CHEM 225 LAB #6 Polyprotic Acid Titration

CHEM 225 LAB #6 – Polyprotic Acid Titration Analysis of Citric Acid in 7-Up

OBJECTIVE: The primary objective of this lab is to determine the 3 Ka values and the concentration of citric acid in 7-Up through the use of a pH meter to generate a titration curve.

INTRODUCTION: In this lab, known volumes of de-gassed 7-Up will be titrated with your standardized NaOH from Lab #5. Through a series of calculations and your knowledge of titrations, you will statistically determine the Ka values for citric acid and the concentration of citric acid in your 7- Up.

CHEMICAL REACTIONS:

Net Reaction

EXPERIMENTAL PROCEDURE:

1. Obtain a pH meter and calibrate it with the following buffers: (Instructions to calibrate will be on the white board in the lab) pH 4 – red ; pH 7 – yellow; pH 10 - blue

2. Take some time to set-up your buret, pH meter, Erlenmeyer flask, and stir plate. You will need the pH meter to be immersed but not to touch the stir bar.

3. Obtain 50.00 mL of degassed 7-Up and dispense into your 250 mL Erlenmeyer flask.

4. Add 2- 3 drops of phenopthalein to your flask.

5. Rinse your buret with your standardized NaOH solution and then fill your entire buret with this solution.

6. ROUGH TITRATION - Titrate with your standardized NaOH until a FAINT PINK COLOR persists for 30 seconds (exposure to air causes this end-point to gradually fade back to a colorless solution). Fractional drops should be added as the end-point is being approached. Record the pH throughout.

7. ACCURATE TITRATIONS – Do three more titrations adding the smallest amounts in mL of your NaOH so that you have enough points around the endpoints to calculate the Ka values for citric acid and the molarity of citric acid in 7-Up. Record the pH throughout. CALCULATIONS (after lab): 1. Plot the following curves for all three accurate trials

(1) pH vs volume NaOH (mL) added (titration curve)

(2) change in pH / NaOH (mL) vs volume NaOH (mL) added (derivative of titration curve)

(3) plot curves (1) and (2) on one graph

2. Calculate the molarity of citric acid in your 7-Up for all three accurate trials. Calculate the average molarity and the standard deviation. **HINT – think about the KHP lab but remember that you are dealing with a triprotic acid!!!

3. Calculate the grams of citric acid in 1 can of 7-Up that you determined from all three accurate trials. Calculate the average grams and the standard deviation.

4. Calculate Ka1, Ka2, Ka3 for the citric acid in your 7-Up for all three accurate trials. Calculate the average for all Ka values and the standard deviation for all. **HINT – For a monoprotic acid, pKa = pH halfway to equivalence. A triprotic acid has 3 equivalence points even if you cannot visualize them on your titration curve.