Acid-Base Titrations
Last edited by Koni Stone on October 22,1998.

In the first part of this experiment you will standardize a solution of sodium hydroxide. Standardize means to accurately determine the concentration. Crystalline potassium hydrogen phthalate will be used as the primary standard acid. After standardizing your NaOH solution, you will titrate a solution of an unknown concentration of a weak acid. Then, you will titrate a dilute solution of a strong acid, HCl and determine its molarity and the molarity of the stock HCl solution.

KHP (Potassium hydrogen phthalate) is a weak monoprotic acid that you will titrate with a strong base, NaOH. At the equivalence point, i.e., the point at which equal moles of acid and base exist, all of the acid is converted to A^-.

HA + OH^- ® A^- + H₂O    (1)

However, A- being the conjugate base of a weak acid will hydrolyze in H₂O to yield a pH higher than 7. [See Equation (2)].

A^- + H₂O <----->HA + OH^-     (2)

Therefore, it is necessary to use acid-base indicators that change color above pH 7. For example, phenolphthalein undergoes a colorless to red transition at a pH of approximately 9. The endpoint of your titration will be given by the phenolphthalein color change.

If you know the number of moles of acid in the flask, and the volume of base added to reach the equivalence point, you can calculate the molarity of the sodium hydroxide solution. You will then use this standardized solution to determine the %KHP in an unknown sample and to determine the concentration of hydrochloric acid in a stock solution.

Note: Students work independently! This lab is graded on accuracy and precision, do everything very carefully.

Stockroom: Things to borrow and return on the same day.

Methods

1. You will want to carefully read the tips and hints for cleaning glassware that are provided by John Burt.  Clean glassware is essential for success in this laboratory exercise.

2. Standardization of NaOH Solution.

From the 6 M NaOH solution available in the lab and distilled water, prepare 1 liter of 0.1 M NaOH solution. Store this in a polyethylene bottle or in a stoppered flask; the plastic container is preferred since basic solutions slowly react with glass. Under no circumstances should a volumetric flask be used. Be sure the solution is well mixed before you use it. At this point you only know that the concentration is approximately 0.1 M. Determine the exact concentration of the solution 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. 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 repeated inversions with shaking and swirling of the inverted flask. Hold the stopper in tightly!

Rinse a buret twice with approximately 10 ml of the 0.1 M NaOH solution and drain the solution through the buret tip. Then 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 at 0.00 or below. Record the level of the base, estimating the reading to the nearest 0.01 ml.

Pipet a 25.00 ml aliquot of the standard KHP solution into a 200 or 250 ml Erlenmeyer flask. Dilute with about 25 ml of water (use your wash bottle), washing down the sides of the flask in the process. Add 2 drops of phenolphthalein indicator solution.

Slowly run base out of the buret into the KHP solution, swirling the flask and contents. As you approach the end point, 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 drop wise with constant swirling. Occasionally wash down the sides of the flask with water from your wash bottle. The end point is where 1 drop (or less) of NaOH solution causes the solution to become permanently pink throughout. Essential ingredients for a successful titration in include care and patience, so don't try to hurry. Record the final buret reading, estimating it to the nearest 0.01 ml.

Sample Buret Reading

Repeat the titration two or three more times using a clean flask each time. After the first titration, the others should go more quickly since you now have some idea of how much base is required per 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. Record your data in your notebook. A proposed format is attached for you to consider in recording your data.

3. Determining the %KHP in an Unknown Sample

To the nearest 0.001 g, weigh a sample of your unknown sample (~1.4 g for a 50 ml buret), and transfer it to an Erlenmeyer flask. Dissolve it in about 50 ml of water, add 2 drops of phenolphthalein indicator solution, and titrate with your standard NaOH solution. If all the sample does not dissolve, begin the titration anyway, adding base very slowly and swirling continuously. By the time you approach the end point, all the sample will have dissolved (if not, do not continue until it has dissolved).

This first sample may be too large or too small to make the best use of your buret. Repeat the determination using your experience from the first titration to decide on a better sample size. Repeat this titration two or three times until your get consistent results.

4. Determine the concentration of HCl.

Take a 25 ml aliquot of a stock HCl solution and dilute it to 250 mL. Titrate a 25 ml aliquot of this dilute solution with your standardized NaOH solution, using phenolphthalein as the indicator. Repeat this titration two or three more times, until you get consistent results.

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 to the Stockroom.

Data analysis

Organize your data in tables using Excel and be sure to include a copy of the table showing the formulas that you used. You must show a sample of every calculation.

Calculate the molarity of your NaOH solution for each titration, and report an average molarity.

Sample dimensional analysis set-up. Put your numbers where the green letters are. Report the values that are in pink. Use blue values for subsequent caluculations.

Calculate the percentage of KHP in your unknown sample for each titration and report an average percentage.

Sample dimensional analysis set-up. Put your numbers where the green letters are.

Calculate the concentration of your dilute HCl solution for each titration and report an average molarity.

Sample dimensional analysis set-up. Put your numbers where the green letters are.

Calculate the concentration of the stock HCl solution for each titration and report an average molarity.

M₁V₁ = M₂V₂

For every average that you report, calculate and report an average deviation.

Conclusion

Restate all of your results. Be sure to report average values ± average deviations.

Answer the following questions in well organized paragraph form: