Titration of Phosphoric Acid and Buffers
May 26, 2006
Objective
The goal of this experiment is:
Introduction
Phosphoric
acid (H3PO4) is a chemical that is commonly found in
everyday products such as soft drinks and cleaning agents. It is called a
polyprotic acid because it can donate more than one proton (H+
ion) per phosphoric acid molecule. Because it can donate up to three protons,
it is sometimes called a triprotic acid.
The released protons combine with water to form hydronium ions (H3O+).
Phosphoric
acid releases its protons in a step-wise manner as follows:
H3PO4
+ H2O « H3O+ + H2PO4- Ka1 = 7.5 ´10-3 (1)
H2PO4- + H2O « H3O+ + HPO42- Ka2 = 6.2´10-8 (2)
HPO42 + H2O « H3O+
+ PO43- Ka3 = 4.2´10-13 (3)
For
example, reaction (2) will not occur until reaction (1) is complete.
The
Ka values listed after each reaction are called acid ionization constants.
They indicate the relative ease with which each reaction occurs. A small Ka
value shows that a reaction does not occur easily. The Ka value for phosphoric
acid’s second donated proton is much smaller than for the first donated proton,
while the third Ka is five orders of magnitude smaller than the second.
To
determine the amount of acid in an unknown sample, you will need to add a
known amount of base until the acid and base are neutralizes. This technique
is known as titration, and it is widely used in chemistry and other
natural sciences.
During
a titration, the pH of the solution is constantly monitored while the known
acid or base (called the titrant) is slowly added to the unknown solution.
The pH of the unknown solution will stay fairly constant until the moles of
titrant added equals the moles of unknown acid or base. When the moles of
acid and base are the same, further additions or titrant will cause a dramatic
change in pH until the pH eventually stabilizes. A graph of pH versus added
titrant is called a titration curve, and the point at which the pH
changes drastically is called the equivalence point.
The
titration curve for a polyprotic acid will have more than one equivalence
point. As the added base completely removes each proton from the acid, the
pH will jump significantly.
By
graphing the pH versus volume of base added during an acid-base titration,
you can easily see the successive ionization steps taking place. To find the
concentration of a polyprotic acid, the volume of base required to reach the
first equivalence point is needed. The half-equivalence points on this graph
can also be used to obtain the Ka values each successive ionization.
Because
most acid-base reactions are exothermic, the temperature of the solution will
rise until the equivalence poins is reached. For reactions involving dilute
solutions of acids and bases, a very small amount of heat is given off. The
LabWorks thermistor, however, is able to detect these small changes of temperature.
SAFETY PRECAUTIONS
General laboratory safety precautions apply to this experiment.
Be sure to wear goggles at all times.
Materials
In addition to the LabWorks, you will need the following materials to complete this experiment:
Experimental
Procedures
1. Obtain a pH probe and connect it
to pH/mV 1 on the interface. Open the Labworks program "Titration_Curves_and_the_Dissociation_Constant_of_Phosphoric_Acid".
2. Enter the calibration program. Select
"pH" in the "Sensor Setup" window and click "Calibrate".
Follow the on-screen instructions in the "Sensor Calibrate" window.
You will be doing one point calibration using a pH 7 buffer. Note that pH
probe should always be rinsed with deionized water and carefully patted dry
before inserting into any solution, so as to avoid cross contamination.
3. Obtain, in a dry beaker, 30 mL of
a phosphoric acid solution. Rinse your 10 mL pipet with this solution and
pipet 10 mL into a 150 mL beaker. Use a graduated cylinder to add 50 mL of
distilled water. Rinse and fill your 25 mL buret with 0.4 M NaOH. The initial
buret reading should be 0.
4.
Place the beaker on the magnetic stirrer and add a stir bar. Position
the buret ready for titration. Insert the pH probe. Turn on the magnetic stirrer
and adjust the stirring rate to moderate speed (without splashing).
5. Enter the "Acquire" program
and follow the on-screen instructions. As the experiment starts, you should
see successive pH readings at halfsecond intervals. Capture a pH value, when
these readings become stable, by pressing switch W on the interface and use
the keyboard to manually enter the initial buret reading. You should be estimating
the buret readings to the nearest 0.01 mL.
6. Enter the "Analyze" program.
The buret reading should appear in column A and the pH reading should appear
in column B.
7. Click on the graph window to activate
it. Select "Properties" from the "Graph" menu. Name the
plot "Titration Curve". Click on the "Data Series" tab.
Select "mL(buret)" under the X column and the "pH" under
the Y column. Click "Add plot" and "Apply". This should
display your titration curve. Name the axis by clicking on the appropriate
tabs in the "Graph Properties" window. Obtain a printout of your
titration curve and attach it to your report sheet.
8. Highlight column C by clicking on
it. Select "Column Setup" from the "Data" menu. Name the
column "First Derivative". Click on the "Formula Entry"
tab. Type "=deriv(B1,A1)" and click "OK". This should
display the derivative values in the C column. Click on the graph window to
activate it. Select "Properties" from the "Graph" menu.
Name the plot "First Derivative Curve". Click on the "Data
Series" tab. Remove the titration curve by clicking "Remove plot"
and "Apply". Select "mL(buret)" under the X column and
"first derivative" under the Y column. Click "Add plot"
and "Apply". This should display your derivative plot. Peaks should
appear corresponding to the points of steepest inflection in the titration
curve. Name the axis by clicking on the appropriate tabs in the "Graph
Properties" window.