Experiment 31 Dissolved Oxygen Levels in Natural Waters The dissolved oxygen levels in natural waters are dependent on temperature and water flow • To develop a proper technique for obtaining a natural water sample • To determine the dissolved oxygen concentration of a natural water sample • To learn the chemical reactions involved in xing and analyzing a water sample for dissolved oxygen using the Winkler method Objectives The following techniques are used in the Experimental Procedure: Techniques Streams, rivers, lakes, and oceans play vital roles in our quality of life They not only are a source of food supplies with the likes of shrimp and salmon but also provide recreational opportunities in the forms of boating and swimming Additionally, the larger bodies of water such as lakes and oceans affect seasonal weather patterns, producing changes in rainfall and snowfall and generating conditions for hurricanes and typhoons The aesthetic appearance of smaller bodies of water such as rivers and lakes indicates an immediate perception of the quality of the water Color, surface growth, and odor are early indicators of the quality of the water and the nature of its marine life As the public water supplies of most larger cities rely on the presence of surface water, water chemists must be keenly aware of the makeup of that water “How must the water be treated to provide safe and clean water to the consumers?” A number of water-quality parameters are of primary interest in analyzing a “natural” water sample: pH, dissolved oxygen, alkalinity, and hardness are but a few A quick test, pH, is generally determined with a previously calibrated pH meter; dissolved oxygen concentrations can be completed with a dissolved oxygen meter (Figure 31.1) although its availability is less likely than that of a pH meter Alkalinity and hardness levels are determined using the titrimetric technique (see Experiments 20 and 21) The concentration of dissolved oxygen in a water sample is an important indicator of water quality Waters with high oxygen concentrations indicate aerobic conditions: clean, clear, and unpolluted Low oxygen concentrations indicate anaerobic conditions: high turbidity, foul odors, extensive plant growth on the surface Dissolved oxygen levels that drop to less than ppm can stress the existing aquatic life The solubilities of oxygen in fresh water (saturated solution) at various temperatures are listed in Table 31.1 Introduction Figure 31.1 Dissolved oxygen meters can be used for determining O2(aq) levels in water samples Experiment 31 343 Table 31.1 Solubility of Oxygen in Freshwater at Various Temperatures Temperature (ЊC) 10 15 18 21 Winkler Method of Analysis ppm O2 Temperature (ЊC) ppm O2 14.6 12.8 11.3 10.2 9.5 9.0 24 27 30 35 40 45 8.5 8.1 7.6 7.1 6.5 6.0 The Winkler method of analysis for dissolved oxygen, developed by Lajos Winkler in 1888, is the standard experimental procedure for determining the dissolved oxygen concentration in water and for the calibration of dissolved oxygen meters The Winker test is performed in two parts: (1) the water sample is gathered in the eld, where the dissolved oxygen is “ xed” with two reagents, and (2) the sample is titrated for nal analysis in the laboratory within a 48-hour period The natural water sample is carefully collected on-site such that no air bubbles remain trapped in the ask after collection The oxygen is xed by an immediate reaction with manganese(II) sulfate in a basic solution: MnSO4(aq) ϩ O2(aq) ϩ NaOH(aq) ϩ H2O(l) l Mn(OH)3(s) ϩ Na2SO4(aq) (31.1) The oxygen is xed as the manganese(III) hydroxide,1 an orange-brown color precipitate—the more precipitate, the greater is the dissolved oxygen concentration While on-site, a basic solution of KI-NaN3 is also added to the sample.2 The manganese(III) hydroxide oxidizes the iodide ion to the triiodide ion, I3Ϫ, while the manganese(III) reduces to the manganese(II) ion: Mn(OH)3(s) ϩ IϪ(aq) ϩ Hϩ(aq) l I3Ϫ(aq) ϩ H2O(l) ϩ Mn2ϩ(aq) (31.2) The resulting solution now has a slight yellow-brown color due to the presence of I3Ϫ ([I2•I]Ϫ) The remainder of the dissolved oxygen analysis is completed in the laboratory (but within 48 hours) The sample is acidi ed with sulfuric acid to dissolve any precipitate A titration of the sample with a standardized sodium thiosulfate solution in the presence of a starch indicator determines the amount of