Experimental Procedure

Part A. Calibration of pH electrode

1. Connect the pH electrode to the pH meter. (The electrode is in a buffer solution. Keep the electrode in the buffer solution as long exposure of buffer outside the solution can cause damage to the probe).
2. Connect the power cord to the meter and turn the instrument on.
3. Allow instrument to warm for 2 minutes.
4. Press the standardize menu on the meter screen.
5. Pour approximately 20 mL of buffer of pH 4.00 solution into a 100 mL beaker.
6. Gently remove the electrode from the buffer solution (electrode is fragile).
7. Rinse the electrode tip and surface with DI water and blot dry tip with KIMWIPE. The electrode tip should be rinsed over the waste beaker.
8. Gently place electrode in the buffer solution to read pH. The probe tip should be submerged into the solution but do not allow the tip to rest on the surface of the beaker. The meter should read pH around 4.00 (+/- 0.02). If the meter reads a value equals to or greater than +/-0.10 pH units of the buffer, consider replacing electrode if this is a viable option or seek sharing electrode with adjacent group (Instructor discretion).
9. Remove the electrode from the 4.00 buffer solution once the pH has been obtained.
10. Rinse the tip and the surface of electrode and blot dry tip with KIMWIPE.
11. Repeat steps 5, 8 and 9 for pH buffer solution 10.00.
12. Place electrode back into its original buffer solution and proceed to part B.

Part B. pH analysis of the weak acids and weak base

1. Pour 20.0 mL of 0.010 phosphoric acid into a 100 mL Erlenmeyer flask.
2. Rinse the electrode tip and surface with DI water and blot dry tip with KIMWIPE. The electrode tip should be rinsed over the waste beaker.
3. Read the pH and record the value and place electrode back into its buffer solution.
4. Pour 0.010 M solution into waste beaker and thoroughly rinse Erlenmeyer flask to be used for step 5.
5. Pour 20.0 mL of 0.10 M phosphoric acid into the 100 mL Erlenmeyer flask used in step 1.
6. Remove the electrode from the buffer solution, rinse tip / blot drive and place into 0.10 M phosphoric acid solution.
7. Repeat steps 1 to 5 for acetic acid (2) and ammonia (3) in the order listed above (Obtain pH of the 0.010 M solution first, 0.10 M solution last).
8. Clean up work area, wash all beakers and flasks, gently place the pH probe back in its original buffer solutions and unplug the meter.

Prelab Questions

*This prelab needs to be completed prior to lab. Obtain a signature prior to leaving class for full credit.

1. Classify each acid as a Bronsted-Lowry or Lewis Acid and explain your classification.a. HNO3 b. Fe+2
   c. C2H2O4 d. FeCl3 e. HCO3-1
2. Write an acid base equilibrium expression and identify the conjugate acid and base pairs for the following in water:a. C6H5COOHb. C6H5NH2
3. A student makes a 0.0450M solution of a weak acid (HA) that has a pH of 3.75. Determine the Ka for this acid
4. A student makes a 0.0450 M solution of a weak base that has a pH of 9.85. Determine the Kb for this base.

Experimental Data and Results

Post Lab Questions

1. From a chemistry textbook or other credible source, obtain the theoretical Ka for acetic acid and phosphoric acid and the Kb for ammonia. Calculate the % error associated with your calculated Ka and Kb values.

2. What effect if any did the concentrations of the measured weak acids and bases have on the accuracy of the determined equilibrium values in this experiment? Account for any differences in calculated equilibrium values in the different concentrations of the weak acids and base measured

3. Based on the pH’s of the weak acids (acetic acid and phosphoric acid) and weak base (ammonia) with respect to their respective concentration, calculate the % acid dissociated for the weak acid or the % proton acceptance for the weak base.