ABSTRACT

From this experiment, we have determined the amount of dissolved oxygen from the lake water in Shah Alam sample that we had collected. Through the observations, we noticed that the lake water sample that we had collected contain the oxygen, dissolved oxygen by it properties showing the orange-brown color with precipitates when added with Manganese Sulphate Powder Pillow with Alkali Iodide Azide Reagent Powder Pillow. When added with sulfonic acid powder, the water sample have a color yellow, which both of the results proved the existence of oxygen in the water sample. Using a few series of calculations, we have determined the amount of dissolved oxygen in the lake water sample.

INTRODUCTION

Dissolved oxygen (DO) level is refers to the amount of oxygen dissolve in water and is particularly important in aquatic ecology. Without an appreciable level of DO, many kinds of aquatic organisms cannot exist in water. A high DO level is needed for the aquatic organism to live and it makes drinking water taste better than usual. High DO level is important to every living organism. However, industries reduce to least possible amount of dissolved oxygen in order to reserve it pipeline from corrode.

The level of DO in water is dependent on many physical, chemical, and biochemical factors—aeration, wind, velocity of water flow, algae, temperature, atmospheric pressure, organic compounds, salt content, bacteria, and animals. Algae produce oxygen during photosynthesis under sunlight. However, this process is really not an efficient means of oxygenating water because some of the oxygen formed by photosynthesis during the daylight hours is lost at night when the algae consume oxygen as part of their metabolic processes. When algae die, the degradation of their biomass also consumes oxygen.

Temperature has a significant impact on the solubility of oxygen in water. Increasing the temperature will normally decrease the DO concentration in water. It is important to distinguish between oxygen solubility, which is the maximum DO concentration at equilibrium, and the actual concentration of DO, which is generally not the equilibrium concentration and is limited by the rate at which oxygen dissolves. Water saturated with oxygen at 25oC contains 8.4 mg/L.

The colorimetric method offers a basic approximation of dissolved oxygen concentrations in a sample. There are two methods designed for high-range and low-range dissolved oxygen concentrations. These methods are quick and inexpensive for basic projects, but limited in scope and subject to error due to other redoxing agents that may be present in the water .The traditional method is the Winkler titration. While this method was considered the most accurate and precise for many years, it is also subject to human error and is more difficult to execute than the other methods, particularly in the field. The Winkler method now exists in seven modified versions which are still used today.

Therefore, in this experiment, we will determine the amount of dissolve oxygen in water by using a selected procedure that is Azide Winkler Method. Here, we had checked the dissolve oxygen in the sample and it must be comply with Malaysian Standard of Water Quality.

OBJECTIVES

1) To learn the specific sampling technique in determining dissolved oxygen concentration in water samples.

2) To determine the dissolved oxygen in water samples.

THEORY

Winkler test is used in this experiment in order to measure the concentration of

dissolved oxygen (DO) in a water sample. In this method, excess manganese (II) sulphate

will react with the alkali-iodide azide reagent that is an iodide ion (I - ) and potassium

hydroxide ion (OH-) ion in water to form a white precipitate Mn(OH)2. Alkali-iodide azide

also composed of NaN3 that is an alkali azide compound that will eliminated the nitrile

interference that can interfere the reaction of reducing or oxidizing substances in this test.

MnSO4+2KOH Mn(OH)2+K2SO4

If oxygen is present inside the water, the Mn(OH)2 will react further to form an

orange- brown precipitate, manganic oxide (MnO(OH)2) but if the oxygen is not present

inside the water sample, the colour of the white precipitate will remain unchanged.

2Mn(OH)2 + O2 2MnO(OH)

Sulfamic acid powder pillow, H3NSO3 is added, which dissolves the manganic oxide

and in together with the potassium iodide, KI added earlier, forms iodine (I2), that had give

the orange colour to the sample:

2Mn(OH)2 + 4H3NSO3 2Mn(SO4)2 + 6 H2O (3)

2Mn(SO4)2 + 4KI 2MnSO4 + 2K2SO4 + 2I2- (4)

Then, by referring table 1, a sample of volume of the solution is taken and poured into

a graduated cylinder.

Range

(mg/L

D.O)

Sample volume

(mL)

Titration Cartridge,

N (Na2S2O3)

Catalog

Number

Digit

Multiplier

1-5 200 0.200 22675-01 0.01

2-10 100 0.200 22675-01 0.02

>10 200 0.200 14401-01 0.10

The quantity of iodine is measured by titrating with sodium thiosulfate (Na2S2O3)

(1)

(2)

until the orange colour from I2 becomes pale yellow as yellow plastic straw.

4 Na2S2O3+ 2I2 2Na2S4O6+ 4NaI (5)

Starch is added near the end of the titration to measure if there is any unreacted

iodine,I2 in the solution as the starch will give a dark blue colour in the presence of I 2 and

gives a more obvious colour of endpoint for the test. Then, it is titrated again with sodium

thiosulphate solution until the colour of the solution becomes colourless

The available quantity of MnO(OH)2 formed in the first step is directly proportional to

the dissolved oxygen, and the amount of iodine formed in the second step is directly

proportional to the MnO(OH)4. So, the titration of sodium thiosulphate solution measures a

quantity of iodine directly related to the original dissolved oxygen concentration. Therefore,

by calculating the amounts of sodium thiosulphate used by recording the digits at the body of

the mechanical titrator and multiply its by the digit multiplier given at a sample volume we

had used like in the table 1.

In test 2, we used oxygen powder pillow 1 and 2 to trap the oxygen that gives the

yellow colour of oxygen and then inserted oxygen powder pillow 3 to dissolve any precipitate

and titrate it with sodium thiosulphate as in test 1.

Digits required X Digit Multiplier = ____mg/L Dissolved Oxygen (6)

Here, we obtained the amount of dissolved oxygen in the sample of water that we had

used in this experiment.

APPARATUS AND MATERIALS

Apparatus:

60ml BOD bottle

300ml BOD bottle

Measuring cylinder

250ml Erlenmeyer flask

Straight-stem delivery tube

Beaker

0.2000 N Sodium Thiosulfate Titration Cartridge

Dropper

Stopper

Reagent :

Sample water

Dissolved Oxygen1 Reagent Powder Pillow

Dissolved Oxygen 2 Reagent Powder Pillow

Dissolved Oxygen 3 Reagent Powder Pillow

0.2000 N Sodium Thiosulfate

ManganousSulfate Powder Pillow

Alkaline Iodide-Azide Reagent Powder Pillow

Sulfamic acid Powder Pillow

Two 1mL droppers of starch indicator solution.