Experiment 31Dissolved Oxygen Levels in Natural Waters • To develop a proper technique for obtaining a natural water sample • To determine the dissolved oxygen concentration of a natural
Trang 1Experiment 31
Dissolved Oxygen Levels in Natural
Waters
• 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
The following techniques are used in the Experimental Procedure:
Objectives
Techniques
Introduction 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,
pro-ducing 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
indi-cates 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
“nat-ural” 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
tur-bidity, foul odors, extensive plant growth on the surface Dissolved oxygen levels that
drop to less than 5 ppm can stress the existing aquatic life
The solubilities of oxygen in fresh water (saturated solution) at various
tempera-tures are listed in Table 31.1
The dissolved oxygen levels in natural waters are dependent on temperature and water flow.
Figure 31.1 Dissolved oxygen
meters can be used for determining O 2 (aq) levels in water samples.
Trang 2The 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:
(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, I3⫺, while the man-ganese(III) reduces to the manganese(II) ion:
(31.2) The resulting solution now has a slight yellow-brown color due to the presence of I3⫺ ([I2•I]⫺)
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 I3⫺generated in the reactions conducted on-site and provides a direct determination of the dissolved oxygen concentration in the water sample:
(31.3) The starch indicator forms a deep-blue complex with I3⫺but is colorless in the pres-ence of I⫺:
I3⫺•starch (deep blue) l 3 I⫺⫹ starch (colorless) (31.4) From equations 31.1–31.3, 1 mole O2reacts to produce 4 moles of Mn(OH)3, of which
2 moles of Mn(OH)3react to produce 1 mole of I3⫺ The I3⫺, which is the result of the xing of the dissolved oxygen, reacts with 2 moles of S2O3⫺in the titration
(31.5)
⫻ 2 mol Mn (OH)2
1 mol I3⫺ ⫻ 1 mol O2
4 mol Mn(OH)3
mol O2⫽ volume (L) S2O3 2⫺⫻ mol S2O3⫺
L S2O3⫺
⫻ 1 mol I3 ⫺
2 mol S2O3⫺
I3⫺(aq) ⫹ 2 S2O32⫺(aq) l 3 I⫺(aq) ⫹ S4O62⫺(aq)
2 Mn(OH)3(s) ⫹ 3 I⫺(aq) ⫹ 6 H⫹(aq) l I3 ⫺(aq) ⫹ 6 H2O(l) ⫹ 2 Mn2⫹(aq)
l 4 Mn(OH)3(s) ⫹ 4 Na2SO4(aq)
4 MnSO4(aq) ⫹ O2(aq) ⫹ 8 NaOH(aq) ⫹ 2 H2O(l)
Winkler Method of
Analysis
Table 31.1 Solubility of Oxygen in Freshwater at Various Temperatures
Temperature (⬚C) ppm O 2 Temperature (⬚C) ppm O 2
Trang 3From 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
dis-solved oxygen concentration expressed in mg/L or ppm (parts per million) O2:
(31.6)
A sodium thiosulfate solution is standardized for the experiment with potassium
iodate, KIO3, a primary standard In the presence of iodide ion, KIO3generates a
quan-ti ed concentraquan-tion of triiodide ion, I3⫺
(31.7) This solution is then titrated to the starch endpoint with the prepared sodium
thiosul-fate solution
(31.8) For the analysis of the dissolved oxygen concentration in a water sample, the standard
Na2S2O3solution should have a molar concentration of 0.025 M or less.
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
solu-tion of KI-NaN3 The samples are stored in the dark on ice and analyzed in the
labora-tory within ideally 6 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 Na2S2O3is available If so, proceed to
Part B of the Experimental Procedure
Create and design your own Report Sheet for this part of the experiment.
1 Preparation and standardization of 0.1 M Na2 S 2 O 3 solution.Refer to
Experi-ment 29, Parts A and B of the ExperiExperi-mental Procedure for the preparation and
standardization of a 0.1 M Na2S2O3solution Prepare only 100 mL of the Na2S2O3
of the solution described in Experiment 29, Part B.1 and standardize the solution
using KIO3as the primary standard solution (Part B.3–4) Calculate the average
concentration of the Na2S2O3solution
2 Preparation of a standard 0.025 M Na2 S 2 O 3 solution.Using a pipet and 100-mL
volumetric ask, prepare a 0.025 M Na2S2O3solution from the standardized 0.1 M
Na2S2O3
1 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
2 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
3 “Fix” the dissolved oxygen Below the surfaceof the water sample, pipet ⬃1 mL
of the basic 2.1 M MnSO4solution into the sample (some over owing will occur)
Similarly pipet ⬃1 mL of the basic KI-NaN3solution A precipitate should form
(equation 31.1)
4 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
Disposal: Dispose of the test solutions as directed by your instructor
I3⫺(aq) ⫹ 2 S2O32⫺(aq) l 3 I⫺(aq) ⫹ S4O62⫺(aq)
IO3⫺(aq) ⫹ 8 I⫺(aq) ⫹ 6 H⫹(aq) l 3 I3 ⫺(aq) ⫹ 3 H2O(l)
mg O2
L sample⫽ ppm O2
Standard Solution of Sodium Thiosulfate 3
Experimental Procedure
A A Standard 0.025 M
Na 2 S 2 O 3 Solution
B Collection of Water Sample
Trang 4b. 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
5 Temperature.Read and record the temperature of the water at the sample site Also, write a brief description of the sample site
6. Analysis should begin within 6 hours of sampling
1 Prepare the titrant.Prepare a clean buret Add 3 to 5 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
solu-tion in the buret (Technique 16A.2), using all certain digits plus one uncertain digit.
