The water samples will come from Boulder Creek at locations upstream and downstream of Boulder's wastewater treatment plant on 75th Street (the discharge from the plant is pictured at right). The water discharged by the plant is low in dissolved oxygen, and it contains organic matter that consumes dissolved oxygen, so downstream of the plant, we expect lower dissolved oxygen concentrations.
The TA will provide the following materials:
a. Dissolve about 2 g potassium iodide (KI) in about 100 mL
deionized water (dH2O)
in a 250 mL Erlenmeyer flask.
b. Add about 10 drops (about 0.3 mL) of concentrated sulfuric acid.
c. Add exactly 20.00 mL of your KH(IO3)2 solution.
d. Dilute to approximately 200 mL with dH2O.
e. Titrate with your Na2S2O3 titrant until
the solution turns
"straw-yellow" in color -- the color change is caused by I3- formation.
f. Add two or three drops of starch solution and titrate until the light blue color (ideally)
caused by the I3--starch complex disappears.
g. Record the volume of thiosulfate solution necessary to titrate your bi-iodate standard.
Measure the dissolved oxygen concentration in
the Boulder Creek samples by the colorimetric
method.
a. Record the
stream temperatures at the sample locations.
b. Use the Hach DR/890 "pocket colorimeter" to measure the
dissolved oxygen concentration of the samples following the procedures
in the Hach manual. Use the high range dissolved oxygen chemicals first and the low
range dissolved oxygen chemicals if the DO is less than 1 mg L-1.
i. open a sample bottle and remove the needed
sample.
ii. pass the sample bottle to a lab partner to
begin step 3 below.
iii. measure DO colorimetrically.
Measure the dissolved oxygen concentration of the
Boulder Creek samples using the modified Winkler titration.
a. Take the open sample bottle (see step b.ii. above) and add the following reagents to the sample:
i. 1 mL of the manganous sulfate solution
ii. 1 mL of alkali-iodide-azide reagent
c. Replace the ground-glass stopper and mix by inverting the bottle
repeatedly.
d. Allow the brown precipitate (MnO2(s)) to settle (allow about 5
min for this step).
e. Open the sample bottle, add 1 mL of concentrated H2SO4,
replace the stopper, and mix by inverting.
f. Allow the brown precipitate to dissolve (this step may require 5-10
min).
g. Into a 250 mL Ehrlenmeyer flask, add 200.0 mL of water, mark the volume
on the flask, and empty the flask.
h. Open the sample bottle, gently pour 200 mL of the sample into the marked Ehrlenmeyer flask,
and add a stir bar.
i. Titrate the sample with the sodium thiosulfate solution:
i. add thiosulfate titrant until the solution
turns "straw yellow" in color
ii. add a few drops of starch to form the blue complex
iii. add more thiosulfate to remove the blue color
iv. record the volume of thiosulfate added.
In the field, dissolved oxygen had typically been measured by dissolved oxygen meters over the past couple of decades; however, the meters have given way to the colorimetric kits like the ones we are using in lab today. The dissolved oxygen meter uses a platinum electrode that reduces oxygen to produce an electric current. The voltage of the current is proportional to the amount of oxygen present in the water. To accurately measure DO in natural waters, the water must be stirred near the electrode to draw in a fresh supply of oxygen to the electrode. Dissolved oxygen meters have fallen out of favor owing to their inability to measure low DO levels (less than about 1 mg L-1 or so). At these low levels, the meter is not sufficiently sensitive to the reduction of oxygen at the electrode. Meanwhile, some of these kits are accurate down to the 0-0.1 mg L-1 range.
(Results) What was the concentration (M) of the sodium thiosulfate titrant? Show the equation used to calculate the thiosulfate concentration, including the stoichiometry of thiosulfate and bi-iodate.
(Results) What were the dissolved oxygen concentrations (M and mg L-1) in the Boulder Creek samples as measured by colorimetry and the Winkler titration? Report these data in a table including the volume of titrant for each Winkler titration. Show the calculations, including the stoichiometry of thiosulfate and dissolved oxygen.
(Discussion) Did the colorimetry and Winkler titration results agree? If not, what are some of the reasons that might cause the difference between the two measurements?
(Discussion) For the stream temperatures recorded by the TA, determine the degree of DO saturation in each sample (show these in the table, too) and compare the measured DO with DO saturation concentrations. Assume that the elevation of the sample locations is 5,350 ft.
(Discussion) Explain the change in DO concentration (if the change is significant) in the Boulder Creek samples.
Last updated on August 22, 2007 at 01:24 PM by Joe Ryan