Laboratory 7– CE 321, Fall 2014
Geochemistry Assessment of the Bushkill Creek Watershed
You may work in groups of three.
Objectives
Measure the alkalinity of a number of samples from the Bushkill Stream and a standardized
sample.
Gain an understanding of the natural buffer system.
Introduction
In this lab you will perform alkalinity titration on a various samples from the Bushkill
Stream. You will combine all of the class information and then relate the difference in the
sample results to the geology of the Bushkill Stream watershed, and its ability to buffer acid
precipitation.
Background
Alkalinity is defined in a number of ways; in general alkalinity is a measurement of the
capacity of water to accept protons. Alkalinity is also defined as the sum of all titratable bases
down to a pH of 4.5; the titratable limit is found by determining how much acid it takes to lower
the pH of water to 4.5. One other common way to define alkalinity is the capacity of water to
neutralize strong acid; every body of water has an “acid neutralizing capacity” (ANC).
In most natural waters the only significant contribution to alkalinity are the carbonate
species and any free H+ or OH-. For practical purposes Alkalinity is defined as:
Alkalinity (mole/L) = HCO3 2 CO32 OH
or
Alkalinity (eq/L) = ( HCO3 ) (CO32 ) (OH )
The more rigorous definition is as follows:
Alkalinity, mole/L = HCO3 2 CO32 OH H
At the final titration endpoint, all forms of carbonate have been converted to the carbonic
acid (H2CO3) form. The final pH of the solution, the solution remaining being H2CO3 and H2O,
is 4.5.
Two indicators1, methyl orange and phenolphthalein are added to a sample at specific pH
points to assist in identifying the critical endpoints where: 1st endpoint - the dominate carbonate
species are converted to bicarbonate, and 2nd endpoint - where the dominate bicarbonate species
are converted to carbonic acid.
1
A compound having a physical property (usually color) that changes abruptly near the equivalence point of a
chemical reaction.
When the pH of a sample is above 8.3 carbonate (CO3-2) and bicarbonate (HCO3-)
coexist, therefore a phenolphthalein indicator is used. Titration is done to endpoint of pH 8.3,
where bicarbonate (HCO3-) becomes the only carbonate form, just below pH 8.3 bicarbonate and
carbonic acid coexist. We will titrate to this endpoint first if the initial sample pH is 8.3. A
methyl orange indicator is used next to indicate the final endpoint point of a pH 4.5. The final
specie remaining will be carbonic acid at pH of 4.5 or lower.
The H+ added is the amount of acid (usually from a burette and at a 0.02M concentration)
required for the following reactions:
H HCO3 H 2 CO3
2 H CO32 H 2 CO3
(Consumes H+)
H OH H 2 O
Procedure for Laboratory Alkalinity Titration Test
1. Using the graduated cylinder provided measure 50 mL’s of each of the four samples plus the
standard (each group 5 samples in total) into a designated 125 mL Erlenmeyer flask.
2. Measure and record the initial pH and conductivity of each sample.
3. If the pH is 8.3 add 4-5 drops of phenolphthalein to each respective sample. If the pH is
less than 8.3 go to Step 5
4. Begin titration, measure and record pH after the addition of each 1 mL of titrant. As you
approach a pH of 5.75 reduce the addition of titrant to 0.25 mL. Below a pH of 5.25 only
add a few drops at a time and finally below pH of 5.0, drop by drop. AGAIN - GO SLOW:
practice without the sample under the burette. GO DROP BY DROP as you approach each
end-point.
***Record the volume of the titrant at the point it becomes colorless and record the pH.
***** If the solution turns colorless before titrating, record the pH again, and proceed to Step
5.*****
5.
Add 5 drops of Methyl Orange indicator using a dropper, and titrate again. When you first
add the Methyl Orange the color should be a yellow color. Measure and record pH after the
addition of each 1 mL of titrant. As you approach a pH of 5.75 reduce the addition of titrant
to 0.25 mL. Below a pH of 5.25 only add a few drops at a time and finally below pH of 5.0,
drop by drop. GO SLOW: practice without the sample under the burette. AGAIN - GO
DROP BY DROP as you approach each end-point. Record the volume of titrant when the
solution becomes a light gold-orange color, and then record its final pH. If the pH is not
quite 4.5, add another drop or two of titrant and see what happens.
2
3
Questions
1) Could anything other then OH-, HCO3- and CO32- contribute to alkalinity? If so what and
how?
