LEC
06.17
Potentiometric pH titration (phosphoric acid in a soft drink)
Related concepts
Galvanic cell, types of electrodes, Galvani voltage, cell voltage,
Nernst equation, potentiometry, volumetry.
Principle
The cell voltage and the Galvani voltage of the electrodes of an
galvanic cell are dependent upon the concentration of the ions
involved in the potential forming process. Thus, conclusions can
be made about the concentration of the ions to be investigated
from the measured cell voltage at a constant potential of a suitable reference electrode (potentiometric titration).
Tasks
Using the Cobra3-System, measure the change in the cell voltage in the titration of
a) diluted phosphoric acid with 0.1 molar sodium hydroxide
solution
b) a sample of a carbonated beverage (Cola) containing phosphoric acid (E 338) with 0.1 molar sodium hydroxide solution
and calculate the beverage’s phosphoric acid content from
the consumption of the standard solution.
Equipment
Cobra3 Chem-Unit
Power supply 12 V/2 A
Data cable, RS232
Software Cobra3 Chem-Unit
pH-electrode, gel-filled, BNC
Immersion probe NiCr-Ni, Teflon
Motor piston burette, 50 ml
12153.00*
12151.99
14602.00
14520.61*
46265.15*
13615.05
36499.93
1
1
1
1
1
1
1
Cable Chem-Unit / Motor piston burette
Rubber stopper, d = 18/14 mm, 1 hole
Magnetic stirrer, mini
Magnetic stirrer bar, l = 30 mm
Glass beaker, 50 ml, tall
Glass beaker, 250 ml, tall
Volumetric flask, 100 ml
Volumetric pipette, 5 ml
Volumetric pipette, 50 ml
Graduated pipette, 1 ml
Pipette dish
Pipettor
Wash bottle, 500 ml
Ortho-phosphoric acid, 85 %, 250 ml
Caustic soda solution, 0.1 M, 1000 ml
Buffer solution, pH 4.62, 1000 ml
Buffer solution, pH 7.01, 1000 ml
Buffer solution, pH 9.00, 1000 ml
Water, distilled, 5 l
PC, Windows® 95 or higher
36501.01*
39254.01
47334.93
46299.02
36001.00
36004.00
36548.00
36577.00
36581.00
36595.00
36589.00
36592.00
33931.00
30190.25
48328.70
30280.70
46271.12
30289.70
31246.81
1
1
1
1
3
2
1
1
1
1
1
1
1
1
1
1
1
1
1
Changes in the equipment required for use of the BasicUnit: (instead of * above mentioned)
Cobra3 Basic-Unit
12150.00 1
Module converter
12150.04 1
Measuring module, pH/Potential
12101.00 1
pH electrode, gel-filled
18450.00 1
Measuring module, NiCr-Ni
12104.00 1
Software Cobra3 pH/Potential
14509.61 1
Cable Basic-Unit / Motor piston burette
36501.03 1
Fig. 1. Experimental set-up.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3061740
1
LEC
06.17
Potentiometric pH titration (phosphoric acid in a soft drink)
Set-up and procedure
Set up the experiment as shown in Fig. 1.
Prepare the solutions required for the experiment as follows:
– 0.08 molar H3PO4 solution: Pipette 0.55 ml of 85% phosphoric acid into a 100 ml volumetric flask and make up to the
mark with distilled water.
– Soft drink containing phosphoric acid: Pour approximately
150 ml of the soft drink into a 250 ml beaker. Place the
beaker on the magnetic stirrer and heat it carefully to remove
dissolved carbon dioxide. Allow to cool to room temperature.
Connect the control input of the motor piston burette to the TTLoutput of the Cobra3 Chem-Unit with the cable supplied for this
particular purpose. Connect the pH electrode to the pH-input ,
and the temperature probe to temperature input T1.
Call up the “Measure“ programme in Windows and enter
<Chem-Unit> as measuring instrument. Set the measurement
parameters as shown in Fig. 2. In <Preferences> under <pH/Potential> select <Potential> as mode and <mV> as unit. In menu
prompt <Displays> set the display range to -300 - 300 mV. Set
Digital display 1 to <Potential>, and select, under Diagrams,
<Line diagram> for Diagram 1, <Potential> for Diagram 1a, and
for the display range <0 - 10 ml> and <no auto range>. Confirm
your entries with <OK>.
Now calibrate your pH electrode using two buffers. To do this,
enter the appropriate pH value in <pH / Potential> under the
menu point <Calibrate>, dip the electrode into the buffer and
save with <Calibrate>. To calibrate the temperature sensor,
either balance it against a temperature measured with a thermometer, or with the level of a temperature probe connected to
T2 or T3.
After having made all settings, press <Continue> to reach the
field for the recording of measured values. Arrange the displays
as you want them.
Measure the potentials of the two buffers used above and a further one, in each case immersing the electrode in the buffer and
recording the value that is shown in the display.
Fill 100 to 150 ml of water into a 250 ml glass beaker , add a
magnetic stirrer bar and place the beaker on the magnetic stirrer. Pipette 5 ml of 0.08 molar phosphoric acid into the beaker.
Cut the rubber stopper with central hole lengthwise from one
side to the hole and fit the temperature probe into it. Fix the dispensing tip of the motor piston burette, together with the pH
electrode and the temperature probe, in the electrode holder.
The sensing diaphragm of the pH electrode must be covered by
the solution. Adjust the magnetic stirrer to a medium stirring
speed and start the measurement with <Start measurement>.
