AP Chemistry
Conductimetric Titration and
Gravimetric Determination
of a Precipitate
Objectives
In this experiment, you will



Measure the conductivity of the reaction between sulfuric acid and barium hydroxide.
Use conductivity values as a means of determining the equivalence point of the reaction.
Measure the mass of a product of the reaction as a means of determining the equivalence
point of the reaction gravimetrically.
 Calculate the molar concentration of a barium hydroxide solution.
Introduction
Gravimetric analysis is a standard classical method for determining the amount of a given
component present in many solution or solid unknown samples. The method involves
precipitating the component of interest from the unknown by means of some added reagent.
From the mass of the precipitate, the percentage of the unknown component in the original
sample can be calculated.
Generally, the reagent causing the precipitation is chosen to be specific as possible for the
component of interest. The process is intended to remove and weigh only the particular
component of interest from the unknown sample. The precipitating agent must also be chosen
carefully so that it completely precipitates the component under study. The resulting
precipitate must have an extremely low solubility, must be of known composition, and must
be chemically stable. Handbooks of chemical data list suggested precipitating agents for
routine gravimetric analysis of many unknowns.
A complete gravimetric analysis includes a series of distinct steps. First, a precise, known
amount of original sample must be taken for analysis. If the unknown is a solid, an
appropriately sized portion is taken and weighed as precisely as permitted by the balances
available (typically to the nearest 0.001 or 0.0001 g). If the unknown is a solution, an
appropriately sized aliquot (a known fraction) is taken in a pipet. The sample of unknown
taken for the analysis must be large enough that precision may be maintained at a high level
during the analysis, but not so large that the amount of precipitate generated cannot be
handled easily.
Next, the unknown sample must be brought into solution (if it is not already dissolved). For
some solid samples, such as metal ores, this may involve heating with acid to effect the
dissolving. Frequently, the pH of the solution of the unknown must be adjusted before
precipitation can take place; this is usually done with concentrated buffer systems to maintain
the pH constant throughout the analysis.
The reagent that causes the precipitation is then added to the sample. The precipitating
reagent is added slowly, and in fairly dilute concentration, to allow large, easily filterable
crystals of precipitate to form. The precipitate formed is allowed to stand for an extended
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period, perhaps at elevated temperature, to allow the crystals of precipitate to grow as large as
possible. This waiting period is called digestion of the precipitate.
The precipitate must then be filtered to remove it from the liquid. Although filtration could be
accomplished with an ordinary gravity funnel and filter paper, this would probably be very
slow. The precipitates produced in gravimetric analysis are often very finely divided and
would tend to clog the pores of the filter paper. Specialized sintered glass filtering funnels
have been prepared for routine gravimetric analysis. Rather than filter paper, such funnels
have frit plate constructed of several layers of very fine compressed glass fibers that act to
hold back the particles of precipitate. Such glass funnels can use suction to speed up the
removal of liquid from the precipitate, can be cleaned easily before and after use, and are not
affected by reagents in the solution.
In this experiment, you will monitor conductivity during the reaction between sulfuric acid,
H2SO4, and barium hydroxide, Ba(OH)2, in order to determine the equivalence point. From
this information, you can find the concentration of the Ba(OH)2 solution. The reaction
between sulfuric acid and barium hydroxide yields an insoluble product, barium sulfate, and
water, as shown in the reaction equation below.
Ba2+(aq) + 2 OH–(aq) + 2 H+(aq) + SO42–(aq) → BaSO4(s) + 2 H2O(l)
In this reaction, the total number of dissociated ions in solution is reduced dramatically during
the reaction as a precipitate is formed. As 0.100 M H2SO4 is slowly added to Ba(OH)2 of
unknown concentration, changes in the conductivity of the solution will be monitored using a
Conductivity Probe. When the probe is placed in a solution that contains ions, and thus has
the ability to conduct electricity, an electrical circuit is completed across the electrodes that
are located on either side of the hole near the bottom of the probe body. This results in a
conductivity value that can be read by the interface. The unit of conductivity used in this
experiment is the micro Siemens per cm, or µS/cm.
