ACID-BASE TITRATION
LAB PH 2.PALM
INTRODUCTION
Acids and bases represent a major class of chemical substances. We encounter them
every day as we eat, clean our homes and ourselves, and perform many other daily
activities. Figure 1 shows the pH values of some common substances. Acids can be
recognized by their sour taste, while bases are known for their bitter taste and slippery
feel. One way to define acids and bases chemically is that acids are substances that
donate protons or H+ ions to a solution whereas bases accept H+ when in solution.
pH units constitute a scale which allows scientists to determine the acid or base content
of a substance or solution. The
pH 0
ionization of water is the basis for the
pH scale. Ionization of water refers
Lemon juice (~ 2.0)
to the breaking apart of water
molecules resulting in hydrogen ions
Acid rain (3.5-4)
(H+ ) and hydroxide ions (OH-). The
ACIDIC
pH scale ranges from 0, the most acid,
Milk (6.6)
to 14, the most basic. Neutral pH,
7.0 NEUTRAL (H+ = OH-)
where the concentrations of H+ and
Blood (7.3 – 7.5)
OH- are equal, is set at 7, in the
middle of the scale. pH is on a
logarithmic scale. This means that a
Tums (~9.0)
BASIC
change of 1 pH unit is equivalent to a
10-fold change in H+ ions. If a
Hair remover (~ 12.5)
solution increases from a pH of 3.0 to
pH 14
a pH of 4.0, then there are 10 times
fewer H+ ions in the solution.
Figure 1. pH values of some common substances
A titration is a process used to determine the volume of a solution needed to react with a
given amount of another substance. In this experiment, you will titrate hydrochloric acid
solution, HCl, with a basic sodium hydroxide solution, NaOH. The concentration of the
NaOH solution is given and you will determine the unknown concentration of the HCl.
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Acid-Base Titration
Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one
ratio to produce water in the overall reaction:
⎯→ H2O(l) + Na+(aq) + Cl–(aq)
H+(aq) + Cl–(aq) + Na+(aq) +OH–(aq) ⎯
When an HCl solution is titrated with a NaOH solution, the pH of the acidic solution is
initially low. As base is added, the change in pH is quite gradual until close to the
equivalence point, when equimolar amounts of acid and base have been mixed. Near the
equivalence point, the pH increases very rapidly. The change in pH then becomes more
gradual again, before leveling off with the addition of excess base.
One consideration in a titration is the ionic strength, or molarity of the NaOH titrant and
the molarity of the unknown HCl. If the molarity of the NaOH is higher than that of the
HCl, it will be able to accept more H+ ions. This means that the volume of NaOH
solution necessary to reach the equivalence point will be less than if the NaOH and HCl
solutions have the same molarity. Differences in molarity between the NaOH titrant and
the unknown HCl are accounted for in the equation given in the “Processing the Data”
section on p. 4.
In this experiment, you will use a pH Sensor to monitor pH as you titrate. The region of
most rapid pH change will then be used to determine the equivalence point. The volume
of NaOH titrant used at the equivalence point will be used to determine the molarity of
the HCl.
PURPOSE
The purpose of this experiment is to find the concentration of a hydrochloric acid
solution. To accomplish this, a graph of pH versus volume of sodium hydroxide solution
added will be plotted and interpreted.
EQUIPMENT/MATERIALS
LabPro interface
Palm handheld
Data Pro program
pH Sensor
deionized water and wash bottle
magnetic stirrer (if available)
stirring bar
50-mL burette
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HCl solution, unknown concentration
0.1 M NaOH solution
ring stand
utility clamps (2 per station)
10-mL pipette and pipette pump
250-mL beaker
goggles
gloves
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SAFETY
•
Always wear an apron and goggles in the lab.
•
Handle the hydrochloric acid with care. It can cause painful burns if it comes in
contact with the skin.
•
Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.
PROCEDURE
1.
Prepare the pH Sensor for data collection.
a. Plug the pH Sensor into Channel 1 of the LabPro interface. Connect the
handheld to the LabPro using the interface cable. Firmly press in the cable ends.
b. Remove the pH Sensor from the pH storage solution bottle by unscrewing the
lid. Carefully slide the lid from the sensor body.
c. Rinse the tip of the sensor with distilled water.
2.
Titration with a Burette
a. Use a pipet bulb (or pipet pump) to pipet 10 mL of the HCl solution into a 250mL beaker. Add 50 mL of distilled water. CAUTION: Handle the hydrochloric
acid with care. It can cause painful burns if it comes in contact with the skin.
b. Place the beaker on a magnetic stirrer and add a stirring bar. If no magnetic
stirrer is available, you need to stir with a stirring rod during the titration.
c. Use a utility clamp to suspend a pH Sensor on a ring stand as shown in Figure 2.
Position the pH Sensor in the HCl solution and adjust its position so that it is not
struck by the stirring bar.
d. Obtain a 50-mL burette and rinse the burette with a few mL of the ~0.1 M
NaOH solution. Before pouring the NaOH into the burette, be sure that the
stopcock is closed (perpendicular to the glass burette).
Dispose of the rinse solution in the extra beaker
provided.
e. Use a utility clamp to attach the burette to the ring stand
as shown in Figure 2. Fill the burette a little above the
0.00-mL level of the burette with ~0.1 M NaOH
solution. Drain a small amount of NaOH solution so it
fills the burette tip and leaves the NaOH at the 0.00-mL
level of the burette. Record the precise concentration of
the NaOH solution in your data table. CAUTION:
Sodium hydroxide solution is caustic. Avoid spilling it on
your skin or clothing.
