Laboratory 7– CE 321, Fall 2012
Geochemistry Assessment of the Bushkill Stream Samples
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.
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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   CO32  H 2 CO3
(consumes H+)
H   OH   H 2 O
Calculations
Alk (eq/L)*Volsample = Nacid (eq/L)*Volacid
Alk (mg/L as CaCO3) = Alk (eq/L)*50,000 mg CaCO3/eq
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: 3x(10)-3 M NaHCO3
Indicators: Phenolphthalein (P) and Methyl Orange (M)
Apparatus for Laboratory Alkalinity Test
pH Meter, 4 Erlenmeyer flasks (one extra for practice), 1 Burette, Pipettes, P & M Indicators
w/droppers, 1 small beaker to hold rinse water, 1 large beaker for w/w
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Procedure for Laboratory Alkalinity Titration Test
1. Using the graduated cylinder provided measure 50 mL’s of each of the threes sample plus
the standard 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 6
4. Begin titration, measure and record pH after the addition of each 1 mL of titrant. GO
SLOW: practice without the sample under the burette. GO DROP BY DROP.
***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
6.*****
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 gold color. Measure and record pH after the
addition of each 1 mL of titrant. GO SLOW: practice without the sample under the burette.
GO DROP BY DROP. Record the volume of titrant when the solution becomes a light
orange color, and then record its final pH. If the pH is not at 4.5 then you may want to add
another drop or two of titrant.
Procedure for Field Test
Perform alkalinity field test on one field-collected sample of your choosing. See instructor
and/or lab technicians for kit.
Questions
1) Is there a difference between the laboratory titration result and the field kit titration result?
If so, WHY?
2) Could anything other then OH-, HCO3- and CO32- contribute to alkalinity? If so what and
how?
3) Why does the phenolphthalein turn clear immediately when added to all the Bushkill
samples? Explain in a clear and precise manner.
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4) 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?
5) For the acid titration performed in the laboratory plot pH vs. Volume of acid. Provide
thoughtful comment for each plot.
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.
Calculations
Total alkalinity (as mg/L of CaCO3) =
B x N x 50,000
mL of sample
B = Volume of Acid used in ml
N= Normality of Acid used as N
Final Data
Sample
Conductivity
Standard (3E-3 M)
Sober/Tatamy
Penn’s Pump/13th St.
DI Water
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pH
Alkalinity
Standard
Volume
0 ml
pH
Sober/Tatamy
P Pump/13th St
Volume
0 ml
Volume
0 ml
pH
5
pH
DI Water
Volume
0 ml
pH
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 one-half 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
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1000
ml sample
Bicarbonate alk. = total alk. - carbonate alk.
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.
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