In addition, you will capture the precipitate, and measure its mass. You will have two
methods, therefore, of calculating the molar concentration of a barium hydroxide solution that
is titrated with a sulfuric acid solution of known concentration.
Figure 1
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Conductimetric Titration and Gravimetric Determination of a Precipitate
Pre-Laboratory Questions
1. Which solution is the better conductor, sulfuric acid or barium hydroxide?
2. Write a balanced net ionic equation for the reaction of aqueous sulfuric acid with
aqueous barium hydroxide.
3. Explain why the conductivity drops as sulfuric acid is added to the barium hydroxide
solution.
4. How many mL of 0.100 M H2SO4 will be required to react with 0.375 g. of
Ba(OH)28H2O?
5. What is the definition of flocculate and how does it improve the experimental result?
6. What is an equivalence point?
Choosing A Method
Method 1 has the student deliver volumes of H2SO4 titrant from a buret. After titrant is
added, and conductivity values have stabilized, the student is prompted to enter the buret
reading manually and a conductivity-volume data pair is stored.
Method 2 uses a Vernier Drop Counter to take volume readings. H2SO4 titrant is delivered
drop by drop from the reagent reservoir through the Drop Counter slot. After the drop reacts
with the reagent in the beaker, the volume of the drop is calculated, and a conductivityvolume data pair is stored.
Materials
Materials for both Method 1 (buret) and Method 2 (Drop Counter)
LabPro interface
TI-83+ graphing calculator
Magnetic stirrer / Hot plate
Stirring bar or Vernier Microstirrer
Vernier Conductivity Probe
Two ring stands
Barium hydroxide, Ba(OH)2, solution
0.100 M sulfuric acid, H2SO4, solution
Utility clamp
Distilled water
Gooch crucible
Gooch crucible holder
Two 250 mL beakers
500 mL side-arm filtering flask
50 mL graduated cylinder
Balance, ±0.001 gram accuracy (or better)
10 mL pipet and pipet bulb or pump
Drying oven (optional)
Materials required only for Method 1 (buret)
50 mL buret
Buret clamp
Materials required only for Method 2 (Drop Counter)
Vernier Drop Counter
100 mL beaker
60 mL reagent reservoir
10 mL graduated cylinder
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Method 1: Measuring Volume Using A Buret
1. Obtain and wear goggles and apron.
2. Use a pipet bulb (or pipet pump) to transfer 10.0 mL of the Ba(OH)2 solution into a 250 mL
beaker. Add 50 mL of distilled water. CAUTION: The barium hydroxide solution is
caustic. Avoid spilling it on your skin or clothing.
3. Place the beaker on a magnetic stirrer and add a stirring bar. If no magnetic stirrer is
available, you will stir with a stirring rod during the titration.
4. Connect a Conductivity Probe to Channel 1 of the LabPro interface. Use the link cable to
connect the interface to the TI graphing calculator. Set the selector switch on the
Conductivity Probe to the 0-20000 µS/cm range.
5. Measure out approximately 60 mL of 0.100 M H2SO4 into a 250 mL beaker. Record the
precise H2SO4 concentration in your data table. CAUTION: H2SO4 is a strong acid, and
should be handled with care.
6. Set up a ring stand, buret clamp, and 50.0 mL buret to conduct the titration (see Figure 1).
Rinse and fill the buret with the H2SO4 solution.
7. Use a utility clamp to connect the Conductivity Probe to a ring stand, as shown in Figure 1.
Position the Conductivity Probe in the Ba(OH)2 solution and adjust its position so that it is
not struck by the stirring bar.
8. Set up the calculator and interface for the Conductivity Probe.
a. Turn on the calculator and start the DATAMATE program. Press CLEAR to reset the
program.
b. Select SETUP from the Main screen. If CH 1 displays MICS, proceed directly to Step 8. If it
does not, continue with this step to set up your sensor manually.
c. Press ENTER to select CH 1.
d. Select CONDUCTIVITY from the SELECT SENSOR menu.
e. Select CONDUCT 20000 (MICS) from the CONDUCTIVITY menu.
