Titration of a weak base with
a weak acid with Cobra4
TEC
Related concepts
True and potential electrolytes, strong and weak acids and bases, law of mass action, dissociation constant and pKa value, potentiometry.
Principle
Ammonia exists in a weakly dissociated condition in aqueous solutions. The location of the dissociation
equilibrium is quantitatively described by the Ka or pKa value which can be determined with potentiometric measurements.
Equipment
1 Cobra4 Wireless Manager
2 Cobra4 Wireless-Link
1 Cobra4 Sensor-Unit Chemistry
Software measure Cobra4, multi-user
1
licence
1 Retort stand, h = 750 mm
1 pH-electrode, gel-filled
1 Storage flask for pH electrodes,filled
2 Holder for Cobra4 for support rod
3 Right angle clamp
1 Cobra4 Sensor-Unit Drop Counter
1 Immersion probe NiCr-Ni, Teflon
1 Magnetic stirrer, mini
1 Magnetic stirrer bar, l = 15 mm
Burette, lateral stopcock, Schellbach,
1
50 ml, graduations 0,1 ml
1 Burette clamp, roller mount.,2pl
1 Funnel,d = .40 mm, f.burettes
1 Volumetric pipette, 10 ml
1 Glass beaker, 150 ml, tall
12600-00
12601-00
12630-00
14550-61
37694-00
46265-15
18481-20
12680-00
37697-00
12635-00
13615-05
47334-93
46299-01
36513-01
37720-00
36888-00
36578-00
36003-00
1 Glass beaker, 150 ml, short
2 Glass beaker, 50 ml, tall
1 Beaker DURAN,short form, 100 ml
Set balance Sartorius CPA 623S and
1
software, 230 V, 620 g / 0.001 g
1 Pipettor
1 Pasteur pipettes
1 Rubber bulbs
1 Volumetric flask, 1000ml, IGJ24/29
1 Wash bottle, 500 ml
1 Buffer solution, pH 4.62, 1000 ml
1 Buffer solution, pH 9.00, 1000 ml
1 Ammonia solution, 25%, 250 ml
1 Acetic acid, 0.1 M sol., 1000 ml
1 Water, distilled, 5 l
36012-00
36001-00
36000-00
49224-88
36592-00
36590-00
39275-03
36552-00
33931-00
30280-70
30289-70
30933-25
48126-70
31246-81
Additional equipment
PC with USB interface, Windows XP or
1
higher
Fig. 1: Experimental set up.
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TEC
Titration of a weak base
with a weak acid with Cobra4
Safety instructions
When handling chemicals, you should wear suitable protective gloves, safety goggles, and suitable
clothing. Please refer to the appendix for detailed safety instructions.
Tasks
1. Measure the alteration of the pH value during a titration of approximately 0.1 molar aqueous solutions of ammonia with a 0.1 molar acetic acid solution at constant temperature using the Cobra4 system.
2. From the neutralisation curve read the pKa value of the base.
Set-up and procedure
Preparation of the required 0.1 molar ammonia solution: Use the Pasteur pipette to weigh out 6.81 g
of the 25% ammonia solution in a 100ml beaker, then quantitatively transfer it to a 1000 ml volumetric flask (rinse the beaker several times with distilled water) and fill the flask to the mark.
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Set up the experiment as shown in Fig. 1.
Combine the Cobra4 Sensor Unit Chemistry and the Cobra4 Drop Counter with the Cobra4 Wireless-Links.
Attach them to the retort stand with the holders for Cobra4 and right angle clamps.
Connect the pH electrode to the pH input of the Cobra4 Sensor Unit Chemistry and the temperature
probe to temperature input T1.
Start the PC and connect the Cobra4 Wireless Manager with a USB socket of the computer.
After the Cobra4 Wireless-Links have been switched on, the sensors are automatically recognized.
Some ID numbers (01 and 02) are allocated to the sensors, which are indicated in the displays of
the Cobra4 Wireless-Links.
Call up the “measure” programme.
Click the “Unknown titration volume” button and confirm with “OK”.
Boot the experiment “Titration of a weak base with a weak acid with
Cobra4” (experiment > open experiment). The measurement parameters for this experiment are loaded now.
For calibration: Pour some buffer solution with pH 4.62 and pH 9.00
in two beakers.
Immerse the well-rinsed probe into one of the solutions.
In the Cobra4 Navigator under “Devices” double-click the “pH” symbol. Now you can change some measurement parameters.
Enter the pH value for the given solution under the menu point “Calibration” (Step 1, see Fig. 2).
Click the “Apply” button.
Repeat this procedure with the other buffer solution (Step 2).
Finish the calibration with “OK”.
Fig. 2: Settings for the calibration mode of
the sensor.
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Titration of a weak base with
a weak acid with Cobra4
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TEC
Procedure: Position the magnetic stirrer under the stand.
Pipette 10 ml of the 0.1 molar ammonia solution in the 150 ml beaker and slip a magnetic stirring rod
in.
Fill the burette up to the 25 ml mark with 0.1 molar acetic acid.