Figure 2. Clamping pH
f. Proceed to Step 3.
sensor and burette
3.
Turn on the handheld. To start the Data Pro program, tap
the Data Pro icon on the Applications screen. Choose
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Acid-Base Titration
New from the Data Pro menu or tap
4.
to reset the program.
Set up the handheld and interface for the pH Sensor.
a. On the Main screen, tap
.
b. If the handheld displays pH in CH 1, proceed directly to Step 5. If it does not,
continue with this step to set up your sensor manually.
c. Tap
to select Channel 1.
d. Press the Scroll buttons on the handheld to scroll through the list of sensors.
e. Select PH from the list of sensors.
5. Set up the data-collection mode.
a. While still on the Setup screen, tap
, then choose Events with Entry from
the list.
b. Enter the x-axis label (Volume). For the unit, enter “mL”. You can enter this
information using the onscreen keyboard (tap “abc”).
c. Tap
twice to return to the Main screen.
6.
You are now ready to perform the titration. This process is faster if one person adds
NaOH titrant, while another person operates the handheld and enters volumes.
a. Tap
on the Main screen to begin data collection.
b. Before you have added any of NaOH solution, tap
and enter “0” as the
burette volume (using the numerical keyboard displayed on the screen). Tap
to store the first data pair for this experiment. Record this in your lab
handout as well.
c. Add the next increment of NaOH titrant (enough to raise the pH about 0.15
units). When the pH stabilizes, tap
and enter the current burette reading
(to the nearest 0.01 mL) and then tap
. You have now saved the second
data pair for the experiment. Be sure to record each data point in the Data Table
provided in the Data Sheet portion of this lab handout. This information will be
used to create a graph.
d. Continue adding NaOH solution in increments that raise the pH by about 0.15
units and enter the burette reading after each increment. When a pH value of
approximately 5.5 is reached, change to a one-drop increment. Enter a new
burette reading after each increment. Note: It is important that all increment
volumes in this part of the titration be equal; that is, one-drop increments. Read
the burette as accurately as possible; two numbers past the decimal for both the
pH and the volume are acceptable in this experiment.
e. After a pH value of approximately 10 is reached, again add larger increments
that raise the pH by about 0.15 pH units, and enter the burette level after each
increment. Continue adding NaOH solution until the pH value remains constant.
7.
Tap
when you have finished collecting data, then tap
of pH vs. volume.
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to view a graph
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Acid-Base Titration
8.
Use the data recorded on your Data Sheet to plot the titration curve on the provided
graph paper. You may compare the graph you plot to the graph displayed on the
Palm.
9.
Use your graph to find the equivalence point—that is the largest increase in pH
upon the addition of 1 drop of NaOH solution. Find the NaOH volume just before
this jump. Record this value in the Calculation Table. Then record the NaOH
volume after the drop producing the largest pH increase was added. To examine the
data pairs on the displayed graph on the Palm, tap or any data point. As you
move to examine the line, the pH and volume values of each data point are
displayed to the right of the graph.
10.
(Optional) Print a copy of the graph of pH vs. volume from the Palm.
11.
Dispose of waste solutions as directed by your instructor. Rinse the pH Sensor and
return it to the pH storage solution.
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Acid-Base Titration
PROCESSING THE DATA
1.
Use your printed graph and data table to confirm the volume of NaOH titrant you
recorded before and after the largest increase in pH values upon the addition of 1
drop of NaOH solution.
2.
Determine the volume of NaOH added at the equivalence point. To do this, add the
two NaOH values determined above and divide by two.
3.
Determine the concentration of the hydrochloric acid solution (in M or mol/L),
using the formula:
V
Macid = Mbase X base
Vacid
where Macid is the concentration of the acid (in M or mol/L), Mbase is the
concentration of the base, Vbase is the volume of the base, and Vacid is the volume
of the acid. Use the precise concentration of the sodium hydroxide solution
provided by your instructor (it is ~0.10 M). The volume of sodium hydroxide
solution at the equivalence point was determined from the experiment. Recall that
you initially added 10.0 mL of the unknown HCl.
References:
Introduction and Laboratory Protocol adapted from: Gastineau, J.; Holmquist, D. D.; DeMoss,
G.S. Science with Handhelds: Acid-Base Titration; Vernier Software & Technology: Beaverton,
OR, 2002; pp 16-1 – 16-5.
Starr, Cecie. Biology: Concepts and Applications. “Chemical Foundations for Cells”.
Brooks/Cole-Thomson Learning, Inc. Fifth Edition. ©2003. pp.30-31.
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DATA SHEET
Name ________________________
Name ________________________
Period _______ Class ___________
Date ___________
ACID-BASE TITRATION
CALCULATIONS TABLE
Concentration of NaOH (M)
NaOH volume added before the largest pH increase
NaOH volume added after the largest pH increase
Volume of NaOH added at equivalence point
Concentration of HCl (M)
QUESTIONS
1.
What is the concentration of the unknown HCl stock solution? (The original 10 mL of
HCl)?
2.
What is the final concentration of the HCl solution (10 mL + 50 mL water)?
3.
What is the difference in the concentration of the NaOH and the final HCl solutions? How
does this affect the titration? (Consider the molarity of the two solutions.)
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Acid-Base Titration
4.
Normal rainwater has a pH of approximately 5.5. In western Pennsylvania, though, rain
with a pH of 4.3-4.5 is recorded regularly. Does this change represent an increase or
decrease in H+? By how much does the H+ ion level increase or decrease?
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DATA TABLE
pH
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Volume NaOH (mL)
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Data Table, continued
pH
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Volume NaOH (mL)
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Acid-Base Titration
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