I3Ϫ generated in the reactions conducted on-site and provides a direct determination of the dissolved oxygen concentration in the water sample: I3Ϫ(aq) ϩ S2O32Ϫ(aq) l IϪ(aq) ϩ S4O62Ϫ(aq) (31.3) The starch indicator forms a deep-blue complex with I3Ϫ but is colorless in the presence of IϪ: I3Ϫ•starch (deep blue) l IϪ ϩ starch (colorless) (31.4) From equations 31.1–31.3, mole O2 reacts to produce moles of Mn(OH)3, of which moles of Mn(OH)3 react to produce mole of I3Ϫ The I3Ϫ, which is the result of the xing of the dissolved oxygen, reacts with moles of S 2O32Ϫ in the titration mol O2 ϭ volume (L) S2O32Ϫ ϫ mol S2O32Ϫ ϫ mol I3Ϫ L S2O32Ϫ mol S2O32Ϫ mol Mn (OH)2 mol O2 ϫ ϫ Ϫ mol I3 mol Mn(OH)3 (31.5) There is uncertainty among chemists as to the oxidation number of manganese in the precipitate— MnO(OH)2, the hydrated form of MnO2, often represents the form of the precipitate Sodium azide, NaN3, is added to eliminate interference in the dissolved oxygen analysis caused by the presence of nitrite ion, NO2Ϫ, common in wastewater samples 344 Dissolved Oxygen Levels in Natural Waters From the data collected and analyzed, the moles of O2 converted to milligrams divided by the volume of the water sample (in liters) that is titrated results in the dissolved oxygen concentration expressed in mg/L or ppm (parts per million) O2: mg O2 ϭ ppm O2 (31.6) L sample A sodium thiosulfate solution is standardized for the experiment with potassium iodate, KIO3, a primary standard In the presence of iodide ion, KIO3 generates a quanti ed concentration of triiodide ion, I 3Ϫ (31.7) IO3Ϫ(aq) ϩ IϪ(aq) ϩ Hϩ(aq) l I3Ϫ(aq) ϩ H2O(l) This solution is then titrated to the starch endpoint with the prepared sodium thiosulfate solution (31.8) I3Ϫ(aq) ϩ S2O32Ϫ(aq) l IϪ(aq) ϩ S4O62Ϫ(aq) For the analysis of the dissolved oxygen concentration in a water sample, the standard Na2S2O3 solution should have a molar concentration of 0.025 M or less Standard Solution of Sodium Thiosulfate Procedure Overview Three water samples are collected from a source that is selected either by the student chemist or the instructor The samples are immediately “ xed” with the addition of a basic solution of manganese(II) sulfate and a basic solution of KI-NaN3 The samples are stored in the dark on ice and analyzed in the laboratory within ideally hours of sampling The dissolved oxygen concentrations are reported in units of parts per million (ppm) O2 Ask your instructor if a standard solution of Na2S2O3 is available If so, proceed to Part B of the Experimental Procedure Experimental Procedure Create and design your own Report Sheet for this part of the experiment A A Standard 0.025 M Na S O Solution Preparation and standardization of 0.1 M Na2S2O3 solution Refer to Experiment 29, Parts A and B of the Experimental Procedure for the preparation and standardization of a 0.1 M Na2S2O3 solution Prepare only 100 mL of the Na2S2O3 of the solution described in Experiment 29, Part B.1 and standardize the solution using KIO3 as the primary standard solution (Part B.3–4) Calculate the average concentration of the Na2S2O3 solution Preparation of a standard 0.025 M Na2S2O3 solution Using a pipet and 100-mL volumetric ask, prepare a 0.025 M Na2S2O3 solution from the standardized 0.1 M Na2S2O3 Disposal: Dispose of the test solutions as directed by your instructor Prepare the ask for sam pling Thoroughly clean and rinse at least three 250-mL Erlenmeyer asks and rubber stoppers to t Allow to air dry Collect the water sample Gently lay the ask along the horizontal surface of the water See Figure 31.2, page 346 Slowly and gradually turn the ask upright as the flask fills being careful not to allow any air bubbles to form in the flask Fill the ask to over owing “Fix” the dissolved oxygen Below the surface of the water sample, pipet ϳ1 mL of the basic 2.1 M MnSO4 solution into the sample (some over owing will occur) Similarly pipet ϳ1 mL of the basic KI-NaN3 solution A precipitate should form (equation 31.1) Secure the sample a Carefully stopper the sample to ensure that no air bubbles become entrapped beneath the stopper in the water sample Again, some over owing will occur B Collection of Water Sample See Experiment 29 for further explanation and Experimental Procedure Experiment 31 345 Figure 31.2 Allow a gentle flow of water into the flask Slowly turn the flask upright as it fills to overflowing b