CE 420/520 Environmental Engineering Chemistry
Laboratory 1 - Equilibrium Constants and Kinetics
Dr. S.K.Ong
Purpose
To determine the equilibrium constants for the solubility of oxygen in water, the effect of temperature on
the solubility, and the kinetics of dissolution of oxygen from atmosphere into water.
Theory
Dissolution and evolution of dissolved oxygen (DO) in air-water system can be represented by:
O2(g) < === > O2(aq)
with an equilibrium constant (assuming that the activity may be approximated by the concentration.)
given by:
K = [O2(aq)]/[O2(g)]
When the partial pressure is used for O2(g), then the equilibrium constant (Henry’s law constant) is equal
to:
KH = [O2(aq)]/PO2
At constant temperature the partial pressure of oxygen is given by the equation
PO2 = XO2(PT - PH2O)
where XO2 is the volume fraction of oxygen in dry air (generally 0.208), PT is the total pressure of the gas
phase, and PH2O is the vapor pressure of the water. Under atmospheric conditions, the correction for
vapor pressure of water is negligible.
The effect of temperature on oxygen solubility is given by the Van’t Hoff equation:
ln (K1/K2) = Ho/R(1/T2 – 1/T1)
where Ki is the equilibrium constant at Ti ( oK), Ho is the enthalpy of reaction in cal/mole and R is the
ideal gas constant (1.99 cal/mole.oK). This equation is based on the assumption that Ho does not vary
with temperature over the range of interest.
Equipment
- Dissolved oxygen meter and probe
- Winkler's method for DO measurement
- Distilled water at approx. 4o C, room temperature (≈20 oC) and incubator temperature (≈ 35 oC)
- 0.5 M NaCl solution to simulate seawater
- Boiled water or nitrogen-purged water (initial DO approx. 0) at approx. 4 oC and 20o C
- Stop watch
- 200 mL and 500 mL beakers
Procedure
1.
2.
Equilibrium DOs at various temperatures and for a salt solution
i.
For the measurement of DO in water, use the dissolved oxygen meter and probe.
Standardize the dissolved oxygen probe as per manufacturer’s guidelines.
ii.
Determine the DO concentration in distilled water at 4oC, room temperature and at
incubator temperature. Volume to be used approximately 100 - 200 mL. Record DO
concentration and temperature of the distilled water.
iii.
Measure the DO in 0.5 M NaCl solution (at room temperature) using the DO probe and
by the Winkler's method (see procedure below).
Dissolution of oxygen into water (reaeration kinetics)
i.
For the distilled water (approximately 250 - 300 mL) with zero DO or close to zero,
gently placed DO probe and thermometer in the water, record DO and initial
temperature. Stir the water (time equal to zero – make sure stirring rate is the same for
all experiments) with a magnetic stirrer until a vortex is formed.
ii.
For every change in 0.5 mg/L of DO, record the time. Continue until the DO
concentration until sufficient data is collected or the DO concentration is close to the DO
measured in 1.ii above. Conduct the same experiment for both temperatures. Record
final temperature of water.
Data Analysis
1.
Compute the Henry’s law constants at various temperatures (units: moles/liter.atm).
Determine the Ho value for oxygen dissolution. Compare the Ho value with a textbook or
handbook value. Comment on the similarity or differences in the values.
2.
Estimate the salting out coefficient for dissolved oxygen.
3.
Plot the change in DO concentration with time. Model the dissolution of oxygen in water with a
first order reaction. Compute the first order rate constants for the dissolution of oxygen for both
temperatures. The effect of temperature on rate constant can be modeled using the following
equation (see lecture notes for limitations of this equation):
k2 = k1(T2-T1)
where k2 and k1 are the rate constants at temperature T2 and T1, respectively.
Determine the value. Compare the value in your book or an environmental engineering
book (e.g., Davis and Cornwell, Introduction to Environmental Engineering [look under
reaeration]).
Comment on any similarity or differences in the value.
Extra Questions for CE 520 Students
4.
Imagine that earth is filled with 100% oxygen instead of air, what would be the dissolved oxygen
(DO) concentration in water?
5.
Explain why the rate of reaeration would be different for different temperatures?
