chemical and biological activities in water body. The solubility of atmospheric oxygen in fresh water ranges from 14.6mg/L at 0°C to about 70mg/L at 35°C under normal atmospheric pressure. Since it is poorly soluble gas, its solubility directly varies with the atmospheric pressure at any given temperature.
Analysis of DO is a key test in water pollution control and wastewater treatment processes.

WINKLER, TITRATION METHOD

PRINCIPLE:

Oxygen present in sample rapidly oxidizes the dispersed divalent manganous hydroxide to its higher valancy, which is precipitated as a brown hydrated oxide after the addition of NaOH/KOH and KI. Upon acidification, manganese reverts to divalent state and liberates iodine from Kl equivalent to the original DO content. The liberated iodine is titrated against Na2S203 (N/40) using starch as an indicator.

APPARATUS:

• BOD bottle, (250-300ml)
  
• Graduated cylinder (50-100)ml
  
• Flask (250-500)ml
  
• Burette(25-50)
  
• Pipette(2-10)ml
  
  
  
  
  Burette Pipette Graduate Cylinder flask BOD bottle
  
  
  
  
  

REAGENTS:

• Manganous Sulphate solution:
  

Dissolve 500g Manganous Sulphate pentahydrate MnSO4.5H20 in distilled water and make up to 1liter

• Alkaline-iodide-azide solution:
  

Dissolve 500g sodium hydroxide (NaOH) or 700g of potassium hydroxide (KOH) and 135g sodium iodide (NaI) or 150g potassium iodide (KI) in distilled water dilute up to 1 liter. And dissolve 10g sodium azide NaN3 in 40 ml distilled water and add to the Mixture of NaOH/Nal

• Starch Indicator Solution
  

Make smooth paste by 1g of starch in 200ml cold distilled water and add 200ml of boiling distilled water an. stirring constantly. Boil For 1 minute and allow to cool.

• Sodium thiosulphate solution:
  

0.025 M for 200ml sample dissolve 6.3g sodium thiosulphate pentahydrate NazS2O3.5H20 in distilled water and make up to liter. Standardize against KI. Add either 1 ml Chloroform or 10 mg mercuric iodide to stabilizer the solution

STARCH INDICATOR MAGANOUS SULPHATE

PROCEDURE

1. Fully Fill BOD Bottle (300ml) with sample
   
2. Add 1ml of MnSO4 in the Sample
   
3. Add 1ml of Alkaline-iodide-azide in the Sample and shake well the precipitates will be formed
   
4. Leave the sample for approximately 10 min to settle down the precipitates.
   
5. Add 1ml of H2S04 in the Sample and Shake Well the precipitates will disappear.
   
6. Fill the Burret with 0.025 M solution of Sodium thiosulphate (Na2S0403) for titration.
   
7. Take 100ml of sample in and pour in Conical Flask
   
8. Add 1-2 drops c Starch indicator the sample will turn into black/ dark brownish color.
   
9. Titrate sample with sodium thiosulphate until the sample becomes colorless. Alter titration the dissolve oxygen in the sample may be calculated from the following formula
   

OBSERVATIONS:

MEMBRANE ELECTRODE METHOD

DISCUSSION

These electrodes are especially useful for taking dissolved oxygen profiles of reservoirs and streams. The electrodes can be lowered to micrometer located at the surface. They can also be suspended in biological waste treatment tanks to monitor DO level at any point. In laboratory investigations, membrane electrodes have been used for continuous DO analysis in bacterial cultures, including the BOD test. These electrodes provide an excellent method for DO analysis in polluted wastes, highly colored waters, and strong waste effluents.

PRINCIPLE:

Oxygen sensitive membrane electrode of the polarographic or galvanic type are composed of two solid metal electrodes in contact with supporting electrolyte separated from the test solution by a sensitive membrane (polyethy l ene and fluorocarbon membranes are commonly used).

For calibration of the instrument, follow manufacturer’s procedure exactly to obtain designed precision and accuracy. Usually these electrodes are calibrated by reading against air or a sample of known Do concentration (determined by iodometric method) as well as sample with zero DO (add excess of Na2S2O3 and a trace of CaCl2 to bring DO to zero). Preferably calibrate with samples of water under test.