CHM 130 Acid-Base Titration Experiment
Introduction: A titration is a method of analysis that will allow you to determine the precise
endpoint of a reaction and therefore the precise quantity of reactant in the titration flask. A buret
is used to deliver the second reactant to the flask and an indicator or pH Meter is used to detect
the endpoint of the reaction.
Fig 1: Weak acid (acetic acid) titrated with a strong base (NaOH)
The reaction is expected to go to completion when the equivalence point is reached. This is when
stoichiometrically equivalent quantities are mixed (equal amount of base and acid are in the
system). Acid/base indicators like phenolphthalein show when this point has been reached. They
show this by changing colors. Phenolphthalein changes from colorless to pink at the equivalence
point.
For this lab, potassium hydrogen phthalate (a weak acid) will be titrated with a strong base, NaOH.
The following equation describes the reaction.
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Laboratory objectives:
1. Measure the initial pH of a solution and the pH after the addition of each volume
of the standard solution.
2. Determine the equivalence point of the reaction from the graph of pH versus
volume of standard solution.
3. Determine the Ka value of the acid used as the unknown.
Materials:
1. 125 ml beaker
2. 50.0 ml Buret
3. Magnetic Stirrer
4. pH Meter
5. Analytical balance
6. chemicals: sodium hydroxide solution (known concentration) and Potassium hydrogen
phthalate
Procedure: This exercise will contain four parts. Download the spreadsheet to record all of
your data ( see Excel File)
1. Preparation of the acid solution and preparing the buret.
2. Trial 1 titration
3. Trial 2 titration
4. Ka determination
Part 1. Preparing the buret and weighting out the potassium hydrogen phthalate:
1. Rinse your buret with three 15 ml portions of R.O. water.
2. Set the buret in the hood and rinse with three 5 ml portions of the NaOH solution.
3. Fill the buret to above the 0.00 level with the NaOH solution. Drain to the 0.00 level to
begin the titration. Record the concentration of the NaOH solution on the data page.
4. Weigh a 0.5100 g sample of KHpht into a clean and dry 250 ml beaker or erlenmeyer
flask.
5. Record the mass of KHpht on the data page.
6. Add 50 ml of water and stir gently with the magnetic stirrer until the sample has
dissolved.
Part 2. Trial 1 titration:
1. Record the pH of the acidic solution before any NaOH is added, record under 0.0 mL
volume.
 If the pH meter does not list 4, 7, and 10 on the display, the meter needs to be
calibrated. Click Here for Calibration instructions.
 The pH meter must be calibrated with a solution whose pH is known before you
can measure an unknown pH with accuracy. These solutions of known pH are
called buffer solutions.
 You should be able to read the pH of a solution about 10 seconds after the
electrode(s) have been immersed. The reading should be steady and not suddenly
changing. If sudden changes do occur, consult your laboratory instructor.
2. Add 1.0 ml of NaOH record the pH.
3. Continue to add NaOH as shown on the data table until all the data boxes are filled.
4. When the pH change is greater than 0.3 units reduce the volume of NaOH from 1.0 ml to
0.10 ml or 0.20 ml. Make the excel spreadsheet match your buret readings.
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5. Continue to add 0.10 ml or 0.20 ml of NaOH until a significant change occurs. Then
continue for an additional 3.0 ml. Now return to adding 1.0 ml of NaOH until a final
volume of 35.0 ml have been added.
Part 3. Trial 2 titration:
1. Obtain a new sample of KHpth and make another solution (see part 1).
2. Refill the buret with NaOH.
3. Record the pH of the acidic solution before any NaOH is added use volume 0.00 ml.
4. Add 1.0 ml of NaOH and record the pH.
5. Continue to add NaOH as shown on the data table until all the data boxes are filled.
6. When the pH change is greater than 0.3 units reduce the volume of NaOH from 1.0 ml to
0.10 ml or 0.20 ml.
7. Continue to add 0.10 ml or 0.20 ml of NaOH until a significant change occurs. Then
continue for an additional 3.0 ml. Now return to adding 1.0 ml of NaOH until a final
volume of 35.0 ml have been added.
Part 4. Determination of the Ka of potassium hydrogen phthalate:
Using the titration graphs from Part 2 and 3 to determine the following values (see calculation
tab in Excel):
1. Equivalence point
2. pH at one-half the volume of the equivalence point
3. pH at the half way point (which equals the pKa)
4. convert the pKa to a Ka
5. Using this experimentally determined Ka value, calculate the expected pH of the
original acidic solution.