Procedures

Experiment 1: Preparing the Primary Standard

PLEASE NOTE: The procedures described in this lab assume that you have already done the Titration Tutorial and are familiar with the technique. If you have not yet done the Titration Tutorial Lab, please do it now.

1. Take a beaker from the Containers shelf and place it on the workbench.

2. Prepare a sodium hydroxide (NaOH) solution of approximately 0.1 M concentration by dissolving 0.8 g of solid sodium hydroxide in 200 mL of water from the Materials shelf. 

3. In your lab notes,  show the calculation for this expected concentration given the moecular weight of NaOH as 40g/mol.

4. To prepare the primary standard, first calculate and record the expected mass of benzoic acid (C₇H₆O₂) (MW = 122.12) required to react in stoichiometric proportion with 20 mL of a0.1 M sodium hydroxide  solution. Record the required mass of benzoic acid in your Lab Notes.

5. Take a clean Erlenmeyer flask from the Containers shelf and place it on the workbench.

6. Add 100 mL water from the Materials shelf to the Erlenmeyer flask. 

7. Add the required mass of benzoic acid to the Erlenmeyer flask. The 100 mL of water will be enough to dissolve the solid benzoic acid.

8. Add 2 drops of  phenolphthalein indicator to the Erlenmeyer flask.

Place a pH Meter from the Instruments shelf onto the flask to attach it. Record the initial pH of the solution in your Lab Notes.

10. Coarse Titration:

(a) Take a burette from the Containers shelf and place it on the workbench. Fill theburette with 50 mL of the prepared sodium hydroxide solution. Record the initialburette reading.

(b) Place the Erlenmeyer flask on the lower half of the burette - this will connect them.

(c) Perform a coarse titration, adding large increments of the sodium hydroxidesolution from the burette by pressing and holding the black knob at the bottom of the burette. Each time you add the sodium hydroxide solution, check the volume remaining in the burette. As the sodium hydroxide is added to the benzoic acid(C₇H₆O₂) solution the pH increases. Watch for a gradual change of thephenolphthalein color in the Erlenmeyer flask. The pink color will appear inErlenmeyer flask when the endpoint is either reached or crossed. Record the burette volume and pH at which this occurs. 

(d) Record both the last volume where the solution was colorless and the first volume where the solution was pink. This gives you the range within which to do the fine titration.

(e) Place the Erlenmeyer flask and burette in the recycling bin underneath the workbench.

11. Fine Titration:

(a) Set up the tiration as before: (i) An Erlenmeyer flask filled with the calculated mass of benzoic acid, 50 mL of water, a pH meter and 2 drops ofphenolphthalein. (ii) A burette filled with 50 mL of the prepared sodium hydroxide solution. Record the initial burette reading. (iii) Place the Erlenmeyer flask on the lower half of the burette.

(b) Click and hold the black knob of the burette to quickly add enough standardsodium hydroxide solution to just get into the range of the coarse titration: 1 mL BEFORE the color change. This is near, but not yet at, the titration's endpoint.

(c) Add sodium hydroxide solution in small increments, down to one drop at a time. Record the pH and volume until several drops after the endpoint is reached.

(d) Place the Erlenmeyer flask and burette in the recycling bin.

(e) Repeat the fine titration twice more, and record the results in your Lab Notes. 

12. With the total volume of sodium hydroxide solution used to titrate the benzoic acidprimary standard, calculate the concentration of the sodium hydroxide solution in your Lab Notes (remember to save them.) The rest of the sodium hydroxide solution can now be used in further lab work as a secondary standard with a reliably known concentration equal to the average of the three titrations.

Experiment 2: Using the Secondary Standard to Determine the Concentration of an Acid

1. In an Erlenmeyer flask, add 25 mL of Unknown #1 concentration ofacetic acid (CH₃COOH) and 2 drops of phenolphthalein indicator and a pH meter.

2. Coarse Titration:

(a) Take a burette from the Containers shelf and place it on the workbench. Fill theburette with 50 mL of the standardized sodium hydroxide solution. Record the initial burette reading. Place the Erlenmeyer flask on the lower half of the burette.

(b) Perform a coarse titration, adding large increments of the sodium hydroxidesolution from the burette by pressing and holding the black knob at the bottom of the burette. Each time you add the sodium hydroxide solution, check the volume remaining in the burette. As the sodium hydroxide is added to the acetic acidsolution the pH increases. Watch for a change of the phenolphthalein color in theErlenmeyer flask. The pink color will appear in Erlenmeyer flask when the endpoint is either reached or crossed. Record the burette volume and pH at which this occurs. 

(c) Place the Erlenmeyer flask and the burette in the recycling bin.