SOP for Determination of Dissolve Oxygen in water

What is Dissolve oxygen in water

Dissolved oxygen (DO) in water refers to the amount of oxygen gas that is present in water and is available for aquatic organisms to breathe. Oxygen dissolves in water through the process of diffusion, which is driven by the concentration gradient between the air and water. The concentration of dissolved oxygen in water is affected by various factors such as temperature, pressure, salinity, and the presence of other dissolved substances. Dissolved oxygen is essential for the survival of many aquatic organisms, and its concentration is often used as a measure of water quality.

Table of Contents

1.0 Scope

This SOP is applicable for the determination of Dissolve oxygen in water sample.

2.0 Responsibility

Chemist

3.0 Principle:

Titrimetric Method

A)Winkler Method:

Divalent manganese salt in solution is precipitated by strong alkali to divalent manganese hydroxide.

It is rapidly oxidized by dissolved oxygen present in the sample to form trivalent or higher valency hydroxide. Iodide ions are added and acidified, which reduce tetravalent hydroxides back to their stable divalent state thereby liberating equivalent amount of iodine. This iodine is, equivalent to dissolved oxygen present in the sample.

4.0 Reagents

4.1 Manganous Sulphate solution:

Dissolve manganese sulphate (480g of MnSO₄. 4H₂0 or 400 g of MnSO₄. 2H₂O or 364 g of MnSO₄.H2O) in freshly boiled and cooled water, filter and make up to 1 000 ml. The solution should not give blue colour by addition of acidified potassium iodide solution and starch.

4.2 Alkaline Iodide Solution:

Dissolve 500 gm. of sodium hydroxide (or 700 g of potassium hydroxide) and 135 g of sodium iodide (or 150 g of potassium iodide) in freshly boiled and cooled water and dilute to 1 litre.

4.3 Sulphuric acid, Concentrated

4.4 Starch Indicator:

Dissolve 2 g of starch and 0.2 g of salicylic acid as preservative, in 100 ml of hot distilled water.

4.5 Sodium Thiosulphate stock Solution(0.1N):

Dissolve approximately 25 g of sodium Thiosulphate (Na₂S₂O₃.5H₂O) in boiled distilled water and make up to 1 000 ml. Add 1 g of sodium hydroxide to preserve it.

4.6 Standard Sodium Thiosulphate Solutions (0.025N):

Dissolve 250 ml of stock solution (4.1.2.5) in boiled distilled water and make up to 1 litre and

Standardize sodium Thiosulphate against known standard before use.

5.0 Procedure:

To the sample collected in 300 ml bottle, add 2 ml of manganese sulphate solution followed by 2 ml of alkaline iodide solution.
Use separate pipettes of 2 ml capacity for each reagent and take care that tip of the pipette in each case is dipped well below the liquid surfaces carefully.
Replace the stopper without the inclusion of any air bubble and thoroughly mix the content by shaking the bottle several times and allow the precipitate formed to settle.
After 2-3 minutes of settling, carefully remove the stopper and immediately add 2 ml of concentrated sulphuric acid by running the acid down the neck of the bottle and mix thoroughly to dissolve the liberated iodine.
Take 200 ml of the solution and titrate immediately against standard sodium Thiosulphate solution, adding 3-4 drops of starch indicator solution. The end point is pale blue to colourless.

Standardization of Sodium Thiosulphate

Take (Dried for 2 hr at 103 deg C) Potassium Dichromate K₂Cr₂O₇ (0.025N) 1.225 gm in 1000 ml volumetric flask and make up the volume by Distilled water. Now dissolve approximately 2 gm of potassium Iodide (KI) in Conical flask with 100 to 150 ml Distilled Water. Add 10 ml of H2SO4 and 20 ml of 0.025N potassium dichromate solution, place in dark for 5 min. dilute apprx. To 400 ml. Titrate with 0.025 N sodium Thiosulphate by using starch indication up to colourless.

6.0 Calculation:

The dissolved oxygen in mg per litre is equal to the volume in ml of 0.025 N Thiosulphate solution used for titration.

B) Azide Modification Method

Azide modification effectively removes interference caused by nitrate which is the most common interference in biologically treated effluents and incubated BOD samples.

1.0 Principle

This method is a modification of the Winkler method for the determination of dissolved oxygen, and the principle is the same. The method depends on the formation of a precipitate of Manganous hydroxide. The oxygen dissolved in the water is rapidly absorbed by Manganous hydroxide, forming a higher oxide, which may be in the following form:

MnSO₄ + 2 KOH + Mn ( OH )₂ + K₂SO₄

2Mn(OH)₂+O₂+2MnO(OH)₂,

Mn(OH)₂ floe acts as a ‘gathering’ agent for oxygen.

Upon acidification in the presence of iodide, iodine is released in a quantity equivalent to the dissolved oxygen present.

H+

MnO(OH)₂+ 2KI + H₂ ——— MnO(OH)₂+ I₂ + 2KOH

The liberated iodine is then titrated with standard sodium Thiosulphate solution using starch indicator.

I₂ + 2S₂O₃ ——– S₄O₆^-2+ 2I^-1

2.0 Reagents

2.1 Manganous Sulphate solution:

2.2 Alkaline iodide sodium Azide solutions

Dissolve 10 g of sodium Azide (NaN₃) in 40 ml of distilled water and add this with constant stirring to the cool alkaline iodide solution prepared as in (4.1.2.2)

2.3 Potassium Fluoride Solution

Dissolve 40 g of potassium fluoride (KF.2H₂O) in 100 ml of distilled water.

2.4 Sulphuric acid, Concentrated

4.5 Starch Indicator:

Dissolve 2 g of starch and 0.2 g of salicylic acid as preservative, in 100 ml of hot distilled water.

3.0 Procedure:

To the sample collected in 300 ml bottle, add 2 ml of manganous sulphate solution followed by 2 ml of alkaline iodide sodium Azide solution.
Replace the stopper without inclusion of any air bubble and mix the contents thoroughly by shaking the bottle several times; allow the precipitate formed to settle.
After 2-3 minutes of settling, carefully remove the stopper and immediately add 2 ml of concentrated sulphuric acid by running the acid down the neck of the bottle; restopper and mix thoroughly to dissolve the liberated iodine.
Take 200 ml of the solution and titrate immediately against standard sodium Thiosulphate solution adding 3-4 drops of starch indicator solution. The end point is the pale blue to colourless.

4.0 Calculation:

The dissolved oxygen in mg per litre is equal to the volume in ml of 0.025 N Thiosulphate solution used for titration.

5.0 Interference

Oxidizing agents give positive interference and reducing agent gives negative interference

6.0 Safety Guidelines

Proper Safety and health precaution are taken while doing the analysis by using safety goggles, Apron, Safety shoes and Using Fuming Chamber for fume generating chemicals.

7.0 References

IS 3025 (P-38): 1989 RA-2019