Place a white sheet of paper beneath the receiving ask
2 Prepare sample 1
a. Remove the stopper from the 250-mL Erlenmeyer ask To the collected water sample, add ⬃1 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 (⬃200 mL, 0.1 mL) to a receiving ask (either a beaker or Erlenmeyer ask) for the titrimetric analysis (Part C.3)
3 Titrate water sample 1.Slowly dispense the Na2S2O3titrant into the water
sam-ple Swirl the ask as titrant is added ( Technique 16C.4) When the color of the
analyte fades to a light yellow-brown, add ⬃1 mL of the starch solution Continue slowly adding titrant—when one drop (ideally, half-drop) results in the disappear-ance of the deep-blue color of the I3⫺•starch complex, stop the titration and again (after ⬃15 seconds) read and record the volume of titrant in the buret
4 Additional trials.Repeat the analysis for the two remaining samples
5 Calculations. Calculate the dissolved oxygen concentration for each sample expressed in ppm O2(mg O2/L sample )
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)
concentra-tion 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
biode-grade 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 Disposal: Dispose of the test solutions as directed by your instructor
C Sample Analysis
Figure 31.2 Allow a gentle flow of water into the flask.
Slowly turn the flask upright as it fills to overflowing.
Read and record the buret to the
correct number of significant figures.
The Next Step
Trang 5Experiment 31 Prelaboratory Assignment
Dissolved Oxygen Levels
in Natural Waters
Date Lab Sec Name Desk No
1. For a natural water sample, what range of dissolved oxygen concentrations may you expect? Explain your reasoning
2. 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)?
3. Experimental Procedure, Part B.3 A solution of MnSO4is 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 (⬃1 mL) of MnSO4is required for xing the dissolved oxygen in the sample Explain why an exact volume is not critical
Trang 64. A water chemist obtained a 250-mL sample from a nearby lake and xed the oxygen on-site with alkaline solutions of MnSO4and KI-NaN3 Returning to the laboratory, a 200-mL sample was analyzed by acidifying the sample with conc
H2SO4and then titrating with 14.4 mL of 0.0213 M Na2S2O3solution to the starch endpoint (This is a calculation similar to the one for this experiment.)
a. Calculate the number of moles of I3⫺that reacted with the Na2S2O3solution See equation 31.3
b. Calculate the number of moles of Mn(OH)3that were produced from the reduction of the dissolved oxygen See equation 31.2
c. Calculate the number of moles and milligrams of O2present 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
5 a. Experimental Procedure, Part A What is the procedure for preparing 250 mL of 0.0210 M Na2S2O3for this
experi-ment 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 Na2S2O3solution using the 25.0-mL pipet What
would be its exact molar concentration?
6. A 100-mL volume of a primary standard 0.0110 M KIO3solution is prepared A 25.0-mL aliquot of this solution is used to standardize a prepared Na2S2O3solution A 15.6-mL volume of the Na2S2O3solution titrated the KIO3solution
to the starch endpoint What is the molar concentration of the Na2S2O3solution?
I3⫺(aq) ⫹ 2 S2O32⫺(aq) l 3 I⫺(aq) ⫹ S4O62⫺(aq)
IO3⫺(aq) ⫹ 8 I⫺(aq) ⫹ 6 H⫹(aq) l 3 I3 ⫺(aq) ⫹ 3 H2O(l)
Trang 7Experiment 31 Report Sheet
Dissolved Oxygen Levels
in Natural Waters
Date Lab Sec Name Desk No
A A Standard 0.025 M Na2 S 2 O 3 Solution
Prepare a self-designed Report Sheet for this part of the experiment Review the Report Sheet of Experiment 29 for
guid-ance Submit this with the completed Report Sheet.
B Collection of Water Sample
Sampling site: Temperature: _C
Characterize/describe the sampling site
C Sample Analysis Sample 1 Sample 2 Sample 3
4. Volume Na2S2O3dispensed (mL) _ _ _
5. Molar concentration of Na2S2O3(mol/L), Part A _
6. Moles of Na2S2O3dispensed (mol) _ _ _
7. Moles of I3⫺reduced by S2O3⫺(mol) _ _ _
10. Dissolved oxygen, ppm O2 (ppm) _ _ _
Appendix B Appendix B
Trang 8Write a short summary based on an interpretation of your analytical data.
Laboratory Questions
Circle the questions that have been assigned
1. 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
2. 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
3. Part B.5 A water chemist measured and recorded the air temperature at 27C when he should have measured the water temperature, which was only 21C As a result of this error, will the dissolved oxygen concentration be reported as being higher or lower than it should be? Explain
4. 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 S2O3⫺titrant, even though a precipitate formed in Part B.4 What can be stated about the dissolved oxygen concentration of the sample? Explain
5. Assuming a dissolved oxygen concentration of 7.0 ppm (mg/L) in a 300-mL water sample,
a. how many moles of Mn(OH)3will be produced with the addition of the MnSO4solution?
b. how many moles of I3⫺will be produced when the KI-NaN3solution is added to the above solution?
c. how many moles of S2O3⫺will be needed to react with the I3⫺that is generated?
d. and also assuming the concentration of the S2O3⫺titrant to be 0.025 M, how many milliliters of titrant will be
pre-dictably used?
6. 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?
7 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
8. 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