9. Set up the data-collection mode.
f. To select MODE, press
once and press ENTER .
g. Select EVENTS WITH ENTRY from the SELECT MODE menu.
h. Select OK to return to the Main screen.
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Conductimetric Titration and Gravimetric Determination of a Precipitate
10.
Select START to begin data collection. Conduct the titration carefully, as described
below.
a. Before you have added any H2SO4 solution, press ENTER and type in “0” as the buret
volume in mL. Press ENTER to save the first data pair for this experiment.
b. Add 1.0 mL of 0.100 M H2SO4 to the beaker. When the conductivity value stabilizes,
press ENTER and enter the current buret reading. You have now saved the second data pair
for the experiment.
c. Continue adding 1.0 mL-increments of H2SO4 solution, each time entering the buret
reading, until the conductivity has dropped below 100 µS/cm.
d. After the conductivity has dropped below 100 µS/cm, add one 0.5-mL increment and
enter the buret reading.
e. After this, use 2-drop increments (~0.1 mL) until the minimum conductivity has been
reached at the equivalence point. Enter the volume after each 2-drop addition. When you
have passed the equivalence point, continue using 2-drop increments until the
conductivity is greater than 50 µS/cm again.
f. Now use 1.0-mL increments until the conductivity reaches about 1000 µS/cm, or 15 mL
of H2SO4 solution have been added, whichever comes first.
11. Press
STO
when you have finished collecting data.
12. Examine the data on the displayed graph to find the equivalence point; that is, the volume
when the conductivity value reaches a minimum. As you move the cursor right or left on the
displayed graph, the volume (X) and conductivity (Y) values of each data point are displayed
below the graph. Record the H2SO4 volume of the point, with the minimum conductivity
value, in your data table.
13. Store the data from the first trial.
a. Select TOOLS from the Main screen.
b. Select STORE LATEST RUN from the TOOLS MENU.
14. Filter and measure the mass of the barium sulfate precipitate.
c. Obtain two Gooch crucibles and two small beakers that can accommodate the Gooch
crucibles and label the beakers 1 and 2.
d. Use a hot plate to warm the 100 mL beaker of mixture containing the BaSO4 precipitate.
Warm the solution to near boiling for about five minutes to help flocculate the particles.
e. While the mixture is heating, set up the 500 mL side-arm flask for vacuum filtration.
Clean the Gooch crucible holder where the crucible will make contact. Record the mass
of a Gooch crucible and set it in the crucible holder.
f. Allow the mixture to cool, and then filter it. The liquid need not be at room temperature
to be filtered. Wash the precipitate out of the beaker with small amounts of distilled
water, if necessary.
g. Dry the precipitate in the Gooch crucible in a drying oven for at least 15 minutes.
h. Cool the precipitate and Gooch crucible to near room temperature. Measure and record
the mass of the Gooch crucible and precipitate.
i. Heat the precipitate again for five minutes, cool the precipitate, and weigh it.
j. Heat the precipitate a third time, for five more minutes, cool the precipitate and weigh it.
If the masses of Gooch crucible and precipitate are the same in the final two weighings,
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dispose of the precipitate as directed. If the final two weighings are not the same, check
with your instructor to see if more drying time is needed.
15. Rinse the Conductivity Probe and the Gooch crucible with distilled water in preparation for
the second titration. Dry the Gooch crucible in the drying oven for 1 hour before the next
trial.
16. Repeat the necessary steps to conduct a second titration. Conduct a third trial, if needed.
Record the results in the data table. Print or sketch the graph of your final trial.
Method 2 Measuring Volume Using A Drop Counter
1. Obtain and wear goggles and apron.
2. Use a pipet bulb (or pipet pump) to transfer 10.0 mL of the Ba(OH)2 solution into a 150 mL
beaker. Add 50 mL of distilled water. CAUTION: The barium hydroxide solution is
caustic. Avoid spilling it on your skin or clothing.