Report Preparation
Lab report should include a brief introduction, purpose of study, description of procedures and
equipment, data analysis and graphs, interpretation of data, answer all questions, and conclusions.
Results
Water Sample
Ice water
Room Temperature
Incubator Temperature
Temperature (oC)
DO (mg/L)
Atmospheric Pressure (PT) = ___________
Water Sample (0.5 M):
Temperature:
_________________
DO (probe):
_________________
DO (Winkler's method): ________________
Kinetics
Sample A=
Initial Temperature
Final Temperature
Time
DO
(mg/L)
_________
_________
_________
Sample B
Initial Temperature
Final Temperature
Time
__________
__________
__________
DO
(mg/L)
Winkler's Method
Principle
The Winkler's method or iodometric test is the most precise and reliable titrimetric procedure for DO
analysis. It is based on the addition of divalent manganese solution, followed by strong alkali, to the
sample in a glass-stoppered bottle. DO rapidly oxidizes an equivalent amount of the dispersed divalent
manganous hydroxide precipitate to hydroxides of higher valency states. In the presence of iodide ions
in an acidic solution, the oxidized manganese reverts to the divalent state, with the liberation of iodine
equivalent to the original DO content. The iodine is then titrated with a standard solution of thiosulfate.
The titration end point can be detected visually with a starch indicator or electrometrically with
potentiometric techniques. Precision is + 50 g/L.
1.
2.
3.
Reagents
1.1
Alkali-iodide-azide reagent:
Dissolve 500 g NaOH and 135 NaI in distilled water and dilute to 1 L. Add 10 g NaN3
dissolved in 40 mL distilled water.
1.2
Sodium Thiosulfate Titrant (0.025 M):
To a 1 L volumetric flask, add 6.205 g sodium thiosulfate to 600-700 mL of nanopure
water. Add 1.5 mL of 6 N NaOH. Bring to 1 liter.
1.3
Standardizing Solution
Add approximately 150 mL nanopure water to a 300 mL beaker.
Add 2 g potassium iodide, 5 drops concentrated sulfuric acid, and 20.00 mL potassium
bi-iodate solution to the beaker. (Use a volumetric pipette to measure the potassium biiodate.) Bring to approximately 200 mL.
Standardize the sodium thiosulfate titrant.
2.1
Prepare 1 beaker of the standardizing solution as described in step 1.3.
2.2
Fill the auto titrator bottle with sodium thiosulfate titrant. Fill the titrator and zero the
reading. Place a beaker of standardizing solution on the stir plate, add a stir bar, and
while stirring rapidly, begin titration.
2.3
When the solution is a light straw color, add 1.5-2.0 mL of the starch indicator
solution and continue titrating to until the solution is clear. Record the mL of titrant
used (should be 19-21 mL).
Prepare the samples for titration (250 - 300 mL).
3.1
Add the following, in order, to the bottles to be titrated:
1 mL manganous sulfate solution and 1 mL alkali-iodide-azide (AIA) solution
3.2
Stopper immediately, rinse bottles under cold water, mix thoroughly by inverting bottles
several times, and allow floc to settle. Repeat mixing and settling once.
3.3
Remove stoppers. Add 1 mL concentrated sulfuric acid just before titrating. Restopper
the bottles and rinse under cold tap water and mix well until dissolution is complete.
3.4
Obtain a plastic 201 mL measuring flask and, with a swirling action, transfer sample
from incubation bottle to the 201 mL flask. Then transfer the sample from the 201 mL
flask to a 300 mL beaker.
Pour the sample that remains in the incubation bottle into the ENU.
3.5
Titrate sample in the same manner as the standardizing solution described in steps 2.2
and 2.3. Titration volumes may vary with sample, but will usually be between 0 and 9
mL. Record the mL of titrant used on the data sheet.
Pour the titrated sample into the ENU.
3.6
For titration of 200 mL sample, 1 mL 0.025 M Na2S2O3 = 1 mg DO/L
3.7
Rinse the beaker and stir bar and repeat titration process for other samples, if any.
Waste from this procedure is characterized as corrosive. Place waste in the “Elementary
Neutralization Unit” (ENU)". Do NOT pour the waste down the sink.