Invert and roll the ask to thoroughly mix the reagents Once the precipitate settles, repeat the mixing process c Label the sample number for each of the asks Store the sample in the dark and, preferably, in a cool or cold location or on ice Temperature Read and record the temperature of the water at the sample site Also, write a brief description of the sample site Analysis should begin within hours of sampling C Sample Analysis Read and record the buret to the correct number of significant figures Prepare the titrant Prepare a clean buret Add to mL of the standard Na2S2O3 solution to the buret, roll the solution to wet the wall of the buret, and dispense through the buret tip and discard Use a clean funnel to ll the buret—dispense a small portion through the buret tip Read and record the volume of Na2S2O3 solution in the buret (Technique 16A.2), using all certain digits plus one uncertain digit Place a white sheet of paper beneath the receiving ask Prepare sample a Remove the stopper from the 250-mL Erlenmeyer ask To the collected water sample, add ϳ1 mL of conc H2SO4 (Caution!) and stir or swirl to dissolve any precipitate The sample can now be handled in open vessels b Transfer a known, measured volume (ϳ200 mL, ע0.1 mL) to a receiving ask (either a beaker or Erlenmeyer ask) for the titrimetric analysis (Part C.3) Titrate water sample Slowly dispense the Na2S2O3 titrant into the water sample Swirl the ask as titrant is added ( Technique 16C.4) When the color of the analyte fades to a light yellow-brown, add ϳ1 mL of the starch solution Continue slowly adding titrant—when one drop (ideally, half-drop) results in the disappearance of the deep-blue color of the I3Ϫ•starch complex, stop the titration and again (after ϳ15 seconds) read and record the volume of titrant in the buret Additional trials Repeat the analysis for the two remaining samples Calculations Calculate the dissolved oxygen concentration for each sample expressed in ppm O2 (mg O2/L sample ) Disposal: Dispose of the test solutions as directed by your instructor The Next Step 346 The biological oxygen demand (BOD) of a water sample is a measure of the organic material in a water sample that is consumable by aerobic bacteria The O2(aq) concentration is measured when a sample is taken and then again ve days later, that period being the incubation period for the aerobic bacteria to consume a portion of the O2(aq) to biodegrade the organic material Research the importance and signi cance of BOD levels in natural waters and develop an experiment to determine the BOD for a water analysis Dissolved Oxygen Levels in Natural Waters Experiment 31 Prelaboratory Assignment Dissolved Oxygen Levels in Natural Waters Date Lab Sec Name Desk No For a natural water sample, what range of dissolved oxygen concentrations may you expect? Explain your reasoning How does the dissolved oxygen concentration in a water sample change (if at all) with a ambient temperature changes? b atmospheric pressure changes? c the volume of the ask collecting the water sample? d the amount of organic matter in the water sample? e the depth of the body of water (e.g., lake, river, or ocean)? Experimental Procedure, Part B.3 A solution of MnSO4 is added to x the dissolved oxygen in the collected sample a What is the meaning of the expression, “ x the dissolved oxygen,” and why is it so important for the analysis of dissolved oxygen in a water sample? b Only an approximate volume (ϳ1 mL) of MnSO4 is required for xing the dissolved oxygen in the sample Explain why an exact volume is not critical Experiment 31 347 A water chemist obtained a 250-mL sample from a nearby lake and xed the oxygen on-site with alkaline solutions of MnSO4 and KI-NaN3 Returning to the laboratory, a 200-mL sample was analyzed by acidifying the sample with conc H2SO4 and then titrating with 14.4 mL of 0.0213 M Na2S2O3 solution to the starch endpoint (This is a calculation similar to the one for this experiment.) a Calculate the number of moles of I3Ϫ that reacted with the Na2S2O3 solution See equation 31.3 b Calculate the number of moles of Mn(OH)3 that were produced from the reduction of the dissolved oxygen See equation 31.2 c Calculate the number of moles and milligrams of O2 present in the titrated sample See equations 31.1 and 31.5 d What is the dissolved oxygen concentration in the sample, expressed in ppm O2? See equation 31.6 