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

Polyprotic acids and bases

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

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!

Glass electrodes E = constant – (0.05916) pH at 25oC

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

pH standard buffers

Finding the End Point with a pH Electrode-example

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.

pK determination by Gran plot