University of Pittsburgh at Bradford
Science In Motion
Chemistry Lab 015
PH TITRATION CURVE
Introduction:
One method a chemist can use to investigate acid-base reactions is a titration. A pH titration is
performed by adding small, accurate amounts of standard base to an acid of unknown concentration. The
pH is recorded methodically and is plotted vs. the volume of base added to the acid solution.. The result
of this plot is an "S" shaped curve. The inflection point of this curve (middle of the "S") is indicative of
the endpoint. This point occurs when the acid and base in solution are stoichiometrically equivalent. An
endpoint can be very useful in determining the concentration of an acid or base.
Purpose:
The purpose of this experiment is to titrate a dilute HCl solution with a dilute NaOH solution and to
plot a titration curve to determine the endpoint and find the concentration of an unknown acid solution.
Equipment / Materials:
HCl (unknown conc.)
0.10 M NaOH
magnetic stirrer and stirring bar
pH meter and electrode
250-mL beaker
100-mL graduated cylinder
250-mL volumetric flask
50-mL buret
pH buffer solutions (pH 4 and 7)
Kimwipes
buret clamp
Safety:
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Always wear apron and safety glasses in the lab.
Avoid contact of solutions with skin and clothing. If there is contact, wash with water.
Sodium hydroxide can cause instantaneous, permanent eye damage.
Procedure:
Standardize the pH meter
1. Plug in pH meter.
2. Press the stdby key, then make sure you are in pH mode. If not, press the mode key until pH mode is
displayed. Press the setup key twice followed by the enter key.
3. Rinse the electrode with distilled water and then immerse it into a buffer solution. Stir gently.
4. Press std key twice to access and initiate the standardize mode. When the stable icon appears, the
buffer is entered. The screen then returns to the measure screen.
5. Repeat steps 3 and 4 with one or two other buffers.
6. Turn the instrument to standby by pressing the stdby key.
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Preparing the experiment
7.
Measure 25.00 mL of unknown HCl into a 250-mL beaker and dilute it with 75 mL of distilled
water.
8.
Place the beaker on the magnetic stirrer and add a stirring bar.
9.
Clean a 50-mL buret and rinse three times with about 5 mL NaOH solution.
10. Fill the buret past 0.00-mL mark with NaOH solution and deliver the excess into a waste beaker. If
the 0.00 mL is passed, just record the initial volume and account for it in the determination of NaOH
titrated. (volume = final volume - initial volume)
11. Immerse the electrode in the HCl solution and turn on the stirring bar at a very slow rate. Higher
speeds may cause the stirring bar to hit the electrode and damage it. Keep the electrode in the
solution but above the stirring bar.
12. Begin titrating, stopping to record the volume of NaOH and the pH on the data sheet at intervals of 1
mL until the pH begins to change significantly. Then add smaller increments. Try to maintain a pH
change of 0.2 pH unit per addition. When the period of rapid change is over, return to the 1 mL
portions.
13. Continue the titration until the titrant in the buret reaches 50.0 mL or a pH of 12 is obtained.
14. Plot a titration curve of pH (y axis) versus volume of NaOH (x axis).
15. Determine the endpoint of the titration (inflection point in the graph).
16. Repeat as instructed.
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Name_________________________________
Name_________________________________
Period_________________________________
Date__________________________________
PH TITRATION CURVE
Student Evaluation
Data Table:
Trial # _______________
pH
mL NaOH
pH
mL NaOH
pH
mL NaOH
Concentration of NaOH: _______________
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Calculations:
1. From the graph find the volume of base required to reach the end point.
2. Find the moles of base. (vol. base (L) x conc base (M/L) = moles).
3. Write and balance the equation for the reaction of HCl with NaOH.
4. Calculate the moles of acid required to react with the calculated moles of NaOH.
5. Determine the concentration of acid in moles/L.
6. Repeat for all trials and average.
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Questions:
1.
How would the following affect the results?
a.
The buret is dirty and drops of NaOH cling to the side walls of the buret as it is drained.
b.
The buret is not rinsed with NaOH prior to filling.
c.
The buret tip is not filled at the start of the titration.
d.
30 mL of unknown acid are used instead of 25 mL.
e.
The buret is not read at eye level. Explain.
f.
The base is added too rapidly in the region of rapid pH change.
g.
100 mL of distilled water is used instead of 75 mL.
h.
The original volume of base is not at 0.00 mL and a student assumes it is.
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