a Experimental Procedure, Part A What is the procedure for preparing 250 mL of 0.0210 M Na2S2O3 for this experiment from a 100-mL volume of standard 0.106 M Na2S2O3? b For the preparation of the 0.0210 M solution in a 250-mL volumetric ask, only a 25.0-mL calibrated volumetric pipet is available Explain how you would prepare the 0.0210 M Na2S2O3 solution using the 25.0-mL pipet What would be its exact molar concentration? A 100-mL volume of a primary standard 0.0110 M KIO3 solution is prepared A 25.0-mL aliquot of this solution is used to standardize a prepared Na2S2O3 solution A 15.6-mL volume of the Na2S2O3 solution titrated the KIO3 solution to the starch endpoint What is the molar concentration of the Na2S2O3 solution? IO3Ϫ(aq) ϩ IϪ(aq) ϩ Hϩ(aq) l I3Ϫ(aq) ϩ H2O(l) I3Ϫ(aq) ϩ S2O32Ϫ(aq) l IϪ(aq) ϩ S4O62Ϫ(aq) 348 Dissolved Oxygen Levels in Natural Waters Experiment 31 Report Sheet Dissolved Oxygen Levels in Natural Waters Date Lab Sec Name Desk No A A Standard 0.025 M Na2S2O3 Solution Prepare a self-designed Report Sheet for this part of the experiment Review the Report Sheet of Experiment 29 for guidance Submit this with the completed Report Sheet B Collection of Water Sample Sampling site: Temperature: _ЊC Characterize/describe the sampling site Sample Sample Sample Sample volume (mL) _ _ _ Buret reading, initial (mL) _ _ _ Buret reading, nal (mL) _ _ _ Volume Na2S2O3 dispensed (mL) _ _ _ C Sample Analysis Molar concentration of Na2S2O3 (mol/L), Part A _ Moles of Na2S2O3 dispensed (mol) _ _ _ Moles of I3Ϫ reduced by S2O32Ϫ (mol) _ _ _ Moles of O2 (mol) _ _ _ Mass of O2 (mg) _ _ _ _ _ _ 10 Dissolved oxygen, ppm O2 (ppm) 11 Average dissolved oxygen, ppm O2 (ppm) _ 12 Standard deviation _ Appendix B 13 Relative standard deviation (%RSD) _ Appendix B Experiment 31 349 Write a short summary based on an interpretation of your analytical data Laboratory Questions Circle the questions that have been assigned Part B The water chemist waits until returning to the laboratory to x the water sample for the dissolved oxygen analysis Will the reported dissolved oxygen concentration be reported as too high, too low, or remain unchanged? Explain Part B.4 No precipitate forms! Assuming the reagents were properly prepared and dispensed into the sample, what might be predicted about its dissolved oxygen concentration? Explain Part B.5 A water chemist measured and recorded the air temperature at 27ЊC when he should have measured the water temperature, which was only 21ЊC As a result of this error, will the dissolved oxygen concentration be reported as being higher or lower than it should be? Explain Part C.3 The color of the analyte did not fade to form the light yellow-brown color but remained intense even after the addition of a full buret of the S2O32Ϫ titrant, even though a precipitate formed in Part B.4 What can be stated about the dissolved oxygen concentration of the sample? Explain Assuming a dissolved oxygen concentration of 7.0 ppm (mg/L) in a 300-mL water sample, a how many moles of Mn(OH)3 will be produced with the addition of the MnSO4 solution? b how many moles of I3Ϫ will be produced when the KI-NaN3 solution is added to the above solution? c how many moles of S2O32Ϫ will be needed to react with the I3Ϫ that is generated? d and also assuming the concentration of the S2O32Ϫ titrant to be 0.025 M, how many milliliters of titrant will be predictably used? A nonscientist brings a water sample to your laboratory and asks you to determine why there was a sh kill in the nearby lake Having recently nished this experiment, what might you tell that person about the legitimacy of a test for dissolved oxygen? What reasoning would you use to maintain the integrity of your laboratory? a Fish kills are often found near the discharge point of water from cooling waters at electrical generating power plants Explain why this occurrence may occur b Fish kills are often found in streams following heavy rainfall in a watershed dominated by farmland or denuded forestland Explain why this occurrence may occur Explain how the dissolved oxygen concentrations may change starting at the headwaters of a river and ending at the ocean Account for the changes *9 Salt (ocean) water generally has a lower dissolved oxygen concentration than freshwater at a given temperature Explain why this is generally observed 350 Dissolved Oxygen Levels in Natural Waters