3. Connect a Conductivity Probe to Channel 1 of the LabPro interface. Lower the Drop
Counter onto a ring stand and plug it into the DIG/SONIC 1 port (see Figure 2). Use the link
cable to connect the interface to the TI graphing calculator. Set the selector switch on the
Conductivity Probe to the 0-20000 µS/cm range.
4. Obtain the plastic 60 mL reagent reservoir. Close both valves by turning the handles to a
horizontal position. Follow the steps below to set up the reagent reservoir for the titration.
k. Rinse the reagent reservoir with a few mL of the 0.100 M H2SO4 solution and pour the
H2SO4 into an empty 250 mL beaker. CAUTION: H2SO4 is a strong acid, and should be
handled with care.
l. Use a utility clamp to attach the reservoir to the ring stand.
m. Fill the reagent reservoir with slightly more than 60 mL of the 0.100 M H2SO4 solution.
n. Place the 250 mL beaker, which contains the rinse H2SO4, beneath the tip of the reservoir.
o. Drain a small amount of the H2SO4 solution into the 250 mL beaker so that it fills the
reservoir’s tip. To do this, turn both valve handles to the vertical position for a moment,
then turn them both back to horizontal.
p. Discard the drained H2SO4 solution in the 250 mL beaker as directed.
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Conductimetric Titration and Gravimetric Determination of a Precipitate
Figure 2
5. Set up the calculator and interface for the Conductivity Probe.
a. Turn on the calculator and start the DATADROP program. Press CLEAR to reset the program.
b. If CH 1 displays CONDUCT(MICS), proceed directly to Step 6. If it does not, continue with
this step to set up your sensor manually.
c. Select SETUP from the Main screen.
d. Select CONDUCT 20000(MICS) from the SELECT A SENSOR menu.
6. Calibrate the drops that will be delivered from the reagent reservoir. Note: If you are using
the stored calibration (20 drops per mL), then skip this step.
a. Select CALIBRATE DROPS from the Main screen.
 If you have done a previous calibration with this particular apparatus, you can choose
CALIBRATE DROPS and manually enter the drops/mL ratio. Select OK and proceed to
Step 7.
 If you want to perform a new calibration, continue with this step.
b. Select CALIBRATE DROPS from the Main screen.
c. Select CALIBRATE NOW.
d. Open the bottom 2-way (on-off) valve to begin releasing drops from the vessel through
the Drop Counter (the top valve finely adjusts the drop rate).
e. Continue to release drops until there are between 9 and 10 mL of liquid in the graduated
cylinder.
f. Close the bottom valve on the vessel to stop the drops.
g. Press ENTER .
h. Type the precise volume of liquid in the graduated cylinder in mL and press ENTER .
i. The number of drops/mL will be displayed. It may be helpful to record the value for that
particular reagent reservoir for future titrations.
j. Select OK to return to the Main screen.
k. Discard the H2SO4 solution in the graduated cylinder as indicated by your instructor and
set the graduated cylinder aside.
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7. Assemble the apparatus.
a. Place the magnetic stirrer on the base of the ring stand.
b. Insert the Conductivity Probe through the large hole in the Drop Counter.
c. Attach the Microstirrer to the bottom of the Conductivity Probe. Rotate the paddle wheel
of the Microstirrer, and make sure that it does not touch the bottom of the Conductivity
Probe.
d. Adjust the positions of the Drop Counter and reagent reservoir so they are both lined up
with the center of the magnetic stirrer.
e. Lift up the Conductivity Probe, and slide the beaker containing the Ba(OH)2 solution onto
the magnetic stirrer. Lower the Conductivity Probe into the beaker.
f. Adjust the position of the Drop Counter so that the Microstirrer on the Conductivity Probe
is just touching the bottom of the beaker.
g. Adjust the reagent reservoir so its tip is just above the Drop Counter slot.
h. Turn on the magnetic stirrer so that the Microstirrer is stirring at a fast rate.
8. You are now ready to perform the titration.
a. Select START. No data will be collected until the first drop goes through the Drop Counter
slot.
b. Fully open the bottom valve—the top valve should still be adjusted so drops are released
at a rate of about 1 drop every 2 seconds. When the first drop passes through the Drop
Counter slot, check the data table to see that the first data pair was recorded.
