Acid-Base Titrations

References:

Chemistry, 4th ed, by J. McMurry and R. Fay. Prentice Hall, 2004, Sections 3.10, 4.5

Purpose:

In the first part of this experiment you will prepare and standardize a solution of sodium hydroxide to use in this experiment and also to use in the next experiment (so don't throw it away yet).  In addition, you will use this solution to determine the concentrations of an unknown sulfuric acid solution.

Method:

Standardize means to accurately determine the concentration of a solution to use for another measurement. You can think of it as calibrating a solution.  Crystalline potassium hydrogen phthalate (abbreviated KHP) will be used as the primary standard acid. After titrating your your NaOH solution against a measured mass of KHP, you can then titrate solutions of acids having unknown concentrations with the NaOH solution to determine their respective molarities.

Potassium hydrogen phthalate is a weak monoprotic acid that reacts with aqueous sodium hydroxide according to the reaction:

KHP(aq) + OH-1(aq) P-1(aq) +  H2O(l) + K+1(aq)       (1)

In order to detect the equivalence point. or endpoint, the volumes when the reactants are exactly neutralized, an indicator dye, phenolphthalein, is added to the reaction mixture.  The endpoints of your titrations will be signaled  by the phenolphthalein color change.

Note: Students must work independently for this experiment! This lab is graded on accuracy and precision, correct use of significant figures and the quality of your report.

Special Equipment: Things to borrow and return on the same day.

250 ml vol flask (~$20)
25 ml pipet (~$15)
1 L polyethylene bottle, 500 mL polyethylene bottle
Pipet pump
Buret and brush (~$75)
 - - also get an unknown sulfuric acid solution (Record the unknown ID's in your notebook! If you do not record it in your notebook, no one knows the true value, so you can not get any credit for accuracy.  Don't let that happen.)

Procedure:

1. Read the tips and hints for cleaning glassware that are provided by John Burt.  Clean glassware is essential for success in this experiment.

2. Standardization of the NaOH Solution.

From the 6 M NaOH solution available in the lab and distilled water, prepare 1 liter of approximately 0.1 M NaOH solution.  Be very sure that this solution is thoroughly mixed.  Rinse the 500 mL bottle with deionized water.  Use approximately 20 mL portions of your sodium hydroxide solution, to rinse the 500 mL polyethylene bottle three times. Fill this bottle with your sodium hydroxide solution.  Put it away in your drawer to use in the next experiment.  For today's experiment, use the remaining sodium hydroxide solution from the 1 L bottle.  When you finish today, rinse the 1 L bottle and return it.  At this point you only know that the concentration of the sodium hydroxide solution is approximately 0.1 M. Determine the exact concentration of the solution by standardization as described below.

To the nearest 0.001 g, weigh a sample of potassium hydrogen phthalate (KHP), KHC8H4O4 (about 5 g), transfer the sample to a clean 250 ml volumetric flask, and add about 100 to 150 ml of distilled water. Be sure to record the mass of the KHP.  Swirl the flask until the KHP dissolves. The dissolution may be rather slow so be patient. When it is all dissolved, dilute to the mark with distilled water and mix the contents by five repeated inversions with shaking and swirling of the inverted flask. Hold the stopper in tightly while you do this!

Rinse a buret twice with approximately 10 ml of the 0.1 M NaOH solution and drain the solution through the buret tip. Then, using a funnel, fill the buret with the 0.1 M NaOH solution; make sure there are no air bubbles in the tip of the buret or just above the stopcock. Run base out of the buret until the level is  just below 0.00 mL. Read the bottom of the meniscus (see below) to determine the initial level of the base, estimating the reading to the nearest 0.01 ml.  Record this value in your Table in your notebook.  Be sure to keep all of the significant figures when you put the value in the Table.

Sample Buret Readings Using a Meniscus

meniscus2.gif (2959 bytes)

 

Pipet a 25.00 ml aliquot of the standard KHP solution into a 200 or 250 ml Erlenmeyer flask. Add about 25 ml of water (use your wash bottle), washing down the sides of the flask in the process.  Next, add 2 drops of phenolphthalein indicator solution.  Don't forget this indicator or the titration will not work and you will spend much longer in the lab than you had anticipated.

The most efficient way to do this experiment is to add base from the buret to the KHP solution fairly rapidly the first time, swirling the flask and contents, as you add the base.  As you approach the endpoint, the area in the KHP solution, where the drop of NaOH falls, will turn pink; then the pink color will disappear as the solution becomes mixed.  From this point on, add the NaOH dropwise, with constant swirling.  Occasionally wash down the sides of the flask with a little water from your wash bottle.  The endpoint is reached when one drop (or less) of NaOH solution causes the solution to become permanently pale pink throughout.  Don't worry too much about getting exactly to the one drop, this time.  This is your "quick and dirty" trial.  Record the final volume, again estimating the reading to the nearest 0.01 ml.  The difference between the initial and final volumes is approximately the volume needed to reach the endpoint.

Repeat the titration three more times using a clean flask each time.  After the first titration, the rest should go more quickly since you now have some idea of how much base is required for each aliquot of KHP solution. The base may be added quickly until you are within 2 or 3 ml of the end point, then change to drop wise addition.  Be sure you read the buret by estimating each reading to the nearest 0.01 ml.  It is a good idea to record the volumes when you are close to the endpoint so that if you add a little too much you have a value of the volume just before that happened.  Be sure to record all of your data, in your Table, in your notebook, as you collect it.  This data will be used to calculate the exact concentration of your standard sodium hydroxide solution.

3. Determine the Concentration of Unknown Sulfuric Acid

Pipet a 25.00 mL aliquot of your unknown sulfuric acid (a diprotic acid) solution into a clean 250 ml Erlenmeyer flask. Titrate this solution with your standardized NaOH solution, using phenolphthalein as the indicator, as described above. Repeat this titration at least three more times.  Record your data as you collect it.  Significant figures are very important!

Wastes and Spills: All the chemicals in this experiment can be put in the trash or down the drain. Be sure the water is running and be careful with the 6 M NaOH. Use wet paper towels to wipe up small spills. Rinse the pipet and buret and other glassware with water before returning them.

Results and Conclusion:

Using the data in your Table, do the following calculations.  You must show a sample of every calculation.

  1. Calculate the molarity of your NaOH solution for each titration, and the average molarity and the standard deviation.
  2. Calculate the concentration of your unknown sulfuric acid solution for each titration and report an average molarity and the standard deviation.

Tabulate your result, including the standard deviations.  Note: all results must be expressed with the correct number of significant figures.