1. Experiments in Analytical Chemistry
-Determination of pK1, pK2, and pK3 of phosphoric acid

2. Polyprotic acids and bases

3. Phosphoric acid
K1 = 7.5 x 10-3
K2 = 6.2 x 10-8
K3 = 4.2 x 10-13

4. Titration of phsphoric acid with strong base
The third equivalence point is invisible because K3 is very small.
How do we experimentally determine K3?
Answer: Titrate Na3PO4 with strong acid!

5. Glass electrodes
E = constant – ( ) pH at 25oC

6. Errors in pH measurements
Standards
Junction potential
Junction potential drift
Sodium error
Acid error
Equilibration time
Hydration of glass
Temperature
Cleaning
Effect of junction drift

7. pH standard buffers

8. Finding the End Point with a pH Electrode-example

10. Titration of phosphoric acid with a pH meter
Check out a pH meter, combination electrode, magnetic stirrer and stirring bar from the instructor.
Read the instructions on the use of the pH meter.
Standardize the pH meter using the buffer supplied.
Clean the electrode thoroughly with distilled water; drying is not necessary.
Immerse the electrode in the solution to be titrated; it should not go to the bottom of the titration vessel.
Start the stirring motor, being careful that the stirring bar does not break the glass electrode. You should allow room for the stirrer to rotate below the tip of the electrode.
Set the mode to pH and begin the titration. Record pH and mL of titrant added.
Watch for the region where the pH begins to change rapidly with each added portion of titrant. As the pH begins to change more rapidly, add the titrant in smaller portions.
When you have passed the equivalence point by several mL, there is no reason to continue any further in the titration.

11. pK determination by Gran plot