Dissolved oxygen and its method of determination

dissolved oxgen its definition and how it is available has been mentioned.also its methods of determination has shortly explained.

1. LAXMINARAYAN INSTITUTE OF TECHNOLOGY, NAGPUR A presentation on the topic of Dissolved Oxygen

2. What is dissolved oxygen? Dissolved Oxygen is the amount of gaseous oxygen (O 2 ) present in the water in its dissolved state. DO is the most important  indicator  of the  health of a water body and its  capacity  to support a balanced aquatic  ecosystem of plants and animals . A higher dissolved oxygen level indicates a better water quality. If  dissolved oxygen levels are too low, some fish and other organisms may not be able to survive .

3. Mostly the dissolved oxygen in water comes from oxygen in the air. A lso it comes from the photosynthesis process done by aquatic plants. Water temperature and the volume of moving water affects dissolved oxygen levels. Oxygen dissolves easier in cooler water than warmer water. Salinity is also an important factor in determining the amount of oxygen a body of water can hold. As the amount of dissolved salts in the water increases, the amount of oxygen the water can hold decreases. Hence ,for lower saline water, it increases. How it is available?

4. Dissolved Oxygen Measurement Methods There are three methods available for measuring dissolved oxygen concentrations in water:- Titration Method Colorimetric Method Optical or electrochemical Sensor.

5. TITRATION METHOD The method for dissolved oxygen analysis is known as “Winkler method”. Also known as the Iodometric method. Chemicals Required : Manganous Sulphate Solution (Mnso 4 .4H 2 O ) Alkali-iodine Oxide Conc. Sulphuric Acid(H 2 SO 4 ) Starch Solution Std. Sodium Thiosulphate Solution(0.025 N) Std. Potassium Dichromate Solution (0.025 N) WINKLER METHOD:-

6. Procedure:- Take 300 ml of water sample in the bottle. A dd 2 ml of manganous sulfate solution and 2 ml of alkali oxide solution to the sample. Mix the solution obtained by inverting the bottle for at least 15 times. Precipitate begin to form and allow it to settle ,leaving a clear supernatant liquid above ,shake again. After two minute of settling ,remove the stopper and add 3 ml of conc. H2SO4 by allowing acid to run from the neck of the bottle. Apply the stopper and mix again until the dissolution is complete. take about 203 ml of the solution from the bottle to flask.

7. Titrate with 0.025 N sodium thiosulfate solution to pale straw colour. add 1.2 ml starch solution and continue the titration until blue color disappears and note down the volume of sodium thiosulfate added which DO value in mg/lit. Calculation:- The formula to calculate amount of DO will be given as :- = * * * Amount of do present in water sample (mg/li.) Total volume of sodium thiosulphate req. Normality of sodium thiosulphate Eq. Wt. of oxygen Volume of sample 1000

8. Chemical reactions :- White precipitate formation- 2.Brown precipitate formation- 3.After adding acid- 4. Titration- Mn 2+ + 2OH - Mn (OH) 2 4 Mn(OH) 2 (s) + O 2 (aq) + 2 H 2 O 4 Mn(OH) 3 (s) 2Mn(OH) 3 + 2I - +6H + I 2 + 6H 2 O + 2Mn 2+ 2S 2 O 3 2- + I 2 S 4 O 6 2- + 2I -

9. COLORIMETRIC METHOD There are two variations of dissolved oxygen analysis by the colorimetric method. They are known as the indigo Carmine method and the R hodazine D method. Both variations use colorimetric reagents that react and change color when reacted with oxygen in the water  . These interactions are based on the oxidation of the reagent, and the extent of the color change is proportional to the dissolved oxygen concentration  . a spectrophotometer, colorimeter or a simple comparator is used for measuring DO.

10. Indigo carmine method The indigo carmine method is used for measuring dissolved oxygen concentrations between 0.2 and 15 ppm (mg/L). This method produces a blue color, the intensity of which is proportional to the dissolved oxygen concentration   this method is not affected by temperature, salinity or dissolved gases 

11. Rhodazine D method The R hodazine D method is used to determine very low dissolved oxygen concentrations i.e. upto parts per billion (ppb) Rhodazine D reagents react with dissolved oxygen to form A deep rose-colored solution . this colorimetric method is not affected by salinity or dissolved gases present in the water sample . In addition, sample color and turbidity can affect the accuracy of the readings . this method is time-dependent, as the analysis must be made within 30 seconds of mixing the reagent

12. Optical sensor Optical dissolved oxygen sensors measure the interaction between oxygen and certain luminescent dyes. When exposed to blue light, these dyes become excited (electrons gaining energy) and emit light as the electrons return to their normal energy state  . When dissolved oxygen is present, the returned wavelengths are limited or altered due to oxygen molecules interacting with the dye. The measured effect is inversely proportional to the partial pressure of oxygen  .  As oxygen crosses the membrane, it interacts with the dye, limiting the intensity of the luminescence   . The intensity of the returned luminescence is measured by a photo detector, and is used to calculate the dissolved oxygen concentration .

13. SIGNIFICANCE OF DO IN WATER The level of oxygen is a much more important measure of water quality . D issolved oxygen is absolutely essential for the survival of all aquatic organisms (not only fish but also invertebrates such as crabs, clams, zooplankton, etc.). Oxygen affects a vast number of other water indicators, not only biochemical but also like the odor, clarity and taste.

14. As dissolved oxygen levels in water drop below 5.0 mg/l, aquatic life is put under stress. The lower the concentration, the greater the stress. Oxygen levels that remain below 1-2 mg/l for a few hours can result in large fish kills. Total dissolved gas concentrations in water should not exceed 110 percent. Concentrations above this level can be harmful to aquatic life. Fish in waters containing excessive dissolved gases may suffer from "gas bubble disease"; however, this is a very rare occurrence. External bubbles (emphysema) can also occur and be seen on fins, on skin and on other tissue. In a community water supply a high do level is good because it makes drinking water taste better. However, high DO levels speed up CORROSION in water pipes. For this reason, industries use water with the least possible amount of dissolved oxygen.

15. Thank you…