Position the beaker containing the ammonia solution on the magnetic stirrer so that the pH measuring electrode dips into the solution (Note: The pH electrode must dip at least so deep in the buffer
solution that the diaphragm is completely immersed in the solution. Add more water if necessary).
Position the tap of the burette so that hydrochloric acid can drop into the beaker. Also ensure that
individual drops will be recorded by the drop counter.
Adjust the stirrer to a medium stirring speed (Note: Do not allow the magnetic stirring bar to hit
against the measuring electrode.)
Start measurement with a click on
in the icon strip.
Add acetic acid drop-wise to the ammonia solution from the
burette. (Note: Take care that the addition of the drops is not
so rapid that the light barrier cannot register individual drops.)
When about 20 ml of hydrochloric acid has been so added,
close the tap of the burette and click on the icon to end
measurement.
Send all data to “measure” (see Fig. 3). Save the measurement (File > Save measurement as…).
Fig. 3: Window which appears after
measurement.
Fig. 4 shows the graph for formic acid as presented by the
programme. To have the equivalence point and the pKa value
displayed use
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Fig. 4: Measurement curve of a titration of 10 ml of an 0.1 molar ammonia solution
with acetic acid (c = 0.1 mol/l).
Theory and evaluation
The illustration shows the typical course of titration curve for titrations of a weak base against a
weak acid.
It is a typical example of the curve that results from titration of a weak base with a weak acid.
The pH value at the start of the titration is relatively low compared to titrations in which a strong base
of the same concentration is used. The reason for this is that ammonia, being a weak base, is only
subject to a low degree of protolysis (approx. 3%):
NH3 + H2O
NH4+ + -OH
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TEC
-
Titration of a weak base
with a weak acid with Cobra4
The pH is further reduced as acetic acid is added during the titration, whereby it first drops quite
quickly but then only to a small extent (buffering region). The following reaction takes place hereby:
CH3COOH + NH4+ +-OH
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NH4+ + -OH + H2O
As the ammonium ions that are formed act together with the remaining ammonia as a buffer, the
addition of acetic acid first only causes a small drop in pH. In the vicinity of the equivalence point,
however, further addition of acetic acid leads to a sudden drop.
The software enables the equivalence point (end point) of the titration to be automatically determined. To have this carried out, click on the
icon in the icon strip of the evaluation programme to
call the “Equivalence point determination”. From this, you can either directly read the value of the
equivalence point or choose to have the equivalence point in the measurement curve (see Fig. 5).
Fig. 5: Equivalence point entered by means of the evaluation programme.
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On comparing the titration of ammonia solution with acetic acid with the titration of ammonia solution
with hydrochloric acid (see the graph in Fig. 6 from experiment “Titration of a weak base with a
strong acid with Cobra4”), the following is apparent.
Fig. 6: Curve of Titration of an 0.1 M ammonia solution with hydrochloric acid (c = 0.1 M).
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The courses of the curves of the two measurements are almost identical at first, but the jump in pH
at the equivalence point is far greater with hydrochloric acid than with acetic acid. This is because
hydrochloric acid is a strong acid and is therefore completely dissociated. Acetic acid however, is a
weak acid and is so subject to only a small degree of protolysis. This results in there being only a
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Titration of a weak base with
a weak acid with Cobra4
TEC
few hydronium ions in the solution and the pH of the acetic acid solution is therefore far higher than
that with hydrochloric acid. The equivalence points found for the two titrations are so at different pH
values. The equivalence point pH found in the example of a titration of ammonia solution with hydrochloric acid is 5.5 (see Fig. 6). The position of this equivalence point in the weakly acid region is due
to the fact that the ammonium chloride that is produced itself acts as a weak acid as it is a salt of a
weak base. The pH of this equivalence point is so dependent on the concentration of the ammonium
ions that are produced. In contrast to this, the titration of ammonia solution with acetic acid should
lead to an equivalence point of pH 7.
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The measurement used as example here gave an equivalence point of pH 7.1 and a volume of
added acetic acid of 9.98 ml, which demonstrates that the procedure described here can lead to
good results.
Disposal
The diluted and neutralised solutions of the used acid and base can be disposed by rinsing into the
drain.
Appendix
Hazard symbol, signal word
Hazard statements
Precautionary statements
Ammonia solution, 25 %
H314: Causes severe skin
burns and eye damage
.H400: Very toxic to
aquatic life.
Danger
P273: Avoid release to
the environment.
P280: Wear protective
gloves/protective clothing/eye protection/face
protection.
P305 + 351 + 338: IF IN
EYES: Rinse cautiously
with water for several
minutes. Remove contact lenses if present and
easy to do. continue rinsing
P309: IF exposed or you
feel unwell:
P310: Immediately call a
POISON CENTER or
doctor/physician.
Acetic acid
H226: Flammable liquid
and vapour.
H314: Causes severe skin
burns and eye damage.
Danger
P280: Wear protective
gloves/protective clothing/eye protection/face
protection.
P305 + 351 + 338: IF IN
EYES: Rinse cautiously
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TEC
Titration of a weak base
with a weak acid with Cobra4
with water for several
minutes. Remove contact lenses if present and
easy to do. continue rinsing.
P310: Immediately call a
POISON CENTER or
doctor/physician.
6
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