2) Why does the phenolphthalein turn clear immediately when added to all the Bushkill
samples? Explain in a clear and precise manner.
3) Explain whether or not your results make sense, based on what you know about the
watershed geology. Which part of the stream is better equipped (i.e., buffered) to neutral
acid from acid rain?
4) For the acid titrations your group performed in the laboratory plot pH vs.Volume of acid.
Provide comment.
5) The 0.02 N solution of sulfuric acid was prepared ahead of time by diluting 2.8 mL of
concentrated H2SO4 ( = 1.835 g/cm3) into 1000 mLs of deionized water, and diluting this
solution by a factor of 5. Show that this procedure results in a 0.02 N solution of H2SO4.
What to Hand In
1) One set of questions and results for each group.
2) Data and calculations neatly presented.
When is it due?
Next lab period
4
Calculation from Alkalinity Measurements Alone:
In this procedure, phenolphthalein and total alkalinity's are determined, and from
these measurements the calculation of three types of alkalinity, hydroxide,
carbonate, and bicarbonate, are made. This can be done by assuming (incorrectly)
hydroxide and bicarbonate alkalinity cannot exist together in the sample. This
permits only five possible situation to be present, which are as follows: 1)
hydroxide only, 2) carbonate only, 3) hydroxide plus carbonate, 4) carbonate and
bicarbonate, and 5) bicarbonate only.
Hydroxide only - Samples containing only hydroxide alkalinity have a high pH,
usually well above 10. Titration is essentially complete at the phenolphthalein end
point. In this case hydroxide alkalinity is equal to the phenolphthalein alkalinity.
Carbonate only - Samples containing only carbonate alkalinity have a pH of 8.3
or higher. The titration to the phenolphthalein end point is exactly equal to onehalf of the total titration. In this case carbonate alkalinity is equal to the total
alkalinity.
Hydroxide-Carbonate - Samples containing hydroxide and Carbonate alkalinity
have a high pH, usually well above 10. The titration from the phenolphthalein to
the methyl orange end point represents one-half of the carbonate alkalinity.
Therefore carbonate alkalinity make be calculated as follows:
Carb. alk.= 2 (titration from phenol. to methyl orange) x
1000
ml sample
and
Hydroxide alk. = total alk. - carbonate alk.
Carbonate-bicarbonate - Samples containing carbonate and bicarbaonate
alkalinity have a pH > 8.3 and usually less than 11. The titration to the
phenolphthalein end point represents one-half of the carbonate. Carbonate
alkalinity may be calculated as follows:
Carb. alk. = 2 (titration to phenol. end point) x
and
Bicarbonate alk. = total alk. - carbonate alk.
5
1000
ml sample
Bicarbonate only - Samples containing only bicarbonate alkalinity have a pH of
8.3 or less, usually less. In this case bicarbonate alkalinity is equal to the total
alkalinity.
*This is an approximate method to determine the type of alkalinity present.
Figure 1 - Graphical representation of titration of samples containing
various forms of alkalinity.
6
Data Sheets
Calculations
A x N x 50,000
mL of sample
B x N x 50,000
Total alkalinity (as mg/L of CaCO3) =
mL of sample
Where:
A = mL of acid to reach Phenolphthalein end point
B = mL of acid to reach Methyl Orange end point
N = Normality of acid
Phenolphthalein alkalinity (as mg/L of CaCO3) =
Reagents
Titrant: 0.02 N H2SO4
Standard Solution: 3 x 10-3 M NaHCO3
Indicators: Phenolphthalein (P) and Methyl Orange (M)
Apparatus for Laboratory Alkalinity Test
pH/conductivity Meter, 6 Erlenmeyer flasks (one extra for practice), 1 Burette, Pipettes, P & M
Indicators w/droppers, 1 small beaker to hold DI rinse water, 1 small beaker to collect samples
(about 75 ml – once collected, measure 50 ml using graduated cylinder), 1 large beaker for w/w
Final Data
Sample
Conductivity
Standard
Tap Water
Sobers Run
L. Bushkill
U. Shoeneck
Shoeneck
B. Shoeneck
13 Street
7
pH
Alkalinity
Standard
Volume
pH
Tap Water
Volume
Sobers Run
pH
Volume
8
pH
Little Bushkill
Volume
pH
Above Shoeneck
Volume
pH
Below
Shoeneck
Shoeneck
Volume
pH
Volume
9
pH
13th Street
Volume
pH
0
