pH TITRATION: Potassium Hydrogen Phthalate (KHP)
(Standardization of a NaOH Solution)
Introduction:
The endpoint of an acid/base titration is the point at which the acid and base are
stoichiometrically equivalent. A graph of pH versus volume of titrant added will exhibit an
inflection point where the reactants are equivalent. In this experiment, a pH meter will be used
to monitor pH during the titration. A graph of pH versus NaOH will be plotted and the EXACT
concentration of the NaOH solution will be calculated.
Substances such as potassium hydrogen phthalate (KHP) which are available pure and are
used to standardize (determine the exact concentration of) solutions of other substances, are
called primary standards. The NaOH solution standardized in this manner would be called a
secondary standard.
Purpose:
To standardize a prepared NaOH solution using KHC8H4O4 ( KHP) as a primary
standard.
Equipment/Materials:
Van Provides:
Teacher Provides:
pH meter and electrode
250 mL beaker
pH buffer solutions (4, 7, and 10)
2 100 mL beakers
magnetic stirrer and stir bar
spatula
potassium hydrogen phthalate (KHP)
analytical balance
Sodium nitrate
weigh boats or weighing paper
Elemental sulfur
buret
2 Liter bottles
buret clamp
device for NaNO3- Sulfur mixture
wash bottle with DI water
Kimwipes
NaOH solution (approximately 0.1 M)
Safety Considerations:
Wear apron and goggles at all times in the lab. Preparation of sulfuric acid should be
completed under a fume hood.
Sulfur Acid Preparation Procedure:
1. Obtain a clean dry 2L drink bottle. Add approximately 10 mL of distilled water to the 2L
bottle.
2. Obtain a special device from instructor to hold sodium nitrate and elemental sulfur
mixture.
3. Mass 1.0 grams of sodium nitrate and place in the bucket of device.
4. Mass 0.5 grams of elemental sulfur and place on top of the sodium nitrate.
5. Using a Bunsen burner, light the sodium nitrate and elemental sulfur mixture.
6. Once a good flame is achieved, carefully place the device into the 2Lbottle. Care should
be taken not to spill the burning mixture. Screw the top of the device to the 2L bottle so
that no smoke exits the bottle.
7. The flame will burn until all oxygen is consumed. Allow the bottle to sit undisturbed
overnight.
Procedure:
1. Plug in pH meter. Allow the instrument to warm up for 10 minutes.
2. Prepare a solution of NaOH approximately 0.10 M in concentration. (This solution may
have already been prepared for you. Ask your instructor.)
3. Accurately mass about 0.4 g of KHP crystals (note the formula on the label) and dissolve
them in about 150 mL of deionized water in a 400 mL beaker. Make sure you record the
mass of the measured KHP to at least 4 significant figures.
4. Carefully place a magnetic stir bar in the acid solution and mount the beaker on a
magnetic stirrer. Turn the stirrer on to a very slow rate (2 or 3). Rinse the buffer off of
the pH electrode and carefully place the electrode of the pH meter in the solution. Be
sure that the electrode is immersed in the solution but is clear of the stirring bar. THESE
ELECTRODES ARE FRAGILE AND EXPENSIVE!!! If a magnetic stirrer is not
available swirl gently by hand.
5. Take an initial reading of the pH and record in the Data Table.
6. Rinse a 50 mL buret three times with about 5 mL of the NaOH solution.
7. Fill the buret past the 0.00 mL mark with the NaOH solution and deliver the excess into a
waste beaker while ridding the buret tip of air bubbles. Adjust the NaOH solution to the
0.00 mark of the buret.
8. Begin adding the NaOH slowly, stopping at frequent intervals to record the volume and
the pH. Gently stir during the addition of NaOH. Collect data at intervals of 0.2 pH units
or 1 mL, whichever occurs first. Be particularly alert as you pass pH 4 as you will find
the pH rising quite fast at this point.
9. Continue the titration until you have added about 50 mL of the base or until the pH
reaches 11, whichever occurs first.
10. In order to increase the accuracy of your calculations, repeat the titration as instructed.
11. Wash the pH probe with water and allow it to sit in a beaker of water while preparing for
titration of the unknown acid.
12. Obtain approximately 10 ml of sulfuric acid of unknown concentration. The acid can be
prepared by your instructor or earlier in a special procedure provided by your instructor.
Record the exact volume of sulfuric acid.
13. Add the sulfuric acid to a 250 mL beaker. Add 100 mL of water to the beaker.
14. Take an initial reading of the pH and record on the Data Table.
15. Follow steps 11-13 to titrate the sulfuric acid with the NaOH solution.
Clean-up:
Wash the buret completely (including the tip) with deionized water at least three times.
Leave the buret open and inverted in the buret clamp so that it will drain optimally. Wash all
used glassware. Be sure to wash your hands well following the lab in order to remove any
residual NaOH or KHP.
Graphing and Calculations:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Plot a titration curve of pH (y-axis) versus mL of NaOH solution (x-axis) using the data
from the NaOH vs.KHP itration.
Mark the equivalence point of the titration and read the volume of NaOH required to
reach this point.
Write a balanced equation for the reaction of NaOH and KHP. Note: KHC8H4O4 is a
monoprotic acid.
Calculate the moles of KHP from the initial mass.
Calculate the number of moles of NaOH required to reach the equivalence point.
Calculate the molarity of the NaOH. Be sure to keep as many significant digits in your
answer as allowable. The molarity calculated will serve as the standard concentration for
NaOH in the sulfuric acid titration.
Plot a titration curve of pH (y-axis) versus mL of NaOH solution (x-axis) using the data
from the NaOH vs. H2SO4 titration.
Mark the equivalence point of the titration and read the volume of NaOH required to
reach this point.
Write the balanced equation for a reaction between NaOH and H2SO4. Note: Sulfuric
acid is diprotic. Carefull attention should be paid to balancing the equation.
Using the volume of NaOH added to reach the equivalence point and the standard NaOH
concentration, calculate the molarity of the unknown sulfuric acid.
HINT: MacidVacid = MbaseVbase ; the formula must be adjusted to reflect the stoichiometry
of the balance equation.
Questions:
1.
2.
3.
4.
5.
Would it have made a difference in your calculations if you dissolved the KHP in 100 mL
of water instead of 150 mL? Why or why not?
Why was the NaOH solution standardized?
Write the equations that reflect the complete dissociation of sulfuric acid.
Would you expect to see multiple inflection points to reflect the dissociation of the
second hydrogen ion in the titration curve?
In terms of acid strength, explain why some polyprotic acids, when titrated, may only
show one inflection point.
pH TITRATION: Potassium Hydrogen Phthalate (KHP)
Data Table
Name(s): ______________________________________
Date: ___________________
________________________________________
Period: _________________
Mass of KHP:
______________________g
Formula of KHP:
______________________
Gram formula mass of KHP: ______________________g/mol
Buret
Reading
0.00 mL
pH
Buret
Reading
pH
Buret
Reading
pH
pH TITRATION: Unknown Sulfuric Acid
Data Table
Volume of H2SO4:
______________________mL
Standardize NaOH concentration:
Buret
Reading
0.00 mL
pH
______________________mol/L
Buret
Reading
pH
Buret
Reading
pH