The first measurement (for V = 0.0) is immediately recorded.
Following this, the motor piston burette dispenses the first portion of sodium hydroxide solution. The titration now proceeds
according to the parameters that have been set, and is automatically stopped after the addition of 10 ml of sodium hydroxide solution. On completion of the series of measurements, save
the data with <File><Save measurement as...>.
Now titrate the soft drink containing phosphoric acid. To do this,
pipette 60 ml of the prepared soft drink into the 250 ml glass
beaker, add approximately 100 ml of distilled water and a stirrer
bar, place the beaker on the magnetic stirrer, fix the burette tip
and pH electrode in position and carry out the titration and saving of data exactly as above.
Figs. 3 and 4 show the graphs as they can be presented when
the measurement is stopped.
Fig. 3:
Cell voltage U and pH value as a function of the volume
of added standard solution for the titration of an
approximately 0.08 molar phosphoric acid with a 0.1
molar sodium hydroxide solution.
Fig. 4:
Titration diagram of the neutralisation of a beverage
containing phosphoric acid (V = 50 ml) with a 0.1 molar
sodium hydroxide solution.
Follow the operating instructions supplied with the motor piston
burette to fill it with 0.1 molar sodium hydroxide solution. Set the
motor piston burette to <Dosing>, <nom.Vol. = 0.2 ml> and
<Speed = 99.9 ml/min>.
Fig. 2:
2
Measurement parameters
P3061740
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
LEC
06.17
Potentiometric pH titration (phosphoric acid in a soft drink)
Theory and evaluation
The titration curves recorded show the cell voltage U as a function of the volume of sodium hydroxide solution added. To
obtain the relationship between voltage and pH, plot the potentials measured for the three buffers against their pH values.
pH of the buffer
Voltage measured
4.62
+147 mV
7.01
+0 mV
9.00
-98 mV
With the help of the best straight line through the three measured
points and the equation for a straight line f(x) = ax + b (with a =
∆pH / ∆U and b = pH at U = 0 mV), you can have the diagram
for the corresponding pH values calculated with <Channel modification> under menu prompt Analysis. For this, set the parameters as given in Fig. 5.
The pH measurement is therefore a potentiometric measurement. It can only be carried out, when the potential of the indicator electrode is measured against that of a reference electrode. Both electrodes are contained in single-rod pH measuring
systems. A glass electrode is used as the indicator electrode.
When this is immersed in an aqueous solution, a swelling layer
(gel) is formed at the pH sensitive glass membrane. This also
occurs at the inner side of the membrane, which is in contact
with a defined buffer solution, commonly of pH 7. According to
the pH of the test solution, hydronium ions diffuse either out of,
or into, the outer swelling layer, whereby the potential of this
layer changes. The pH, and so also the potential, of the inner
side of the glass membrane remains constant while this occurs.
With regard to the pick-up electrode, the same system is chosen
as for the reference electrode (AgCl), so that the potential difference between the two electrodes is equal to zero. The voltage
measured with the single-rod measuring system therefore
results from the potential difference between the inside and outside swelling layers of the glass electrode.
When the system is dipped into a test solution of pH 7, then a
potential of 0 mV should be given, as the inner potential is equal
to the outer potential. Theoretically, when the pH changes by a
single unit, the voltage should change by 59.16 mV. This can be
calculated using the Nernst equation, which is as follows for
hydrogen:
Two equivalence points can be clearly seen in the titration curve
for pure phosphoric acid. They correspond to the following dissociation equilibria:
H3PO4 + Na+ + OH- S H2PO4- + Na+ + H2O
H2PO4- + Na+ + OH- S HPO42- + Na+ + H2O
The second equivalence point is only weakly shown in the titration curve for the soft drink. This is because, in addition to phosphoric acid, further acids are present here which also take part
in the titration with sodium hydroxide solution and exert a buffering effect in the region of the second equivalence point. A
knowledge of the first equivalence point suffices for the calculation of the quantity of phosphoric acid contained in the soft
drink.
The concentration c2, and so the content m2 of acid-forming
substance in the sample of volume V2, can be calculated from
the corresponding consumption V1 of standard solution of
known concentration c1.
c1V1 = c2V2 = m2 / M2
where
M
Molar mass of phosphoric acid (= 98 g/mol)
Data and results
The first point of inflection of the titration curve gives
V1 = 3.34 ml NaOH, from which a phosphoric acid concentration
c2 = 6.68 · 10-3 can be calculated, and from this a content of
m = 32.73 mg H3PO4 in the volume of the sample tested
(V2 = 50 ml).
A 1.5 l bottle of the soft drink tested therefore contains 981.9 mg
of phosphoric acid.
Fig. 5: Parameters of the channel modification
E = Eo + 2.3 · RT/F · log a (H+)
where
E0
=0
log a (H+)
log c(H+) = - pH
2.3 · RT/F = 59.16 mV (at T = 298.15 K)
From this it follows that: E = - 59.16 mV · pH
T
R
F
Absolute temperature
Universal gas constant
Faraday constant
This rise in the pH-characteristic line is called the slope of the
single-rod pH measuring system. The slope of real pH measuring systems deviates like the zero point from the theoretical
value given by the Nernst equation.
You can have the equivalence point calculated with <Detect
point of equivalence> under menu prompt <Analysis>.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3061740
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LEC
06.17
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P3061740
Potentiometric pH titration (phosphoric acid in a soft drink)
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen