Abstract In this experiment, the Winkler Method was used to measure the dissolved oxygen (d. o. ) in a water sample from the pond in the Arch of the Centuries which had the owl statues. Then, the amount of oxygen is determined through a series of reaction. Usually, Winkler bottles or dissolved oxygen bottles are used in this method; but in this laboratory analysis, 50mL syringes were used instead.
The significance of a Winkler Method is that it can be used to determine the health or cleanliness of a lake or stream, to know the amount and type of biomass a freshwater system can support, and to measure the amount of decomposition occurring in the lake or stream. Also, if there is sufficient dissolved oxygen, a body of water can sustain life; but if there is depletion in dissolved oxygen, this may cause major shifts in the kinds of aquatic organisms found in water bodies. Introduction The amount of dissolved oxygen present in water or wastewater is crucial for most forms of life. Dissolved oxygen is a good indicator of water quality.
Oxygen dissolves into water by means of the atmosphere and plants. The primary source of oxygen for a body of water is from microscopic algae or submerged plants. The concentration of oxygen is greatest during the daylight hours due to photosynthesis. Also, Dissolved Oxygen is affected by temperature. As the temperature of the water goes up, the lower the concentration of dissolved oxygen gas. Simply stated, the water temperature helps determine the maximum amount of oxygen gas that water can dissolve. This dissolved oxygen concentration, in return, helps determine water’s ability to support oxygen consuming creatures.
Cooling a system down by 10 degrees slows down the rates of such reactions by a similar factor. During warm summer months, competition among water inhabitants for dissolved oxygen can become quite severe with rising water temperatures, bacteria and fish require. Aerobic substances and aquatic life such as fish must have dissolved oxygen to survive. According to Hitchman, Aerobic waste water treatment processes use aerobic and facultative bacteria to break down the organic compounds found in wastewater into more stable products that will not harm the receiving waters.
Wastewater treatment facilities such as lagoons or ponds, trickling filters and activated sludge plants depend on these aerobic bacteria to treat sewage. If the amount of DO present in the wastewater process becomes too low, the aerobic bacteria that normally treat the sewage will die. The process will not operate efficiently and septic conditions will occur. Also, anaerobic substances develop causing rivers and lakes to turn dark black and smell foul. The DO test is used to monitor the process to guarantee that there is enough dissolved oxygen present to keep the process from becoming diseased.
The objective of this activity is to determine the primary standard used for the titrant and indicator used in each case. Experimental Section A. Preparing the Buret The buret was rinsed with liquid detergent and water. It was noted that water should not form beads inside the buret. The buret was rinsed with distilled water. Then, it was rinsed with the titrant. B. Preparation of 0. 1 M Na2S2O3 titrant 250mL of distilled water was boiled for at least 5 minutes. The water was then cooled and approximately 6-7 grams of Na2S2O3•5H2O was added. After which the solid was dissolved by stirring. A pinch of Na2CO3 was added.
The solution was stored in an amber bottle reagent. C. Preparation of the Primary Standard Approximately 1-2 grams of KIO3 was dried at 160? C for an hour. This was done by placing the KIO3 in a glass which was weighed and heated in an oven without any cover. After drying, 0. 6-0. 7 grams was quantitatively transferred to a 250. 0mL volumetric flask. The technique used for this operation was “subtractive weighing. ” Then, the solution was diluted to volume. D. Standardization of the Na2S2O3 titrant (Determination of the EXACT concentration) 2 grams of KI was approximately weighed in three 250mL beakers.
10. 00mL aliquots of KIO3 standard solution was added to each beaker. Approximately 10mL 1M HCl was added to each beaker. When the solution turned to violet in color, it was gently swirled. The resulting solution was titrated until the liquid turned to light yellow. Then, 5mL of starch was approximately added. The titration mixture turned to color blue. Titration was continued until the blue color disappeared. The concentration of the Na2S2O3solution was then computed. The calculated concentration was the standardized Na2S2O3.
In this method, it was noted that the addition of the indicator (starch) was delayed until the reaction was almost complete. This is because starch is used to improve the sensitivity of the reaction to know whether or not iodine is still present. E. Preparation of the Titrant for D. O. Determination 5. 00mL of the standardized Na2S2O3 was transferred to a 250. 0mL volumetric flask. Then, the solution was diluted to volume. The solution was used as the titrant for dissolved oxygen determination. It was immediately used. After a day, the solution was disposed. F. Dissolved Oxygen Determination.
The water sample taken from the pond in the Arch of Centuries which had the owl statues was carefully and slowly drawn in a 50mL syringe. It was made sure that no bubble was trapped inside the syringe. The water sample was drawn until it reached 50mL mark of the syringe. After the sample has reached the 50mL mark, the water sample was slowly expelled until it reached the 30mL mark. The outside of the syringe was then rinsed with distilled water and wiped until it was dry. The syringe was then dip into the manganese reagent. The manganese reagent was then carefully drawn until it reached the 35mL mark.
It was made sure that no bubble was trapped inside the syringe. The reagent in the syringe was slowly mixed by carefully tipping the syringe repeatedly. The outside of the syringe was then rinsed with distilled water and wiped until it was dry. The basic iodide reagent was drawn in up to the 40mL mark in a similar manner as the manganese reagent. The outside of the syringe was then rinsed with distilled water and wiped until it was dry. Finally, the H2SO4 reagent was drawn until it reached the 50mL mark in a similar manner as the two reagents. The solution was mixed by tipping over repeatedly until all brownish particles dissolve.