9. Continue watching your graph to see when a large increase in conductivity takes place; this
will be the equivalence point of the reaction. When this jump in conductivity occurs, let the
titration proceed for several more milliliters of titrant, then press STO . Turn the bottom
valve of the reagent reservoir to a closed (horizontal) position.
10. Examine the data on the displayed graph to find the equivalence point—that is, the volume
when the conductivity value reaches a minimum. As you move the cursor right or left on the
displayed graph, the volume (X) and conductivity (Y) values of each data point are displayed
below the graph. Record the H2SO4 volume of the point, with the minimum conductivity
value, in your data table.
11. Leave DATA DROP and print a copy of the graph of conductivity vs. volume of H2SO4 for
the first trial.
a. Press ENTER and select RETURN TO MAIN SCREEN.
b. Leave the DATADROP program by selecting QUIT from the Main screen.
c. Transfer the data to a computer, using Logger Pro software. Print a copy of your graph of
conductivity vs. volume of H2SO4.
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Conductimetric Titration and Gravimetric Determination of a Precipitate
12. Filter and measure the mass of the barium sulfate precipitate.
a. Obtain two Gooch crucibles and two small beakers that can accommodate the Gooch
crucibles and label the beakers 1 and 2.
b. Use a hot plate to warm the 100 mL beaker of mixture containing the BaSO4 precipitate.
Warm the solution to near boiling for about five minutes to help flocculate the particles.
c. While the mixture is heating, set up the 500 mL side-arm flask for vacuum filtration.
Clean the Gooch crucible holder where the crucible will make contact. Record the mass
of a Gooch crucible and set it in the crucible holder.
d. Allow the mixture to cool, and then filter it. The liquid need not be at room temperature
to be filtered. Wash the precipitate out of the beaker with small amounts of distilled
water, if necessary.
e. Dry the precipitate in the Gooch crucible in a drying oven for at least 15 minutes.
f. Cool the precipitate and Gooch crucible to near room temperature. Measure and record
the mass of the Gooch crucible and precipitate.
g. Heat the precipitate again for five minutes, cool the precipitate, and weigh it.
h. Heat the precipitate a third time, for five more minutes, cool the precipitate and weigh it.
If the masses of Gooch crucible and precipitate are the same in the final two weighings,
dispose of the precipitate as directed. If the final two weighings are not the same, check
with your instructor to see if more drying time is needed.
13. Rinse the Conductivity Probe and the Gooch crucible with distilled water in preparation for
the second titration.
14. Repeat the necessary steps to conduct a second titration. Conduct a third trial, if needed.
Record the results in the data table. Print or sketch the graph of your final trial.
Data Table
Trial 1
Trial 2
Trial 3
Equivalence point (mL)
Mass of Gooch crucible + precipitate (g)
Mass of Gooch crucible (g)
Mass of precipitate (g)
Molarity of H2SO4 (M)
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Data Analysis
1. Use the titration results to calculate the moles of H2SO4 that were used to reach the
equivalence point in each trial.
2. Use your titration results to calculate the molar concentration (molarity) of the Ba(OH)2
solution using the molar amount of H2SO4 used in each trial.
3. Convert the mass of the barium sulfate precipitate, formed in each trial, to moles.
4. Use the moles of BaSO4 from 3 above to calculate the molarity of the Ba(OH)2 solution.
Post Laboratory Questions
1. What ions were present in the solution at the beginning of the experiment? What ions were
present at the equivalence point? What ions were present at the end?
2. Compare the results of your calculations from the titration and the mass of the precipitate
with the actual molarity of the Ba(OH)2 solution. Which method of analysis, equivalence
point or gravimetric determination, was more accurate in your experiment? Why?
3. If precipitations in gravimetric analysis are performed too rapidly, or if the precipitate is not
allowed to “digest”, the precipitate may occlude ions from the other substance present in the
system. Use a textbook of analytical chemistry to explain what the occlusion of ions is, and
how it can affect the mass of the precipitate determined.
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