The contents of the syringe was the then expelled into a 250mL beaker. The inside of the syringe was cleansed with distilled water. The rinsing was included in a beaker. Approximately 5mL of starch was added to the beaker. The solution was then titrated with the Na2S2O3 for dissolved oxygen determination. The value of the dissolved oxygen was reported in ppm O2. Discussion of Result In the Winkler Method, the addition of Mn2+ (manganese reagent) is needed to a measured volume of water sample. The manganese reagent will react with oxygen but is still needed to be in alkaline.
Under alkaline conditions, the Mn2+ is converted to Mn3+ by the dissolved oxygen present in the solution: O2 (g) + 4 Mn(OH)2 (s) 2 H2O > 4 Mn(OH)3 (s) Mn(OH)2 appears as a brown precipitate. The second part of the Winkler test reduces (acidifies) the solution. A solution of KI is added. The Mn3+ present oxidizes the I-1 to I2 under acidic condition: 2 Mn(OH)3 (s) + 2 I-1 + 6H+ > 2 Mn2+ + I2 +6H2O The precipitate will dissolve back into solution. The acid facilitates the conversion by the brown, Manganese-containing precipitate of the Iodide ion into elemental Iodine.
Thus, the iodine is dissolved with water creating the light yellow color. The I2 produced is titrated by standardized Na2S2O3: I2 + 2 S2O32-(aq) > 2 I-(aq) + S4O62-(aq) From the volume and concentration of the Na2S2O3, the amount of dissolved oxygen can be computed based on the balanced equation shown above. Mass of Na2S2O3 used = 6. 5585 Table 1: Dissolved Oxygen by Volumetric Analysis Computation | Trial 1| Trial 2| Trial 3| M Na2S2O3| 0. 1027 M| 0. 1040 M| 0. 1040 M| Average M Na2S2O3| 0. 1036 M| Trial 1: MNa2S2O3= 1. 41g KIO310 mL500 mL KIO3 214. 00gmolKIO3(61)7. 7 x 10-3L= 0. 102682.
Mwhere Standardized KIO3 = 1. 41g KIO3 V KIO3 used = 10 mL500 mL KIO3 Moles of Na2S2O3 = moles of KIO3 (61) MW KIO3= 214. 00gmolKIO3 V Na2S2O3 used = 7. 7 x 10-3L Trial 2 and 3: MNa2S2O3= 1. 41g KIO310 mL500 mL KIO3 214. 00gmolKIO3(61)7. 6 x 10-3L= 0. 104033 M Average M Na2S2O3: 0. 1036 M Table 2: Data for the computation of Dissolved Oxygen Determination | Trial 1| Trial 2| Trial 3| Initial Volume| 0| 15. 00 mL| 20. 00 mL| Final Volume| 15. 00 mL| 38. 60 mL| 40. 40 mL| | 0. 015 L| where Average M Na2S2O3 = 0. 1036 M V1 Na2S2O3 used= 5mL V2 Na2S2O3 vol. flask = 250mL 0. 0236 L| 0. 0204 L|.
C1V1 = C2 V2 C2 = C1V1 V2 C2 = (0. 1036 M)(0. 005 L) = 2. 072 x 10-3 0. 25L Trial 1: 2. 072 x 10-3. 015L1 mol O24 mol S2O3-231. 9988g O21 mol O21000mg O21 g O21 0. 03L= 8. 29 DO ppm where M S2O3-2= 2. 072 x 10-3 V S2O3-2 = 0. 015 L MW O2 = 31. 9988g V sample = 30mL Trial 2: 2. 072 x 10-3. 0236L1 O24 S2O3-231. 9988 O21 mol O21000mg O21 g O210. 03L=13. 04 DO ppm Trial 3: 2. 072 x 10-3. 0204L1 O24 S2O3-231. 9988 O21 mol O21000mg O21 g O210. 03L= 11. 27 DO ppm Average DO ppm: 10. 87 mg/L Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste.
If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria present working to decompose this waste. When BOD levels are high, dissolved oxygen (DO) levels decrease because the oxygen that is available in the water is being consumed by the bacteria. Since less dissolved oxygen is available in the water, fish and other aquatic organisms may not survive. The Department of Environment and Natural Resources (DENR) limits the amount of BOD at a maximum of 10 mg per litre over a period of five days. So since our sample had an average DO ppm of 10.
87 mg/L, there was oxygen from the air that was trapped in our sample. Conclusion The primary standard used for the titrant was Na2S2O3. From the three trials that were completed, the concentrations of the standardized Na2S2O3 solution were 0. 1027, 0. 1040 M, and 0. 1040 M, respectively. The average M Na2S2O3 was 0. 1036 M. Starch which is used to improve the sensitivity of the reaction to know whether or not iodine is still present was used as an indicator. The DO was then computed. The average dissolved oxygen in parts per million was 10. 87 mg per litre.
Since the computed average of DO in ppm was greater than the 10 mg per litre which is the limit amount of BOD, therefore oxygen from air was trapped in our sample. Acknowledgment I would like to thank her co-associates Miguel Antonio Isada and Margarito Mana-ay Jr. , for taking part to accomplish the activities in this experiment. Also, I would like to the College of Science Chemistry Department for funding the reagents and equipments needed in this analysis. I would also like to thank my parents who provided all my needs and necessities to complete this experimentation.
References 1. Kroner, R. C. , Longbottom, J. E. , Gorman, R. A. , “A Comparison of Various Reagents Proposed for Use in the Winkler Procedure for Dissolved Oxygen”, PHS Water Pollution Surveillance System Applications and Development, Report #12, Water Quality Section, Basic Data Branch, (July 1964). 2. Annual Book of ASTM Standards, Part 31, “Water”, Standard D1589-60, Method A, p 373 (1976). 3. Standard Methods for the Examination of Water and Wastewater, 14th Edition, p 443, method 422 B (1975).